During the last ice age about 15,000 years ago, an enormous glacier pushed down from British Columbia and blocked the Clark Fork River in northern Idaho, forming an ice dam that created Glacial Lake Missoula, the largest known glacial lake. Its waters backed up into western Montana, creating a body of water comparable to Lake Ontario, and as the lake deepened at the dam, the lighter glacial ice floated and eventually broke apart, unleashing floods of epic proportions that rushed through the Clark Fork drainage toward the Pacific Ocean. These torrents scarred eastern Washington and created scablands that still define the landscape. The geologic record indicates that the lake filled and emptied cyclically over about 2,000 years, with the large road cut where the Interstate 90 bridge crosses the Clark Fork River at Nine Mile, ten miles east of here, preserving evidence of at least 36 separate fillings, while ancient ice age shorelines remain visible on the mountains around Missoula. In 1860, 150 men under Lieutenant John Mullan carved a wagon road through the Precambrian mudstones on the mountainside north of here in six weeks using explosives; this Point of Rocks Segment of the Mullan Road still traces the mountainside above. In 1908, the Chicago, Milwaukee, St. Paul & Pacific Railroad built its transcontinental line through the Clark Fork canyon toward Seattle, excavating tons of rock on its way to St. Paul Pass, and the old railroad grade later known as the Route of the Hiawatha remains evident along the north side of Interstate 90. In 1914, the Yellowstone Trail passed through this canyon, became U.S. Highway 10 in 1926, and was bypassed here by Interstate 90 in 1963.