Geology

Thick layers of sandstone, limestone and dolomite, deposited 600 million to 430 million years ago during Cambrian and Ordovician times, originally covered all of the Lower Wisconsin River region. Through time, forces of erosion cut a deep, V-shaped gorge down through the layers. This gorge was the start of the Lower Wisconsin River valley. Four miles wide at Sauk City, the gorge narrows down to 2 miles at Muscoda and only about a mile near Wauzeka. The funnel shape of the gorge can be explained by the differences in the uppermost layers of rock found on the bluffs as one proceeds from east to west. Relatively soft Cambrian sandstones dominate the valley walls at the east end of the valley, while harder Ordovician age dolomites dominate the bluffs toward the west end. The river was able to carve a wider gorge in the softer rock.

Beginning about 1 million years ago, glaciers in the northern and eastern parts of the state leveled hills and valleys, covering the bedrock surface with a great amount of rock debris (called drift or glacial till) of varying composition and thickness. However, the southwestern portion of Wisconsin, including the Lower Wisconsin River valley, was not covered by glaciers. This "driftless" landscape is unique in the state. The prominent hillsides of the river valley were not smoothed off by glacial ice, nor was the valley filled in with drift.

The Lower Wisconsin River valley (gorge) was not totally unaffected by the glaciers. When the glaciers finally receded 10,000 years ago, meltwater from glaciers to the north found its way into the Wisconsin River. This river of meltwater carried glacial sand and gravel (glacial outwash) south and deposited it up to 150 feet deep in the river valley. The river later cut down through this deposit of glacial outwash to form a series of terraces that tend to run parallel to the river. Although the valley floor is fairly flat, the elevation rises slightly with each of the terraces. The ancient sides of the gorge not covered by glacial outwash deposits can still be seen: They are the steep hillsides (bluffs) with bedrock outcrops that rise abruptly 300 to 400 feet above the valley floor.

In contrast to glaciated areas of the state, naturally occurring lakes are few in number in Wisconsin's Driftless Region. Thousands of years of uninterrupted erosion formed a drainage pattern that allows water to rapidly run off the steep hillsides. However, the meandering characteristics of the river have allowed several shallow, "oxbow" lakes to form in backwater areas. The river itself approaches 1,500 feet in width in places. It slowly descends at an rate of 1½ feet per mile on its way toward the Mississippi River.