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.
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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 forma on 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 con nent. Chapter 1 The Formation of the Bedrock of the Hudson River Valley
It is hard to find another place on the North American con nent 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 ac vity 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 iden fy these specific forma ons, 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 Coun es, that push started in Connec cut near Candlewood Lake. The folding and faul ng 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 forma on of the Hudson River Valley and the last two glacial ice ages to provide perspec ve.
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Figure 3: Geologic Era with Hudson Valley Milestones Included
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As shown on the previous page, about 480 million years ago, the Taconic Mountains started to form as the ancient ocean closed and the con nental plates collided. These mountains must have looked like a collec on 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 con nental 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 Connec cut. In some places the folding was intense enough to melt and deform the rocks, which concentrates the mineral content. Figure 4: Geologic Cross‐Sec ons 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 Connec cut 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 faul ng.
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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 bo om 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 hea ng. A er careful examina on of the faults of the area, you would see that rock layers in Poughquag match up with rock layers in Connec cut. This “over thrust” fault shows the rocks have been displaced westward by miles. The folding and faul ng 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 faul ng.
Figure 14: Sec on showing the Once Sedimentary Rocks between Squantz Pond and Lake Mohonk
Figure 15: Cross‐Sec on of the Geology from Lake Mohonk to the Connec cut 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 iden fy 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 condi on. 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 direc on.
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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? A er the forma on 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 s ll, but easily iden fied 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 depos- its. 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 forma ons here in the Hudson Valley very difficult. First, the intense folding, hea ng and deforma on we described above easily destroyed any exis ng 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.
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CHAPTER 2 The Coming of the Ice Age Glaciers to Create the Hudson Valley
A er the forma on 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. This epoch was followed by the current stage, called the Holocene Epoch. We can tell that the Mid-Hudson suffered 4 advance and retreat phases because we can see four sets of scratches in the rocks all over the area. Some of these “gouges” are quite deep and caused by hard rock frozen in the glacier bo om and acted like sandpaper.
The last glacial advance extended down the Hudson and far out to sea. Evidence exists that as it passed the present site of the Walkway, it extended upward from the river bed to a height of 1 mile. Although glaciers move slowly, they act like sandpaper, or chisel skewering all the ground it’s base touches. It is hard to find a mountaintop that is not well rounded and all sharp peaks have been rounded
When a glacier retreats it leaves a broad “U” shaped valley. Streams form in the hanging valleys and troughs. Young streams are swi and can move eroded materials quickly and easily. Eventually, the valley flor fills with loose material as in “C”. It was these streams that turned paddle wheels in early industry. The ord in “D” will be discussed later.
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At the height of the last Ice Age, so much of the oceans water was locked up in ice on the land, the margins of the con nent extended seaward for many miles. The mouth of the Hudson was south and east of its current place at Manha an. In fact, even today, a canyon extends far out to sea (as shown in the figure at the right). As the glacial ice melted and retreated at the end of the age, the outlet of the Great Lakes was blocked and much of the melt water found it’s way to the sea through the Hudson River. The Hudson we see today is just a shad- Figure 8: Hudson Canyon, Extending out into the Atlan c ow of former condi on. A er the ice retreated to its current loca on, the ocean flowed back up the river, making it an arm of the sea, subject to des and a salt water front. This glaciated estuary meets the defini on of a ord. The delta of the Hudson River can be found north of Kingston to this day. It differs from most ords in the depth of the water where it meets the sea. The depth of the Hudson Harbor is much deeper than most ords. On the West shore, the Storm King Mountain and Breakneck Ridge on the east shore made a barrier to the flow of water, consequently the water was backed up to form very large lakes between what is now Albany and Beacon. Then, much of the western side of Dutchess County was at the bo om of a glacier lake. The Hudson’s deepest part is where the High- lands constrict the river’s course. Much of the smoothing of the land surface that we see from the Walkway is a result of the abra- sive ac on of glaciers and the deposit of sediments in glacial lakes. Drainage generally flows from the northeast to the south- west. The Fallkill Creek, which is beside the Upper Landing, is a stream with cataracts that are a legacy of the ice age glacier.
Figure 9: Drainage Basins of Dutchess County
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Up North, as a great ice age glacier melted, there were mes when the mouth of the St. Lawrence River was blocked by ice. Much of the meltwater made its way to the sea through the Hudson River. At the Hudson Highland's, the water hit a bo leneck and water backed up to flood much of Dutchess County. As these ice blocks melted, they dropped their sediment, covering the bedrock with thick layers. In some places, this resulted in deep piles of gravel; in other places, the glacier deposited sand. O en the fine ma- terial that fell to the lake bo om was des ned to become clay. It was the great deposits of clay and silt that made the brickmaking industry one of economic engines of the Upper Landing. Much of the land was sculpted by the block of ice that was over 5,000 feet think. Like sandpaper it re- moved all the rough edges and le piles of unconsolidated material, burying the bedrock by many feet of very fer le soil. When glaciers advanced again, the sediments were shaped into hills, which appear as inverted spoons. The side toward the glacial advance is steeper. This type of hill is called a drumlin. If you stand in the middle of the Walkway and look due East, you see an example of a drumlin; it is College Hill in Poughkeepsie. Dutchess County has very many drumlins. Figure 10: Side View of a Drumlin with Contour Lines Glaciers do much of their work when some ice melts during the day then freezes at night. In this it can “pluck” rock away from rock walls that confine it. In this way a glacier becomes a huge conveyer belt with depos- i ng its load at the front, margins and middle where ice lobes meet. The resul ng deposits are called ll, unsorted rock material dropped by the receding glacier. Long a er the glacier retreated to the Arc c, the Hudson Valley’s landscape is s ll crowded with its remains and many of these features are visible from the Walkway. The twin peaks of Storm King Mountain (on the West shore) and Breakneck Mountain (on the East) are the very fea- tures that obstructed the glacial meltwater and caused the Mid-Hudson Valley to become site of a great lake for a very long me.
Figure 11: Overview of Glacial Till
Much of the resul ng clay, the fine silt of the glacial lake bo om, became the resource that made the Hud- son Valley a leader in brickmaking at the turn of the Century. Near the Upper Landing Park, there was an extensive brick making industry based on this bounty of the Ice Age glacier. Any inves ga on of the nature of the rock and mineral courses that were those de- posits were used by those brick makers or by the early se lers, much of what has value (gravel, clay for brickmaking, sand) has been lost to our use. What a Geologist Sees from the Walkway Over the Hudson 9
Want to know more? Here are some resources that give a great insight on the Walkway, history and natural. The blog for my CLS course “Impact of the Hudson River in American History and Culture” has a segment on the Walkway. It contains notes from Joyce Tommiselli, Kevin SSSS and others about the history of the walkway. There are some YouTube videos of trains, con- struc on of wat walkway and other related topics, Point your favorite browser at: h ps://hudsoncourse.wordpress.com/walkway/
The best source for data like the Climate, Geology, Soils, Water resources and other infor- ma on about Dutchess County can be found in the Dutchess County Resource Inventory. It has 12 chapters wri en by local experts. h p://www.co.dutchess.ny.us/countygov/departments/planning/16138.htm
The best website for discovering informa on about the Earth is at the United States Geolog- ical Service can be found at: h ps://www.usgs.gov/
An excellent “interac ve” site for learning more about the Earth Science resources of New York State can be found at: h p://mrdata.usgs.gov/sgmc/ny.html
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