ICE AGES
The fixed amount of water on the Earth has many important geological consequences. From time to time, much of the Earth´s water supply becomes locked into glaciers that advance across land and from the poles–a period called an ice age. The total amount of water available to fill the ocean basins decreases and the sea level drops. During the most recent advance of glaciers some 20¸000 years ago, the eastern coast of what is now the United States was about 250 km (402 miles) father east than it is today. The land bridge joining western North America and eastern Asia made it possible to walk from Alaska to Siberia. This land bridge provided a route that many of the ancestors of Native Americans used to get here from Siberia.
The period onset of massive glaciers is one of the most intriguing features of the Earth´s climate. We are now in the middle of an ice age, a period of several million years during which glaciers repeatedly advanced and retreated. Within the present ice age we are living in an interglacial period between two major advances of glaciers. About 20¸000 years ago, massive glaciers spread down from eastern and central Canada, covering a good deal of North America, and then receded to Greenland about 10¸000 years ago. Glaciers have come and gone many times, and periods of cyclic glaciers like the one in which we now live occurred relatively often during the past 2 million years of Earth history.
