Rocks of the Ridge

No single feature so defines our region as does the Shawangunk Ridge. The long white battlement of this escarpment is a constant reminder of the bedrock that underlies our lives, mostly out of sight and out of mind. Here in New Paltz we have an opportunity to encounter naked rock, to gain an intimacy with the raw stuff of the planet that is unparalleled in most of eastern North America.

Many may have found the study of geology in high school or college, with its specialized vocabulary reflecting the earth’s long and complex history, an intimidating subject. But all children enjoy the feel of stones, their heft, texture and mass. Sensory experience with so elemental a material as rock naturally leads children toward wonder and curiosity, just as it led the first geologists to ask questions that were at once simple and profound. Men such as William Smith and James Hutton, working in the British Isles just over 200 years ago, found that the questions prompted by looking at landscapes and the rocks that formed them required answers more complex than church doctrine of the time could provide. In this way geology, youngest of the sciences, was born. Geology has come a long way in two centuries towards unraveling the mysteries of earth history, hence the mass of information that seems overwhelming to the student. But it starts with observation, and curiosity, with turning a stone in the hand, or puzzling over the tilt of rock layers sighted across a ravine, or wondering about scratches on stone surfaces. And that basic experience is more available to all of us here where we live than it is in many other places.

To begin with, people in the immediate vicinity of New Paltz, and the ridge itself, are likely to find just two rock types, both sedimentary: shale and conglomerate. Our valley, through which the Wallkill River flows, is underlain by up to 10,000 feet of black shale. Shale is basically ancient clay, or mud, compressed into rock by the weight of sediment above it. Our shale, called the Martinsburg formation, was formed from clay and silt laid down at the bottom of a sea that covered the region during the Ordovician period, about 465 million years ago, long before any dinosaurs roamed the earth. In all that time, the thin, originally flat-lying, layers of this shale have been squeezed into folds, and tilted steeply, by three separate mountain-building episodes. This contorted rock can be seen in road cuts throughout the area, such as the western end of the Mid-Hudson Bridge. A great place to see an exposure of the shale is the Undercliff carriage road at Mohonk Preserve, at the Trapps. Here you can pick up and examine flakes of shale, which was used to help pave many of the Minnewaska and Mohonk carriage roads. You’re not likely to find any fossils in them, though, as the environment in which the shale was deposited was not favorable to their preservation.

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From Undercliff Road the cliff of Near Trapps, higher up on the ridge, stands out in dramatic contrast to the valley below, where erosion has worn down the soft shale. The Shawangunk conglomerate, younger than the shale and deposited on top of it, is one of the hardest rocks known, its pebble-studded sand cemented together by quartz. For this reason it was quarried for millstones (like those seen in fragments on some Shawangunk trails), which were chiseled by hand, and sent by canal boat and then river barge to port cities along the Atlantic seaboard in the 19th Century. You can find sheer cliff faces of this rock, favored by rock climbers, a little further along the same carriage road, where it is cleft by fissures formed by stress cracks, or joints, and wedged further apart by the action of freezing water. Cycles of freeze and thaw help to pry great slabs of rock loose from the ridge top that tumble down to form massive talus slopes at the base of the escarpment.

A good place to see how the structure of the Shawangunk beds has given us the landscape we admire today is the Millbrook ridge trail at the Preserve. To walk along the crest of the ridge here is to feel underfoot, as well as see, the westward tilt of the Shawangunk conglomerate, Called the “dip slope” by geologists, it is always opposite to the steeper escarpment, or cliff slope, which faces east. The same forces that thrust up the Appalachian mountain range to the south tilted these beds. The resulting escarpment is the longest continuous geological formation in North America, and has been softened by a mantle of soil and vegetation in the southern Appalachians, where it is known as the Blue Ridge. Here, however, the continental glacier that receded from our region 10,000 years ago after the last Ice Age sharpened the ridge and heightened its features. This great ice sheet, thousands of feet thick, stripped away soil, and left polished rock surfaces, with parallel scratches, called striations, and crescent-shaped “chatter marks,” for us to notice, and ponder, on our walks today. It’s just the latest chapter in the long history of our landscape. Perhaps the most valuable resource we can mine our rocks for now is the story they have to tell us, about the earth and how we are related to it.

 

Richard Parisio is a lifelong naturalist, educator and writer. He currently leads field trips for school classes at Mohonk Preserve, teaches courses about John Burroughs and conducts tours of Slabsides and the John Burroughs Sanctuary for groups and individuals by request. Rich is New York State coordinator for River of Words, a national poetry and art program on the theme of watersheds, and teaches River of Words programs for school classes, grades K-12, by request. Contact Rich (richparisio@gmail.com) with questions, comments, or suggestions for Nature at Your Doorstep.

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