Thinking Deep: The Short Story of Yahara’s Geologic Past, Part One

Post by Eric Booth

To help encourage long-term thinking in the Yahara Watershed, WSC is using storytelling, as Steve Carpenter explained in last week’s post. Storytelling is not only a helpful way to think about the long-term future, but it is also a common way to think about the past and how we got to where we are today. Our understanding of the past can help inform us about possible futures.

Geology is a field in which thinking into the past gets really deep. Geologists struggle with how to make geologic time, or deep time, tangible to the public. A metaphor courtesy of John McPhee’s Basin and Range helps put our 4.5 billion years of history into perspective:

“Consider the Earth’s history as the old measure of the English yard, the distance from the King’s nose to the tip of his outstretched hand. One stroke of a nail file on his middle finger erases human history.” (quoted from Stephen Jay Gould’s Time’s Arrow, Time’s Cycle: Myth and Metaphor in the Discovery of Geological Time, p. 3)

So, nearly the entire story of any particular place on Earth has actually occurred before humans even existed. This is where geology becomes the storyteller.

A deep perspective of time can also fill our minds with awe and wonder about the amazing changes that have taken place under our feet.

Credit: USGS

Image source: USGS

Deposition and erosion, or how Yahara’s geologic story was written

What is present under our feet in Yahara today is the result of billions of years of geologic history. Everything from volcanic eruptions, to shallow tropical seas, and to massive continental glaciers is part of this story. Needless to say, I will go into only very brief detail here. For a more in-depth look, check out the Landscapes of Dane County.

Two primary geologic processes are responsible for what we know as the Yahara Watershed: deposition and erosion. Deposition is the creation of layers of sediment and rock, and erosion is the process of dissecting and removing these layers. During a given time period, these two processes often switch back-and-forth in dominance, but one thing is constant: the watershed is always changing in geologic time.

While other regions of the United States were (and still are) more geologically active (e.g., earthquakes, volcanoes, and mountain building), southern Wisconsin has always been relatively stable. As a result, the layers of rock and sediment are relatively flat.

One can think of these layers as a cake that is continuously being devoured and topped with more layers. The first layer of cake might be made of limestone and the frosting above it of sandstone. During an era of erosion, these layers are “bitten into” from the top wherever water collects and forms streams and rivers.

Image Credit: Sean Ragan, Makezine.com

A geologic layer cake. Image source: Sean Ragan, Makezine.com

But erosion is not uniform for each layer. The varying chemistry of the different rocks causes varying levels of erosion between the layers; for example, sandstone erodes more easily than the harder dolomite. What’s more, the next era of deposition might even fill in a valley that was eaten away. After hundreds of millions of years, what is left behind can be quite complicated.

Geologic stories can help us make sense of our world today and provide context for our plans for the future. They can give us perspective on current rates of environmental change, help us understand the importance of infrequent geologic events, and provide answers to questions about why our landscape looks and behaves like it does today.

In practical terms, the geologic history of a place puts limits on what kind of land use can be sustained. For example, and relevant to the Yahara Watershed, steep areas are unsuitable for row crop agriculture, since this practice makes land more susceptible to erosion.

Yahara through the eras

But any good geologic story needs to start 4.5 billion years ago—i.e., the beginning of the Earth. While the Yahara Watershed did not exist at this time, the geologic foundations of it were beginning to form.

The earliest era in geologic time is known as the Precambrian and includes everything up to 500 million years ago, or 8/9 of Earth’s history! While we don’t know as much about this era as we do about more recent ones, we do know that Precambrian Earth was a much different place (think of Mordor from Lord of the Rings). During this time molten material welled up to the surface of the earth and was eroded, broken apart, contorted, and fused together, eventually creating the earliest continents.

Erosion was likely the dominant geologic “author” of the end of the Precambrian era in the region that is now Wisconsin.  Water wore away any mountains that had once existed, except for a few outliers, such as the Baraboo Hills. Eventually this eroded jumble of rock became the hardened core or “basement” rock of the North American continent. This rock is also referred to as the Canadian Shield, since it is exposed at the earth’s surface mostly in eastern Canada. The Shield is also exposed in the highlands of north-central Wisconsin, where it hits its peak elevation within the state. The Shield’s surface slopes downward from there and is buried under younger rocks throughout the rest of the state.

Image Credit: USGS

Image source: USGS

The younger rocks above the Shield were laid down between 500 and 350 million years ago, in an era called the Paleozoic. During this relatively short time span (geologically speaking), the vast majority of the bedrock in the Yahara Watershed was formed. This is also when life became much more evolved, as evident in the higher abundance of fossils in the rock record.

Paleozoic rocks consist of limestones, dolomites, sandstones, siltstones, and shales. These rocks, also known as sedimentary rocks, all started as minerals and sediment carried by rivers. The rivers then deposited them in areas close to the shores of ancient seas. Yes, that’s right. During the majority of this era, the Yahara Watershed was under a shallow sea. It was also thousands of miles away from its present-day location, thanks to plate tectonics.

After deposition, and with the addition of pressure from additional overlying layers and chemical reactions (i.e., cementation), the relatively soft sediments eventually became hardened Paleozoic rock. The occurrence of, for example, sandstone over limestone shows that the level of the shallow sea was constantly changing, as were the conditions for deposition and erosion (the latter occurred when sea levels receded).

The oldest of these Paleozoic rock layers in the Yahara is called the Mount Simon sandstone, which was laid down during the Cambrian period, the oldest period of the Paleozoic era (501–488 million years ago). This rock layer also happens to be an aquifer and contains the Madison metropolitan area’s main source of drinking water. This highly permeable sandstone yields an abundant supply of clean freshwater for all of our daily uses.

Throughout the remainder of the Cambrian period, additional layers of the cake— dolomites, sandstones, and shales—joined the party. The Eau Claire shale, which is atop the Mount Simon aquifer, is an important impermeable barrier that keeps contaminants out of the groundwater. Sandstone in the Tunnel City group, which can be seen in the bluffs along Lake Mendota’s shores at Maple Bluff and Raymer’s Cove, are the oldest exposed rocks in the Yahara Watershed.

The next period was the Ordovician (488–444 million years ago), and the main rock type deposited during this time was dolomite. The first rock group of this period was the Prairie du Chien dolomite, which can be seen throughout the watershed where the land has been cut away for roads—e.g., Hwy 12 just north of Middleton. This layer also contains fossils of ancient stromatolites.

Fossil records indicate the Yahara Watershed was once submerged by a shallow sea. Image credit: Matthew Ong, www.thefossilhunter.weebly.com

Fossil records indicate the Yahara Watershed was once submerged by a shallow sea. Image source: Matthew Ong, http://www.thefossilhunter.weebly.com

The layers above the Prairie du Chien dolomite are less commonly seen across the Yahara Watershed today. However, that does not mean they were not once prevalent on the landscape. In fact, several additional rock layers that were formed as far back as the Silurian and Devonian periods (444–359 million years ago) once covered the entire watershed. These were the top layer of our geologic cake—a layer that took 150 million years to be whipped together.

Once the seas receded for good (after the Devonian period), erosion once again took control. Water from precipitation devoured the nearly flat (even by Midwest standards) top layers of the complex layer cake. But again, the water did not feast on the rock layer uniformly. Where it took repeated bites, small gullies formed, which eventually became broad stream valleys. Bite by bite, eventually entire layers disappeared.

The youngest and top-most layer, which has not yet completely disappeared, is the Maquoketa shale. This late-Ordovician layer was recently mapped on the southeast side of Madison, the only spot where it can still be found. Despite these leftovers, the Prairie du Chien dolomite is the main course on erosion’s plate in the Yahara Watershed today.

Over millions of years, erosion created a deep bedrock valley in the Yahara Watershed, cutting all the way down to the Mount Simon sandstone, roughly 500 feet deep. If you had stood on the edge of Picnic Point two million years ago, you would have been peering down into a deep valley. The topography would have looked similar to the Driftless Area of southwestern Wisconsin. Some geologists have even hypothesized that the ancient Mississippi River once flowed through the Yahara valley, but this idea has recently been challenged. Either way, the deep Yahara valley would have been impressive to see.

Even more impressive are the protagonists of our story’s next chapter: mile-high glaciers! We’ll explore this chapter next week in part two.

These topographic maps, with identical scales, provide a glimpse of what the Yahara Watershed was like prior to (left) and after (right) glaciation. Notice the deep valley snaking its way around the Madison area (light green to light blue colors) and then disappear after glacial materials filled it in, increasing the land’s elevation. Image created by Eric Booth; Data courtesy of Wisconsin Geological & Natural History Survey and the U.S. Geological Survey

These topographic maps, with identical scales, provide a glimpse of what the Yahara Watershed was like prior to (left) and after (right) glaciation. Notice the deep valley snaking its way around the Madison area (light green to light blue colors) and then disappear after glacial materials filled it in, increasing the land’s elevation. Image created by Eric Booth; Data courtesy of Wisconsin Geological & Natural History Survey and the U.S. Geological Survey

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