Post by Eric Booth
Erosion and Deposition were the protagonists of part one of this version of Yahara’s geologic history. Here in part two, massive mile-thick ice sheets that descended from the Arctic carry the story.
Around 2 million years ago, the Pleistocene epoch began and the dawn of the Ice Age arrived. Several rounds of continental glaciation, or the formation of ice sheets, dramatically re-shaped the landscape of the Yahara Watershed, smoothing out the land, shaving off uplands, and filling in valleys. The pre-glacial Yahara valley was covered by the Green Bay Lobe, a piece of the Laurentide Ice Sheet, which covered all of Canada and most of the northern United States.
But the glacier did not reach too far beyond the edge of the Yahara Watershed, which we know by reading today’s landscape. The western side of the watershed consists of hills known as end moraines, or ridges oriented perpendicular to the direction in which the glacier moved, just like what a bulldozer would create after backing away from a newly pushed load of earth. The end moraine during the last glaciation—named the Wisconsin glaciation because the state has so much glacial evidence—is known as the Johnstown moraine.
The landscape near the Johnstown moraine often appears bumpy because it originally consisted of glacial material—a mix of gravel, sand, silt, and clay known as till—and large chunks of ice. The top layer of the deposited glacial material was often smooth, but once the ice melted and the water percolated deeper underground, the material collapsed. The resulting landscape has what is known as “hummocky collapse topography.”
As you go further east away from the end moraine, smaller moraines dot the Yahara landscape, which the glacier created as it started receding. Although the glacier was mostly moving back to the northeast, it wasn’t a continuous process. The glacier would constantly stop and start, and even re-advance, leaving behind the moraines. Many other glacial landforms were created during this time, including elongated hills called drumlins and thin ridges called eskers.
When glacial material filled the Yahara valley, it dammed the Yahara River’s previous course, allowing large glacial lakes to form as glacial melt water collected on the landscape. These lakes were much larger than today’s Yahara lakes. They even covered places such as the present location of Greenway Station, a shopping center in Middleton.
Eventually the lakes’ waters broke through some of the dams, and the supply of melt water decreased as the glacier retreated, leaving behind the Yahara lakes we see today as reminders of their chilly creators.
Overall, the glaciers almost completely wiped the topographical slate clean, smoothing over a landscape that had been previously eroded and dissected by millions of years of rain and creating a new, “younger” landscape. This smoothing effect enabled the formation of one of the most common ecosystems in the watershed: wetlands. Since water does not easily drain from flat land, it collects in places, which can eventually lead to wetland ecosystems.
But the glaciers didn’t erase the watershed’s entire pre-glacial topography. The watershed’s higher elevation areas–namely near its western and eastern boundaries–are where the land escaped much of the glacier’s reconstruction and the hard pre-glacial bedrock was not covered by as much glacial debris. Appropriately named places such as High Point and Valley View roads in southwest Madison, Middleton’s Pleasant View Golf Course, and City View Drive on Madison’s far northeast side provide good viewing points for gazing down at the valley filled by the glaciers.
What is now left of the Yahara Watershed, after the rounds of deposition, erosion, and glaciation, is a landscape still influenced by its eroded bedrock foundation and pre-glacial valleys, but with topography that has been subdued by the flattening, scouring, and valley-filling power of ice.
The Newest Chapter: The Anthropocene
Even though the geology of the Yahara Watershed seems like a story from the distant past, it is important to recognize that geologic activity is still taking place today. Humans have now become the primary agent of change by cutting into bedrock for road construction projects, digging out glacial sands and gravels for building material, and constructing dams for flood prevention, navigation, and recreation.
A notable example of human-driven geological change is the former glacial moraine known as Dead Lake Ridge, which was once between Lakes Monona and Wingra. This high ridge offered a great view of the Madison area before, starting in the 1870s, it was quarried and leveled to help build the growing city.
Some geologists have argued that we have entered a new geologic epoch called the Anthropocene, in which human activity (hence “anthro”) has dramatically increased rates of environmental change.
Now with a working knowledge of Yahara’s deep past, we can move forward into the future with a greater appreciation for how the watershed we see and depend upon came to be.