What a Geologist Sees from the Walkway Over the Hudson

What a Geologist Sees from the Walkway Over The Hudson By John DeGilio, Walkway Ambassador A Layman’s overview of the geologic forces that shaped the Hudson River as seen from the Walkway over the Hudson. What a Geologist Sees from the Walkway Over the Hudson 1 Overview of the Geology of the Hudson River Valley The geologic history of the Hudson River Valley is best understood as a story in two chapters; the first is the formaon of the Valley’s bedrock and the second is the carving of the valley at the start of the last Glacial Age. This story began about 900 million years ago with some of the oldest rock structures on the North American connent. Chapter 1 The Formation of the Bedrock of the Hudson River Valley It is hard to find another place on the North American connent with bedrock as diverse as the Hudson Val- ley. Here, from the Walkway over the Hudson, you can see samples of every major type of rock – igneous (formed by the fires of volcanoes), sedimentary (deposited under bodies of water), and metamorphic (rocks altered by heat and pressure). Al- most every corner of this Valley contains loose glacial deposits mixed with sandstones, shales, and lime- stones. More to the point, they are sandy shales, limy shales, hardly any are purely one type of rock. And in most areas, these rocks are not found lying in flat layers the way they were originally formed. Because of the age of the deposits (some 500 million years old), they have been heated and compressed many mes. This acvity has converted them to a wide variety of metamorphic rocks. In some places this bedrock has been heated enough to be melted and redeposited. The map above shows the Taconic Uplands and Adirondack Mountains, which are among the oldest in North America. When we idenfy these specific formaons, we can easy tell that the region we see from the Walkway is very complex. There have been many changes over a short distances. To understand why it looks this way, think of the ground receiving a great “shove.” For those of us in Dutchess and Ulster Counes, that push started in Conneccut near Candlewood Lake. The folding and faulng forces acted from southeast pushed the bedrock to the northwest. The result was to align the hills and uplands from the East to the South- west. The next page shows an overview of the Earth’s geologic eras and points out the period of the rock formaon of the Hudson River Valley and the last two glacial ice ages to provide perspecve. What a Geologist Sees from the Walkway Over the Hudson 2 Figure 3: Geologic Era with Hudson Valley Milestones Included What a Geologist Sees from the Walkway Over the Hudson 3 As shown on the previous page, about 480 million years ago, the Taconic Mountains started to form as the ancient ocean closed and the connental plates collided. These mountains must have looked like a collecon of volcanic islands, similar to how Hawaii or Japan looks today. As the ocean became shallower, rock sediments collected as the island arc forced its way along the connental crust. It is likely at the end of the mountain building period, the Taconics were as high as the European Alps. Many of the rock layers were transformed as we see them today when we drive across Interstate 84 going east to Conneccut. In some places the folding was intense enough to melt and deform the rocks, which concentrates the mineral content. Figure 4: Geologic Cross‐Secons of Eastern America When you cross the FDR Mid-Hudson Bridge, you can see this type of intense folding along the wall of the western approach to the bridge. In Dutchess County, the rocks of the southeast corner are harder and more resistant to erosion. From Southeast to Northwest, you can see that the layers are very so, almost flat at the hairpin turn on Route 199, crossing the Shawangunk Mountains. On the eastern border of Dutchess county are many layers of rock older than rocks below them. This is a result of a low angle fault in which rocks from Conneccut were “over thrust” the onto younger rocks. In some areas the layers were displaced many miles. At the rock wall in the Upper Landing Park you can see clear evidence of the compression and faulng. What a Geologist Sees from the Walkway Over the Hudson 4 To fit the Walkway into a larger view, review the map to the right. We drew a line from Candlewood Lake through the Walkway over the Hudson to the Shwangunk Mountains in Ulster County. If you examined the rocks that make the lake boom and surrounding areas, you would see that they are highly metamorphosed: mostly quartzite, marbles, and slates. They would show evidence of being subjected to extreme pressure and heang. Aer careful examinaon of the faults of the area, you would see that rock layers in Poughquag match up with rock layers in Conneccut. This “over thrust” fault shows the rocks have been displaced westward by miles. The folding and faulng would decrease as you reach the Walkway. In the exposed rocks of the Upper Landing Park, you can see the consequence of the folding and faulng. Figure 14: Secon showing the Once Sedimentary Rocks between Squantz Pond and Lake Mohonk Figure 15: Cross‐Secon of the Geology from Lake Mohonk to the Conneccut Border Imagine a mountain range at least as impressive as the Alps in Europe, formed over 580 million years ago. This is a period long before the dinosaurs (170 million years ago). There are be few flowering trees and land plants, but the creatures in the sea you could idenfy today. Then, comes an immense period of me where the rocks are subjected to catastrophic geologic forces many mes. Many of the rocks were modified from their original condion. The net effect would be an environment with a profoundly impacted geology of outstanding beauty. If you wan to see something different, you just look in a different direcon. What a Geologist Sees from the Walkway Over the Hudson 5 But, where did the Taconic Mountains go? Figure 5: Modern Catskill Mountains If the Taconic's were as high as the Alps in Europe where are they now? Aer the formaon of the Taconic Mountains, the sea was to the west not as it is today as we see the Atlan- c Coast. The materials eroded from the Taconic Mountains were deposited on a shallow sea to the west. What we see as the Catskill Mountains formed as a series of deposits that spread from the Hudson River Valley. You can see these Catskill Mountains from the east side of the Walkway only. We know the rocks of Catskills spread out to the west as the Allegheny plateau. They consist of flat lying bands of sedimentary rock that get thinner as you travel west to Ithaca famous for its gorges. layers going West. By the me these layers appear in Ithaca’s gorges, the layers have thinned to inches. By the me you reach the Buffalo area, they are thinner sll, but easily idenfied with the rocks in the Upper Hudson Valley. As a general rule for rocks forms under water, “...the further from shore the finer the sedi- ment.” Just before entering the Walkway elevator, you can see a set of displaced beds of very fine deposits. In at least one layer, you can see a hint of ripple marks on the upper surface of an embedded layer. A Note about the Fossils in the rocks of the Hudson Valley Two factors make finding fossils in the formaons here in the Hudson Valley very difficult. First, the intense folding, heang and deformaon we described above easily destroyed any exisng fossil evidence. Secondly, the marine life of the early Cambrian Sea was very delicate to begin with, resembling Man-o-War or Jellyfish today. Many of the creatures of the Cambrian and Early Ordovician sea had few hard parts that could leave impressions in the sea floor. This makes their remains scarce. Five hundred million years later, there are very few Figure 6: Rendering of Fossil collected fossils. Once they were easily found in the rocks near the Upper Landing, but as you will see in the next chapter, Nature had other plans for the rocks that were to become the Hudson River Valley. What a Geologist Sees from the Walkway Over the Hudson 6 CHAPTER 2 The Coming of the Ice Age Glaciers to Create the Hudson Valley Aer the formaon of the bedrock, in the period between 100,000 and 10,000 years ago, the most dominant influence on the Walkway’s view was the direct result of the Ice Age. The glacier that slid down our State carved the Hudson River Valley and the river we see now is just a shadow of what it once was. There have been at least five, major ice ages documented during the 4.6 billion years since the Earth was formed — and most likely many more before humans came on the scene about 2.3 million years ago. There is evidence of that the ice advanced and retreated at least 4 mes in the last 20,000 years. Figure 7: Geologic Era with Hudson Valley Milestones Included The Pleistocene Epoch is the first in which modern man evolved, and by the end of the it, humans could be found in nearly every part of the planet.

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