Perrot State Park's Geology

What's now Perrot State Park was shaped by millions of years of natural forces.

Seas

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 shallowest 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).

Erosion

Since Silurian times, this "sandwich" has been above sea level, but it has undergone erosion for nearly 400 million years. This very long period of erosion developed a great treelike drainage pattern on the surface of the rock, with the Mississippi River the main trunk, smaller rivers as branches, and creeks as twigs.

This drainage system, changing shape through the ages, has cut downward into the rock here more than 1,000 feet. It has removed all the Silurian rocks and most of the Ordovician rocks which once covered the park.

Hiking up Brady's Bluff Trail, you first pass Cambrian rock layers and then, in order, 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 in the park, is about 425 feet high. The other bluffs in Perrot State Park have some points 500 feet above the Mississippi River.

Glaciers

Before the glacial period that started about a million years ago, the Mississippi Valley was deeper than now, and the Mississippi River flowed in the 5-mile-wide flat valley just north and east of Perrot Park. As the glacier melted, its waters ran down the old Mississippi Valley and plugged it solid. This forced the river to flow in its present valley south of the park. Glacial deposits raised the river about 150 feet from its pre-glacial level.

For more information, ask Perrot State Park, (608) 534-6409.