The Wisconsin Dells
The Dells (term used in the singular) 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 Area that was bypassed 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 feet (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 feet (15 m) and cutting deep, narrow gorges and unusual rock formations into the sandstone seen today.