The common picture of North America at the height of the last ice age is of a vast, nearly treeless expanse, with two gigantic lumps of ice in the north: one centered over northern Canada, and the other lying atop the western mountains.  The maximum extent of these two ice sheets was attained about 20 to 15 thousand years ago, at the Last Glacial Maximum (LGM), when they may have been joined to make a single ice cap.  The amount of ice in the North American ice caps was comparable to that covering Antarctica today.
Then, as the ice began to melt about 14 thousand years ago, a gap began to open between the two ice sheets.  At some time there appeared a corridor between the sheets that was relatively free of ice during the summers.  It has been thought by many anthropologists that this Ice Free Corridor would make an ideal route for early humans to reach the interior of the continent.
At the LGM there was no sea between Alaska and Siberia; the vast amounts of water locked up in the ice sheets lowered the sea level by over 100 meters.  The two land masses were joined to make a sub-continent: Beringia.  Beringia extended as much as several thousand miles southward from the present coast of Alaska.  For some reason, perhaps a lack of moisture, Siberia and Beringia remained mostly free of ice; though the winters must have been horrendous.  Many animals, including humans, from Siberia crossed the Beringia land bridge, and spread southward over North and South America.  In many cases the invadors displaced native populations.  Before the last Ice Age, there were apparently no humans in the Americas, so the invading humans found no competition.  The small numbers of humans eventually spread over the entire continents, as far south as Tierra del Fuego., as early as 14 to 15 thousand years ago.
How did people cross the land bridge?  They might have endured the intense cold of the LGM and come across the vast plains in search of mammonths and other large animals.  Or they could have moved along the coast, subsisting mainly on fish and products of the sea.  Unfortunately any evidence for early settlements on the coast has long ago disappeared under the sea, after the sea rose again to present levels.
Two principal routes were have been suggested for the southward migration:
Many of the issues regarding the environment of North America during the LGM, and during the warming period that followed, are related to the extent of the ice sheets.  Researchers have addressed this problem by looking for evidence of moving ice on the ground, and by modelling the climate and accumulation of ice.  I've taken a simpler, less rigorous approach by simply looking at landform maps for evidence of glaciation.  Much of the evidence can be seen at resolutions of 5 or more kilometers.  The great southern lobes, the James River Lobe, the Red River Lobe, the Lake Superior Lobe, and the Lake Michigan are obvious on topographic maps.
For a review of the issues involved in reconstructing former Ice Ages, see the book by Bennett and Glasser cited below.  They describe many clues that I have deliberately ignored.  For instance, one can look at close-up views of the terrain and rocks, to see whether they have been scratched or abraded by glaciers.  The approach here is based entirely on the largest scale features.
First, let's see what the land looks like today.  This is
a portion of the USGS Atlas of North America.  The glaciers
have done a lot of "damage." They left the north cluttered with lakes.
They rearranged rivers and drainage systems.  They flattened out
the terrain.  They left the shallow Hudson's Bay, which resulted
both from depression of the land and removal of a relatively thin layer
of soil and rock.
This web site has been created and maintained by Gerald Davidson,
Most recent revision April, 2010.
Please send comments, suggestions, and corrections to