Some cool free animals images:
Ice Free Corridor Viability
Image by Travis S.
This shows the long-standing, original theory of how North America was inhabited by humans. The green is an estimate of sea level during the Late Pleistocene. The Aleutians look a bit funny, but other than that the map is more or less representative.
That is, until it comes to the ice. The small northern ice sheet in Alaska is one that sits atop the Brooks Range. To the east is the large Laurentide Ice Sheet. Along the coast is the Cordilleran Ice Sheet.
The theory stands that people "waited" for the larger ice sheets to melt before they passed between them. However, today there is more geology pointing to the fact that this ice sheet didn't split until about 11,000 years ago. There is very convincing proof that the Clovis culture had been well-established in the Southwest, Southeast, and Plains areas of what is today the United States at 11,300 years ago. This brings up the question of how they got there if the ice sheets hadn't yet allowed for the travel.
Today we also know that the Cordilleran Ice Sheet along the Pacific Ocean didn't always cover the entire coast. There is evidence of bear and seal remains in certain areas upwards of 20,000 years ago which insinuate that, at least for the bear, that there was enough of the landscape not covered by ice for the animal to forage for the amount of resources necessary to keep alive for several years. These ice-free pockets are known as refugia.