Perrot State Park Geology
What's now Perrot State Park was shaped by millions of years of natural forces.
In the Cambrian period, beginning about 600 million years ago, Wisconsin slowly sank beneath a shallow inland sea. Eventually, it rose and the sea drained. Then, in the Ordovician and Silurian periods, roughly 400 million to 500 million years ago, other shallow seas invaded and retreated from Wisconsin.
The shallow seas deposited thick layers of sand and deeper waters deposited mud and limy ooze. Piling atop one another, these sediments became hundreds of feet thick. By pressure and natural cementation, the layers in this giant "sandwich" became sedimentary rocks called sandstone, shale and dolomite (limestone).
The seas retreated about 400 million years ago and erosion became the next natural force shaping the landscape. Water and wind cut downward into the rock more than 1,000 feet. This process still continues today.
Hiking up Brady's Bluff west trail, the layers in order include Cambrian rock, Wonewoc sandstones, Lone Rock sandstones, St. Lawrence dolomite and Jordan sandstone. The highest rocks, capping the bluff, are early Ordovician dolomites called Prairie du Chien dolomites.
Trempealeau Mountain, standing alone across the Trempealeau River, is about 425 feet high. The other bluffs in Perrot State Park are in some points 500 feet above the Mississippi River.
Perrot is located in a unique geological region called the Driftless Area. When the last glacier swept down from the north, it dramatically changed the landscape of Wisconsin. Southwestern Wisconsin escaped that force, leaving the bluffs of sedimentary rock and the unique topography which you can see at Perrot.
Before the glacial period that started about a million years ago, the Mississippi Valley was deeper than it is now and the Mississippi River flowed in a 5-mile wide flat valley north of the park. As the glacier melted, its waters ran down the old valley and plugged it solid with glacial debris. This forced the river to change its flow to the current valley south of the park. Glacial deposits raised the river about 150 feet from its pre-glacial level.