Saturday, August 11, 2018

The Geology of Watkins Glen State Park

Watkins Glen Rainbow Falls
Years ago, I saw a photo of a spectacular place called Watkins Glen State Park, and I decided that I would—at some point in my life—go visit the park. That chance came this past June, as I was traveling from Maine to Ohio to start a new job. I made a detour into the Finger Lakes region of New York, and spent a morning exploring the park that lies just on the edge of the town of Watkins Glen. Watkins Glen State Park—one of the 215 parks and historic sites operated by the state of New York—sits at the very southern edge of Lake Seneca, the largest and deepest of the Finger Lakes. The main attraction of the state park is a deep shale gorge with over a dozen waterfalls. The geological history of this gorge is incredibly interesting, and this post will dive into that history. We'll explore the geological context of the rock layers that the gorge cuts through, while also examining what forces eroded those rocks to form the gorge itself. After this post, I'll be posting a pictorial walk-through of the gorge, so stay tuned for that post as well for more photos and information about the modern-day park. (UPDATE: Here is the second post: A Journey Through the Watkins Glen Gorge)

When trying to understand and appreciate any present-day geological formation, you have begin with understanding the rocks that the formation is made of. The history of the rocks gives you context for how erosional forces ultimately created the formation in question, and understanding the gorge at Watkins Glen State Park is no different. In a quick-and-dirty summary, fine-grained sediments were deposited in a shallow sea, and these sediments formed a thick layer of a soft rock called shale. Large-scale tectonic forces then exerted pressure on this shale layer, creating thousands of fractures called joints. These joints represent weak areas in the shale, and when glacial and post-glacial forces met these joints, water was able to easily erode the shale and cut a deep gorge over the time span of several thousand years.

Geology of Watkins Glen State Park
Map courtesy of Woudloper, from Wikimedia Commons.
Let’s look at this geological story in greater detail though. Our story begins approximately 380 million years ago in the Late Devonian Period. As you might expect, the Earth looked quite different at this time. Terrestrial plants were just beginning to take root across the world, and early amphibious tetrapods like Tiktaalik were first venturing onto land for short spurts of time. But also occurring at this time—and of importance to this story—was the Acadian Orogeny (orogeny meaning a period of mountain-building). The two smaller landmasses of Avalonia and Baltica had been colliding with the continent of Laurentia, which is the ancient landmass that the modern North American Plate is built around. The collision of these landmasses pushed up layers of rock, forming an ancient mountain range called the Acadian Mountains.

History of Watkins Glen Gorge
Map courtesy of Dennis C. Murphy via The Devonian Times.
These Acadian Mountains were flanked on the southeast by the ancient Rheic Ocean, and on the northwest side by a shallow inland sea called the Kaskaskia Sea. The Kaskaskia Sea covered what is now western New York, western Pennsylvania, Ohio, Michigan, West Virginia, and more. It is in the Kaskasia Sea that our story continues. As all mountain ranges experience, the Acadian Mountains faced erosional forces like rain and wind, and eroded sediments began moving downhill. On the northwestern side of the Acadian Mountains, many streams and rivers were transporting these sediments down the mountains and toward the Kaskaskia Sea. Once these waterways hit the Kaskaskia Sea, they formed a large series of deltas. Geologists refer to these deltas collectively as the Catskill Delta, and the rocks that were subsequently formed from these delta sediments underlay many parts of what was to become western New York, western Pennsylvania, and parts of Ohio and West Virginia.

Linguoid ripples from Kaskaskia Sea in Genesee Group Shale Watkins Glen
If you pay close attention while hiking in the gorge, you can actually see remnants of ripples that formed from the Catskill Delta sediments after they were deposited on the floor of the Kaskaskia Sea. Although these "fossilized" ripple marks are common in the overall rock layers, they can be hard to notice. Luckily, a section of the Gorge Trail exposes some of these ancient hardened seafloor ripples! The photo above shows these ripple marks, and they can tell us a bit about the environment in which they were formed. Due to the fact that the ripples are lobe-shaped, we can tell that the water at this specific point in time and at this specific area had a good deal of energy in it, as the higher energy current created linguoid ripples.

Many of the sediments that were deposited in the Catskill Delta—especially the area that would become western New York—were fine-grained (i.e. small). These fine-grained sediments eventually became fine-grained rocks like shale and siltstone, although there are some thinner layers of limestone and sandstone mixed in as well. The rock layers of the Catskill Delta are incredibly thick, ranging from several hundred feet thick to several thousand feet thick, depending on your location. This large formation is subdivided into smaller layers, and the rocks that are exposed at Watkins Glen State Park belong to a specific slice of the Catskill Delta formation called the Genesee Group.

Genesee Group Black Shale
The Genesee Group is primarily made up of what are known as black shales. As one might guess, black shale is called such due to its dark hue. This dark coloration is thought to be the result of its depositional environment lacking oxygen. Also to note—as this will come into play later on—shales are relatively soft rocks, and are consequently easy to erode.

As the Permian Period began around 300 million years ago, another orogeny took place. This orogeny—termed as either the Appalachian or Alleghenian Orogeny—occurred as Gondwana (specifically the part of Gondwana that would become Africa) collided with Euramerica (the continent that was made of Laurentia plus the newly acquired Avalonia and Baltica from the Acadian Orogeny). A giant mountain range was formed that included the Appalachian Mountains, Ouachita Mountains, Atlas Mountains, and several other associated ranges. The ocean that used to separate Gondwana and Euramerica was also closed up during this orogeny. The resulting Euramerica-Gondwana landmass is known today as Pangea, a name many of you have undoubtedly heard.

Joints Watkins Glen State Park
When this giant tectonic collision occurred, it exerted a tremendous deal of pressure on the rocks of the Catskill Delta formation, including the rocks of the Genesee Group. This pressure cracked these rock layers in thousands upon thousands of areas, creating a multitude of small fractures in the rocks known as joints. These joints come into play later on, as a joint represents a weak area in a rock that can be more easily eroded by water. When you hike through the gorge today, you can see these joints nearly everywhere you look.

Dissected Allegheny Plateau
The dissected Allegheny Plateau.
After the Permian Period, the areas that were to become western New York, Pennsylvania, and West Virginia, as well as eastern Ohio and parts of eastern Kentucky, experienced various cycles of sediment deposition and erosion. By the time that the Cretaceous Period ended, and the Paleogene Period began around 64 million years ago, the land in this area had been eroded down to a broad, flat plain that was criss-crossed by rivers and streams. However, this all changed during the Neogene Period around 20 million years ago. This part of the world experienced a period of uplift, and this flat plain suddenly found itself raised up as a plateau, which we call the Allegheny Plateau. The rivers and streams that criss-crossed the Allegheny Plateau began eroding down into the plateau. Over the next several millions of years, the Allegheny Plateau began to resemble less and less of a broad flat plain, and began to look more like a hilly, dissected plateau.

Geological Formation of Watkins Glen State Park Gorge
Map adapted from Google Maps.
If you were to look at western New York at this time, you would see it was rather hilly with stream valleys in between the hills. Some of these stream valleys ran nearly north-south. These north-south running valleys ultimately became the Finger Lakes that you can see today. But how? Glaciation!

Beginning around 2.5 million years ago, the Earth began a cyclical period of cooling, and a large ice sheet formed at the North Pole. Thus began the Pleistocene Ice Age. During this ice age, the polar ice sheet advanced and retreated in a cyclical fashion nearly two dozen times. Each time this ice sheet advanced southward, it would travel up those north-south running valleys in western New York, making them wider and deeper.

Watkins Glen State Park Hanging Valley
Remnants of the hanging valley carved by Glen Creek as it empties out into the main Seneca Valley. The valley no longer "hangs" above Seneca Valley.
The last glacial advance of the Pleistocene Ice Age reached its max extent around 20,000 to 18,000 years ago. This glacial advance was termed the Wisconsin Glacial Period, and it’s the effects of this glacier which set the stage for the actual formation of the Watkins Glen Gorge. When this glacier was at its extent, the glacial ice had completely filled the Seneca Valley. The glacier did not, however, cover the tops of the ridges that flanked either side of this valley (at least this is what is currently thought by the majority of geologists from what I’ve gathered). A small stream, now called Glen Creek, formed on the western ridge of Seneca Valley, and the water in the creek emptied out either onto the top of the glacier or into a hole in the glacier (we will never know for sure). As the glacier began retreating from the valley, the small stream valley that the Glen Creek had began carving found itself perched above the main Seneca Valley, which was now becoming ice-free. When a smaller valley is perched above a main valley that was carved by a glacial, the smaller valley is known as a hanging valley. (For a visual of what a hanging valley, check out this diagram.) And this is how the Watkins Glen Gorge began.

When it comes to liquid water, it’s really important to know that gravity is forcing water to travel to the lowest point of land it can possibly get to. This is why water flows downhill. This feature of liquid water can create spectacular erosional features, especially if a stream finds itself having to cover more vertical distance than it previously had to. When a stream finds itself in such a situation, it begins to incise (which is eroding downward in a narrow fashion). Stream incision has led to formations like the Grand Canyon in Arizona, or the New River Gorge in West Virginia. And in western New York, stream incision led to the creation of Watkins Glen Gorge.  

Glen Creek Eroding Genesee Group Formation
Glen Creek as it begins cutting into the top portion of the Genesee Group layers.
Starting approximately 12,000 years ago, the water in Glen Creek had to travel from atop a ridge down to a lake—now called Seneca Lake—which had formed at the bottom of the valley. This is a vertical distance of roughly 1,000 feet. As it traveled downhill, it began cutting into the bedrock. The creek soon hit the Genesee Group, which, if you remember, was that layer made of soft shales and filled with joints. This is when things got interesting.

Glen Creek began incising deeply into the shales of the Genesee Group. The joints made it easier for the water to do so, as well. As we mentioned earlier, joints are an area in a rock layer that can be more easily eroded than a solid piece of rock. Essentially, joints allow for water to access and impact a greater surface area of the bedrock than a solid block of rock would allow. Liquid water in a joint can mechanically and chemically weather and erode the rock, and water in the joint can freeze and further crack and break apart the rock as the water expands.

Waterfall Formation Watkins Glen New York
Consequently, many of the awesome formations in Watkins Glen Gorge occur where a joint is or used to be. If you look at the photo above, you can see several joints running up and down the exposed bedrock. You'll also notice that a waterfall formed along one of the joints.

For the past 12,000 years, Glen Creek has been carving its way down through the black shale and siltstones of the Genesee Group, exploiting the hundreds of joints found throughout the bedrock layer. A gorge dotted with waterfalls, plunge pools, and other features has since taken shape. This is Watkins Glen Gorge as we currently know it.

Watkins Glen State Park Nature
When you start digging into the geological history of any place, you soon realize that the place only looks the way it does now due to a specific combination of factors that came together over a long past. The gorge at Watkins Glen owes its existence not to any one factor, but a whole host of them: the sediments in the Catskill Delta laid the foundation and set the scene; the Allegheny Orogeny shattered the resulting rocks and filled them with joints that would be of utmost importance later on; the uplift and consequent dissection of the Allegheny Plateau brought the Genesee Group layers to a perfect location for a gorge to form; the glaciers shaped the area as a whole and the retreat of the last glacier put Glen Creek in the right circumstances for incising; and finally, Glen Creek was able to form the Watkins Glen Gorge due to the erosional power of water meshing with all those other factors that had led up to this moment when the creek met the rocks. Without all these factors and circumstances coming together the way they did, Watkins Glen Gorge wouldn't exist.

Behind Rainbow Falls Watkins Glen State Park
Now that we've done a somewhat deep-dive into the geological history of Watkins Glen State Park, it's time for a pictorial journey through the gorge as it is today. Check out Part 2 of my Watkins Glen posts at this link: A Journey Through the Watkins Glen Gorge

No comments:

Post a Comment