Large ice sheets (glaciers) advanced down from the Arctic in episodes called glaciations, followed by warmer climate called interglacials when glaciers retreated.

The last glaciation (called Wisconsinan) began about 100,000 years ago.

Two vast ice sheets extended over much of the North American continent, with an ice-free corridor in between.

With much water locked in ice, sea levels dropped, exposing a land bridge across the Bering Strait between Siberia and Alaska.