The Bering Straits between North America and Asia, showing the shallow, submerged shelf that would become land with a drop of sea level of only 150 feet. During the Ice Age and at various times during the Tertiary Period, this land bridge was open to migrations between the Americas and Eurasia. In many ways, it is the most important land bridge on the earth. Land areas are in grey, water over 150 feet is in black and water less than 150 feet deep in white. (Stokes, 1960, p. 405).