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TABLE
OF CONTENTS Highland Scenery Magma Intrusions Peneplain Banding & Folding Slickenslide Dip Slopes Joints Dikes & Sills |
THE ROCK OUTCROPPINGS seen in road cuts, on mountain tops and in cliff faces not only add to the scenic beauty of the Hudson Highlands, but also allow geologists to determine the history of the region. The rocks that make up these low, rugged mountain ranges extending from Pennsylvania to Vermont include the Reading Prong, New Jersey Highlands, Ramapo Mountains, Housatomic Highlands, Berkshire Mountains and Green Mountains. These are very ancient, Precambrian, rocks which are among the oldest in the United States and have been folded, faulted (broken), uplifted, eroded (worn down), invaded by several igneous (melted rock) phases and metamorphosed (changed by heat and pressure). (A detailed history of the region is shown on the Geologic Timetable.)
The rocks of the Highlands are igneous granites and metamorphic gneisses and schists. Bear Mountain is composed of Storm King granite, a medium to course grained gray material, which is very resistant to weathering. This rock was cooled and solidified from molten magma that was intruded into the Precambrian, metamorphosed Canada Hill granite/gneiss. The remainder of the rocks of the region are referred to as the Highlands Complex. This large variety of rocks have been intensely metamorphosed, deformed and broken during the several igneous invasions and intervals of regional deformation.
Much of the spectacular scenery that can be viewed throughout Bear Mountain and Harriman State Parks and the surrounding Hudson Highlands is the result of erosion. The combined processes of erosion and weathering have produced the soils that are able to maintain the beautifying vegetation that is so abundant throughout the region. Erosion by glacier ice has cut the Hudson River fiord, formed lakes and shaped the rounded mountains. Rapidly flowing brooks and rivers have cut valleys into the bedrock adding to the scenic beauty of this geologically diverse region. Now it is the responsibility of all of us to maintain and preserve this natural wonderland for future generations.
When magma is intruded into cracks, or joints, in rocks that are buried miles below the surface of the earth the molten rock cools slowly. This slow cooling process allows time for relatively large crystals of the rock forming minerals to grow. The mineral composition of the granite that results from the solidified magma includes: feldspar, quartz, mica and hornblende.
The talus block, shown above, was plucked from the rock knob at the Indian rock shelter near Lake Tiorati. The gneiss bedrock, from which this boulder is formed, is a fine grained, gray matrix. The intruded granite in this boulder is much courser grained resulting in the rough exposed surface that is easily seen in the photograph. In this rock sample the intruded magma resulted in both horizontal and near vertical bands through the original bedrock layers.
From the Cretaceous Period of the Mesozoic Era about 150 million years ago, to the Oligocene Epoch of the Tertiary Period of the Cenozoic Era about 50 million years ago, was a time during which little crustal activity occurred. (See Geologic Time Scale) The local Appalachian Highlands, that had been uplifted during the Permian Period of the Paleozoic Era about 250 million years ago, were slowly worn away by the processes of weathering and erosion. The land was reduced to an almost flat surface, near sea level, called the Schooley Peneplain.
As one stands on the observation deck in Perkins Tower, at the top of Bear Mountain, and looks to the west or north the very flat horizon that is observed represents the remains of a peneplain formed in ancient times. Renewed uplift of the region during the Miocene Epoch of the Tertiary Period of the Cenozoic Era, resulted in erosion cutting into this level surface, which was further modified by glaciers and recent water erosion.
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Gneisses of the Hudson Highlands are metamorphic rocks formed from granite, sandstone, shale and molten magma deep beneath the surface of the earth. During the metamorphic process the minerals composing the rocks may separate into layers or bands. After separation these layers may be folded as forces are applied to the rock while it is in a plastic state.
The left hand photograph above shows banding in gneiss along Seven Lakes Drive south of Lake Kanawauke Circle and the right hand picture is a close-up of the portion of the outcrop that most clearly shows the folded bands. These banded and folded rocks are associated with the slickenside outcrops that indicate faulting in the region.
Faulting (breaking) of subsurface rocks results in one rock surface rubbing against another. During this process blocks of bedrock sometimes become polished and groves are cut on their surfaces. The slickenside surfaces thus produced are preserved underground for millions of years as long as they are protected from weathering. When these rocks are exposed in road cuts or on cliff surfaces, as rocks break free, the smoothed and grooved surfaces provide a record of ancient earth movements.
Pictured above is one of several slickenside surfaces which are located along Seven Lakes Drive, south of the Kanawauke Circle. The almost vertical striations that were once clearly visible on this slickenside surface have been almost completely removed by weathering of the rock.
The Precambrian sedimentary rocks that today make up the hills of the Hudson Highlands have been folded, faulted, buried, metamorphosed, uplifted, and eroded several times during their long geologic history. The Canada Hill granite/gneiss that forms much of the bedrock still shows some evidence of this past history. Erosion of the fairly uniformly resistant, sloping layers has developed a number of southeasterly facing hillsides. These dip slopes are the sides of the ancient sedimentary folds whose axes usually ran southwest to northeast.
The photograph above shows one such dip slope that lies west of Seven Lakes Drive between Lake Tiorati and Lake Askoti. During the process of erosion, plate exfoliation has helped to maintain the original contour of the folded surface which has resulted in the rounded hilltop and steeply sloping hillside.
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As the igneous rock, that was formed from molten magma, and the metamorphosed gneisses cooled they contracted and cracks or joints were formed. The jointed rocks shown here are located near Perkins Tower in the picnic area at the top of Bear Mountain [left photograph is granite] and along an unmarked trail leading to Pine Swamp Mine from Lake Tiorati [right photograph is a gneiss]. The joints in most rocks are only a few feet apart but in some locations on top of Bear Mountain they may be separated by 20 to 30 feet. Once exposed to air, chemical weathering widens the joints which allows water to enter. Freezing pries the cracks wider and tree roots add to the prying action as shown.
When magma is forced into cracks in the bedrock that overlies a mass of molten rock dikes or sills are formed when solidification takes place under the earth's surface. Dikes are formed when the molten rock has cut across older rock layers and sills result when the magma is squeezed between existing layers of rock. These molten intrusions may be from part of an inch to many feet thick and extend for miles. The Hudson Palisades are partly a sill and partly a dike which are composed of dark colored basaltic rock. The most common dikes in the Highlands are composed of light colored, large crystal granite intrusions into fine grained gneisses.
The photograph above shows a dike composed of a dark band of basaltic rock which cuts across the lighter layers of gneiss. This dike is located on the west side of the Palisades Interstate Parkway about 2.5 miles south of the Visitor Center.
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