The Ice-Free Corridor Route
During the Late Wisconsinan Glaciation (about 25,000 – 10,000 years BP) two massive ice sheets, the Laurentide in the east and the Cordilleran in the west, covered most of Canada and the northern United States. Supporters of the ice-free corridor hypothesis propose that an area of land between the ice sheets on the eastern flank of the Rocky Mountains remained unglaciated and open for human occupation and migration during all or part of the Ice Age. According to this hypothesis, the ancestors of the Clovis hunters crossed the Bering Land Bridge from Siberia, traversed unglaciated regions in Alaska and the Yukon, and then followed this ice-free corridor to settle in the lands to the south of the ice sheets by 11,500 years BP.
The ice-free corridor hypothesis was first proposed in the 1930s and for many years was widely accepted as the most likely means of entry for the first North Americans. The corridor was thought to have been open and accessible throughout most or all of the Late Wisconsinan Glaciation, allowing early settlers to make the journey even during the height of the Ice Age. More recent evidence, however, indicates that the ice-free corridor did not open until after the glacial ice began to recede, about 12,000 years BP, and so was too late to account for the arrival of the first peoples.