Overview of New York Geology

THIS PUBLICATION

This series of maps was designed for general readers who are curious about the diversity of landforms and geology of the northeast U.S., especially New York State, and for students enrolled in earth science courses in New York. The geological highway map serves as a practical guide to the geological variety of the State. The accompanying image of New York taken from space can be positioned directly over the geologic map, permitting the user to flip back and forth between the two and observe the match between bedrock geology and landscape.

OTHER PUBLICATIONS

The geologic map is a reduced and simplified version of the "Geologic Map of New York," published in six separate sheets at 1:250,000 scale. These are available as Map and Chart Series number 15 from the New York State Museum. A similar set of maps showing faults and fracture systems of the State is available as Map and Chart Series number 31, and a set showing surficial deposits as Map and Chart Series number 40. Gravity anomaly maps are also available at'1:250,000 scale as Map and Chart Series number 17 and at 1: 1,000,000 scale as Map and Chart Series number 32. A bla@k-and-white radar image at 1:1,000,000 scale showing the landforms, major highways, and cities of New York, Pennsylvania, and adjacent areas is available as Map and Chart Series number 42. Those seeking a more detailed discussion of New York's geology should get the companion book, "Geology of New York" (Educational Leaflet number 28). This illustrated book provides an explanation of the State's geology in termz; understandable to the general reader. To order these and numerous other maps, charts, and reports, request a free List of Publications from the Geological Survey, New York State Museum, Room 3136, Cultural Education Center, Albany, NY 12230. Phone (518) 474-5816.

THE COVER

The scenes on the cover are Whiteface Mountain, Adirondacks; Chimney Bluffs, Lake Ontario; Lighthouse at Montauk Point, Long Island; Kaaterskill Fans, Catskill Mountains; and Niagara Falls. Patricia A. Mulligan designed the cover and painted the scenes from photographs.

ACKNOWLEDGEMENTS

These maps and diagrams were compiled using maps and related data prepared by generations of geologists. We are especially grateful to our Geological Survey colleagues for helpful ideas provided during the compilation and to T. Engelder, R. T. Faill, D. P. Murray, and R. S. Stanley for critical reviews of the tectonic map. Our special thanks go to Donna L. Jomov and Judith M. Lauber for their assistance with this publication, and to the many earth science teachers who offered suggestions and illustrations. Generous support was provided by the National Science Foundation (MDR-8651656).

FIELD TRIP GUIDEBOOKS

Eighty guidebooks to geological field trips throughout the State are currently available. Two or three new ones appear each year. Each book generally includes several different trips in one area of the State. Each trip has a road log and descrip-

tions of the geology at designated stops. The map to the left, titled "Routes Of Geological Field Trips," is a key to areas covered by each book. A list of the guidebooks and their sources accompanies this map.

ADDITIONAL READING

Jaffe, H., and Jaffe E., 1986. Geology of the Adirondack High Peaks Region: A Hiker's Guide. Glens Falls, NY: The Adirondack Mountain Club.

Jensen, D. E., 1978. Minerals of New York State. Rochester, NY: Ward Press.

Molitor, L. L., 1988. Classic Field Sites for Teaching Geology in the Northeast, Northeastern Geology, Vol. 10, No. 1.

Roseben-y, C. R., 1982. From Niagara to Montauk: The Scenic Pleasures of New York State. Albany: State University of New York Press.

Roy, D. C., ed., 1987. Northeastern Section of the Geological Society of Amer.ca: Centennial Field Guide Volume 5. Boulder, CO: Geological Society of America.

Schuberth, C. J., 1968. The Geology of New York City and Environs. New York: The American Museum of Natural History.

Tesmer, 1. H., ed., 1981. Colossal Cataract.- The Geological Histoiy of Niagara Falls,. Albany: State University of New York Press.

Van Diver, B., 1976. Rocks and Routes of the Noilh Country, New York. Geneva, NY: W. F. Humphrey Press Inc.

Van Diver, B., 1980. Field Guide.- Upstate New York. Dubuque, IA: Kendall/Hunt Publishing Company.

Van Diver, B., 1985. Roadside Geologv of New York. Missoula, MT: Mountain Press Publishing Company.

Von Engeln, 0. D., 1961. The Finger Lakes Region.- Its Origin and Nature. Ithaca, NY: Comell University Press.

The crust of the earth is solid rock, tens of kilometers thick, made up of individual rock bodies that vary in size, shape, orientation, composition, color, and texture. Together they make up the bedrock, which is present everywhere, although commonly masked by surficial deposits.

The bedrock geologic map gives a vertical view of the pattern made by the eroded edges and surfaces of the rock bodies that crop out in the State. It is, however, only a two-dimensional view of three-dimensional rock bodies. The cross sections below the map show samples of the third dimension as inferred from the surface configuration of rock bodies and other information that may be available from drill holes or geophysical measurements.

Map patterns result from the intersection of topography and individual rock bodies. Most rock bodies are originally tabular and horizontal, but deformation changes their orientation and shape by tilting, folding, crumpling, and breaking. Map patterns can tell us much about the three-dimensional configuration of bedrock in different regions. For example, the rock bodies in western New York are layers of sedimentary rock, of greatly different thicknesses, that are tilted down to the south less than 1 degree. The broad bands on the map in that area are the patterns made by stacked layers that have been beveled at a low angle by erosion. (Visualize a layer cake sliced at a low angle instead of the usual vertical.) Widths of outcrop bands are controlled by the thicknesses of individual map units, the topography, and the low dip. Stream valleys account for the jagged details in the pattern. Steeply dipping faults are omitted from the geologic map except where they separate rock bodies of greatly different ages. This avoids unnecessary congestion on the map, especially within the Adirondacks where such faults abound.

In the Adirondack Mountains the map pattern shows once-tabular rock bodies now swept into broad folds. This deformed-rock pattern is typical of highly metamorphosed "basement" rock. One can easily visualize this rock pattern continuing in the basement as it passes beneath the blanket of sedimentary strata that surrounds the Adirondacks. The pattern of small blocks along the eastern border of the Adirondacks results from faulting that dropped crustal blocks down into a giant staircase.

The Taconic Mountains east of the Hudson River Valley are huge slices of crust that were thrust into that area from the east. The heavy toothed lines show the edges of these thrust sheets. The earth's crust in this region was "telescoped" when a volcanic island arc collided with the edge of the continent, causing the Taconian orogeny. This collision compressed the layered rock and sediment of the intervening sea, thrusting them westward onto the continent as huge, stacked -slices. The slices, which generally dipped east in a shingled arrangement, were contorted considerably in the process. When completed, the stack extended from New England past the western edge of the Hudson Valley. In the Catskill Mountains, the western edge of this transported rock remains buried beneath Devonian rock. Erosion has reduced the original thrust sheets to patches, creating windows to the rock beneath.

The legend on the facing page shows the formations and rock types in each map unit. The legend has two major parts based on divisions of geologic time, Proterozoic and Paleozoic-Mesozoic. Highly deformed Proterozoic rock bodies of the basement are buried beneath younger Paleozoic rock in most of the State, but crop out in the Adirondacks and the Hudson Highlands. Southeast of the Highlands, basement relationships become more complex as one approaches the deformed and metamorphosed root zone of the ancestral Appalachian Mountains (see cross section FGH beneath the geologic map). The Paleozoic-Mesozoic part of the legend applies to the wide expanse of sedimentary rock formations that blanket the State west of the Hudson River and south of the Adirondacks. The blank areas with yellow tint represent gaps in the rock record-geologic times not recorded by rock because of erosion or non-deposition.

Surficial material is ignored on generalized bedrock geologic maps, but it is shown here in several areas of the Adirondacks and over all of Long Island, where is it so thick that it masks all clues to the bedrock geology.

Bedrock generally is covered by a skin of soil and other loose material, especially in regions with humid climates. This cover material results as weathering breaks down the surface rock. The loose materials may remain in place or be eroded, transported, and deposited by water, wind, or glacial ice. In 90 percent of New York State, bedrock is buried by surficial deposits that are more than one meter thick. Most of these deposits were left by a continental glacier-an ice sheet that was perhaps 2 km thick.

Till is the most abundant glacial deposit. It is an unsorted mixture of mud, sand, gravel, cobbles, and boulders that the glacier spread over the countryside. Till can be up to 50 meters thick. It is generally thickest in valleys and thinnest over highlands. Moraines are elongate ridges or strings of hills that formed at the edge of the glacier and are composed of sand, gravel, or till. The Ronkonkoma and Harbor Hill moraines on Long Island dominate that landscape. The Valley Heads moraine dams the south ends of the Finger Lakes. Glacial lake beds are broad ontal layers of mud (deep water) and sand (shore zone) that were deposited in that formed in front of the glacier as the ice melted. Outwash is sand and gravel deposited by meltwater streams that flowed from the front of the glacier. these kinds of deposits have a wide range of thicknesses. In places, they be piled one on top of the other.

ROCKS AND THEIR MINERALS: Igneous and metamorphic rocks make up the Adirondacks and Hudson Highlands. Because the names of such rocks may be unfamiliar, here is an introduction to how they are named.

The minerals that form the rock of the earth's crust have eight chemical elements as their main ingredients. In order of abundance they are: oxygen (0), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), magnesium (Mg), and potassium (K). Less abundant elements in the rocks include hydrogen (H), carbon (C), and titanium (Ti). The metamorphic rocks that dominate the Adirondacks and Hudson Highlands were transformed from pre-existing igneous and sedimentary rocks at high temperature and pressure when they were deep within the earth's crust beneath ancient mountain ranges. This "pressure cooking" recrystallized the original rock-forming minerals into new coarser-grained mineral assemblages. The ingredients (proportions of chemical elements in the original rocks), and the cooking conditions (pressure, temperature and time) determine the mineral combinations that form, and these determine the rock. The mineral assemblages in the Adirondacks and Hudson Highlands indicate temperatures of formation up to 800'C and pressures that required 25 km of overlying rock! The common rock-forming minerals i n the earth's crust, arranged in order of abundance, are:

Mineral	Approximate Chemical Composition
Plagioclase feldspar	NaAlSi3O8 to CaAl2Si2O8 (a chemical series)
Potassium feldspar	KAISi3O8
Quartz	SiO2
Pyroxene	(Mg,Fe)SiO3 and Ca(Mg,Fe)Si2O6
Biotite (black mica)	K(Fe,Mg)3AISi3O10(OH)2
Homblende	(Mg,Fe)5(AlSi7O22)(OH)2
Olivine	(Fe,Mg)2SiO4
Magnetite Garnet	(Fe,Mg)3Al2Si3Ol2
Calcite	CaCO3
Dolomite	CaMg(CO3)2

This list includes the most abundant rock-forming minerals found in the rock of the Adirondacks and Hudson Highlands. Other common minerals include Sillimanite (Al₂SiO₅),Ilmenite (FeTiO₃), Graphite (C), and Pyrite (FeS₂). Chlorite (Mg, Fe, AI)₆(AI,Si)₄0₁₀(OH)₆ is common in faulted areas.

Notice that both of the two most common chemical elements, oxygen and silicon, are present in most of these minerals. Groups of four oxygen atoms clustered about one silicon atom form the building blocks of these minerals, which are called silicates. Quartz is composed only of these silicon-oxygen groups. In calcite and doloiite, the building blocks are three oxygen atoms around one carbon atom; these minerals are called carbonates. If enough water is present when rock is formed, minerals with oxygen-hydrogen (OH) groups, such as biotite or homblende, will be common.

ROCK NAMES: The names that geologists give to rocks are a shorthand or code that describes the rock in some detail. The name indicates the major minerals found in the rock, their size, arrangement, and relative abundances. For example"granite," as defined by geologists, is an igneous rock, consisting of grains of quartz, potassium feldspar, and plagioclase feldspar. Igneous rock types with an abundance of these light colored minerals are called felsic. Those with dark minerals predominating are called mafic. Granite may contain small amounts of dark-colored minerals such as homblende, pyroxene, biotite, or magnetite. If dark minerals are absent, the rock is called leucogranite, leuco meaning light-colored. Metamorphic rocks may have their own names, as in the chart below, or they may have.an igneous name with the prefix "meta," as in "metagabbro." Sometimes an igneous name is used as a modifier to a metamorphic name; thus when igneous leucogranite is metamorphosed, it becomes "leucogranitic gneiss." Mineral names are also used to modify metamorphic rock names, listing the minerals in order of increasing abundance. '1'hus in "biotite-quartz-plagiociase gneiss," plagioclase would be the dominant mineral. The classification of major igneous and metamorphic rocks is shown below.

To read this diagram, visualize a vertical line running through the diagram to represent the composition of a rock in question. As an example, granite ranges in composition from 75% potassium feldspar plus small amounts of quartz, plagioclase, and dark (mafic) minerals (left edge of the granite field) to only 25% potassium feldspar, 40% quartz, 20% plagioclase, and 15% mafic minerals (right edge of granite field).

Thrust faulting has made the structure of the Taconic region so complex that a different kind of map unit is required. Therefore, structural units (thrust slices) are shown here rather than the stratigraphic units, which are used elsewhere on the map for sedimentary rock. A given thrust slice may contain parts of several formations. Other slices contain different parts of those same formations. Individual thrust slices are identified by color. The slices are numbered on the legend. These numbers refer to the list of slice names below the legend. The thrust slices shown on the map are erosional remnants of once-larger sheets. Their boundaries are thrust faults shown by toothed lines.

This physiographic map illustrates the rich variety of landforms in the northeastern United States and adjacent Canada. The various landforms are mainly the result of weathering and erosion, which attack different types of rock at different rates. Lowlands form on easily erodible rocks, highlands on resistant ones, with all gradations between. Thus, the distribution of rock types in a region strongly influences its physiography. The matching tectonic map shows the diverse structure of the region's bedrock. A comparison of the physiographic and tectonic maps shows clearly how much the physiography depends on bedrock geology.

Continental glaciation played a modifying role in the development of New York's landscape in the recent geological past. On its advance south, the glacier removed and transported existing soils and eroded the surface of the bedrock. As the ice melted, this debris (mud, sand, gravel, and boulders) was left at new sites in a great variety of depositional landforms. Melting caused the glacier to retreat across the State from south to north between 20,000 and 10,000 years ago.

The hilly area in the north-central part of the map is the Grenville Province of the Canadian Shield, an area of exposed basement rocks, which extends across the narrow Frontenac Arch into.the Adirondack Mountains. The resistant rocks in the arch eroded more slowly than those around them and formed the Thousand Islands in the St. Lawrence River. Present-day drainage features in the Adirondacks illustrate well glacial erosion, transportation, and deposition. The continental ice sheet converted many pre-glacial river and stream valleys into chains of lakes by carving rock basins during ice advance and by depositing earth dams during ice retreat. The prominent northeast-trending rivers, streams, and lakes occur where faults and fracture zones greatly weakened the bedrock to make it easily eroded.

In the low plains south and west of the Canadian Shield, bedrock is covered extensively by glacial till and by layers of sand and mud deposited in meltwater lakes. This area is underlain by flat layers of sedimentary rock and is part of the Interior Lowlands, which extend westward to the Great Plains. Immediately south of Lake Ontario is a remarkable field of streamlined hills of glacial till called drumlins, some of which are shown west of Syracuse. Their beautiful alignment shows the direction of ice movement in this area.

East of Lake Ontario, elevations increase to form the Tug Hill Plateau. South of the lake and across the Mohawk Valley, the land surface rises to form the Allegheny Plateau. This plateau forms the northern end of the extensive Appalachian Plateaus, which extend to the southwest. The plateau boundary curves eastward across New York south of Rochester and Syracuse, to the Helderberg Escarpment southwest of Albany. The plateau surface rises in this direction until it becomes the Catskill Mountains. Rivers and their tributaries have cut the originally level Appalachian Plateaus into hilly uplands. The branching drainage pattern typically is developed by streams eroding horizontal layers of rock. Some of the north-south stream valleys were broadened and deepened by glacial ice, then dammed by glacial debris to form lakes. The Finger Lakes south of Lake Ontario were formed in this way. The two largest Finger Lakes, Seneca and Cayuga , are labeled.

Southeast of the Appalachian Plateaus is the Appalachian Valley and Ridge Province, a belt of sinuous ridges that curves northward through Virginia and most of Pennsylvania. Here the carpet of sedimentary rocks,was buckled into tight folds during the last Appalachian mountain-building episode. Farther southeast is the Great Valley, a lowland created largely by groundwater and surface water slowly dissolving the carbonate bedrock. This valley merges northeastward with the one occupied by the Hudson River and Lake Champlain. Southeast of the Great Valley is tfie hilly Piedmont Province. The Piedmont passes northward through the Hudson Highlands and merges with the hilly and mountainous New England Province and with the Taconic Mountains along the eastern New York border. The basement rocks that make up the Piedmont extend eastward beneath the younger sedimentary layers of the Atlantic Coastal Plain and the continental shelf. Immediately southeast of the Hudson Highlands are the Newark Lowlands. These lowlands formed on layers of sedimentary and volcanic rock of Triassic-Jurassic age. The northeastern end of the Lowlands is bounded by the Palisades, a striking rampart on the west side of the lower Hudson River.

Included within the Atlantic Coastal Plain are Long Island, Fishers Island, Block Island, Martha's Vineyard, Nantucket, and Cape Cod. These are parts of glacial moraines-lorig ridges of clay, sand, gravel, and boulders deposited at the edge of the continental glacier. On Long Island, the Ronkonkoma moraine marks the southernmost advance of the glacier in this region. During the last ice age, the growing mass of ice on the continents depleted the ocean waters enough to lower sea level by 100 meters. As the ice melted, the rising sea made parts of these moraines into islands. Waves and currents have been modifying them ever since.

The continental shelf, slope, and rise lie seaward of the Atlantic Coastal Plain. The continental shelf is nearly level; the continental slope (much exaggerated on the diagram) has a slope of 2^o to 4^o; the continental rise slopes less than 1^o. They are made of material eroded from the land, carried by rivers to the ocean, and distributed there by marine currents. During the period of low sea level, the continental shelf was exposed as part of the coastal plain, and rivers cut valleys across it to the shelf edge. Most of those valleys have since been filled with sediment, but a vestige of the Hudson Shelf Valley still remains. The Hudson Canyon and other, large canyons are cut into the shelf edge and continental slope. Much of this canyon-cutting occurred when rivers, swollen with glacial meltwater and laden with glacial sediment, flowed across the exposed shelf and met the sea at the top of the continental slope. The sediment that the rivers poured into the ocean at those points formed density currents. These currents cut the canyons into the slope.

This mosaic shows the rich variety of landfonns in New York and surrounding areas. The hilly area in the northwestern part of the image is the southern edge of the Canadian Shield. It is an area of exposed crystalline rock that extends across the narrow Frontenac Arch into the Adirondack Mountains.

East of Lake Ontario, land elevations increase toward the Tug Hill Plateau. South of the Tug Hill Plateau and the Mohawk Valley, the land surface rises to form the northern end of the Appalachian Plateaus. The Appalachian Plateaus have been much dissected by rivers and their tributaries. The northern escarpment of the plateau curves eastward across New York and the plateau increases in elevation until it becomes the Catskill Mountains.

Southeast of the Appalachian Plateaus is the Valley and Ridge Province. Here the carpet of layered sedimentary rock was buckled into tight folds during the last of the Appalachian mountain-building events. Farther southeast is the Great Valley, which merges northeastward with the Hudson-Champlain Lowlands. Southeast of the Great Valley are the New Jersey-New York Highlands. Northeastward, these highlands extend into the hilly and mountainous New England Province, with the Taconic Mountains along the New York border. Southeast of the New Jersey-New York Highlands, the Newark Basin, an area of uplands and lowlands, forms a transition to the Atlantic Coastal Plain. At the northeastern end of the Newark Basin, the Palisades form a ridge on the west side of the Hudson River.

This view is a mosaic of 21 winter scenes taken by NASA's Landsat 1 satellite as it passed 913 km (567 mi) above the earth in roughly north-to-south orbits. The satellite repeated each scene every 18 days as it flew in successive orbits that shifted westward each day. The mosaic uses the clearest cloud-free images selected from those taken in January through April 1973.

The satellite's orbit was sun-synchronous, with all images taken in midmorning when the sun was at a low angle. This procedure enhances the topography by producing highlights on east-facing slopes and shadows on west-facing slopes. This results in a three-dimensional appearance in areas of high relief, such as the Adirondack Mountains. The resolution of the image is about 80 m (260 ft).

The scenes were recorded by a multispectral scanner in four spectral bands: green, red, and two infrared bands just outside the visible spectrum. The digitized data were then transmitted to earth by radio waves and converted to false color composite images. The composite is called false color because each of the four bands is assigned a color. Clear water appears black because it does not reflect light in the green, red, and infrared bands. Growing vegetation gives off infrared energy and appears as red on the false color image. Clouds and snow are white and urban areas are blue-gray. The state borders are dropped along some drainages, to avoid hiding details on the image. Place names are omitted for the same reason. Cities can be located by referring to the matching geologic map.

	High-angle normal fault, hachures on downthrown side.
	Low-angle thrust fault, barbs on overthrust block.
	Boundary along which a terrane was welded to North America.
	Clarendon-Linden monocline; draped over buried normal fault. Displacement is down to the west.
-500-	On land, depth to basement in meters below sea level (not shown below 5000 m due to complexity of basement contours); offshore,, depth of water in meters. Circle shows location of drill hole to basement.

	Sedimentary rocks of the North American platform, Late Proterozoic to Paleozoic in age. Deposited in shallow seas on the stable North American craton. Tectonically passive.
	Rift basins of Late Triassic to Early Jurassic age, onshore and offshore, filled with non-marine sedimentary rocks as well as basaltic lava flows and their diabase feeders. Offshore basins are buried beneath younger marine sediments. Records a period of crustal stretching and rupture during opening of the Atlantic Ocean.
	Mesozoic plutons, mainly granitic, of Jurassic and Cretaceous age. Includes the White Mountains in New Hampshire and the Monteregian Hills in southern Quebec. Plutons were intruded late in the crustal stretching related to opening of the Atlantic Ocean.
	Marine and continental sediments of the Atlantic Coastal Plain, Cretaceous to Recent in age. Deposited near sea level as part of a passive continental margin on the east coast of North America.
	Offshore, submarine sediments and sedimentary rocks deposited since the North Atlantic Ocean began to open about 200 million years ago in the Jurassic Period. Passive continental margin.

	Archean volcanic and granitic rocks of the Superior Province, metamorphosed at low grade about 2.7 billion years ago. Lines show the trends of rock bodies.
	Middle Proterozoic basement of the North American craton, last deformed and metamorphosed to high grade during the Grenville orogeny about 1 billion years ago. Exposed in the Grenville Province of the Canadian Shield and the Adirondack Mountains, elsewhere buried beneath sedimentary rocks of the platform.
	Middle Proterozoic basement deformed at high metamorphic grade during the Grenville orogeny, and again, at medium grade, during the Taconian orogeny. Some areas also deformed during the Acadian and Alleghanian orogenies.
	Marine sedimentary rocks of an ancient continental shelf and slope, but also includes rift volcanic and rift sediments. Late Proterozoic to Ordovician in age. Primarily deformed during the Taconian orogeny, with some areas deformed by the Acadian orogeny in New England, and the Alleghanian orogeny in the Mid-Atlantic Region. Low to high metamorphic grade. Toothed areas are remnants of Taconian thrust sheets that were transported westward onto the platform.
	Early Paleozoic ultramafic rocks. Slivers of serpentinized oceanic crust welded onto North America as the Iapetus Ocean closed during the Taconian orogeny. Hachured areas show metamorphosed pre-Taconic mafic Baltimore Complex in Maryland, and undeformed post-Taconic Cortlandt Complex in southeastern New York.
	Connecticut Valley Synclinorium. Metamorphosed volcanic and marine sedimentary rocks of Cambrian to Devonian age. Primarily deformed during the Acadian orogeny at medium metamorphic grade.
	Bronson Hill Anticlinorium in New England, and the Charlotte and Carolina belts to the south. Metamorphosed volcanic island arc rocks, associated marine rocks, and exotic Proterozoic basement. Low to high metamorphic grade. This terrane was added to North America with the Taconian orogeny. Later deformed by the Acadian and Alleghanian orogenies.
	Merrimack Synclinorium. Metamorphosed volcanic and marine sedimentary rocks of Cambrian to Devonian age. Deformed primarily during the Acadian orogeny at medium to high metamorphic grade.
	Avalon Terrane. Granite and associated volcanic rock of Late Proterozoic m age, overlain by metamorphosed marine and continental platform rocks of Late Proterozoic to Ordovician age. Low to medium metamorphic grade. The Avalon microcontinent was added to North America with the Acadian orogeny. Deformed by the Alleghanian orogeny. Small pieces of West Africa (not shown) remained attached to the Avalon terrane.
	Igneous and metamorphosed granitic rocks. Mostly Devonian through Pennsylvanian in age and intruded during the Acadian and Alleghanian orogenies. Includes both strongly deformed and undeformed rock.
	Mississippian and Pennsylvanian marine and terrestrial sedimentary rocks m deposited in basins that formed after attachment of the Avalon terrane to North America. The basins were created, filled, and deformed during a sideways-slip phase of the Alleghanian orogeny.
	Valley and Ridge Fold Belt. (1) Anthracite grade rocks of Mississippian and Pennsylvanian age. (2) Platform rocks of Middle to Late Paleozoic age. (3) Platform rocks of Cambrian and Ordovician age. Deformed by compression during the Alleghanian orogeny.
	Paleozoic sedimentary rocks of the North American Platform. Deformed into broad open folds during the Alleghanian orogeny. These folds gradually diminish into unfolded strata that glided northwestward on layers of salt. Compression has shortened the unfolded strata parallel to layering by 10 percent.

The earth's crust is of two major types, continental (35 km thick) and oceanic (10 km thick). Continental crust is less dense than oceanic crust, which causes the continents to stand higher than the ocean floors.

Three major classes of rock make up the earth's crust: igneous, metamorphic, and sedimentary. Continental crust is divided into crystalline "basement" rock and overlying layers of sedimentary rock or layered volcanic rock. The basement is a complex of metamorphic and igneous rock bodies that were generated in the rootsof mountains. These record long, repeated cycles of submergence; burial by sedimentation; mountain-building with metamorphism, igneous intrusion, and voicanism; and uplift and erosion. Sedimentary rock blankets the basement over three quarters of the earth's continental area. This blanket ranges in thickness from a feather edge to more than 14 km. It includes many individual layers that began as widespread horizontal accumulations of sediment such as sand and mud. As sediment piles up, the lower part is compressed under the load. Water is squeezed out of the pores, and eventually the sediment is cemented into rock. These processes and cycles are summarized by the Rock Cycle diagram below.

Plate Tectonic Mechanisms

The earth's crust is deformed by a mechanism called plate tectonics. There are two types of crust, oceanic and continental. Oceanic crust is made of basalt and is the denser of the two, hence low-standing (below sea level). Continental crust is less dense, thicker, more buoyant, and therefore high-standing. The crust is the outer part of earth's rigid outer shell, the lithosphere. The lithosphere is broken into about a dozen large and several small segments or plates, most of which are made up of both types of crust. These plates float and glide about on a thick layer of denser material, the asthenosphere, which. is plastic rather than rigid. Heat from the decay of radioactive elements causes this plasticity and creates convection currents in the asthenosphere. These currents break the overlying lithosphere into plates and move them about. This movement causes three types of tectonic deformation at the edge of the plates: 1) Stretching. Plates break apart. This process may stretch a continent until rift basins form or may split one into two new continents with an ocean between them that grows as they move apart. Lava rising along the break adds new basaltic (oceanic) crust to the plate edges. 2) Compression. Converging plates force the edge of one plate below the other and may cause continents to collide. Such continent collision causes folding, faulting, volcanism, and mountain-building (orogeny), and may add new terrane to the edge of a continent. 3) Sideways slip. Plates move sideways past each other, grinding their edges together and forming local uplifts and basins.

The northeastern United States has undergone more violent tectonic events over a longer period of time than any other region of the country. The basement rocks in this region record a long, complex Proterozoic history beginning with crustal stretching, ocean formation and marine sedimentation about 1.3 b.y. (billion years) ago and ending with the Grenville orogeny about 1.0 b.y. ago. Erosion followed for a very long period; then a sequence of plate tectonic events began that were to add considerable territory to North America. Major events were the Taconian orogeny, caused by a collision with a volcanic island arc 460-440 m.y. (million years) ago; the Acadian orogeny, caused by a collision with a microcontinent called Avalon 410-380 m.y. ago; and the Alleghanian orogeny, caused by a collision with West Africa 330-250 m.y. ago. At this point, the earth's continents were assembled as a supercontinent, Pangea. About 220 m.y. ago, a stretching event began that split Pangea and eventually formed the Atlantic Ocean, which is still growing as the modem continents continue to separate.

Map Description

In contrast to a geologic map, which shows the kinds and ages of rock in a region, a tectonic map displays the kinds and ages of deformation. The region shown on the map extends across a number of tectonic provinces. Compare the physiographic and tectonic maps to see how strongly the tectonic provinces influence the development of the landscape.

The Grenville Province of the Canadian Shield (salmon color on map) with its extension into the Adirondack Mountains consists of greatly contorted metamorphic rocks that are 1.3 to 1.0 b.y. old. These abut 2.7 b.y.-old rocks of the Superior Province to the west (beige) at the east-dipping thrust zone known as the Grenville Front. The Grenville Province extends southward in the subsurface as geological basement all the way to Texas and into Mexico. The depth to basement in the map area is known from drill holes and inferred from geophysical measurements.

Overlying the basement are horizontal layers of sedimentary rock (pale yellow), which extend west to the Rocky Mountains and south to the Gulf of Mexico and make up what geologists call the North American platform. Toward the east, these layers become progressively more compressed, first into gentle folds (ocher) within the Appalachian Plateaus, then into sharp tight folds in the Valley and Ridge Province (orange), and finally into the multiply deformed fold and thrust belt (dark blue and red). Still farther east are several terranes that have been added to the North American continent since the early Paleozoic by continental collisions. These are the Connecticut Valley synclinorium (light green); the Bronson Hill anticlinorium (dark green); the Merrimack synclinorium (yellow-green); and the Avalon terrane (brown) with the Narragansett and Boston Basins (blue-green). Areas shown in lavender are mainly granite intrusions of Devonian or Carboniferous age.

The areas shown with dot pattern, both on land and offshore, are Mesozoic basins, formed during plate tectonic stretching, that are filled with undeformed sedimentary material. The heavy lines with hachures are faults along which these rift basins were down-dropped. Shown in gray is the Atlantic Coastal Plain. It is composed of unconsolidated sediments and sedimentary rocks that merge eastward with those of the continental shelf, slope, and rise.

The structure of the present continental margin of eastern North America is inferred from scattered drill holes and geophysical measurements. This structure developed in the stretching event that formed the Mesozoic basins mentioned above. At the Basement Hinge Zone the basement drops steeply to the east along a series of fault blocks. Between this zone and the East Coast Boundary Fault the crust gradually sagged, forming an elongate sedimentary basin, the Baltimore Canyon trough, where 15 km of sediment gradually accumulated during the Mesozoic Era. The basement in this zone is stretched and rifted continental crust that has been highly fractured and much intruded by oceanic basalt, making it transitional between continental and oceanic crust. Between the East Coast Boundary Fault and the J-3 fault scarp, the crust consists of lava flows interbedded with sedimentary material. East of the J-3 scarp, this marginal oceanic crust grades into true oceanic crust, formed entirely by intrusion and extrusion of basalt at a mid-ocean ridge.

	Glacial and alluvial sediments (Pleistocene to Recent) Till, gravel, sand, mud
	Rocks of Middle Proterozic Age (1.3-1.0 billion years old)
	Leucogranitic gneiss (lacks dark minerals); biotitic gneiss in Vermont, Connecticut, and Massachusetts
	Metanorthosite (metamorphosed anorthosite)
	Olivine-bearing granitic gneiss
	Metagabbro (metamorphosed gabbro) and amphibolite
	Hornblende granitic gneiss and pyroxene-hornblende granitic gneiss (charnockite)
	Pyroxene-(hornblende) syenitic gneiss (mangerite)
	Mixed rock: mangerite or charnockite with plagioclase crystals from anorthosite
	Interlayered hornblende granitic gneiss and amphibolite
	Tonalitic gneiss
	Metamorphosed sedimentary rocks: dominantly calcitic and dolomitic marble, calcsilicate rock, quartzite, interlayered gneisses; also in Vermont
	Biotite-quartz-plagioclase gneiss and migmatite; may contain garnet

	Leucogranitic gneiss (lacks dark minerals)
	Interlayered hornblende granitic gneiss and amphibolite
	Hornblende granitic gneiss
	Biotite granitic gneiss, hornblende granitic gneiss
	Pyroxene-hornblende granitic gneiss (chamockite)
	Garnet-quartz-feldspar gneiss, minor marble, amphibolite, rusty gneiss
	Rusty and gray biotite-quartz-feldspar gneiss with variable amounts of garnet, sillimanite, cordierite, graphite, sulfide; minor marble and calcsilicate rock
	Biotite-quartz-plagioclase gneiss with subordinate biotite granitic gneiss, amphibolite, calcsilicate rock
	Garnet-biotite-quartz-feldspar gneiss, quartzite, quartz-feldspar gneiss, calcsilicate rock
	Calcitic and dolomitic marble, calcsilicate rock, interlayered gneisses

	Normal geological contact
	Intrusive contact, dashed where uncertain; on legend only
	High angle fault, generally between rock bodies of greatly different ages
	Thrust fault, barbs on overthrust plate; Middle Ordovician age
	"Cameron's Line," separates ancient North America from an accreted island arc terrane; Middle Ordovician age
	Carthage-Colton Mylonite Zone; ductile normal fault, west side down; Middle Proterozoic age
	Location of cross-section line

Geologic cross sections show hypothetical vertical slices through the earth's crust. These two cross sections permit the reader to compare the geology of three sharply different types of mountains, the domed Adirondacks, the residual Catskills, and the folded and thrusted Appalachians.

Cross section ABCDE (located on the geologic map) extends from the St. Lawrence Lowlands, across the Adirondack Mountains, and into Vermont. It shows the surface of the earth in profile and the inferred configuration of the rock bodies below the surface. Notice how subdued the surface profile appears when drawn at true scale. The profile above the cross section, drawn with a 5-fold vertical exaggeration, provides a more familiar (although distorted) picture of the topographic relief. The areas shown in shades of blue represent a variety of metamorphosed sedimentary rocks. These rocks were originally deposited in a shallow sea as horizontal layers, and later deformed into tight folds during the Grenville orogeny about 1.1 billion years ago. The Taconic Mountains and Green Mountains east of the Adirondacks are part of the Appalachian Mountain chain. Bedrock there was thrust westward tens of kilometers along faults during the Taconian orogeny about 450 million years ago. Several faults not shown on the geologic map are shown in the cross section as examples of their character and importance.

Geologic cross section FGH extends southward from the Mohawk River to the Catskill Mountains, then southeast across Long Island. In order to show the thin rock units, it was necessary to use a 5-fold vertical exaggeration for this cross section. The use of vertical exaggeration amplifies not only topography and rock thickness, but the tilt of the rock layers as well; the strata between F and G are actually nearly horizontal! At the Shawangunk Mountains, near the western edge of the Appalachian fold belt, they are slightly upturned. Proceeding eastward Appalachian deformation steadily increases. The strata of Bellvale Mountain have been folded into a large syncline, and faults have dropped this structure down even more. Farther east, these same Paleozoic rock units become sufficiently deformed and metamorphosed to constitute "Appalachian Basement." This basement has been overridden by the "Grenville Basement" of the Hudson Highlands, but continues to the east beneath the downfaulted Newark Basin. It comes to the surface east of the Palisades diabase sill and Hudson River where, along with Proterozoic basement, it makes up the bedrock of Westchester County and the New York City area. The terrane east of the "Cameron's Line" fault was transported westward great distances as a volcanic island arc until it collided with the North American continent causing the Taconian orogeny. Seaward of New York City, the deeply eroded island arc terrane forms the basement beneath Cretaceous and overlying Pleistocene deposits. The small fault basin beneath the Cretaceous deposits is of the same age and character as the Newark Basin.