This geologically unique area is one of only a handful of sites in the world where the terrain was sculpted by immense floods. During the last ice age, about 15,000 to 12,000 years ago, the Purcell Lobe of the great Cordilleran Ice Sheet moved south from Canada, blocking the mouth of what is now the Clark Fork River and forming Glacial Lake Missoula in western Montana. When the lake reached about 2,000 feet in depth (with a volume as great as Lakes Erie and Ontario combined) the seal at the bottom of the glacier was broken, channeling beneath the glacier expanded exponentially, and the entire lake--about 500 cubic miles of water--drained in two to three days across eastern Washington and down the Columbia River Gorge. This flow was ten times the combined flow of all the rivers in the world. The glacier continued to advance, re-blocking the Clark Fork valley and reforming the lake. This cycle of filling then outburst-flooding occurred probably more than sixty times. One might expect rather a bit of erosion.