Glaciers form when snow accumulates faster than it melts over a long period and is compressed into ice, and eventually the weight and pressure become so great that the ice oozes slowly downslope. Although volcanism, uplift, erosion, and human endeavor have indelibly etched the Columbia River Gorge, cataclysmic floods during the twilight of the last Ice Age, 12,000 to 15,000 years ago, affected the landscape more than any other terrestrial force, creating these scablands called the Rowena Plateau. As a massive ice sheet advanced from Canada, it dammed rivers with up to 2,000 feet of glacial ice, creating huge lakes across Idaho and Montana. The largest of these, 3,000 square mile glacial Lake Missoula, held over 500 cubic miles of water, half the volume of Lake Michigan. When rising waters undermined these ice dams, tremendous floods swept across the land and down the Columbia River. Torrents of water and ice raged downstream as many as 100 times, scouring away soils at elevations up to 1,000 feet.