Mill Bluff State Park - Natural History

Natural History

The bluffs in the park, and others nearby, are formed of Late Cambrian sandstone. Stratigraphically, the stone of the bluffs is part of the Galesville Member of the Dresbach Group. Sandstone from the Ironton Member of the Franconia Formation, which is more solidly concreted, tops each bluff. This capping layer helped protect the softer stone below it from erosion. Similar structures elsewhere in Wisconsin would have been bulldozed away by glaciers, but these bluffs lie in the Driftless Area; that part of the American Midwest which was never glaciated. The bluffs are all outliers of the Franconia Cuesta to the south.

During the last ice age a tongue of ice dammed the Wisconsin River, causing the water to back up into Glacial Lake Wisconsin. It is estimated that the lake was about 60 to 80 feet (18 to 24 m) deep in this area, so the taller bluffs became islands while the shorter ones would have been entirely underwater. For the 300 years of the lake's existence waves eroded the edges of the bluffs, giving them their distinctive steep sides. Thus in a geological sense the bluffs are sea stacks because they formed in a body of water. In practice the larger formations are described as mesas, the medium-sized ones as buttes, and the smallest as a pinnacle.

The flat ground in the park consists of finely sorted sediments that settled to the bottom of Glacial Lake Wisconsin. Glacial erratics have been found on the former lakebed, which are explained as rocks which were embedded in icebergs that melted as they were floating in the lake.

The park is mostly forested, with northern pin oak, jack pine, red pine, eastern white pine, and white oak dominant.