Between 15,000 and 13,000 years ago, a Pleistocene Ice Age glacier moved through the Purcell Trench in northern Idaho, damming the Clark Fork River. This glacial dam created Glacial Lake Missoula, a lake larger than Lake Erie and Lake Ontario combined. As the lake continued to grow, pressure increased on the glacial dam, ultimately causing the dam to fail. Water from the lake rushed westward at speeds between 30 and 50 miles per hour across Montana, Idaho, and Washington. The violent flow created by the draining of Glacial Lake Missoula left shoreline marks that are still visible on the face of Mount Sentinel and Mount Jumbo, a neighboring mountain to the north.