Written by Stalac-Tate
Being in the Driftless Area, many who come to visit ask…
“What is it?”
“Why is it here?”
“How is this related to Cave of the Mounds?”
I hope to explain a little bit of that today. But we are going to have to travel back in time.
The story of the geologic formation of the Cave of the Mounds begins with the creation of the rock in which the Cave is found. The Cave was formed within the sedimentary rock, a limestone made of compacted seashells. This rock dates back over 400 million years to the Ordovician Period of the earth’s geologic history.
During this Ordovician Period, warm shallow seas covered the continent where we find Wisconsin today. Abundant shell life could thrive in these seas. Layers and layers of calcium carbonate shell debris accumulated and slowly hardened into limestone. Thousands of feet of limestone and other sedimentary rocks were laid down during this Ordovician Period.
Millions of years ago, the seas receded leaving these layers of rock behind. Erosion began to wear them down. Today the exposed rock in Blue Mounds is a limestone called Galena dolomite, which is a specific kind of limestone containing some magnesium. Cave of the Mounds itself began to form about a million to a million and a half years ago
During the last ice age, the Laurentide Ice sheet worked its way down from the north to cover most of Wisconsin. The Driftless Area is the portion of Wisconsin (and Illinois, Minnesota, and Iowa, although most of the region is in Wisconsin) that was never covered by glaciers. During the ice age roughly 80,000 years ago to 11,000 years ago, glaciers covered the northern and eastern parts of Wisconsin. These glaciers acted as bulldozers, picking up rocks and sediment as they scraped across the landscape. All those rocks and sediment are what we call drift, and as the glaciers retreated and melted they left their drift across the landscape. But glaciers never touched Southwest Wisconsin, so they were never able to leave their drift behind. Thus, we call this area the Driftless Area because glaciers were never there.
Cave of the Mounds is located in the Driftless Area. There are many caves in the Driftless area. Some are just small enough to fit a human and others are over 4,000 feet long. These caves were left untouched by the glaciers, not covered by up to a mile of heavy ice. It is very possible that there were other limestone caves outside the Driftless, but glaciers erased evidence of them when they collapsed or filled them in. Luckily Cave of the Mounds was not one of these caves.
Guests can tour Cave of the Mounds all year round! Since the temperature of the cave is a constant 50 degrees Fahrenheit, it is a great activity to do in the winter or summer. Learn more about the Driftless Area on our Karst Trail.