Nineteen thousand years ago, a great glacier extended from the north to within 4 miles of the Dells of the Wisconsin River. Then, 15,000 years ago the glacier began to melt, forming Glacial Lake Wisconsin, about the size of the Great Salt Lake in Utah. The 150 foot deep lake was held in place by ice dams which eventually melted, releasing a torrent of water which inundated southern Wisconsin. The meltwaters cut deep, narrow gorges and unusual rock formations into the sandstone and formed the steep-sided canyons and bluffs that are seen today along the Wisconsin River. The gorges in the Dells were likely formed in a matter of days or weeks as the water eroded the soft sandstone.