Asia, too, is the closest great land mass to this continent, and from it there are more practicable means of access than from any other area. Even today the Bering Strait could be crossed by rafts, for islands at the middle cut the open water journey into two 25-mile stretches. Eskimos make the trip in their skin boats, or in winter by dog sled over the frozen surface of the strait. In the past, the journey must have been even simpler. During the several worldwide glaciations of the Pleistocene Epoch, a geological period which began more than 600,000 and ended about 10,000 years ago, great masses of ice spread across the surface of the continents in the higher latitudes. Since the growth of these ice sheets was nourished by falling snow, the seas, which supplied the necessary moisture, were reduced in volume as the ice expanded. The maximum drop in sea level has been calculated as between 200 and 400 feet, but the floor of Bering Strait is so shallow that a drop of as little as 120 feet would have been sufficient to create a dry land bridge between the continents. Further lowering must have increased the area and elevation of this passage, but the main effect of this was simply to extend the length of the interval during which the bridge remained open. This may have continued well into the period of milder climate after the time of maximum ice advance. Another peculiar condition in this region at this time was the presence of considerable areas untouched by glacial ice. These included the foothills and coastal plain along Alaska’s northern coast as well as the great central Yukon Valley. This surprising situation was probably due to the small amount of moisture left in the winds which had passed over the high and cold mountain chains bordering the southern coast and the second great mass of the Brooks Range to the north. Furthermore, the broad Mackenzie Valley, leading south along the eastern slopes of the Rockies, was the area latest to be covered by glacial ice and first to open up with the return of warmer conditions. It may even be that the ice failed to cover this region during the last one or more of the minor advances which together make up the latest, or Wisconsin, glacial period.