Heldeberg Escarpment Planning Guide

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Go to Mohawk Hudson Land Conservancy to purchase the complete book.

A Project of the Helderberg Escarpment Planning Committee

February 2002

Daniel A. Driscoll and Lindsay N. Childs, Editors

II. Geology

The Helderberg plateau consists predominantly of layers of shale. sandstone and limestone, all lying on top of older shale and sandstone layers which extend north across the Capital District. The exposed cliff face of the Escarpment has revealed this layering to generations of scientists, providing evidence of nearly half a billion years of geological history of the region.

The exposed layers of sedimentary rock contain a rich array of fossils that have made the Escarpment an important scientific and cultural resource. In fact, the Helderberg region is the birthplace of American paleontology (the science of the study of fossils). Many fossils of worldwide distribution were first found in this area: such fossils provide a basis for the time correlation of geologic events around the world (Landing, 1991).

Water flowing through the layers of limestone on the plateau above the Escarpment has dissolved and eroded the rock, leading to a geological landscape known as karst terrain. The characteristic features of karst areas are the existence of caves, sinking streams, sinkholes, complex underground drainage, and springs. The caves found in karst terrain have important ecological and recreational significance. The karst terrain as a whole poses significant development problems, because of the uncertain quantity and quality of ground water.

Sections A and B of this chapter will survey the geological history of the region, in order to explain the presence of the fossils, the karst terrain. and the Escarpment. A reader not interested in the details of geological history may with to skip to Section C, which focuses on the karst topography and its implications for development.

A. Geological History of the Region

The 'basement" rock throughout most of New York State is a metamorphic rock, the residue of the Grenville orogeny (orogeny = great mountain building event), which occurred about 1,300 million years ago. According to the theory of plate tectonics, the outer thin shell of the earth is broken into a small number of enormous plates (perhaps 12) that resemble slabs of Arctic sea ice floating on the ocean. These plates are constantly moving, probably as the result of convection currents in the molten interior of the earth. Over long periods of time, some of these plates collide, some grind past each other, and some pull apart. For example. The interaction of two plates along the Pacific coastline of North America is the cause of the frequent earthquakes in California and Alaska (Van Diver, 1992).

The mountain building event known as the Grenville orogeny occurred when the North American plate collided with the African plat, This created a mountain range stretching from Labrador to Mexico, including the ancestral Adirondacks. The rock in the Adirondacks is 1,100 million years old. During this time animal and higher plant life did not exist, but the fossil record shows evidence of algae.

The African and North American continents separated around 650 million years ago. By that time, the mountain range had been reduced by erosion to a relatively flat surface. Large parts of New York were covered by the sea during the latter part of this period and much of the subsequent 200 million years. As the high land was worn down by wind and rain, freezing and thawing, the sand, grit and silt grains eroded from the high land were carried by streams and rivers and deposited in the seas. The pressures caused by the buildup of these deposits caused the bottom layers to turn into sedimentary rock. With sufficient pressure and heat, for example, during mountain building events, sedimentary rock will change into metamorphic rock. The Grenville basement rock became metamorphic by this process.

The erosion and sedimentation continued during the Cambrian period (between 540 and 505 million years ago), when the first animals with shells appeared, and during the Ordovician period (505 to 438 million years ago), when the first vertebrates, the fishes, appeared. Shales and sandstones from the middle Ordovician period form the bedrock in the Albany area northeast of the Helderberg Escarpment. Table 2-1 shows the approximate thickness of the layers of bedrock in the study area, their names, and the geologic time period when the sediments were deposited. Table 2-2, reprinted with permission of the New York State Museum from Educational Leaflet N. 28 (Isachsen, 1991) graphically depicts the relative time scale, geological events, and the fossil record.

The next important mountain-building event in the area, the Taconian orogeny, began about 460 million years ago (Figure 2- 1). Huge slices of the earth's crust were thrust into the area east of the present Hudson River to form the Taconic Mountains (Figure 2-2). The slices generally dipped towards the east in a shingled arrangement in a stack ranging from New England past the western edge of the Hudson Valley. One slice, the Livingston thrust, is the primary bedrock south of Feura Bush between the Helderbergs and the Hudson River (Rogers, 1990).

West of the Taconic Mountains was a shallow inland sea, which was gradually filled in by erosion of the Taconics around 440 million years ago (late Ordovician period). This Queenston delta plain, in turm, was itself uplifted and subjected to significant erosion. A layer of this Ordovician sandstone and dark shale. named the Indian Ladder Formation, remains in the Albany area and is found at the base of the Helderberg Escarpment.

Most of the layers of limestone, Dolomite, sandstone and shale which make up the Helderberg Escarpment were formed between 435 and 387 million years ago during the Silurian and early Devonian periods (after plants and animals had moved onto the land and winged insects appeared). One layer, known as Oriskany Sandstone, is well known to paleontologists because of the fine collections of remarkable fossils that have been obtained from it (Goldring, 1935).

Table 2-1 Geologic Section of the Study Area
Middle Devonian Hamilton Group Shale & Sandstone 2000 plus
Middle Devonian Onondaga Limestone 140
Early Devonian Tristates Group Schoharie Grit 25
Early Devonian Tristates Group Esopus Shale 175
Early Devonian Tristates Group Oriskany Sandstone 5
Early Devonian Helderberg Group Becraft Limestone 15-50
Early Devonian Helderberg Group N. Scotland Limestone 100
Early Devonian Helderberg Group Kalkberg Limestone 60
Early Devonian Helderberg Group Coeymans Limestone 20-35
Early Devonian Helderberg Group Manlius Limestone 55
Early Devonian Helderberg Group Rondout Dolostone 0-3
Middle Ordovician Schenectady Beds Indian Ladder Shale 0-410
Middle Ordovician Schenectady Beds Schenectady Shale 2000 plus
(from Generalized Bedrock Geology of Albany Co.; Fickies 1982)

Between 410 and 380 million years ago, the Acadian mountain-building event created a much loftier range of mountains in New England, east of the heavily eroded Taconics in New York. Erosional sediment from these mountains spread west and covered much of Albany County, displacing a shallow inland sea and forming the Catskill Delta between 380 and 345 million years ago (middle and late Devonian period). During this time the earlier strata were tilted towards the west and mildly folded. The present bedrock of southwestern Albany County, southwest of Thompsons Lake and Clarksville, is predominantly Hamilton shales from the middle Devonian period, except near streams. For example. the Onesquethaw Creek has eroded the shales and left a broad expanse of Onondaga limestone (and more downstream, Helderberg group limestone) adjacent to and extending outward from the creek bed.

At the site of the Gilboa dam, 40 miles west-southwest of Albany, a petrified forest of middle Devonian origin was found, suggesting that the area in the western foothills of the Acadian mountains included swampy terrain that supported a substantial tropical forest.

The Catskill Delta in New York. with the underlying early Devonian formations, has yielded one of the most complete fossil records of the Devonian period found anywhere in the world.

Around 250 million years ago, all the continents in the world united in one super-continent, called Pangaea. Around 200 million years ago Pangaea began to break apart to form the modern continents and ocean basins. The Atlantic Ocean basin began at that time, and continues to widen even today, continually splitting apart along the mid-Atlantic rift system at the rate of about 2 centimeters per year (V. Diver, 1992).

The modem New York landscape is largely the product of slow erosion during the past 200 million years. The broad outlines of the modern mountains. valleys and plains were carved especially by the rush of water to the seas during the last 65 million years (the Cenozoic Era), just after the extinction of the dinosaurs (Scheffel, 1973).

The Helderbergs started to take shape when the northeast began arching upward at the beginning of the Cenozoic Era. The center of the uplift appears to be the dome of the Adirondack Mountains; they seem to have been lifted straight up from below, as if the Earth's crust were being pushed from beneath by a giant fist. This process continues to this day, at rate of close to 3 meters every thousand years in the center. This upward arching has tilted the layers of rock in the Helderberg plateau towards the south. The current dip of the rock layers is between 1 and 2 degrees (about 135 feet per mile) in a south-south west direction (about south 20 to 25 degrees west) (Baker, 1976 and Mylroie, 1977).

The Helderberg Escarpment was formed primarily during the early Cenozoic Erie; the general topography resulted from erosion along outcrop belts of weak rock. The layered rock formations found on the Helderberg plateau (see Table 2-1) originally stretched northward across the plains of the Capital District to the Adirondacks and east to the Taconics. Through stream and atmospheric erosion, the various layers were gradually worn away, beginning at their contact with the mountains to the north and east. This erosion created a rock cliff that was the beginning of the Helderberg Escarpment.

As the Adirondack uplift continued, erosion caused the Escarpment to move towards the south and west and to grow higher. Rivers formed and began etching through the accumulated sedimentary rock on the Helderberg Plateau, and then through the early Devonian limestone layers. The lowland formed by this recession of the Escarpment is now occupied by the Mohawk and Hudson River valleys (Figures 2-2 & 2-3). The Escarpment survives as a cliff in places such as Thacher Park because of the highly resistant layer of Coeymans limestone that tops the cliff the Escarpment has retreated southward through erosion of the underpinning of the Coeymans layer.

Where rivers and streams on the Helderberg and cut through the sedimentary layers atop the early Devonian limestone layers (Figure 2-4), the stage was set for the formation of underground waterways and caves such as near the Escarpment face and near Onesquethaw Creek

B. Glacial Geology

While the large-scale topographic features of the greater Helderberg region were formed by uplifts and erosion, the small-scale modern-day topographical features of the region were molded by glacial advances and retreats during the Ice Age (Pleistocene Epoch) which began in North America about 2 million years ago and ended only about 6000 years ago. In New York the latest, or Wisconsin glaciation swept away clues of earlier glacial advances, so only features of the Wisconsin advance are well preserved (Van Diver, 1992).

According to one account the branch of the last glacier that covered Albany County moved southward between the Adirondacks and the Green Mountains along the great trough formed by the basin of Lake Champlain and the Hudson River valley. The glacier was forced diagonally across the lowland of the upper Hudson basin and against the rather formidable barrier of the Helderberg Escarpment. Confinement of the ice to the lowlands, however, was only temporary until its thickness built up sufficiently to mount the Helderberg plateau and move southward over it (Goldring, 1935).

The direction of a glacier's movement can be determined by the effects it had on the landscape. Drumlins, for example, are hills of glacial debris that were molded into streamlined forms by overriding ice. They are elongated in the direction of ice movement.

The axis of the glacier as it moved across the Helderberg Plateau is a line through Dunnsville, the hamlet of Knox. and West Berne (approximately West 40 degrees South).

A boulder pushed by the glacier would have moved in a straight line only along this axis: to the right and left of this line, the flow swung away in long sweeping arcs. Southeast of the axis the ice flow curved to the south along the Hudson River Valley. Northwest of the axis the ice flow tended to diverge to the west along the Mohawk Valley until it merged, somewhat beyond Utica, with another branch of the glacier flowing south along the western edge of the Adirondacks.

The net result is the unusual Juxtaposition of east-west drumlins at Duanesburg, and north-south drumlins a few miles to the southeast atop the plateau in the Helderberg Mountains. The dense field of drumlins near Duanesburg marks one of the few places in New York where Wisconsin ice moved from east to west (Van Diver, 1992).

Approximately 14,700 years ago, the Wisconsin ice sheet receded from the Helderberg Plateau (the first evidence of man in America is from about 11,700 yeas ago). "Probably the comparatively clean ice which swept across the region and its position far above sea level served to prevent the floods due to the melting of the ice from mutilating or burying under sediments the exceptionally fine example of glaciated land which this very last movement of the latest ice sheet created here" (Goldring, 1935). There are scattered heaps of moraine (glacial till) and some crude terraces associated with the dissipation of the ice; most of them are located within the drainage basin of the Foxenkill.

The till left by the melting glacier (made up of boulders, rocks gravel, sand and silt) is not the kind of soil that is ideally suited for farming. "The ubiquitous stone walls in the northeast were not built only to mark boundaries of fields: they were also repositories for the boulders removed from fields in an almost endless attempt to create plowable domains. No wonder northeast farmers picked up and moved west when less stony land opened up beyond the Appalachians. When the Erie Canal and later the railroads were built, cities in the northeast had access to rich farmlands to the west, and no longer were farmers forced to coax crops from an ice age's dumped detritus. Stone walls that were laboriously built to clear and mark fields now wander through woodlands like the silent remnants of a lost civilization" (Raymo 1989).

During the close of the glacial period, a body of water known as Lake Albany covered the Albany lowland. As a result, the bedrock in the Albany area is more or less covered with boulder clay, sands, gravels. etc. deposited by the ice sheet, and stratified clay deposits laid down at lower levels of Lake Albany. The Pine Bush Preserve in Albany, Colonie and Guilderland protects a small remaining portion of a 25,000-acre ecosystem that developed on the sand dunes of the lakebed of Lake Albany. "The clay and loam beds of this old lake bottom have made the very rich, level farm lands which stretch from Albany westward to the Voorheesville and New Scotland areas" (Goldring 1935).

Prior to its departure, the glacier left behind a unique signature of its presence in Clarksville. This is found in the form of a dry gorge west of Stove Pipe Road. This old streamflow channel, referred to as Stove Pipe Paleogorge, was carved by glacial meltwaters draining from north to southeast during the retreat of the Wisconsin glacier (Rubin 1990). Of more significance is the likelihood that Stove Pipe Paleogorge is related to the karstic drainage network in the Clarksville area. One part of this network is Diddly Cave. The presence of small wave-like features in the cave indicates rapid streamflow along the base of the Wisconsin ice sheet. Rounded stream cobbles in the cave provide further evidence of former stream action. The recent finding of the bones of a varying hare and the extinct passenger pigeon in clay deposits in the cave may provide important scientific information on the latter recolonization during or following deglaciation, and it may help to date regional deglaciation. The Stove Pipe Paleogorge together with its related karstic drainage network may be a geologically unique feature (Rubin 1990).

C. Karst Geology

In many regions on the earth where limestone rocks underlie surface soils there is a landscape known widely as karst, so named for a limestone area along the coast of Yugoslavia; the Slovene word kras indicates bare, stony ground. Karst terrain exists on the Helderberg Plateau wherever the bedrock is Helderberg group limestones or Onondaga limestone (Figure 25). Karst areas, with their characteristic features such as caves, sinking streams, sinkholes, limestone pavement and complex underground drainage systems, are created by the chemical interaction of acidic water and carbonate rocks.

When rainwater combines with carbon dioxide from the soil a weak carbonic acid is formed. As this acid solution flows through cracks and crevices in the limestone, it literally dissolves away the rock. These cracks and crevices enlarge steadily as mom limestone is dissolved. When the limestone lies at the surface, or is only protected by a thin layer of soil, the area is called a limestone pavement; such an area has the appearance of a giant irregular checkerboard of limestone blocks. Large areas of limestone pavement exist in the Helderbergs, the spaces between the limestone blocks can be one-half to three feet wide. three a three hundred feet long, and one-half to sixty feet deep (Mylroie 1977). Figure 2-6 shows areas in the Helderbergs where limestone pavement is likely to be found; these are areas of very shallow soils over limestone bedrock.

In areas where the limestone layers lie below the surface, the cracks and crevices enlarge to form conduits or tunnels. Figure 2-4 illustrates the development of such a conduit. Ultimately, the solution of the limestone bedrock can form large caves. Several well-known caves in the study area are Haile Cave (which has an opening on the face of the Helderberg Escarpment), Clarksville Cave, and Knox Cave. Once formed, these complex systems allow groundwater to flow rapidly, unimpeded by soils. Water flowing through karst systems does not, therefore. have the advantage of being filtered and purified by soil. In karst areas, surface water is frequently diverted into underground routes. Areas where water enters underground drainageways are called insurgences. Insurgences may be small features, such as sinkholes, or may cover large areas, for example, limestone pavements. Contaminants placed in or around insurgences can be carried long distances by water entering the underground conduits.

All karst aquifers have at least one discharge point. Some caves have multiple discharge poi.. These points are called resurgences or karst springs.

Wells on karst plateaus are likely to have highly variable yields. Well data indicate that water in the fractured stone is drawn down toward the cave conduits. The only high-yield wells are those penetrating a conduit or a water-filled fracture (Baker 1976).

Go to |Mohawk Hudson Land Conservancy to purchase the complete book.

Used by permission of Dan Driscoll