FORMATION OF THE DELLS

The Dells of the Wisconsin River are located at the boundary of the Central Sand Plains and Western Uplands of Wisconsin. The Central Plain was formed by a glacial lake - Glacial Lake Wisconsin - while the Western Uplands were untouched by glaciers.

The Dells are characterized by nearly vertical rock cliffs along the river, rising over one hundred feet above the water at some points. The cliffs were shaped by erosive processes of water and wind. A number of steep-sided tributary canyons cut through the rocks on the east side of the river.

The Cambrian Period

The sandstones exposed in the cliff of the Dells, as well as nearby bluffs, were formed about 520 million years ago during the Cambrian period. The sandstones in the cliffs were deposited as windblown dune sands. During the period when the dunes were formed, the shore of an inland sea lay to the west, between the Dells and the Mississippi River. The sea gradually eroded eastward to flood the Dells area, and as this happened, the region became part of the sandy shore of a sea. Then as the shoreline moved north, sandstones containing fossils were deposited in the area.

The Ordovician Period

About 510 million years ago, during the Ordovician period, conditions changed in the inland sea. Sand deposits gave way to an accumulation of limestone and dolomite, which cap the bluffs and hills - Coon Bluff and Rattlesnake Bluff - a few miles west of the Dells.

Then a long period of geological quiet set in. No deposits appear to have been made from the end of the Ordovician until the glaciers advanced on the Dells, roughly 20,000 years ago.

Glacial Lake Wisconsin

The Laurentide Ice Sheet flowed southwest from Green Bay to the Baraboo Hills, coming within three miles of the Upper Dells. Roughly 15,000 years ago, the ice front dammed the Wisconsin River at a point one mile east of what is now Lake Delton, to form Glacial Lake Wisconsin, which stretched north along the Central Sand Plains to roughly Wisconsin Rapids.

A Catastrophic Flood

As the glacier began melting about 14,000 years ago, the dam failed and Glacial Lake Wisconsin drained rapidly southward. Flow from Glacial Lake Wisconsin was blocked by sandstone ridges to the west, and a low range of glacial moraines to the south and east of the Dells. As the huge volume of water gushed southward through a gap less than a mile wide, the water carved the Dells.

Glacial Lake Wisconsin's water level dropped as much as 100 feet almost overnight, and the resulting flood carved the Dells of the Wisconsin River, the tributary canyons, and other drainage flows in the Dells area. Scientists estimate that the Dells were carved in a few days as the flood waters rushed through the sandstone.

Since the retreat of Glacial Lake Wisconsin, a thin layer of sand and silt has been deposted in the area, creating the Dells area's characteristic sandy loam soil.