Dells of the Wisconsin River: Encyclopedia II - Dells of the Wisconsin River - Description and formation

Dells of the Wisconsin River - Description and formation

The Dells was formed during the last ice age approximately 15,000 years ago, although the rock itself is much older, dating from the Cambrian approximately 510-520 million years ago when the area of Wisconsin was at the bottom of a shallow sea.

Approximately 19,000 years ago, the Dells was at the extreme eastern margin of the continental glacier. However, the Dells itself was never covered by glacial ice sheets - it was part of the large Driftless Zone that was by-passed by the ice. The melting of the glacier formed Glacial Lake Wisconsin, a lake about the size of Great Salt Lake in Utah and as deep as 150 ft (45 m). The lake was held back by an ice dam of the remaining glacier. The eventual bursting of the ice dam unleashed a catastrophic flood, dropping the lake's depth to 50 ft (15 m) and cutting deep, narrow gorges and unusual rock formations into the sandstone seen today. The Dells was probably formed in a matter of days or weeks.

Adapted from the Wikipedia article "Description and formation", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki