************************************************************ FIRST 1997 INTERNET EDITION OF 10^9 YEARS: A GUIDE TO THE GEOLOGY OF WESTCHESTER COUNTY, NEW YORK Thomas McGuire (BA, MAT Geology) Briarcliff High School & The State University of New York at Purchase With a local Geologic History by Dr. Yngvar Isachsen, Senior Research Scientist, New York State Geological Survey First Edition, 1991 Special Publication #10 Rochester Mineralogical Symposium Rochester, New York First Edition, 1991 Special Publication #10 Rochester Mineralogical Symposium Rochester, New York Internet Edition of 01/97 ************************************************************ Dedication: This book is dedicated to those who will use it, out of their love of geology, science, nature, education, or, just for fun. ************************************************************ Cat. # 557.47m, QE145 ************************************************************ Original produced on an Apple Macintosh SE, using WriteNow and SuperPaint, and printed at 300 dpi on a Hewlett -Packard DeskWriter. ************************************************************ GUIDE TO THE INTERNET EDITION This form of transmission/preservation is one that will be readily available to anyone with Internet access, regardless of their computer brand/platform. Most of the formatting and type styles are lost in transformation. (Unfortunately, Cinderella had to change back to a pumpkin.) Important breaks in the text are shown by lines of asterixis. ( ****...) All mileage references have been omitted from the field trip logs, although the directions used in conjunction with the strip maps (available from the author) or a local highway map should permit you to find these locations without difficulty. If you would like copies of any of the figures/maps/diagrams, please send your request to Thomas McGuire, 14 Whittier Court, Yorktown heights, NY 10598. You must include a self addressed large envelope and a first class stamp for each 4 sheets requested. The author will supply photocopies. (There are a total of 42 pages of illustrations.) If you choose to print this document, you may wish to look at a paper copy of this book for the suggested formatting. (They are available through the Westchester County Library System.) You are also welcome to photocopy diagrams from these copies of the book. Titles printed in enlarged or bold type in the original are here represented in UPPER CASE. I would appreciate your help in identifying errors in typing or in factual material. To communicate with the author, send e-mail to < tmcguire@Computer.Net > or to my mailing address two paragraphs above. If you have Internet access, try the New York Geology Resource Page: http://133.30.8.1.8080/=@=:nethomes.com/newyorker/ This site has impressive color graphics with links to professional organizations as well as to geologists. Check out the Hudson Highlands web page and other links available from the URL above. ************************************************************ PREFACE TO THE 1991 TEXT: Geology is sometimes called a Òbackward scienceÓ. This characterization is not a reference to primitive methods, but a realization of the nature of investigations in science. A successful field geologist must be a good detective. In many other domains of science, practitioners perform experiments in order to discover the results. But in geology, we find ourselves examining the results of millions of years of changes, with the goal of reconstructing the events that produced the features we are observing. We start with the Òend productsÓ and we attempt to infer the processes that created them. The Westchester region has a rich natural history and remarkable geologic features. My object in writing this publication has been to relate modern interpretations of the geologic past with the various forms of local evidence that support these hypotheses. The core of this publication is the field trip section in Chapter 6. The essence of modern geology is a synthesis of past events based upon observations in the field, scientific inquiry, laboratory analysis, and applications of scientific principles. This publication is written both for readers with an academic background in geology, and for the general public. Our local geology is remarkably complex and many details are the subject of ongoing professional debate. My own experiences lead me to see this contention as an exciting window into the work of professional scientists, rather than rhetoric that obscures an inaccessible and remote problem. While I have tried to make my writing clear without undue use of jargon, some geological terms that you encounter may require explanation. You will find many of these terms in the glossary section. The general reader may also need a textbook of physical geology and a reference about rocks and minerals. (See the Suggested Readings section.) This guide is intended for those who like to do science more than for those who like to read science.The author would be more honored by knowing of well used, tattered and annotated volumes, than knowing of copies preserved in pristine condition (except, of course, for library or archival copies). This publication is to be used and enjoyed. Science is not static. It is likely that some of the ideas presented in this book will become obsolete in the future. It is the role of a good scientist to give full consideration to alternative and new interpretations as they evolve. In that vein, the author welcomes comments and suggestions from readers. ************************************************************ ACKNOWLEDGEMENTS Thanks are extended to the many individuals who helped me in producing this publication. Special credit is due to Steven C. Chamberlain of The Rochester Mineralogical Symposium for his role in coordinating professional reviews and editing, his contribution of the mineralogy chapter, and his support for publication. I thank Yngvar Isachsen of the New York State Geological Survey for permission to use his writing about the local geologic history, which I adapted from his State Museum pamphlet about the geology of the New York metropolitan region. Pamela Chase Brock of Queens College has shared her recent research and her insights into the geologic history of the north-eastern Westchester region. Charles Merguerian of Hofstra University and Robert Tracy of the Virginia Polytechnic Institute have sent me their recent field publications. Paul Steinek of SUNY Purchase helped with the Highland Mills fossil trip. Stephen Maslansky of Geo-Environmental, Inc. of White Plains provided support and information with the economic geology and the geology of southern Westchester. Nancye Dawers and Leonardo Seeber of the Lamont-Doherty Geological Observatory shared their work on the Dobbs Ferry fault zone and helped with the seismology chapter. Jerome Thaler of Mercy College contributed editorial comments. I also thank the many other people who responded to my inquiries for information. Credit for many of the insights in this publication belong to these people. The errors, however, are my own. No listing of credits would be complete without acknowledgement of the professional and educational contributions of the late Professor Leo Hall of the University of Massachusetts at Amherst. Dr. HallÕs pioneering structural work in Westchester and his professional and personal dedication were an inspiration to all who knew him. I also offer my gratitude to Elaine, Kristen and Erin, who learned what a word processor is: ÒIt's Òwhere you look first when you want to find Daddy!Ó Thomas McGuire, Briarcliff High School, Briarcliff Manor, NY 10510 tmcguire@computer.net ************************************************************ Figure I-1. Physiographic Diagram of the New York Region (Raisz, 1930) ************************************************************ ************************************************************C O N T E N T S: ************************************************************PREFACE ACKNOWLEDGMENTS LIST OF FIGURES The Geographic and Geologic Setting of Westchester County, New York CHAPTER 1: The Geologic History of Westchester (by Dr. Yngvar Isachsen) CHAPTER 2: The Economic Geology of Westchester CHAPTER 3: Minerals of Westchester County (by Dr. Steven C. Chamberlain) CHAPTER 4: Rocks of the Westchester County Region CHAPTER 5: Rock and Mineral Localities In and Near Westchester County CHAPTER 6: Geologic Field Trips in the Westchester County Area Trip A: Bedrock Formations of the New York City Series and the Hudson Highlands Trip B: Evidence of Cameron's Line Trip C: Features of Glaciation in Westchester County Trip D: The Geology of Croton Point Trip E: Fossil Collecting, Highland Mills, Orange County Mini Trips/Family Trips Trip F: The Mianus River Gorge & Hobby Hill Quarry Trip G: Cranberry Lake and the Kensico Dam Trip H: Silver Lake, North White Plains Trip I: Larchmont Manor Park/Umbrella Point Trip J: The Croton and Catskill Aqueducts Trip K: Geological Faults In and Near Westchester Trip L: George's Island, Montrose Trip M: Lenoir Nature Preserve, Yonkers Trip N: Marshlands Conservancy, Rye Other Nearby Localities Of Geologic Interest CHAPTER 7: Earthquake Activity in the Westchester County Area CHAPTER 8: Radon in the Westchester Area (A Preliminary Report) APPENDIX A: The Kitchawan Mammoth APPENDIX B: Suggested Readings REFERENCES GLOSSARY ************************************************************ LIST OF FIGURES Physiographic Diagram of the New York Region Erwin J. Raisz, Physiographic Diagram of the New York Region, The Geographical Press of Columbia University, Hammond Inc., 1930 The Geologic Time Scale with References to New York State Earth Science Reference Tables and Charts, NYS Education Dept.,1990 The Geologic Time Scale with References to Westchester, adapted by the author from the Earth Science Reference Tables, NYS Education Dept., 1990 Block Diagram of Southeastern New York, Yngvar Isachsen (after Raisz) Continental Collisions and Ancient Volcanoes, NYS Museum, 1980. Geological Map of the Westchester Area, Yngvar Isachsen, Continental Collisions and Ancient Volcanoes, NYS Museum, 1980. Floor of the Spreading Atlantic Ocean, Yngvar Isachsen, Continental Collisions and Ancient Volcanoes, NYS Museum, 1980. Cross sectional diagrams showing the Tectonic History of Westchester, Yngvar Isachsen, Continental Collisions and Ancient Volcanoes, NYS Museum, 1980. Timetable of Major Geologic Events, Yngvar Isachsen, Continental Collisions and Ancient Volcanoes, NYS Museum, 1980. Map: Generalized Bedrock Geology of New York State Earth Science Reference Tables and Charts, NYS Education Dept., 1990 A Geologic Map of the Manhattan Prong Leo M. Hall, Bedrock Geology in the Vicinity of White Plains, NY Guidebook of the 40th Annual Meeting of the New York State Geological Association, 1968 A Schematic Diagram of the Rock Cycle, adapted from Robert J. Foster, Physical Geology, Charles E. Merrill Publishing Co., 1971. Identification of Common Sedimentary Rocks Earth Science Reference Tables and Charts, NYS Education Dept., 1990 Texture and Mineralogy of Common Igneous Rocks Earth Science Reference Tables and Charts, NYS Education Dept., 1990 Texture and Mineralogy of Common Metamorphic Rocks Robert J. Foster, Physical Geology, Charles E. Merrill Publishing Co., 1971 Origins of Common Metamorphic Rocks, adapted from Robert J. Foster, Physical Geology, Charles E. Merrill Publishing Co., 1971 Map: Rock and Mineral Localities In and Around Westchester County, adapted from New York State South (map), New York State Department of Transportation, 1974 Road Map of Westchester and Nearby Localities adapted from New York State South (map), New York State Department of Transportation, 1974 Map: Trip A, Rock Formations of the New York City Series and Hudson Highlands, by the author Diagramtic Illustration of the Interpreted Stratigraphic History of the Manhattan Prong, Leo M. Hall, Studies of Appalachian Geology; Northern and Maritime, E-an Zen et al, 1968 Geologic Map of the Cortlandt Complex, Robert Tracy, Nicholas Ratcliffe, John Bender, Igneous And Contact Metamorphic , Rocks Of The Cortlandt Complex, Westchester County, New York, Geological Society of America Centennial Field Guide, Northeastern Section, 1987 Stratigraphic Correlations in Westchester and Southern New England, Charles Merguerian, Geological Society of America - Northeastern Section, 1987 Tectonic Origin of the Hudson Valley & Hudson Highlands, Arthur N. Strahler, The Earth Sciences, Harper & Row, 1963 Map: Relict Structures of the Evolution of the Northern Appalachians, Walter Sullivan, Landprints, Times Books, (Adapted from Peter Robinson and Leo Hall in The Calidonides in the USA), 1984 Map: Trip B, A Look at Cameron's Line, by the author Generalized Geologic Map of a Part of Pelham Bay Park in the Bronx, Carl K. Seyfert & David J. Leveson, Structure and Petrology of Pelham Bay Park, New York State Geological Association Field Guide, 1968 The Bowen Reaction Series, by the author Map: Trip C, Features of Glaciation, by the author A Map of Croton Point Showing the Sand Hills Known as Islands, R. G. Markl, Pleistocene Geology of Croton Point, NY, Transactions New York Academy of Sciences, Submitted: 1/7/71, Croton Point Before Landfill Activities, Cecil H. Kindle, Pleistocene Geology of Croton Point, New York State Geological Association Field Guide, 1958 Hypothetical Course of the Hudson River in Schooley Time, Douglas W. Johnson, Stream Capture Along the Atlantic Slope, Columbia Univ. Press, 1931 Some Invertebrate Fossils of New York, Adapted from J. G. Broughton et al, Geology of New York State, New York State Museum & Science Service, 1962 Map: Other Field Trips In and Around Westchester, adapted from New York State South (map), New York State Department of Transportation, 1974 Map of Cranberry Lake Park, Westchester County Department of Parks, Recreation and Conservation A Geologic Map of the area around Silver Lake, Leo M. Hall, Bedrock Geology in the Vicinity of White Plains, NY New York State Geological Association Field Guide, 1968 Map: Major Earthquakes in the United States in Recent History, United States Coast and Geodetic Survey, Department of Commerce Map: Epicenters and Generalized Geologic Structures in the Manhattan Prong, L. Seeber & N. H. Dawers (Lamont-Doherty Geological Laboratories of Columbia University), in press, Characterization of an Interplate Seismogenic Fault in the Manhattan Prong, Westchester County, NY, Seismological Research Letters, Special Issue, Seismogenesis in the Eastern United States, Used by permission of the authors, 1988 Likely Sources of Radon Contamination in the Home New York State Department of Health, Paths of Nuclear Decay New York State Power Authority, Location of Mastodon and Mammoth Remains in New York State, Judith Drumm, Mastodons and Mammoths, Educational Leaflet # 13, NYS Museum & Science Service, 1963 Wooly Mammoths of Post-Glacial New York, Simon Schaffel, New York City and the Ice Age, Naho, New York State Museum, Spring-Summer issue of 1981 Typical Bog Stratigraphy in New York, Judith Drumm, Mastodons And Mammoths, Educational Leaflet # 13, NYS Museum & Science Service, 1963 ************************************************************ THE GEOGRAPHIC AND GEOLOGIC SETTING OF WESTCHESTER COUNTY, NEW YORK The rolling topography of Westchester is a southwestern extension of the crystalline highlands of western New England. Complex structures, extensive erosion, and glacial features make the landscape difficult to classify as plains, plateau or mountains. It is best characterized as a very mature landscape of ancient mountain roots. Elevations range from sea level along Long Island Sound and the Hudson River, to maximum elevations nearly 900 feet above sea level in the northern parts of the county. Westchester lies within the Manhattan and Reading Prongs of the New England Highlands. The land area of Westchester is about 450 square miles. Putnam County, to the north, lies within the Hudson Highlands, an upthrust fault block of ancient PreCambrian high grade metamorphic rocks. Westchester is bounded on the west by the Hudson River. Across the Hudson in Rockland County is the Triassic Newark fault basin, which collected sediment and igneous rock (including the Palisades intrusion) in the Mesozoic era. To the south are the Bronx and Manhattan, another part of the same geologic and physiographic province as Westchester. Long Island Sound borders Westchester on the southeast and separates it from Long Island. On the east, the hills of Westchester blend into the metamorphic terrain of western Connecticut. Most of the local rock units have been dated with radiometric techniques as late PreCambrian and early Palaeozoic ages. The rare fossil finds in local rock units, and correlations with adjacent regions of lower metamorphism are consistent with this age determination. ************************************************************ Figure I-2. The Geologic Time Scale with References to New York State (New York State Education Department, 1990) ************************************************************ Figure I-3. The Geologic Time Scale with References to Westchester (adapted from the New York State Education Department, 1990) ************************************************************ CHAPTER 1: The Geologic History of Westchester County (Text contributed by Dr. Yngvar Isachsen, Senior Research Scientist, New York State Geological Survey, Albany) The geologic history of Westchester County is largely the geologic history of the Manhattan Prong, which extends from New England through Westchester to the southern tip of Manhattan. (See figures 1-1 and 1-2.) The landscape and rock types of this region are a result of complex geologic processes that began more than 1.3 billion years ago and continue today. Rocks exposed in Westchester record at least three episodes of mountain building and two major periods of volcanic activity. Millions of years of continuing erosion by water, wind and ice extensively eroded each mountain range, so that only the deep roots of these mountains now remain. The hills and hollows of Westchester are largely a result of the rocks that underlie them. The higher ground is composed of the Fordham Gneiss and the Manhattan Schist, both highly resistant to erosion. Inwood marble underlies many of the valleys, now occupied by our small rivers including much of the Croton, Bronx and Saw Mill rivers. These valleys made excellent locations for the dams built in the last century to impound fresh water for the growing city of New York. Even the island of Manhattan owes its isolation to the low resistance of the Inwood Marble. The oldest rock unit in the Manhattan Prong is the Fordham Gneiss, named after its type locality in the Fordham Heights section of the Bronx. The determination of the age of this formation is a complex problem. Fossils are rare in PreCambrian rocks. Additionally, the oldest rocks of our region, which could have originally contained fossils, have been intensely deformed and undergone mineral changes, as a result of heat and pressure deep within the earth. Therefore, our best methods of age determination rely upon the use of radioactive nuclides that decay at a measurable rate. Douglas Mose of George Mason University in Virginia has established a PreCambrian age of 1350 million years for the deposition of the ocean bottom sedimentary and volcanic rocks that were subsequently buried, heated and deformed to make the Fordham Gneiss. Further understanding of the geologic events that shaped this region require some knowledge of the basic concepts of plate tectonics. Recent investigations of the ocean floors and the continental crust have reshaped our ideas about the major processes that affect our planet. They have shown that the solid earth is literally a heat engine fueled by energy from the decay of radioactive elements within the earth. The moving parts of the engine are the structural units of the earth itself. The cross section of a plum makes a good model. Like the pit of the plum, the earth has a solid inner core composed mostly of metallic iron, which is surrounded by an outer core of molten iron. Like the edible part of the plum, the earth's mantle, composed of dense minerals rich in oxygen, silicon, iron and magnesium, is the thickest layer of the earth. Some of these rocks are exposed at the surface near Peekskill. ************************************************************ Figure 1-1. Block diagram of Southeastern New York showing its four physiographic provinces, (Isachsen [1980] after Raisz, XVI International Geological Conference, Guidebook 9, 1936) ************************************************************ Figure 1-2. Geologic Map of the Westchester Area. (Isachsen 1980) Note how the geological boundaries conform to with the landscape boundaries as shown in Figure 1.1 ************************************************************ The crust of the earth is like the skin of a plum. Our deepest mines and bore holes have only penetrated to a depth of about 16 kilometers (10 miles) within the earth's crust. The upper portion of the mantle, along with the crust, acts like a rigid unit which is known as the lithosphere. The lithosphere is roughly 100 km (60 miles.) thick. The lithosphere has been broken into about a dozen major plates and numerous smaller fragments by movements of the earth's surface. Most of the major plates underlie both oceanic and the less dense, more buoyant continental crust. Heat within the lower mantle (the aesthenosphere) allows it to flow like plastic at rates of as much as 15 cm (6 inches) per year. These motions are driven by huge convection currents within the mantle. These currents cause the plates to interact in three ways at their boundaries: (1) Plates may split and drift apart, allowing a new ocean to form. The Red Sea, between Egypt and Arabia, is a modern example of a very young, rifting ocean basin. A much older rift zone runs below the middle of the Atlantic Ocean from Iceland to Antarctica. (Figure 1-3) As this rift zone continues to open , the whole Atlantic Ocean is widening at a rate of about 2Ð3 cm (2 in.) per year, as North and South America move away from Europe and Africa. This separation of the plates is responsible for the numerous volcanoes, earthquakes and hot springs of Iceland and other mid-Atlantic islands. In fact, a whole underwater mountain chain extends along the Mid-Atlantic Ridge. The Icelanders make good use of their natural resources by using the steam and hot water from the ground to heat a large portion of their homes, and to provide abundant electrical energy. (2) Collisions of the plates crumple and destroy the crust. The Pacific Plate is now descending under North and South America, adding oceanic terrain to the western edges of these continents. This subduction carries crustal rocks with their water content deep into the earth where they are heated and often melted. These lower density crustal magmas then return to the surface as violent volcanic eruptions, like to one that blew the top off Mt. St. Helens in 1980. The water content in these magmas makes the eruptions of these particular volcanoes very explosive and destructive. When one oceanic plate descends beneath another, a similar process occurs. The Aleutian Islands and Japan were created in this kind of collision and subduction. The process may swallow up a whole ocean, leading to a collision of continents. The Himalayan Mountains were produced when the Indian subcontinent drifted north and collided with Asia. The contorted folds of the Himalayan rocks and the oceanic crust found in the Indus Valley north of the mountains record the violence of this event. Continued collision is causing a thickening of the crust under the high Tibetan Plateau. (3) Elsewhere, plates grind past one another in Òtransform faultsÓ. A prime example is the San Andreas Fault in California. The Pacific Plate is rotating counterclockwise, carrying the western edge of California northward toward Alaska. Stress builds along the faults of California, and then suddenly releases as the rock moves along fractures. This sudden movements causes earthquakes, such as the great San Francisco earthquake of 1906. These are the global processes that have shaped our land and continue to influence the landscape of Westchester. ************************************************************ Figure 1-3. Floor of the spreading Atlantic Ocean. (Isachsen 1980) ************************************************************ About 550 million years ago an early ocean, the ÒProto-AtlanticÓ, began to open as a great continent split apart. As the ocean widened, the edge of the land mass to the west, now a part of North America, was flooded by a shallow tropical sea. (At that time what we now call Westchester was located near, perhaps even south of, the equator.) Scattered patches of beach sand were deposited on the rock basement. These were overlain by shell beds and coral reefs in a process similar to the present day formation of the Bahamas. Today, we see the remains of these coral reefs and warm water shell deposits as the Inwood Marble and the lower portion of the Manhattan Formation. Nicholas Ratcliffe of the United States Geological Survey reported pelmatozoan fossil remains near Verplanck in 1969, which have been helpful in the determining the geologic age of these rock units. Yngvar Isachsen observed deformed stromatolite remains in the north eastern corner of Westchester in the course of a Geological Association field trip in 1989. Unfortunately, unlike the pelmatozoans, the stromatolites make poor index fossils due to a lack of documentation of structural changes through geologic time. By Middle Ordovician time the ocean had begun to close (Figure 1-4). The eastern margin of the ancestral North America began to slide under the ancestral Africa, causing an offshore arc of volcanic islands. Erosion of these islands contributed their own distinctive sediments, black muds rich in iron and magnesium, to the Ordovician sea. Continued closing of the ocean caused these sediments to stack up above the continental shelf carbonates in huge overlapping slices. These slices are known today as the Taconic Allochthon. They produced the Taconic Mountains which once stretched from Newfoundland to Alabama. Depositional ages established in studies outside the Westchester area have been very helpful in determining the ages of local rock units. In some cases, the time sequence of rock units, folding, and faulting, established in western New England, have been extrapolated into Westchester. Today, a few highly altered remnants of the crust of the Proto-Atlantic can be seen as pods and slivers composed of the light green mineral with a greasy luster, serpentine (Figure 1-5). One prominent mass of this rock forms the backbone of Staten Island. Other pods occur in Manhattan, near New Rochelle, and in Port Chester. These unusual rocks help geologists to locate the weld between the crustal plates. That suture is known today as ÒCameron's LineÓ (The dark line near the right side of Figure 1-2). The Harrison Diorite Gneiss bodies appear to be parts of the root zone, or lava source of these offshore Ordovician eruptions. During the later stages of the Taconic Orogeny (435 million years ago) molten rock intruded at depth near Peekskill and Croton Falls to form a series of (mostly) very dark, dense igneous rocks. The rock types in these deep intrusions include gabbro, norite, pyroxenite, peridotite and dunite. Some screens of the Manhattan Schist that fell into the molten mass were converted by heat and chemical changes to a very hard and useful rock known as emery. Erosion of the Taconic Mountains to the east deposited great thicknesses of mud, sand and gravel from the Ordovician through the middle of the Devonian period (figure 1-5). In its westward motion, the ancestral continent of Africa smashed against the island arc that had earlier been welded to North America (Figure 1-6). With this collision, the Proto-Atlantic became history and the continents were welded to form a great super continent known today as Pangaea. ************************************************************ Figure 1-4 Reconstructed cross section for the beginning of Middle Ordovician time, showing North American and an offshore volcanic chain on a collision course. The Westchester area is shown by the dashed box. (Isachsen 1980) ************************************************************ Figure 1-5. Reconstruction of the conditions prevailing after the collision which caused the Taconic Mountain building event. The relict piece of oceanic crust along the weld represents the serpentinites of Port Chester and other nearby localities. Note the approaching ancestral African Plate. The dashed boxes show the Westchester area. (Isachsen 1980) ************************************************************ This collision buckled the rocks once again causing the Acadian episode of mountain building. Buckling of the earthÕs crust thrust the earlier sediments into the earth to a depth of about 15 kilometers (9 miles) where temperatures of about 600¡C and pressures of millions of atmospheres changed the sandstone into the Lowerre Quartzite, the limestones into the Inwood Marble, and mixed shales and sandstones into the Manhattan Schist. Concurrently, the underlying Fordham Gneiss was once again deformed and recrystallized. The entire rock package was squeezed into a sweeping pattern of tight folds known today as the Manhattan Prong. Erosion has removed the ancestral Acadian Mountains, and, in this area, the Silurian and Devonian sedimentary rocks as well. The subsequent Carboniferous Period is represented only by a very small occurrence of black shale, rich in plant fossils. It was discovered during excavations for a building near Pelham Bay. This site appears to be a tiny remnant of a coal basin, analogous to the Naragansett Basin in Rhode Island. Additional folding of these older rocks continued during the Carboniferous and Permian Periods. Then, renewed erosion reduced the landscape to a lower relief. The present Appalachian Mountains are the result of a more recent episode of uplift. Discovering these events of the past has been a slow process. It has involved the work of dozens of highly prominent laboratory and field geologists. Their theories have undergone evolution and modification over the past century. However, predicting future events is even more precarious. Clearly, the Atlantic Ocean will continue to widen as North America slides away from Africa. Erosion will continue to wear down the hills and deposit sediments in the oceans. It is even possible that the build up of weight on the crust may cause a wedge of the ocean to sink beneath the East Coast, initiating yet another episode of mountain building. The only certainty is that change will occur as the dynamic forces, on and within our planet, continue to shape an ever evolving landscape. ************************************************************ Figure 1-6. Reconstruction of events at the end of the Devonian Period, when ancestral Africa collided with North America to close the Proto-Atlantic Ocean and build the Acadian Mountains. The dashed box shows the Westchester region. (Isachsen 1980) ************************************************************ Figure 1-7. Splitting of the welded Òsuper-continentÓ produces the juvenile Atlantic Ocean, which has continued to expand since its formation 200 million years ago. (Isachsen 1980) ************************************************************ Figure 1-8. Timetable of Major Geologic Events. ************************************************************ Figure 1-9. Generalized Geologic Map of New York State. ************************************************************ Figure 1-10. A Geologic Map of the Reading Prong in New York. (Based upon field mapping by Hall, Ratcliffe and personnel of the New York State Geological Survey.) ************************************************************ CHAPTER 2: The Economic Geology Of Westchester County The local geology has played an important part in the economy of Westchester, both indirectly, as it has shaped our landscape, and directly in terms of geological deposits of economic importance. With a world center of industry and commerce at our southern flanks, we too easily forget that Westchester has its own considerable economic base, including an unusual variety of rocks and minerals of economic importance. Pegmatite The Bedford pegmatites have been well known for centuries. The Hobby Hill Quarry, on the banks of the Mianus River, was a source of minerals in colonial times. Eight Bedford pegmatite quarries were commercially excavated from the 1878 until 1949 (Tan, 1966). The Baylis and Kinkel quarries became particularly popular among mineral collectors because of the approximately 40Ð50 mineral species identified there. Feldspar crystals up to five feet in length have been found and large books of mica as much as a foot across have also been recovered. (Both quarries have recently become inaccessible due to residential development.) The primary product of mining in the early part of this century was a very high quality of feldspar that was used in the production of ceramics and glass. Milky, smoky and rose quartz were taken from the cores of these quarries, particularly the Baylis quarry. It is reported that rose quartz was shipped from Westchester to China, where it was carved by artisans, and returned to the United States. These carvings were then sold to American consumers as authentic Chinese figurines. Granite Two Peekskill Granite quarries are located near US Route 202 between Yorktown and Peekskill. The southern quarry was used in the construction of the Croton Dam, which was completed in 1905. Another quarry, north of Route 202, has supplied the golden granite of the Peekskill Savings Bank in Peekskill, the facing stone of the approaches to the George Washington Bridge, and the facing of the Cathedral of St. John the Divine, along Amsterdam Avenue near the north end of Central Park in Manhattan. This cathedral has been in construction for about a century and it is the largest Gothic cathedral in the world. Until about 1940, quarrying was actually the largest industry in Yorktown (Morril, 1981). Two other Westchester ÒgraniteÓ quarries are actually located in the Yonkers gneiss. Both supplied an attractive crystalline rock for building and ornamental purposes. The Lake Street Granite quarry in Harrison (near Silver Lake) may be changing to a landscape and garden center after 65 years of quarry operation. The DiRienzo Brothers Stone quarry on Fullerton Avenue in Yonkers is the only Westchester ÒgraniteÓ quarry still in operation. The owners of the Yonkers quarry are presently trying to sell their property (Retsky, 1988). Marble The Inwood marble outcrops throughout the County, but the County's earliest commercial marble quarries are in the town of Eastchester. Tuckahoe marble has been quarried since about 1820. Until 1850, it was the largest source of white marble in America. Inwood Marble was used on buildings throughout the East coast including the Lyndhurst estate in Tarrytown, the Brooklyn Borough Hall, and Federal Hall in Lower Manhattan. A plaque at the now closed Tuckahoe quarry on Fisher Avenue in Eastchester commemorates the importance of the Tuckahoe marbles. This is one of several quarries between the Bronx Parkway and Route 22. The Snowflake quarry in Thornwood operated until 1973. Crushed marble was used for terrazzo and stucco. A finer powder from this quarry was used in paint, soap, plastic and as a filler in asbestos products (Shoumatoff, 1979). (A shopping center now occupies the Thornwood quarry and the high marble and schist rock face is attractively exposed behind the plaza.) Convicts at the Sing Sing were recruited to cut the marble on the prison site, which was used to construct the prison buildings in the 1820s. Other marble quarries were located in Scarsdale, Verplanck, and Hastings-on-Hudson. Iron The Hudson Highlands supported a large number of iron mines that began production in the 1750s. The iron chains that were strung across the Hudson River to stop British ships at Stony Point, Bear Mountain and West Point, were forged from the local iron ores. Numerous small open pits can be found throughout Harriman State Park. Many of these were discovered by early surveyors. Their compasses sometimes failed to align with the earth's poles due to the localized magnetic fields around the magnetite deposits. An old mine near the summit of Manitoga Road, just east of the Bear Mountain Bridge, is notable for the occurrence of a number of interesting minerals including pyrite and radioactive ores. Former mining at this location produced iron sulfide, which was used in the production of sulfuric acid (Johnson, 1976). The Tilly Foster magnetite mine, near Carmel, was the largest of the local iron mines, producing about a million tons of iron ore in half a century of operation. It was opened in 1853. The ore body was a narrow wedge that dipped steeply into the earth. The mining company tried to excavate the ore body with tunnels, but the need to leave a large portion of ore as pillars and the failure of these supports eventually necessitated a large open pit operation. The mine was plagued by numerous disasters and loss of life resulting from collapses. The Tilly Foster mine operated until 1897, when a tragic rock fall killed about a dozen miners, and the mine was closed. The nearby Brewster Magnetite District includes four former iron mines between Brewster and Somers. The Croton Magnetic Iron Mine, north of Croton Falls, was the largest of these, producing about 100,000 tons of ore. Emery Among the few commercial emery quarries in the United States are two mines located in the town of Cortlandt. Emery is a mixture of corundum and magnetite. Among natural minerals, corundum is second only to diamonds in hardness. Therefore emery is widely used as an abrasive. The emery boards sold as finger nail files were formerly made from this mineral, although modern ceramics have largely replaced emery in this application. Emery from the Croton Avenue quarry was recently sold as an aggregate to be used in concrete for high impact industrial areas. (This, the DeLuca quarry, is featured in field trip A.) The DiRubbo quarry is located off Mt. Airy Road near Croton. Both mines have recently been purchased by a Westchester construction company for their high quality crushed stone to be used in highway construction. However, the local home owners, to preserve their quiet residential environment, have sustained zoning ordinances against renewed commercial mining. Clay High quality clay was excavated on Croton Point, where there were at least five brick factories. The depressions just south of Squaw Cove, in the central part of the point, are probably the remains of clay pits, where a layer of clay 1Ð3 meters thick was excavated for the brick works. The remains of bricks can be seen scattered along the Hudson shore at Squaw Cove, south of the main parking lot. Clay exposures can be observed in many locations along the shore of Croton Point, just above the normal high tide water line, from Squaw Cove southward to Teller's Point. The clay was probably deposited as rock flour in glacial lake Hudson while the the Hudson River was dammed by a terminal moraine at the Verranzano Narrows. Many of the exposures exhibit varves (thin seasonal layers of lighter and darker colored clays). The clay layers form an impervious barrier that prevents the infiltration of ground water and results in numerous springs that flow onto the beach. These high quality clays are still sought by local potters. Similar clay deposits have been reported near Ossining, Montrose and Peekskill (Markl, 1971). Sand and Gravel Abundant glacial outwash deposits (from the melt waters of the receding glaciers) have provided gravel for landfill, and permeable materials for road beds. Although they are common, sand and gravel deposits have been among the most important materials economically because they are so widely used. Many such deposits have been excavated in the stream valleys of Westchester. With escalating real estate values and a desire for aesthetic restoration of the land surface, most of the old sand and gravel pits have been covered and/or used for real estate development. Croton Point was extensively excavated for sand and gravel. Some of it was used to cover refuse in the extensive sanitary landfill areas on the point. Today, sand and gravel for highways and other construction projects are more often transported from upstate sources. ************************************************************ CHAPTER 3: Minerals Of Westchester County (Text contributed by Dr. Steven C. Chamberlain, Professor and Chairman, Department of Bioengineering, Syracuse University, Syracuse) Introduction The mines, quarries, and other excavations of Westchester County have produced an impressive range of minerals of interest to amateur and professional geologists. Several of the localities are among the most notable mineral sources in New York State. Some of them are represented in the collections of major museums throughout the United States. Other localities are less well known, but they are important because of the rarity or quality of the minerals they have produced. Many of the mineral localities in the county have supplied commercially important products such as ores of metals or crushed stone for construction. Although some of the locations listed below are no longer accessible for collecting, or even observing, documenting these sites is important becauseÊsimilar occurrences could be revealed in future road cuts or excavations. The Most Common Minerals In Westchester County This list is provided to help the reader identify the most common minerals in the local rocks. The information below has been simplified to provide a quick guide for readers without a working background in mineralogy. Both photographs and more comprehensive information can be found in a field guide to minerals, such as Frederick H. PoughÕs Peterson Field Guide to Rocks and Minerals. FELDSPAR: (Al, Ca, Na Silicates) This is the most common family of minerals in the local rocks. Two varieties are commonly recognized: plagioclase and potassium feldspar. Potassium feldspar is white to pink. Plagioclase is commonly white to green. Both are opaque to translucent, and relatively hard, with cleavage at nearly right angles. Plagioclase shows striations on some cleavage surfaces. QUARTZ: (SiO2) Quartz is usually clear to milky, but it can also be gray, pink, or almost any color. Fresh faces show a glassy luster and it usually splits along curved (concoidal) surfaces. Quartz weathers slowly and often forms rounded pebbles. MUSCOVITE: (Hydrated K, Al Silicate) The mica minerals readily split into thin, flexible, transparent sheets. In reflection muscovite mica may look silver. Local muscovite crystals are commonly 1/4 to 1 cm across, but much larger in pegmatites. BIOTITE: (Hydrated K, Mg, Fe Silicate) Biotite mica is found in thin, flexible, black sheets. Local crystals are commonly 1/4 to 1 cm across, but much larger in pegmatites. AMPHIBOLE: (Ca, Na, Mg, Fe, Al Silicates) The amphibole family includes mostly dark colored minerals that form crystals and fibers. A common example is hornblende which is usually dark green or black. Amphiboles have prismatic cleavage at an oblique angle. PYROXENE: (Ca, Mg, Fe Silicates) The pyroxenes are a family of dark colored minerals that form stubby crystals and granular masses. Augite is a common example. Pyroxenes display prismatic cleavage at nearly right angles. CALCITE: (CaCO3) Calcite is the most common mineral in marble and limestone. Calcite may form white or clear crystals that sometimes weather to a sugary powder. Susceptibility to chemical weathering can make outcrops relatively scarce. GARNET: (Ca, Mg, Fe, Mn Silicates) Commonly dark red with a glassy or waxy luster, garnet is the January birthstone and the official New York State mineral. (Quarries near Gore Mountain in the Adirondacks contain large pods of near-gem quality garnet. The Barton Mine at North Creek, NY, is open to the public.) Local pods of garnet are commonly less than 1 cm across. PYRITE: (FeS2) Pyrite is fairly common in local marbles as small, shiny metallic yellow crystals, which can be cubic in shape. Pyrite is popularly known as fool's gold. It weathers to a rusty residue. LOCAL MINERAL LOCALITIES Anthony's Nose - Pyrrhotite was mined for a number of years at Anthony's Nose, about a mile east of the Bear Mountain Bridge, south of Manitoga Road. TheÊore was used locally for the manufacture of sulphuric acid. Zodac (1933) collected a number of minerals from the dumps including albite, amphibole, apatite, aragonite, barite, biotite, calcite, chalcopyrite, copiapite, epidote, goethite, gypsum, hematite, langite, magnetite, melanterite, oligoclase, opal, orthoclase, pyroxene, pyrrhotite, quartz, serpentine, and titanite. The dumps are extensively weathered due to the decomposition of the sulfides. Other localities in the vicinity may be the source of some older specimens labelled ÒAnthony's NoseÓ. Of these, the mostÊinteresting specimens were excavated many years ago and consist of plates of interlocked tabular calcite crystals coated with drusy colorless quartz crystals. Bedford Pegmatites (Kinkel, Baylis, Speranza, Bullock, Hobby, McDonald, BŸresch, and Kelt Quarries) - PreCambrian pegmatites in the vicinity of the village of Bedford produced commercial quantities of feldspar, quartz, and mica and an enormous variety of mineral specimens for collectors. One particularly famous specimen of tourmaline was collected at the Kinkel Quarry in 1928 by James Manchester. When completely reassembled, the specimen consisted of 32 black prismatic crystals and weighed 19 kilograms. The specimen is now in the collection of the American Museum of Natural History. A large specimen of columbite weighing 5 kilograms was collected from the Baylis quarry and is now in the Vaux collection at the Philadelphia Academy of Sciences. Other noteworthy specimens from these pegmatites include asteriated rose quartz, smoky quartz crystals, trapezohedral almandine garnet crystals, and sharp crystals of microcline and albite. Davenport's Neck, New Rochelle - Serpentine and associated minerals, such as brucite, chromite, magnesite, and titanite, are exposed on the north shore of Davenport's Neck. Bright green spinel crystals were reported from this locality around 1900. Ossining - The prison quarry at Sing Sing exposed a number of interesting minerals in the marble, including graphite, pyrite, quartz, rutile, calcite, diopside (malacolite), dolomite, and tremolite. Of these, the white prisms of diopside and theÊcyclic twins of deep red rutile on white dolomite crystals are particularly prized as fine specimens. Rye & Port Chester - Serpentine and associated minerals, calcite, magnetite, microcline, olivine, spinel, tourmaline, and tremolite, are exposed about 1.6 kilometers north of the village of Rye. Sparta - The old copper mine at Sparta, about 1.6 kilometers south of Ossining produced aÊvariety of copper and lead minerals including galena, cerussite, pyromorphite, vanadinite, anglesite, vauquelinite, chalcopyrite, azurite, and malachite. The occurrence of vauquelinite was the first reported in the United States. Valhalla - The construction of the Kensico dam at Valhalla utilized rock from a quarry about 1.6 kilometers east of the dam. Pegmatites exposed during quarrying yielded the only significant crystals of the blue-green amazonite variety of microcline ever found in New York State. These microcline crystals reached a maximum size of about 20 cm, but the amazonite color was usually confined to the outer 5 to 6 mm of the crystals with the interior being grayish-green. These and associated peristerite (albite) yielded fine gemstones early in the 20th century. (See Field Trip G.) SPECIES LISTING (following nomenclature of Fleischer, 1987 and Roberts et al., 1990) albite (NaAlSi3O8): cleavelandite, Bedford quarries; peristerite, granite quarry at Valhalla. allanite (Ce) [(Ce,Ca,Y)2(Al,Fe+3)3(SiO4)3(OH)]: Bedford quarries; emery mines in Cortlandt Township; near Peekskill. amphibole group Anthony's Nose pyrrhotite mine; hornblende, Bedford quarries; Crugers; tremolite, quarry at Eastchester; hornblende, tremolite, Harrison; tremolite, marble quarry at Hastings; hornblende, Larchmont; actinolite, hornblende, tremolite, New Rochelle; tremolite, Sing Sing prison quarry, Ossining; Peekskill; tremolite, Rye-Port Chester district; actinolite, Todd mine, Cortlandt; hornblende, granite quarry at Valhalla; actinolite, south side of Verplanck Point; tremolite, West Farms; tremolite, excavations on aqueduct, 2.5 miles north of Yonkers. analcime [NaAlSi2O6.H2O]: excavations on aqueduct, 2.5 miles north of Yonkers. andalusite [Al2SiO5]: emery mines in Cortlandt Township. anglesite [PbSO4]: old copper mine at Sparta. apatite group Anthony's Nose pyrrhotite mine; Bedford quarries; Larchmont; Peekskill; West Farms; excavations on aqueduct, 2.5 miles north of Yonkers. autunite [Ca(UO2)2(PO4)2.10-12H2O]: Bedford quarries. azurite [Cu3(CO3)2(OH)2]: Orchard Hill shaft of the Delaware aqueduct; old copper mine at Sparta; White Plains. barite [BaSO4]: Anthony's Nose pyrrhotite mine; Shaft 5, New Croton aqueduct, Whitson. bertrandite [Be4Si2O7(OH)2]: Bedford quarries. beryl [Be3Al2Si6O18]: common, yellow, golden, aquamarine, Bedford quarries; Ossining; Shaft 5, New Croton aqueduct, Shaft 5, New Croton aqueduct, Whitson. biotite [K(Mg,Fe+2)3(Al,Fe+3)Si3O10(OH,F)2]: Bedford quarries; Crugers; Harrison; Larchmont; granite quarry at Valhalla. bismuthinite [Bi2S3]: Bedford quarries. brucite [Mg(OH)2]: Harrison; New Rochelle. calcite [CaCO3]: Anthony's Nose pyrrhotite mine; Crugers; New Rochelle; Sing Sing prison quarry, Ossining; Veins 0.5 miles south of Sing Sing prison, Ossining; Rye-Port Chester district; old copper mine at Sparta; south side of Verplanck Point; Shaft 5, New Croton aqueduct, Whitson; excavations on aqueduct, 2.5 miles north of Yonkers. cerussite [PbCO3]: old copper mine at Sparta. chabazite [CaAl2Si4O12.6H2O]: West Farms. chalcopyrite [CuFeS2]: Anthony's Nose pyrrhotite mine; quarry at Eastchester; old copper mine at Sparta; marble quarry at Tuckahoe. chlorite group: emery mines in Cortlandt Township; Harrison; clinochlore (ripidolite), Pleasantville; Rye-Port Chester district; clinochlore (ripidolite), marble quarry at Tuckahoe. chromite [Fe+2Cr2O4]: New Rochelle; Rye-Port Chester district. copiapite [Fe+2Fe+34(SO4)6(OH)2.20H2O]: Anthony's Nose pyrrhotite mine. cordierite [Mg2Al4Si5O18]: emery mines in Cortlandt Township. corundum [Al2O3]: Crugers; emery, emery mines in Cortlandt Township; emery, Peekskill. datolite [CaBSiO4(OH)]: in veins near Yonkers. dolomite [CaMg(CO3)2]: quarry at Eastchester; Sing Sing prison quarry, Ossining; Pleasantville; marble quarry at Tuckahoe. dumortierite [Al7(BO3)(SiO4)3O3]: granite quarry at Valhalla. epidote [Ca2(Al,Fe+3)3(SiO4)3(OH)]: Anthony's Nose pyrrhotite mine; Bedford quarries; Todd mine, Cortlandt; West Farms; excavations on aqueduct, 2.5 miles north of Yonkers. epistilbite [CaAl2Si6O16.5H2O]: Bedford quarries. feldspar group Harrison; Larchmont; granite quarry at Valhalla. ferrocolumbite [Fe+2Nb2O6]: Bedford quarries. fluorite [CaF2]: granite quarry at Valhalla. Pegmatites [PbS]: Sing Sing prison quarry, Ossining; old copper mine at Sparta. garnet group Anthony's Nose pyrrhotite mine; almandine, Bedford quarries; Crugers; Davenport's Neck; emery mines in Cortlandt Township; New Rochelle; Peekskill; marble quarry at Tuckahoe; granite quarry at Valhalla; south side of Verplanck Point; West Farms; White Plains; excavations on aqueduct, 2.5 miles north of Yonkers. goethite [Fe+3O(OH)]: Anthony's Nose pyrrhotite mine; Bedford quarries; Pleasantville. graphite [C]: Bedford quarries; Sing Sing prison quarry, Ossining; Peekskill; granite quarry at Valhalla. harmotome [(Ba,K)1-2(Si,Al)8O16.6H2O]: veins in gneiss at aqueduct excavations, Ossining; Shaft 5, New Croton aqueduct, Whitson. hercynite [Fe+2Al2O4]: Crugers; emery mines in Cortlandt Township. heulandite [(Na,Ca)2-3Al3(Al,Si)2Si13O36.12H2O]: veins in gneiss at aqueduct excavations, Ossining; West Farms; Shaft 5, New Croton aqueduct, Whitson. hšgbomite [(Mg,Fe+2)2(Al,Ti)5O10]: emery mines in Cortlandt Township. ilmenite [Fe+2TiO3]: menaccanite, washingtonite, Bedford quarries. kaolinite [Al2Si2O5.(OH)4]: Bedford quarries. kyanite [Al2SiO6]: Crugers; emery mines in Cortlandt Township; Golden's Bridge; Peekskill. langite [Cu4(SO4)(OH)6.2H2O]: Anthony's Nose pyrrhotite mine. magnesite [MgCO3]: New Rochelle; Rye-Port Chester district. magnetite [Fe+2Fe+32O4]: Anthony's Nose pyrrhotite mine; Bedford quarries; Crugers; emery mines in Cortlandt Township; Larchmont; Peekskill; Pleasantville; Rye-Port Chester district; Todd mine, Cortlandt; Yorktown. malachite [Cu2(CO3)(OH)2]: old copper mine at Sparta; White Plains. melanterite [Fe+2SO4.7H2O]: Anthony's Nose pyrrhotite mine. mica group New Rochelle. microcline [KAlSi3O8]: Bedford quarries; Rye-Port Chester district; amazonstone, granite quarry at Valhalla; White Plains. molybdenite [MoS2]: Anthony's Nose pyrrhotite mine; Peekskill. muscovite [KAl2(Si3Al)O10(OH,F)2]: Bedford quarries; Crugers; Pleasantville; granite quarry at Valhalla; White Plains; excavations on aqueduct, 2.5 miles north of Yonkers. natrolite [Na2Al2Si3O10.2H2O]: Anthony's Nose. oligoclase [Na,Ca)Al(Al,Si)Si2O8]: Anthony's Nose pyrrhotite mine; Bedford quarries; sunstone, Chappaqua; Crugers. olivine group emery mines in Cortlandt Township; chrysolite, Rye-Port Chester district; chrysolite, south side of Verplanck Point; chrysolite, Shaft 5, New Croton aqueduct, Whitson; olivine-hypersthene condrite meteorite fell at Yorktown in September, 1869. opal [SiO2.nH2O]: hyalite, Bedford quarries; hyalite, White Plains. orthoclase [KAlSi3O8]: Bedford quarries; Larchmont. pectolite [NaCa2Si3O8(OH)]: veins in gneiss at aqueduct excavations, Ossining; Shaft 5, New Croton aqueduct, Whitson. phlogopite [KMg3Si3AlO10(F,OH)2]: Pleasantville; marble quarry at Tuckahoe; south side of Verplanck Point; White Plains. phosphuranylite [Ca(UO2)3(PO4)2(OH)2.6H2O]: Bedford quarries. pyrite [FeS2]: Anthony's Nose pyrrhotite mine; Bedford quarries; quarry at Eastchester; Sing Sing prison quarry, Ossining; Pleasantville; old copper mine at Sparta; Todd mine, Cortlandt; marble quarry at Tuckahoe; Shaft 5, New Croton aqueduct, Whitson; excavations on aqueduct, 2.5 miles north of Yonkers. pyrolusite [MnO2]: dendritic, Bedford quarries; Sing Sing prison quarry, Ossining. pyromorphite [Pb5(PO4)3Cl]: old copper mine at Sparta. pyroxene group Anthony's Nose pyrrhotite mine; Bedford quarries; hypersthene, emery mines in Cortlandt Township; Crugers; marble quarry at Hastings; enstatite (bronzite), New Rochelle; diopside (malacolite), Sing Sing prison quarry, Ossining; augite, hypersthene, Peekskill; diopside (malacolite), Pleasantville; south side of Verplanck Point. pyrrhotite [Fe1-xS]: Anthony's Nose pyrrhotite mine. quartz [SiO2]: Anthony's Nose pyrrhotite mine; asteriated rose, citrine, milky, rock crystal, smoky, Bedford quarries; Crugers; smoky, quarry at Eastchester; emery mines in Cortlandt Township; Harrison; Larchmont; chalcedony, rock crystal, New Rochelle; chalcedony, Sing Sing prison quarry, Ossining; smoky, granite quarry at Valhalla; south side of Verplanck Point; rose, rock crystal, White Plains; Shaft 5, New Croton aqueduct, Whitson. rutile [TiO2]: Bedford quarries; Sing Sing prison quarry, Ossining; veins in gneiss at aqueduct excavations, Ossining; Pleasantville; Shaft 5, New Croton aqueduct, Whitson. sapphirine [(Mg,Al)8(Al,Si)6O20]: emery mines in Cortlandt Township. serpentine group Davenport's Neck; Harrison; antigorite, bowenite, deweylite, New Rochelle; Sing Sing prison quarry, Ossining; antigorite, Rye-Port Chester district; south side of Verplanck Point. sillimanite [Al2SiO5]: fibrolite, Croton Lake; fibrolite, Crugers; emery mines in Cortlandt Township; Peekskill; fibrolite, Yorktown. silver [Ag]: old prospect near Sing Sing prison, Ossining. sphalerite [ZnS]: quarry at Eastchester; marble quarry at Tuckahoe. spinel [MgAl2O4]: pleonaste, Crugers; emery mines in Cortlandt Township; Peekskill; pleonaste, Rye-Port Chester district. staurolite [Fe+2,Mg,Zn)2Al9(Si,Al)4O22(OH)2]: Crugers; emery mines in Cortlandt Township; Peekskill; south side of Verplanck Point. stilbite [NaCa2Al5Si13O36.14H2O]: Anthony's Nose; Shafts 3 & 4, New Croton aqueduct, 4 miles southeast of Croton Landing; veins in gneiss at aqueduct excavations, Ossining; Peekskill; West Farms; Shaft 5, New Croton aqueduct, Whitson; excavations on aqueduct, 2.5 miles north of Yonkers. talc [Mg3Si4O10(OH)2]: Sing Sing prison quarry, Ossining. thomsonite [NaCa2Al5Si5O20.6H2O]: Peekskill; Piermont. titanite [CaTiSiO5]: Bedford quarries; Davenport's Neck; Larchmont; New Rochelle; Peekskill; West Farms. torbernite [Cu(UO2)2(PO4)2.8-12H2O]: Bedford quarries. tourmaline group schorl, elbaite, Bedford quarries; brown, quarry at Eastchester; emery mines in Cortlandt Township; Harrison; Peekskill; schorl, Rye-Port Chester district; yellow, Shaft 5, New Croton aqueduct, Whitson; excavations on aqueduct, 2.5 miles north of Yonkers. uraninite [UO2]: Bedford quarries. uranophane-beta [(H3O)2Ca(UO2)2(SiO4)2.3H2O]: Bedford quarries. vanadinite [Pb5(VO4)3Cl]: old copper mine at Sparta. vauquelinite [Pb2Cu(CrO4)(PO4)(OH)]: old copper mine at Sparta. wulfenite [PbMoO4]: old copper mine at Sparta. zircon [ZrSiO4]: cyrtolite, Bedford quarries. ************************************************************ BIBLIOGRAPHY OF THE MINERALS OF WESTCHESTER Agar, W. M. (1933) The pegmatites of Bedford, New York. 16th International Geological Congress 9:123-128. Beck, L. C. (1842) The Mineralogy of New York, White & Visscher, Albany, 536p. Berkey, C. P., and Rice, M. (1919) Geology of the West Point quadrangle. New York State Museum Bulletin p. 225-226. Black, D. (1948) Some minerals of Bedford, New York. Rocks and Minerals 23:710-712. Chase, P. J., and Brock, P. W. G. (1976) Sillimanite and sillimanite-orthoclase isograds in the Croton Falls quadrangle, Southeast New York. Geological Society of America, Northeast section, Abstracts 11th Annual Meeting 8. Dana, J. D. (1892) A System of Mineralogy 6th edition, Wiley, New York, 1134p. Fleischer, M. (1987) Glossary of Mineral Species 5th edition, the Mineralogical Record, Inc., Tucson, 227p. Fluhr, T. W. (1931) The malachite of White Plains, N.Y. Rocks and Minerals 6:54-55. Friedman, G. M. (1952) Sapphirine occurrence of Cortlandt, New York. American Mineralogist 37:244-249. Friedman, G. M. (1952) Study of hšgbomite. American Mineralogist 37:600-608. Friedman, G. M. (1956) The origin of spinel-emery deposits with particular reference to those of the Cortlandt Complex, New York. New York State Museum Bulletin 351. Kerr, P. F. (1935) U-Galena and uraninite in Bedford, New York, cyrtolite. American Mineralogist 20:443-450. Luquer, L. M. (1896) The minerals of the pegmatite veins at Bedford, N.Y. American Geologist 18:259-261. Manchester, J. G. (1931) The Minerals of New York City and Its Environs. Bulletin of the New York Mineralogical Club 3, 165p. Mason, B., and Wiik, H. B. (1960) The Tomhannock Creek, New York chondrite. Mineralogical Magazine 32:528-534. Philips, A. H. (1924) Thompsonite from Peekskill, New York. American Mineralogist 9:240-241. Pough, F. H. (1936) Bertrandite and epistilbite from Bedford, New York. American Mineralogist 21:264-265. Pough, F. H., (1976) A Field Guide to Rocks and Minerals: 4th Edition, Houghton Mifflin Company, Boston, 317p. Roberts, W. L., Campbell, T. J., Rapp, G. R., Jr., and Wilson, W. E. (1990) Encyclopedia of Minerals 2nd edition, Van Nostrand Reinhold, New York, 979p. Shand, S. J. (1942) Phase geology in the Cortlandt complex, New York. Geological Society of Americal Bulletin 53:409-428. Tan, L.-P. (1966) Major pegmatite deposits of New York State. New York State Museum Bulletin 408. Torrey, J. (1848) Discovery of vauquelinite, a rare ore of chromium, in the United States. Annals of the New York Academy of Science IV:76-79. Waite, E. (1940) New localities in Westchester County, N.Y. Rocks and Minerals 15:327-329. Weidhaas, E. (1959) The large Bedford tourmaline group. Rocks and Minerals 34: 390-392. Whitlock, H. P. (1903) New York Mineral Localities. New York State Museum Bulletin 70. Zodac, P. (1933) The Anthony's Nose pyrrhotite mine. Rocks and Minerals 8:61-76. ************************************************************ CHAPTER 4: Rocks of The Westchester County Region A BRIEF GUIDE TO ROCK IDENTIFICATIOn (Please note that it is important to use a fresh, unweathered surface when you attempt to identify rocks and minerals.) ************************************************************ Figure 4-1. Identification of common sedimentary rocks, (New York State Education Department, 1990) Sedimentary Rocks: Sedimentary rocks form at or near the surface of the earth.They are commonly composed of the remains of weathered rock that have been naturally cemented in layers. ************************************************************ Figure 4-2. Texture and Mineralogy of Common Igneous Rocks (New York State Education Department, 1990) ************************************************************ Figure 4-3. Texture and Mineralogy of Common Metamorphic Rocks. (New York State Education Department, 1990) ************************************************************ Figure 4-4. Origins of Common Metamorphic Rocks.Chapter 5 ************************************************************ CHAPTER 4: Rock and Mineral Localities In and Near Westchester County Three points should be made very emphatically. Many of these sites are on private land, or land to which access is restricted. In many places, rocks should be observed without collecting, as the collecting of samples may destroy a valuable natural resource. At each location, the tact and good judgement of the reader is essential. Unfortunately, as time passes, outcroppings become weathered or made unavailable by land development. At the same time, new sites become exposed by construction of roads and buildings. Updating this document is an ongoing activity and the help of the reader is requested. The author would appreciate hearing of new exposures. Please be careful. Watch your footing. Proper footwear is very important. Also, whenever a rock hammer is used, be sure to use goggles to protect your eyes and the eyes of those nearby. Flying rock chips are a very real danger to the person using the rock hammer, as well as to companions. 1. Croton Point: (County Park) This location is easily accessible and it contains an unusual variety of rock types; igneous, metamorphic and sedimentary.Visit this location at low tide. (Check the weather report section in a local newspaper for the time of low tide.) It is about 1 mile west of the Croton-Harmon railroad station. From Route 9 at the railroad station, follow the viaduct and the road west to the main parking lot. (In the summer, there is a parking fee.) Samples may be found along the shore either north or south of the parking lot. Some of the best samples can be seen along the beach 100 meters south of the large grassy picnic area. (See field trip D.)Samples that may be observed include good specimens of 2. Tilly Foster Mine: (Private land; may be posted) The Tilly Foster mine was a source of magnetite iron ore in the early part of this century. The rocks were extracted from a very deep pit along the east side of the entry road. This location is famous for a great variety of common and unusual minerals, including some displayed in the American Museum of Natural History in New York City. A variety of minerals can be found in the piles of spoils, mostly to the south of the mine pit. (Tilly Foster was named after a local farmer (Tillingham Foster) who was an early settler of the area.) The Tilly Foster mine is located near the south side of US Route 6 about half way between Brewster and Carmel in Putnam County. It is on a point in the Middle Branch Reservoir just east of the intersection of Route 6 and County Road 57. Samples of GNEISS, AMPHIBOLITE, MAGNETITE, CLINICHLORE CHLORITE (rust stained), and MILKY QUARTZ can be found here. A total of about 80 different minerals have been identified from the Tilly Foster Mine. 3. Baylis Quarry: (Private land.)This pegmatite body was mined primarily for high grade feldspar. Used in the manufacture of ceramics. large books of biotite and being developed) muscovite mica, and masses of pink rose quartz can be observed in the quarry walls. About 1/2 mile east of Bedford Village on the road to Greenwich, CT, turn left (east) onto Oliver Road. About 1/4 mile up this road a small road leads up to the right (south). The pegmatite quarry is about 75 meters up this road. (At the time of writing the quarry was closed to visitors because it was at the center of a residential development project. However, it is the understanding of the writer that the quarry pit is to be preserved.) A wide variety of minerals have been identified from this location, but the most common minerals include FELDSPAR (plagioclase and orthoclase), ************************************************************ Figure 5-1. Rock and Mineral Localities In and Around Westchester County. (The numbers key to the entries in the text.) ************************************************************ 4. Road Cut At Goldens Bridge:(Public access) Small pods of garnet are common in this exposure of the Manhattan formation. About 1/4 mile north of the intersection of Route 138, along New York Route 22, stop at a large exposure along the east side of Route 22. A car dealership is at the top of the outcrop.Samples of SCHIST, QUARTZ (milky) and small GARNET : pods are common here. 5. Peekskill Granite Quarries:(Private land) A quarry in the north side of Route 202 was used to ) supply some of the facing stone for the Cathedral of St. John the Divine in northern Manhattan. The quarry south of US 202 was used to mine the facing stones for the Croton Dam, about 5 miles to the south. A railroad led from the quarry down grade to the dam site. About half way between Peekskill and Yorktown Heights, one quarry is on the west side of a hill opposite Curry Chevrolet, about 1/4 mile north of Route 202. The other quarry is located about a mile to the west, along the north side of a hill 1/4 mile south of 202 and about 1/4 mile east of Croton Avenue. Attractive samples of gray and rusty GRANITE can be found at these quarries 6. Migmatite on I 684 near Mt. Kisco: (Public access) This road cut exposes an attractive pink granite pegmatite, gneiss and amphibolite. Park along I 684 southbound just before the exit at NY 172, or along New York Route 172 at the southbound exit from Interstate 684. (From route 172 only, walk about 1/4 mile north to the road cut.) See Field Trip B for a description. Good samples of GRANITE PEGMATITE, GNEISS, and AMPHIBOLITE can be found here. 7. Serpentinite Along I 287: (Public access)This exposure, under the Ridge Street bridge, is a rock type called serpentinite. It was formed from a mafic intrusion near the suture of two tectonic plates. (See Field Trip B.) Also, note the Origins of Common Metamorphic Rocks diagram. SERPENTINITE is a retrograde metamorphic rock formed by the slow cooling of a rock rich in minerals of iron and magnesium. ************************************************************ Figure 5-2. Westchester Road Map ************************************************************ CHAPTER 6: Geologic Field Trips in the Westchester County Area Each of the first five excursion features a particular phase of the local geology: bedrock types, plate tectonic features, glacial geology, landforms, structures, and fossil collecting. They have been organized so that a driver, with a person to navigate, can readily find each stop. All mileage figures are measured from the start of the trip to the bold type location. Trips AÐC include schematic road maps that will help the navigator. Trips E - M are less extensive in scope and travel time. Please observe the cautionary rules the the beginning of the Rock and Mineral Localities (Chapter 4). Although these excursions may be done at any time of the year, spring and autumn are particularly recommended, when the weather is favorable and there is a minimum of foliage to obscure viewing. ************************************************************ TRIP A: Bedrock Formations of the New York City Series and the Hudson Highlands Materials: Lower Hudson Sheet of the New York State Geologic Map, Rock Hammer, Hand Lense, Eye Protection ************************************************************ Stop A-1: Fordham Gneiss Start at NY Route 35, at a stop light 1/4 mile west of Interstate 684 (Katonah). Park at the rock cliff near the southwestern side of the intersection. The banded Fordham Gneiss is named for its type locality in the Fordham section of the Bronx, in New York City. This is the oldest member of the New York City Series. The Fordham formation formed from PreCambrian sediments and igneous rocks deposited about 1300 million (1.3 billion) years ago. Features: The readily visible minerals include Feldspar, Quartz, Biotite, and Hornblende Amphibole. Also note Banding, Intrusions, and Glacial Polish & Striations at the top of the outcrop. Proceed west on NY Route 35, about 6 miles to Yorktown Heights. Turn left (east) into the Triangle Shopping Center. (This parking lot is located about 100 yards northeast of the main intersection in Yorktown Heights.) Stop A-2: Inwood Marble. Park at the west end of the parking lot. The type locality of this formation can be found in New York City, in the Inwood section of Northern Manhattan. It was formed from Ordovician limestone and dolostone deposited about 500 million years ago. Outcrops are relatively rare due to the rapid chemical weathering of marble. The brown color is caused by the rusty weathering of accessory minerals. Features: Small dolomitic calcite crystals CaMg(CO3), Rapid weathering to sugary residue, Staining ************************************************************ Figure 6-1. Schematic map of Field Trip A: ************************************************************ Bedrock Formations of the New York City Series and the Hudson Highlands Continue 1/4 mile south along NY Route 118, and turn right at third stop light onto Underhill Avenue. 2 miles ahead, turn left onto the Taconic Parkway South. Drive 3 miles to the NY Route 134 exit. (Note the white pegmatite intrusion on the right side of the Parkway, about 1/4 mile before the exit.) Exit right at Route 134 and park along the exit ramp. Stop A-3: Manhattan Schist. Southbound exit of the Taconic State Parkway at NY 134. The Manhattan Schist formed from a variety of Ordovician sediments deposited about 450 million years ago. The shiny mica crystals and the thin foliation that they produce distinguish the Manhattan Schist from Fordham Gneiss, which you observed at Stop 1. Schist and gneiss have a common origin in sedimentary rocks, but schist represents a lower grade of metamorphic alteration. Features: Muscovite Foliation, Variable Composition, Pods of Garnet, Veins and Intrusions Turn around and take the Taconic Parkway northward, back to the Underhill Avenue exit. Turn left. Continue 1 mile down the hill to NY Route 129. Turn right. 2 miles ahead you will cross a long bridge. At the end of the bridge, turn right onto Croton Avenue. Then bear right at the three way intersection to stay on Croton Avenue. Stop at the piles of quarried trap rock on the right. The quarry on the left includes the contact between the Cortlandt Complex and the Manhattan Formation. The rock was used for fill within the Croton Dam. The next stop will allow you to observe the core of the most recent pluton. The Cortlandt Complex has a mineral composition similar to the earth's mantle. Stop at a bedrock cut and a small hill on the right just before a marsh on the left. An old roadbed leads 45¡ to the left. Walk along the roadbed about 1/2 km (1/4 mile). (Stay right at the ÒYÓ intersection) Stop at an underground cable right of way and rock cut on the hill to the left. Stop A-4: Cortlandt Ultramafics. (You are in the Town of Cortlandt.) This is rock primarily composed of the dark, iron and magnesium rich silicates, pyroxene and olivine. The Cortlandt Complex is a late Ordovician intrusion (about 440 million years ago) at a depth of 24Ð27 km within the earth. From geochemical studies, Kim Waldron and Robert Tracy of Yale University have estimated the average rate of Palaeozoic erosion prior to deposition in the Mesozoic era. They have calculated an average erosion rate 0.2 mm (0.007 inches) per year for 200 million years. ************************************************************ Figure 6-2. Diagramtic Illustration of the Interpreted Stratigraphic History of the Manhattan Prong (Hall, 1968) ************************************************************ Key to a Diagrammatic Illustration of the Interpreted ************************************************************ Stratigraphic History of the Manhattan Prong The late Professor Leo Hall's interpretation of the local rock layers designates the PreCambrian Fordham Gneiss as the local basement rock (Hall, 1985). Radiometric dating has established the age of the Fordham at about 1300 million years (Mose, 1982). Hall has split the Fordham into four mappable units, each distinguishable by its mineral content. In general, the Fordham is a highly resistant and well banded gneiss with intricate folds. In the late PreCambrian Grenvillian period of mountain building, the Fordham Gneiss was metamorphosed and, later, extensively eroded. The associated Pound Ridge Granite Gneiss and Yonkers Gneiss have been found to be about half of that age (Long, 1969). These later gneisses are probably metamorphosed intrusions or volcanic deposits. The Yonkers Gneiss generally shows mineral foliation, but less banding than the Fordham and it contains more pink feldspar. The (Ordovician) Lowerre Quartzite was deposited as an arkose sandstone which was extensively eroded before the deposition of limestone. That limestone survives as the Inwood Marbles and Dolostones. The Lowerre Formation is a tan or buff colored granofels quartzite and feldspathic schist that outcrops very sparsely. The Inwood Marble is highly variable in character, but it is commonly white, tan or gray with swirls of impurities (including small pyrite crystals). Leo Hall has established five sub-units of the Inwood Marble. It weathers to a friable sugary tan or gray sand. Pelmatozoan (echinoderm) fossils have been found by USGS geologist Nicholas Ratcliffe at Verplanck within either the Inwood or Manhattan formation. (It's hard to tell which formation it is at that location.) This find has supported the Ordovician age determination of these units. The Manhattan Schist is a highly variable formation of schist, gneiss and amphibolite units. It commonly shows fine mica foliation and it is more resistant to weathering than the Inwood Formation, but not quite as hard as the Fordham Gneiss. Hall has divided the Manhattan into three sub-units. This formation probably originated as a variety of sedimentary and igneous strata. Charles Merguerian has suggested that the upper two units of the Manhattan, where they are exposed in New York City, are actually complex east- over-west thrust slices where older rock layers have been pushed over younger layers (Merguerian, 1987). The Hartland Gneiss (see excursion B), south-east of Cameron's line, is considered to be equivalent in age to the Manhattan Schist, but of deeper ocean and volcanic island arc origin. It is separated from the bedrock of the New York City Series by a major thrust fault along Cameron's line. ************************************************************ Figure 6-3. Geologic Map of the Cortlandt Complex ************************************************************ 1/4 mile ahead, turn left to stay on Croton Avenue. 1 mile beyond, turn left onto Maple Avenue. Proceed west on Maple Avenue about a mile to the blinking light at Furnace Dock Road. Turn right and drive about 1 mile to another blinking light at Croton Avenue. Turn left. Continue north 1 mile to Croton Park Road on the right. Park along Croton Avenue. The most convenient way to park may be to turn around and park along the west side of Croton Avenue, just south of the Catskill Aqueduct .) (It is prudent to obtain the permission of the land owner before entering the next stop.) Walk 1/4 mile east along the Catskill Aqueduct, and then follow a path up the hill (south) 100 meters to the granite quarry. (The quarry is on posted land.) Stop A-5: Peekskill Granite. (Quarry near Croton Avenue & Route 202) The Peekskill Granite is a Devonian intrusion of about 350 million years ago. This quarry was used to supply facing stone for the Croton Dam. A small railroad followed a graded slope from the quarry, south to the dam. Another quarry, north of Route 202, was used for the facing stones of the Cathedral of St. John the Divine in uptown Manhattan. (See Chapter 2.) Features: Granitic Texture (no foliation), Light Color due to Felsic composition, Rusty weathering of iron rich minerals. The dominant minerals are Quartz, Feldspar, and Biotite. Proceed back south 1/2 mile along Croton Avenue to the road that leads to the Emery-Crete quarry . (The permission of the owner is required at this stop.) Proceed 100 yards to the mill. Stop A-6: Old Emery-Crete quarry. Crompond section of Cortlandt. Emery is a mixture of corundum (Al2O3) and magnetite (Fe3O4). (Ruby and sapphire are pure forms of corundum.) The emery formed when sections (screens) of the surrounding Manhattan schist fell into the still molten Cortlandt intrusion. This rock was used as an aggregate in concrete to make it wear resistant. The material was sold for the construction of industrial floors that have constant, abrasive traffic. Purer foreign emeries were used for emery paper and high quality abrasives, but even in these applications, emery has been replaced by ceramic abrasives. ************************************************************ Figure 6.4. Stratigraphic Correlations in Westchester and Southern New England ************************************************************ Proceed 1/2 mile north along Croton Avenue to US Route 202. Turn west (left). 100 yards ahead, bear right onto the Bear Mountain Parkway. 3 miles ahead turn right onto US Route 6 and across the Annsville Bridge. (This stream follows the Ramapo-Canopus fault zone. (You may be able to see the fault scarp as a ridge line across the Hudson River to the left, and its continuation along Canopus Creek, as it enters the Hudson Highlands to the right.) Continue across the traffic circle but stay on Route 6 north. (At this point you will enter the Hudson Highlands.) Follow Route 6 about 4 miles and turn left to cross the Bear Mountain Bridge. Go through the toll booths and, at the traffic circle, keep right onto US 9W and proceed 4 miles to the second Route 218 exit, north-west of Highland Falls. Exit right, and take two left turns to cross US Route 9W. Park in the road cut. Stop A-7: Complex Metamprphic Rocks. Deep rock cut at NY 218 and US 9W.PreCambrian gneiss and Paleozoic intrusions of Hudson Highlands. This gneiss probably formed from PreCambrian sediments of about 1100 million years ago. The dark colored intrusions are probably related to the intrusion of the Cortlandt Complex about 440 million years ago. Features: (Walking west from 9W along the southbound exit). Gray Gneiss intruded by Pegmatite. Basaltic Dike with chilled margins, and a gabbroic interior. The upper part shows intense and Spheroidal weathering, and, at the top, Residual Soil development. Note the steeply dipping Normal Fault with Drag Folds and Rotated Blocks and Slivers END OF TRIP A. ************************************************************ Figure 6-5. The Tectonic Origin of the Hudson Valley and Hudson Highlands (Strahler, 1963) ************************************************************ TRIP B: Evidence of Cameron's Line The purpose of this trip is to show evidence of a major thrust fault. The fault line was named after University of Wisconsin geologist, Eugene N. Cameron, who mapped a change in rock types in Western Connecticut in the 1950s. Most geologists now consider CameronÕs Line to be a weld between metamorphic rocks derived from continental shelf deposits to the west, and metamorphosed deep oceanic deposits and a volcanic island arc from the Proto-Atlantic to the east. The origin of our landscape includes millions of years of relentless geologic processes. The oldest rock unit in this region is the Fordham Gneiss with its associated Pound Ridge and Yonkers intrusions. The Fordham formation probably began as sedimentary deposits of sandstone, siltstone and shale at the borders of PreCambrian ÒNorth AmericaÓ about 1300 million years ago. (Actually, the continent was much smaller than it is today, and it was located south of the equator.) Marine life of that time, like algae, had no hard parts and no evidence of ancient life has been identified in these units. Following an extensive period of marine deposition, the deeply buried rock units were metamorphosed and intruded by granitic magmas, then uplifted in the Grenvillian orogeny approximately 1000 million years ago. After extensive erosion of the PreCambrian gneiss, limestone was deposited in the shallow tropical seas of the early Palaeozoic. With the advent of the Taconian episode of mountain building, shale, siltstone and sandstone were deposited over the limestone, and buried about 20 kilometers below the surface. These units were thereby changed into the Inwood and Manhattan marbles and schists. The upper units of the Manhattan Formation were moved into place as east-over-west thrust sheets. Extensive folding, burial and metamorphism happened while the ocean in which these sediments were deposited was closing up. A final stage of closure was the thrusting of an 8-10 kilometer thick slice of oceanic crust over the region about 450-440 million years ago. The fault surface is known as Cameron's Line. It has been mapped from Manhattan through the Bronx and south-western Westchester, where it enters Connecticut through the towns of Rye and Pound Ridge. Charles Merguerian of Hofstra University has mapped Cameron's line to the south through Manhattan Island. Merguerian has examined the underground construction site of a new water tunnel being constructed along the East River. There he has observed a zone of crushed and fractured bedrock (mylonites) that he has inferred to be the trace of this fault line. Ancient movement along the thrust is documented in the 1986 doctoral thesis of Kim Waldron at Yale University. Her work shows that while the rocks around the Cortlandt Complex were metamorphosed about 650 million years ago at a depth of about 15Ð20 kilometers, later contact metamorphism caused by the intrusion of the Cortlandt gabbros about 440 million years ago occurred at a depth of 23Ð27 kilometers. She feels that this thickening of the crust was the result of the movement of an east-over-west thrust sheet estimated to be about 8 kilometers (5 miles) in thickness (Waldron, 1986). ************************************************************ Figure 6-6. CameronÕs Line in New York and New England. Cameron's line, first recognized as a suture boundary between deep oceanic and continental shelf crustal plates in Western Connecticut, has now been extended from Massachusetts through the New York metropolitan area. (Sullivan, 1984) ************************************************************ We know that deposition has occurred since the Ordovician, but erosion has eradicated nearly all of these later rocks in the Westchester region. The only known exception is the tiny exposure of black shale referred to in Chapter 1. The opening of the present Atlantic Ocean 200 million years ago is documented by extensional down-faulting of the Triassic basin to the west in Rockland County and Northern New Jersey. It is also marked by the intrusion of the Palisades and Watchung basalts. The most recent advance and retreat of the continental glaciers (perhaps 100,000 years ago and about 20,000 years ago respectively) left a cover of glacial till with associated outwash deposits covering much of Westchester. ************************************************************ Figure 6-7. Schematic map of Trip B: Evidence of Cameron's Line ************************************************************ Materials: Lower Hudson Sheet of the New York State Geologic Map, Rock Hammer, Hand Lense. Stop B-1: Orchard Beach. Start at the northeast corner of the main parking lot at Orchard Beach in the Bronx. (To get there, take the Hutchinson River Parkway to the City Island/Orchard Beach exit, and follow the signs to Orchard Beach.) Walk to the rock outcrops at the northern end of the public beach. Then follow the shore around the northern tip of the point. This is the Hartland Formation, which outcrops more extensively in Western Connecticut. The abundance of dark schist and amphibolite lends credence to the hypothesis that these rocks were derived from deep ocean sediments and intrusions in an off-shore island arc. Japan and the Aleutian Islands are modern island arcs. (Note the geologic map of this stop in Figure 6-8.) Minerals: Muscovite and Biotite Mica, Amphiboles, Veins of Quartz (glassy) and Feldspar (light colored and opaque), Pods of Garnet , Epidote (green alteration products of feldspar), Amphibole, and Pyroxene. Structures: Intrusions (some cross-cutting), Faults (small), Veins, Glacial Polish & Striations Return to the Hutchinson River Parkway North. Proceed 1/2 mile, then exit right onto the New England Thruway ( I 95) north toward Connecticut. Drive about 8 miles and exit right at the Cross Westchester Expressway (I 287). About ß mile ahead, and stop just past the second underpass (Ridge Street). The rock has weathered to a pale green or gray color. Stop B-2:Port Chester Serpentinite along I 287. This serpentinite is an alteration product of of an intrusion of mafic (iron and magnesium rich) composition. Serpentinite is a low grade metamorphic rock which has formed as the original deep marine intrusion adjusted to conditions of lower heat and temperature closer the surface. This is an example of retrograde (reverse) metamorphism. These mafic intrusions are often associated with plate boundaries and orogenic (mountain building) belts. Although this serpentinite is entirely contained within the Hartland Formation, the geologically mapped trace Cameron's Line which separates the Hartland from the Manhattan Schist is only about two miles to the northwest along Interstate 287 . ************************************************************ Figure 6-8. Geologic Map of Pelham Bay Park ************************************************************ Figure 6-8. Generalized Geologic Map of a Part of Pelham Bay Park in the Bronx (Seyfert & Leveson, 1968) Proceed northwest on I 287 to the I 684 exit. Note exposures of the highly variable Hartland Formation. Exit right onto Interstate 684 north. According to the New York State Geological Survey map, this is the approximate location where you will cross the Cameron's Line thrust fault. The fault may be difficult to locate because, as a zone of fracturing, the eroded fault zone is likely to be concealed under weathered sediments. It may also be difficult to differentiate between the Hartland and Manhattan formations since both contain a variety of felsic and mafic metamorphic rocks. Continue about 10 miles north on Interstate 684. Note exposures of Manhattan Schist on the right just before the Anderson Hill Road bridge.Then, see exposures of Manhattan Schist on both sides of I 684.Later you will see exposures of the Fordham Gneiss on the left. Byram Lake, on the left, is probably bounded on the west by a fault. Note intricate folding in this large exposure of the Fordham Gneiss.Exit at NY 172. Exit on the right, then turn left at the traffic light. Park near the southbound exit, just beyond the overpass. Walk 200 meters north along the exit ramp to a large outcrop on the left (west). Stop B-3: Migmatite. (east of Mt. Kisco) Complex exposure just north of NY 172 along Interstate 684. This is the PreCambrian Fordham formation. A migmatite is a metamorphic rock that has been intruded by igneous rock and/or experienced partial melting. The Fordham is the basal unit of the New York City Series. The overall light color indicates its continental (shelf) origin. Minerals: Quartz (clear), Potassium Feldspar (pink), Plagioclase Feldspar (white), Hornblende (black, splintery), Biotite (black, thin sheets) Rock types: Gneiss, Amphibolite, Granite, Pegmatite (both pink and white varieties).Structures: Igneous veins (Are they intrusions, ionic migration, or both?), Banding and Foliation. Xenoliths in the granitic pegmatite, Drill Holes and Drill Stems from road construction About three miles east of this point are the Bedford pegmatite quarries. About eight quarries were mined from 1878 until 1949, producing primarily potassium feldspar, which was used in the production of high quality ceramics. Mineral zonation from plagioclase feldspar at the outer rim to rose quartz at the center is especially evident in the Baylis Quarry, off Oliver Road. These zones were caused by minerals crystallizing at at different temperatures as the magma cooled, according ************************************************************ Figure 6-9. The Bowen Reaction Series ************************************************************ THE BOWEN REACTION SERIES As molten rock (magma) cools, different minerals crystallize at different temperatures. Geochemist N. L. Bowen demonstrated in 1922 that a single magma can yield very different igneous rocks at different times. The Bowen series is a model that displays the order in which common minerals crystallize (solidify) from a heterogeneous melt to make igneous rock. Progressive crystallization can result in rocks that differ greatly from the overall composition of the magma from which they originated. The first rock that solidifies will be relatively rich in the minerals at the top of the Bowen series, while the remaining magma becomes progressively depleted in those minerals. As time passes, the remaining melt becomes more concentrated in minerals that crystallize at lower temperatures. In this way, a single body of magma can yield a progression of igneous rock types from the dark colored basaltic rocks, rich in olivine, pyroxene, amphibole and plagioclase, to granitic rocks, rich in potassium, feldspar, muscovite and quartz. to the Bowen reaction series (Figure 6-9). About 50 minerals have been identified in these pegmatites, some of which have yielded specimens now on display at the American Museum of Natural History in New York City. The last two quarries that were open to the public became inaccessible due to real estate development in the late 1980s. The Baylis Quarry, noted for its concentric mineral zones, is located 1/2 mile east of Greenwich Road and 75 meters south of Oliver Road, The Kinkel Quarry, the largest in the area, is adjacent to a new housing development along Middle Patent Road, 1/4 mile south of Pound Ridge Road. The Hobby Hill quarry (Field Trip F) is still accessible, but very weathered. END OF TRIP B. ************************************************************ TRIP C: Features of Glaciation in Westchester County A great debate took place in the field of geology in the middle of the last century. In many ways, it was like the more recent controversy over plate tectonics. Geologists observed gigantic boulders far above present water courses. They observed parallel scratches on polished rock surfaces and great mounds of unsorted sediments in areas like Westchester. A traditional explanation was that these features were a result of NoahÕs flood in the Bible. The unattached rocks were thought to have been rafted by icebergs or other debris in the great flood. As a result of observations in the Alpine areas of Europe, and enthusiastic support of such geologists as Louis Agassiz (who came from France to join the faculty at Harvard in 1846), the theory of continental glaciation gradually became accepted among scientists. In fact, American geologists found evidence of four advances of the glaciers in the past million years, interspersed with interglacial periods of relatively warm weather, like the present. More recent research has shown that periods of glaciation are not just a feature of modern climates. Based upon analysis of ocean bottom sediments and other sources, glacial advances have been traced back into the PreCambrian era. Glacial cycles seem to be common for at least the last half of the earth's 41/2 billion year history. The most recent period of glaciation is known as the Wisconsin stage, because features of this advance are particularly well displayed in the state of Wisconsin. The glaciers started when more snow accumulated in eastern Canada during the winter, than could melt in the summer. The reason for this climatic cooling is not universally accepted. One reasonable theory holds that the earth's axis was tilted more than its present 23.5¡ relative to the plane of the earthÕs orbit around the sun. This change in the tilt of the earthÕs axis is a part of a natural cycle known as the precession of the earth's axis. The greater tilt led to colder winters, which coated the ground with a thick cover of snow and ice that reflected the warming rays of the summer sun. As a result, much of the summer warmth was reflected back into space and the winter snow accumulated year after year. Under its own weight, the snow on the bottom changed to ice. It then started to flow outward in all directions. While it is not known how thick the ice was at its source, it must have been at least a mile thick over southeastern New York, because event the highest summits of the Catskills have been rounded and scratched by the movement of the glaciers. The ice initially moved southward into our region following the Hudson River Valley. The earliest date of this advance into Westchester is not known, but it has been estimated to be about 100,000 years ago (Schaffel, 1981). As it thickened, it flowed over the adjacent highlands, rounding and polishing the summits. The ice continued to move southward until it reached central New Jersey and Long Island. At that point, the ice front stagnated while the ice melted back as quickly as it advanced. The ice pushed, dragged and carried great amounts of rock and soil. Much of this debris was pushed into an eastÐwest line of hills known on Long Island as the Ronkonkoma Moraine. This moraine can be followed from central Long Island through Martha's Vineyard and Nantucket to the east and into central New Jersey and Northern Pennsylvania to the west. Another terminal moraine can be traced along the northern shore of Long Island. This, the Harbor Hill Moraine, marks a later stage of ice front stagnation. The fish tail shape of Long Island is a result of these two moraines which extend into the Atlantic Ocean as Montauk and Orient Points. A remarkable contrast can be seen between the beaches on the north and south shores of Long Island. The north shore beaches are composed of rocks, boulders and sand that has settled out of the nearby bluffs. But most of the south shore beaches are composed of well sorted sand that was washed into place by meltwater. The south shore beaches have no high bluffs of glacial till, just the sandy berms created by the ocean waves, the prevailing winds and the longshore currents. Radiometric dating of organic materials left in post-glacial bogs has established withdrawal of the glaciers at roughly 15,000 years ago. At that time, a climatic change caused the ice to melt back faster than it advanced. The glaciers left a dam of soil and rock across the Verrazano Narrows creating a fresh water lake along the Hudson River. The outlet to this lake was probably via the Sparkill Gap and the New Jersey Meadowlands. Initially, sea level was much lower than it is now. Enough water was tied up in the ice caps to lower sea level as much as 500 feet . The Hudson River carried great amounts of meltwater from the glaciers as they receded northward. Eventually, the bulk of the meltwater was able to exit to the Atlantic through the St. Lawrence River. The Verrazano gap was breached by the Hudson River about 12,000 years ago. About 9000 years ago sea level rose enough to invade what we now know as Long Island Sound (Schaffel, 1981). It is very difficult to predict future advances of the glaciers. Considering the recent geological history , further ice ages seem almost certain. However, predicting when ice will again accumulate in Canada and begin its relentless flow southward is not possible at present. Although we can work with the analysis of hundreds of years of weather records, it doesnÕt seem possible to see beyond the natural variations from year to year, and the longer cycles of warm and cold years. Such insight would be essential in order to make the accurate long range predictions needed to estimate the time of a new ice age. However, given our knowledge of the past million years of earth history, it is certainly conceivable that 100,000 years from today, Westchester could be in the throes of a new ice age. ************************************************************ Figure 6-10. Schematic map of Trip C: Features of Glaciation. ************************************************************ FIELD TRIP C: Materials: USGS Mohegan Lake Quadrangle Map, Small Shovel Stop C-1: Pocantico Hills. Start in the hamlet of Pocantico Hills, 2 miles west of Route 9, North Tarrytown. Stop C-1 is at a rock knob 200 feet behind (east of) the Pocantico Union Church along Route 448. (Please note that this site is not accessible without prior permission on weekends and holidays.) Features: Whalebacks, Glacial Polish, Striations, Crescent Gouges (for definitions, see the glossary), Farm implement Scratches The grinding action of rocks carried and pushed by the glacier smoothed and scratched the shallow north facing slope, and plucked rock from the steeper southern slope. This shape is typical of many local rocky knobs and hills. The semi-circular gouges were made as the glacier pushed rocks that it was carrying into the outcrop, popped out pressure slabs, and then rolled the rocks forward to dig them in again. Drive west on NY Route 448 to North Tarrytown. In North Tarrytown turn about 70¡ right onto US Route 9 North. (Don't turn too sharply, or you will enter the wrong road.) 2 miles ahead turn right onto NY Route 117. After another mile ahead, turn right into the entry road to Rockefeller State Park. Keep right and proceed to the far right corner of the main parking lot. Find a walking path (an old road bed) parallel to the north side of the parking lot. Follow this path to the left (west) about 1/4 mile. Then, opposite a marsh on the left, look for a very small path to the right that leads through some large locust trees. (The small path is about 60 meters before a major trail junction.) Follow this path about 30 meters north to a large boulder about 6 meters high. Stop C-2: Glacial Erratic. (Perhaps the largest in Westchester) This rock was moved into its present position by the advancing glaciers. It is composed of Fordham Gneiss, which forms the nearby bedrock. The great size and angularity of this rock, as well as its rock type indicate that it was probably transported a relatively short distance. Return to the parking lot. Drive back to Route 117, then turn right. (no left turn is possible.) 11/2 miles ahead make a U-turn at the stop light to head back west on Route 117. Bear right 3 miles ahead, then turn right onto Route 9. Continue north 13 miles through Ossining and Croton. Exit 21. right and turn left at Welcher Avenue, Peekskill. After crossing under Route 9, proceed straight ahead toward Charles Point. (Do not follow the main road which turns south.) Turn right onto South Street. 1/4 mile ahead turn left toward Charles Point. Continue 1/4 mile west and Park on the right with a view up the Hudson River to the north. Stop C-3: View of Hudson River Fjord, Truncated Spurs, Fault Valley(?) Geologists think that the Hudson River has followed its present course for a very long time. As it eroded into the hard, resistant crystalline rocks of the Hudson Highlands, it probably continued it's downward erosion along fault zones. This accounts for the angular path of the Hudson River through the Highlands. The advancing glaciers also followed this route as they pushed southward. The glaciers both deepened and widened the valley as they flowed toward the ocean hundreds of feet lower than the present sea level. As the glaciers moved through this channel, they cut off (truncated) the ridges on either side. Although the glaciers eventually covered the whole landscape, the greatest erosion happened in the Hudson River channel. When the glaciers melted, sea level rose hundreds of feet , drowning this scoured valley and making it a fjord. A fjord is a sea level channel that passes between steep mountain slopes. This is one of the few fjords on the east coast of North America. Make a U-turn back toward Peekskill. Turn left (north) onto South Street. Proceed through several intersections to the ÒTÓ intersection. Then turn right 24.2 onto Hudson Street and another right onto the ramp to US Route 9. Park along the right side of the entry ramp. Stop C-4: Glacial polish, Rounding, Striations. The polished rock face on the left was exposed in the construction of this exit/entrance to Route 9 about 1983. Weathering will destroy the sheen of this polished surface in a few decades. The scratches (striations) show the southward motion of the ice. The rounded and plucked surface also shows the direction of the ice flow. This exposure is especially spectacular when the sun is low in the eastern sky and the rock surface is wet. Sunlight reflections show the polished surface especially well in these conditions. Keep right as you proceed onto Route 9 North. 1 mile ahead, keep right.as Route 9 turns left. Proceed eastward onto the Bear Mountain Parkway. Bear left after 4 miles onto US Route 202. (About 2 miles ahead, note the gentle rises and dips as the road traverses a series of drumlins.) Turn left onto NY 132 at the Yorktown Presbyterian Church. 11/2 miles ahead, park on the right at the bottom of the dip. Stop C-5: Through Valley. Find this valley on the United States Geological Survey Mohegan Lake Quadrangle map. Note how the stream flowing west from Osceola Lake seems to split into north and south branches in the marsh north of your present position. This bifurcation of a stream as it flows downhill is an unusual feature. It is likely that a former divide separating a north flowing stream from a south flowing stream was breached by a tongue of the continental glacier as it moved southward. This erosion created a Òthrough valleyÓ. With the divide leveled and glacial sediment filling the valley to the north, the division of drainage was made possible. Continue 1 mile north on NY 132. Bear left at the ÒYÓ intersection as you turn to the west. Continue straight onto the Old Route 6 after the stop sign. Just past the town library, bear right onto New Road and proceed through right and left 90¡ turns. Continue straight through the stop sign. Park by the soil cut on the right.. Stop C-6: Esker. Climb to the top of the bank and note its shape. This landform probably originated as deposits within a tunnel under the ice, or as debris filling an ice crevasse. The cut into the bank exposes a coarse gravel of rounded cobbles that were probably transported and tumbled by glacial melt water. Note the esker's relationship to the higher land just to the east. Perhaps the esker marked a stream that carried sediment toward a kame terrace delta along the higher portions of New Road. In that case, the bank you descended along the road marks an edge of the glacial ice.Proceed straight ahead to the cross road. Make a U-turn to head back east. Take the first left that crosses US Route 6. Carefully continue straight north across Route 6. (Please watch for oncoming vehicles. This is a dangerous intersection, particularly in heavy traffic!) Bear right right to cross a small bridge. Park on the right where a grassy ramp leads to the left, down to a small community beach. Walk down the ramp and stop at the large boulder. Stop C-7: Erratic of exotic lithology. (Wappingers Limestone) The nearest bedrock exposures of this rock type occur about 10 miles to the north of this location, indicating glacial transport of at least that distance. Note the small fossils on the weathered surface of the erratic. Turn around and drive back to Route 6. Watch for traffic and carefully turn left onto Route 6. 0.1 mile ahead, if vegetation permits, note the kettle lakes on the left. At the Jefferson Valley Mall, Route 6 has been cut through a drumlin. (The right (Southern) portion of the drumlin was used for fill in the construction of the mall and parking lots.) Note Osceola Lake through the trees on the left. This is a large kettle lake. Route 6 crosses several drumlins west to east. Turn right at the blinker light to drive south along Mahopac Avenue. Note the drumlin on the right and other drumlins on the left. A mile ahead, at the stop sign, turn right onto Granite Springs Road . The hills and dips in the next several miles are also drumlins. At the next stop sign, turn right to stay on Granite Springs Road. 1 mile ahead proceed straight onto Broad Street as Granite Springs Road turns right. A mile ahead, turn left onto Whipoorwill Road. Turn left again onto Trelawn Street. Stop at the dead end at the north end of Trelawn Street. Stop C-8: Drumlin. Observe the blunt north end of this hill, the steep sides, and the trailing south end. (The shape shows well on the Mohegan Quadrangle map.) This hill was made when the moving glacier rode up and over debris that it was pushing southward. Most of the local hills are either drumlins or glacially sculpted hills of bedrock. The drumlins in this area usually align north-south, while the bedrock hills more commonly have a northeastÐsouthwest long axis. Make a U-turn to return to Whipoorwill Road. Turn right back toward Granite Springs Road. Turn left (south) onto Granite Springs Road. Turn right onto Routes 35 & 202. The road cuts through a drumlin just west of the intersection. Continue straight through the traffic light at the center of Yorktown Heights and onto Route 118 South. OPTIONAL STOPS: (not on the road log) A. A large erratic is located just south of the Ò7-11Ó store on Veterans Road between Commerce Street and Downing Drive. B. A Kettle lake can be seen about a mile to the east at the intersection of Moseman Avenue and Pines Bridge Road. C. Mohansic and Crompond kettle lakes are 1 mile west, in Franklin D. Roosevelt State Park, via Route 202 and Baldwin Road.Continue south to Locke Avenue, which leads to Turkey Mountain Park. D. 1/4 mile west of here is a town maintained parking lot. A rigorous 20 minute walk leads to the summit from which the New York skyline is visible on clear days. This whole hill is very large whaleback structure. Turn left (east) at the ÒTÓ intersection to stay on NY 118. Turn right at the traffic light onto NY 100 South. Cross the reservoir. Pull over to the right just past the old railroad underpass. Stop C-9: Outwash (Kame?) in Pogact gravel pit. (Contact the owner if you would like to make a closer approach.) Note the layers in the exposed bank about 80 yards to the right. These sediments are glacial outwash, deposited by melt water. (Deposits created directly by glacial ice do not show layering.) This is one of the diminishing number of places in Westchester where outwash deposits have remained visible, rather then being covered by construction and development soon after use. Drive ahead to the gravel pit entry road on the left,.and make a U-turn.. Head back north. Continue north about 10 miles on NY 100 through Somers. As NY 100 turns left, bear right onto NY 116, then follow Route 116 as it turns right. Continue on NY 116 through Purdys and Salem Center to North Salem. Look for a large boulder on the right in North Salem. It is marked as a natural point of interest with a small sign. Stop C-10: Perched erratic. This great bounder, suspended off the ground, has been a subject of speculation for many years. How did it get there? Before the theory of continental glaciation was accepted, there was not a clear answer. Now, the answer is quite simple. This erratic was left perched on a rocky till deposit when the ice melted. Subsequently, the soil and smaller particles were washed away by natural erosion. But the larger rocks, including those on which the boulder sits, were left in place. The boulder has been estimated to have a mass of about 60 tons (Shoumatoff, 1979). END OF TRIP C. (The direct distance from North Salem back to Pocantico Hills is about 30 miles via Interstate 684 and the Saw Mill River Parkway.) ************************************************************ Other Nearby Glacial Features Of Interest: 1. Whalebacks: A whaleback is located in the Pound Ridge Reservation at the Kimberly Bridge picnic area, near the east end of the main road. It is a small tree covered, rocky hill about 30 meters south of the bridge, and next to the stream. Note the severely plucked south end, accessible by a path on the right. Two of these structures can be seen along the west side of Albany Post Road (US 9A) between the intersections with Furnace Dock Road and Route 9, north of CrotonÐonÐHudson. An island near the north-west shore of Canopus Lake in Fahnestock State Park, Putnam County, is an ice sculpted rock knob. It is visible across the lake from NY Route 302. Another is located along Breakneck Ridge above Route 9, north of Cold Spring. (About 100 meters up a steep hill above the road tunnel) The very top of Timp Torn, about a mile north of the summit of Bear Mountain, is a whaleback. Access is via a steep hike the hill from Mine Road, west of Fort Montgomery. 2. Outwash Gravels: These fluvial deposits, found in many of the local valleys, have been used extensively for road construction. Landscaping, roads and buildings have covered many of the remaining layered gravel locations. It is now cheaper to transport road gravel from upstate locations. Occasionally, construction in valley locations will temporarily expose outwash deposits. 3. Erratics: Many large erratics can be seen in the wooded regions of Westchester. A resident of Mark Road in Yorktown has reported a small erratic of weathered iron ore, probably transported from Putnam County, in her back yard. 4. Perched Erratic: In addition to the boulder at North Salem (Trip C), another perched erratic can be seen along Willowcrest Drive in Somers. (To the best knowledge of the author, this Somers balanced rock is a natural formation.) 5. Chattermarks and Crescent Gouges: Parabolic marks and gouges can be found at the summit of Bear Mountain. They are about 30 meters south of the observation tower. ************************************************************ Figure 6-11. A Map of Croton Point showing the Sand Hills known as Islands. (Markl, 1971) ************************************************************ TRIP D: The Geology of Croton Point Plan this trip at low tide, as the beach may be under water at high tide. Daily newspapers commonly print the schedule of the tides on the same page as to the daily weather map. (This trip is partially adapted from ÒPleistocene Geology of Croton PointÓ, R.G. Markl, 1971) From the entrance to the Croton-Harmon railroad station near US Route 9, drive west along the viaduct. The first stop is at the end of the viaduct. (Pull over at the bottom of the ramp.) Stop D-1: Landfill: Note the hill just to the right of the ramp. The unforested northern portion was built up from the level of the road with refuse, and then covered by fill obtained elsewhere on Croton Point. Only the far (west) end of this hill is natural. (See Figure 6-11 and 6-12.) The natural terraced top along the western end of the hill probably shows the higher water level in post glacial Lake Hudson. (Hanging terraces at the same level have been discovered along the shore in nearby Hudson River communities.),Proceed about 1/2 mile, past the toll booth, and drive to the far left (southwest) corner of the large parking lot. A parking fee is charged in the summer. Stop D-2: Overview. (The remainder of the trip will occur on foot from this location.) The following events are probably responsible for the formation of the Croton Point landscape: ************************************************************ 1. The Wisconsin glacier left a longitudinal, recessional moraine as the ice melted back to the north about 15,000 years ago. 2. Varved clays were deposited in a post-glacial lake created when a moraine dam blocked New York harbor at the present position of the Verrazano Narrows Bridge. The outlet to this lake was probably through Sparkill Gap, just south of the Tappan Zee Bridge. (See figure 6-13.) Streams entering this lake left hanging delta terraces about 100' above present sea level in nearby river front communities. 3. Fossil evidence indicates that the Verrazano gap in the Harbor Hill moraine was breached about 12,000 years ago (Newman, 1969), making this a brackish water estuary instead of a fresh water lake. 4. Tombolos (sand spits) formed connecting a series of islands to each other and to the shore. 5. A delta was formed where the turbulent waters of the Croton River entered the relatively calm waters of the Hudson River. 6. The Croton River may have carved the depression between the two hills (ÒislandsÓ) near the center of Croton Point.. (See figure 6-11.) 7. Flooding left sediment in mouth of the Croton River the when the original (earthen) Croton Dam broke during construction in 1840. 8. Clay was excavated from several locations near Squaw Cove to make bricks in the 1800s. 9. Excavation and fill to were moved to cover great volumes of Westchester County garbage buried here until 1986. ************************************************************ Walk south, through the picnic area, to the river front retaining wall. Stop D-3: Landscape Features. From the retaining wall, look across the Hudson River. Note the Palisades Sill with the Triassic Lowlands behind it. The Ramapo fault scarp and Hudson Highlands are visible to the northwest. Walk south onto the beach. Stop D-4: Rocks transported by the glaciers, the Croton River, and wave action. Observe the rocks along the beach. Due to glacial deposition and commercial uses of Croton Point, an unusual variety of rock types can be observed along the beach. The list below includes some of the rock types that can be readily found. Igneous: Gabbro, Granite, Basalt Metamorphic: Gneiss, Schist, Slate, Quartzite Sedimentary: Sandstone, Conglomerate, Limestone, Coal (imported), Breccia Stop D-5: Walk south along the beach to Teller's Point. Look for exposures of varved clay just above the high water mark. (Varves are layers of fine clay deposited in glacial lakes.) The winter sediments are rich in dark organic material, while the summer sediments are lighter in color. Counting the layers enables geologist to estimate how long the lake existed. In some places, the clay layer has created a perched water table, and springs can be seen flowing from above the clay layer onto the beach. The clay from Croton Point is valued by potters for its working characteristics. Stephen Maslansky has reported trees toppling near Teller's Point as the soil creeps downhill in wet conditions. Consider the problems of leaching and runoff that may be carrying dangerous chemicals from the landfill into the river. At Teller's point, note the boulder train that extends from the point along shallows to the south. The shallow water here indicates that the point once extended farther to the south, but has been eroded northward by wave action. END OF TRIP D. ************************************************************ Figure 6-12. Croton Point Before Landfill Activities. (Kindle, 1958) ************************************************************ Figure 6-13. Hypothetical Course of the Hudson River in Schooley Time. (Johnson, 1931) ************************************************************ Figure 6-14. Some Invertebrate Fossils of New York. (Broughton, et al, 1962) ************************************************************ TRIP E: Fossil Collecting Near Highland Mills, Orange County (Although this location is well outside Westchester, it is the closest place where abundant fossils can be readily observed in sedimentary bedrock. The text was contributed by Dr. Paul Steinek, Department of Natural Sciences, SUNY Purchase) The directions start at the Harriman exit of the New York Thruway in Orange County. Follow New York Route 32 north to Highland Mills. In Highland Mills, turn right onto Park Avenue at the Exxon service station on the left (and the stone gazebo on the right.) Where the road curves to the right, park on the left just past a small stream. (The road name changes to Pine Hill Road.) Walk up the rise to the railroad tracks and then 200 yards north. Fossils can be found in the middle of the rock outcrop on the right (east) side of the railroad tracks. Small brachiopods are especially easy to find. This outcrop consists of sandstone and shale of Devonian age. Abundant body fossils (mostly shells), trace fossils (imprints from movements, burrows and trails), and sedimentary structures indicate high energy, shallow water marine conditions. Body fossils are absent in most of the layers, but they are very abundant in two pale yellow weathering limestone shell beds. These include several species of brachiopods, pelecypods, snails and (rarely) trilobites. (See the diagrams on the next page.) Note that the original calcarious skeletons have been dissolved by acidic groundwater leaving only casts, molds and surface impressions. The shell beds may be storm generated shell mounds which now form on modern tidal flats. At this location most of the sand layers were extensively burrowed by soft bodied invertebrates in search of food. The burrows then filled with sediment and now stand out in relief on weathered surfaces. Look closely at the exposed surfaces of the beds. Symmetrical ripple marks were formed by tidal currents and longshore currents. These ripples show up especially well where erosion has exposed the layers in cross-section. Mountain building and regional folding of Acadian age has tilted these strata to a high angle from their original horizontal position. This trip can be combined with a hike up Schunemunk Mountain via the attractive Jessup Trail. To find the trail head, drive 5.1 miles north from the Park Avenue-Route 32 intersection in Highland Falls. At the Black Rock Fish and Game Club, turn left, with a second left across Woodbury Creek onto Taylor Road. 0.3 miles beyond, park on the right. The Jessup Trial, marked by a sign, begins 0.4 miles ahead on the left in a corn field. The trail follows the rapids of Baby Brook to a long ridge of pebble conglomerate with sweeping views of the Hudson Valley to the north. Follow the yellow markers. The distance up to the ridge crest is about 2 miles with about a 1000' climb. (The hike up Schunemunk Mountain is abridged from ÒA Geological Climb of Schunemunk MountainÓ by William J. Tucci and Robert Kalin in the New York State Geological Association Guidebook of 1989.) END OF TRIP E. ************************************************************ Figure 6-15. Other Field Trips In and Around Westchester ************************************************************ MINI-TRIPS / FAMILY TRIPS These brief excursions require less time and include less driving than most of the earlier trips. Some of them will take you to places that are particularly attractive, regardless of the geological features. They are therefore recommended as either geological tours, or casual family excursions. TRIP F. The Mianus River Gorge and the Hobby Hill Quarry (A Colonial Pegmatite Quarry) Directions: Drive 1.1 miles east of the village of Bedford along Pound Ridge Road. Turn right onto Long Ridge Road, which leads south toward Stamford, Connecticut. 0.6 miles ahead, turn right onto Miller's Mill Road. (The old mill, on the right, has been converted into a private home.) At the old mill, make a quick right and a left turn onto Mianus River Road. The wildlife refuge parking lot is located 0.6 miles south, on the left. This reserve is maintained by a local committee of The Nature Conservancy. It was registered as a Natural History Landmark by the Secretary of the Interior in 1964. The lands are controlled and maintained through private donations. The preserve is closed from December 1 through March 31. This is a very attractive area, which includes virgin forests bordering the Mianus River for about 3 miles. A network of well maintained hiking trails leads from the shelter and parking area southward above and through the gorge. There are many designated natural attractions. The Hobby Hill quarry is about 3/4 of a mile south on the main trail along the gorge. At this point, a small trail leads to the left, down the slope to three weathered quarry faces. The Hobby Hill Quarry has been mined since the 18th century for muscovite, quartz, and feldspar. This intrusive pegmatite is probably related in origin to the pegmatites of the Baylis and Kinkel quarries south of the village of Bedford. As this is a private nature preserve, mineral collecting is not allowed. The walk from the parking area to the quarry and back is about 11/2 miles, including some short, but vigorous hills. If you choose to walk to the south end of the trail and Havemeyer Falls, the total round trip distance is about 5 miles. The preserve is attractive in any season, but the rush of the Mianus River and the flowing tributary streams in the spring, and the peaceful blaze of autumn color make this a particularly enjoyable excursion in those seasons. The river follows a linear zone of weakness in the early Palaeozoic Bedford gneiss. It is likely this is fault zone, as it is parallel to other faults that have been mapped in southern Westchester. The river may or may not pre-date the recent glacial advance of the Wisconsin, but the carving action of the glaciers and the rush of meltwater probably helped to deepen its path. TRIP G: Cranberry Lake and the Kensico Dam: An Early Twentieth Century Construction Site: Directions: Drive 1.3 miles north of the Kensico Dam traffic circle along the Bronx River Parkway, along NY Route 22. Turn right onto Old Orchard Street and, 0.1 mile later, make another right turn into the Cranberry Lake Park. Drive 0.3 miles to the parking area. (The major source of information for this field trip is a paper by Ronald Robins, SUNY Purchase, 1984.) Cranberry Lake is a 135 acre Westchester County Park. This location was developed for quarrying, rock crushing and cement making for the nearby Kensico Dam from 1912 until about 1915 . The present dam was constructed on the site of an earlier earthen dam that was built in 1885. When the Catskill reservoirs were developed in the early part of the twentieth century, Kensico was designated as an impounding reservoir to hold a two month supply of Catskill water. At the time of construction, the Kensico Dam was the tallest masonry dam in the world. Most of the water is transported to this reservoir through the Catskill Aqueduct from a number of large reservoirs in the Catskill Mountains. Cranberry Lake Park, established in 1975, features hiking and cross country ski trails, along with several bogs and a small lake. The Kensico Reservoir was planned for this location due to its elevation above all parts of New York City, the natural valley behind the dam, the strong rock foundation at the dam site, and the availability of high quality granitic gneiss about a mile east of the dam site. The Kensico Dam was one of the world's largest construction projects of its day. Three large quarries were excavated. Most of the rock was used to make the ÒcyclopianÓ concrete, composed of very large rock chunks. Other rock was crushed to make a finer traditional aggregate for concrete. Particularly attractive blocks were saved for the facing of the dam. The rock was transported from the quarry and the crushers to the dam over a temporary railroad system. After looking at the exhibits at the nature lodge, take the main trail south about 1/4 mile to the old railroad bed, where the trail veers left over a causeway between two bogs. This is one of two main rail lines that carried rock for the dam. The trail veers right at the end of the causeway, where there is a small cascade over the rocks on the left. (Note a trail that leads back along the east side of the bog and lake.) The trail ahead may contain pieces of plagioclase crystals known as moonstone. (See the Valhalla entry in Chapter 3.) This mineral shows an iridescent play of blue known as chatoyancy. Along the next part of the trail you can see a concrete retaining wall, and bore holes where the rock was blasted away from this rock face. The rock crushers were located in galleys along the upper part of the retaining wall. About 100 meters ahead the trail splits. Take the left (uphill) fork to look down into the crusher galleys. The water drains from the washing operation can still be seen in the galleys. Some of the rock was carried directly from the quarries over a 200 meter trestle that was built 10 meters above the bog. However, the crushed rock went by train from below the crushers over the causeway that you followed between the bogs. These two railroad lines met along the hill west of the bog. ************************************************************ Figure 6-16. Map of Cranberry Lake Park ************************************************************ Return north to the small cascade where the causeway crosses the bog. Follow the wooded trail along the eastern side of the bogs to a point where you can view Cranberry Lake. At that point, another higher trail leads back to the south, along the ridge, to one of the three quarries used to supply stone for the dam. Two rock formations provided excellent building materials for the dam: the Fordham Gneiss and the Yonkers Gneiss. The Fordham Gneiss is found along the western ridge, while the Yonkers gneiss is on the eastern slopes, where the quarries are. These formations may be hard to tell apart without careful observation. The Yonkers Gneiss generally has more pink feldspar, and it may show less dark and light banding than the Fordham. The Yonkers Gneiss quarry also shows exfoliation cracking. This fracturing is finer near the top, where the rock has separated into layers that are thinner than those closer to the bottom of the quarry face. It is not clear whether the cracking is a chemical weathering phenomenon from the absorption of water by the feldspars, or an off-loading and stress relief process resulting from the withdrawal of the glaciers. The trail north along the east side of the lake leads to attractive views of Cranberry Lake (really a small, eutrophic pond). This lake is, in part, a result of the scouring action of the glaciers. Crossing the north end of the lake, you should turn right the main trail, which leads northward, back to the parking area. ************************************************************ TRIP H: Silver Lake, North White Plains: A Geological Syncline Directions: Silver Lake is located about 1.5 miles north-east of the business district of White Plains, along Lake Street. Parking is available in a municipal recreation area near the south end of the lake. (Adapted from Leo Hall's field trip in the 1968 guidebook of the New York State Geological Association) Silver lake is a man made reservoir in a valley that reveals a sequence of local rock formations. Use the text below and the stratigraphic diagrams (Figures 6-2 and 6-4) for information about the major rock units of the New York City Series. The map that follows can be used to plan a geology walk in order to identify the rock formations, to observe their characteristics, and to infer the syncline structure at the lake site. A walk around the lake, starting at the parking lot near the south end and proceeding north along the east side of the lake, is recommended. The following rock formations are listed from youngest, which outcrop near the lake, to oldest, which can be observed uphill from the lake. A. Lowerre Quartzite. This is a buff colored, sandy feldspar rich quartzite or, a muscovite and feldspar rich migmatite. Professional geologists have questioned whether or not the Lowerre constitutes a true formation, or if it is just occasional sandy lenses at the base of the Inwood Marble. Correlation with other locations in the New York an