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:

1. Early people could have reached what is now central Alaska, and waited for the Ice Free Corridor to open.  In the meantime, they might have had several thousand years to exist in the harsh climate of treeless Beringia.  The environment probably couldn't sustain more than several thousand people there, so several exceptionally bad years could have wiped them out.  Then, when the climate began to warm, they would have had to move southeastwards, over land which in many places remained barren and devoid of any vegetation.  Just how bad it might have been is hinted at in the following pages.
   
   Another potential problem the first settlers might have faced was the rapid regrowth of forests.  Open tundra would have been great hunting territory, because they could have seen the mammoths and other prey animals for miles.  But as the glaciers melted the forests quickly reclaimed the land.  Hunting in forests demands another level of skill.  If the people didn't reach the plains to the south (about the latitude of Kansas) before the forests reclaimed the land, their progress would have been greatly slowed.  This is an issue that demands further research, to determine how fast the forests advanced, and whether human settlements predated the forests.
   
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2. Early people might have proceded along the coast as far as present-day Washington, where they could have spread inland in an environment much more hospitable than the barrens of central Beringia.  The sea level was low, so there would have been much broader coastal plains to provide places for settlements in many places.  This would, of course, have demanded a high level of skill at navigating many broad stretches of open water—a point on which we have no evidence, except that Native People living on that coast today can cope easily with ocean passages of many tens of kilometers.  But the climate and weather on the coast would have been relatively hospitable compared with the climate inland.  At best the climate on the coast might have been not much worse than the climate on the western coast of Alaska today.
   
   Once the people had reached relatively southern latitudes, they would still have found much of the interior blocked by glacier-covered mountains.  Carl Sauer long ago suggested that the best route to the interior might have been via South Pass, after crossing the relatively open Columbia Plateau and the Snake River Plain.  Once they reached the interior, early people would have found good hunting on the tundra to the south of the ice sheets. 

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.

Further Reading and Bibliography