4.  THE CORDILLIERAN ICE SHEETS


Map
of Mountain Glaciers

Most of the ice on North America was in the Laurentide ice sheet, lying over eastern Canada and the northern United States.  To the west was another great mass of ice, whose extent is not as well understood.  During the cold of the Ice Ages the mountains must have received huge amounts of snow.  Large expanses of the mountains in the western part of North America would have been covered by ice sheets.  Even today there are some remnants of such ice sheets in the Canadian Rocky Mountains.  The thickness and the extent of the ice sheets that formed on the mountains are, however, highly dependent on the air circulation.  Altitude alone is no guarantee of accumulation of ice, for the Tibetan Plateau remained relatively free of ice throughout the Last Glacial Maximum (LGM). 

I have overlayed the outlines of the high regions with another shade of blue-gray.  These mountains must have been covered by nearly continuous ice caps, broken only by the few high peaks that stood above the ice.  Though most of the valleys must have received huge amounts of snow, it is not certain whether the snow accumulated at lower altitudes to become a continuous ice cap.  The higher mountains might have caused a rain shadow reducing the moisture reaching the lower valleys; as happens today in some relatively arid valleys in British Columbia. If the snow in the valleys did not accumulate to sufficient depth to form ice, it would have melted quickly in the occasional warm periods.  During the colder part of the year, the view from space throughout much of the year would of course have shown both lowland and mountains with a solid white blanket of snow and ice.

Map of Traces of Mountain GlaciersIt is possible that some of the large intermountain basins remained free of permanent ice, just like the tundra of Beringia.  One further piece of evidence that might be pertinent is in the occurrence of long valley glaciers.  In Canada, particularly along the coast, are many U-shaped troughs that suggest the former presence of active glaciers.  The inference is that many of the valleys contained active glaciers rather than a solid ice sheet  These are found as far south as the light blue line in the detail map.  To the south, in the United States, there are many troughs up to 20 km in length, particularly in the Sierra Nevada Mountains of California, but almost none of the immensely long ones found further north.  There are many long troughs in northern Washington, and on the Olympic Penninsula.  There are some spectacular glacial troughs in Northwest Montana, where they form the backdrop of Glacier National Park.  South of Glacier Park they become less prominent.  There is a cluster of glacial troughs surrounding the Yellowstone Plateau and the Wind River Mountains of Wyoming, which had their own ice caps.  Elsewhere there is not much evidence of glaciers advancing down from the mountains.  This can be posed as evidence that much of the interior of western North America remained free of permanent ice.  There is abundant evidence, however, of large lakes in the interior basins of the western United States.

In the far north we know that Beringia and inland Alaska were largely free of ice (except in the mountains).  Just how free is not known; conditions during the winter were probably very cold and snowy.  The extent of ice cover inland, particular to the south of the Mackenzie Mountains (marked with an "M") is critical to the viability of the Ice Free Corridor.  The maps here show an ice-free corridor to the east of the Rocky Mountains, extending southward from the Mackenzie Mountains.  At the Peace River ("P") there is a relatively low corridor through the mountains, which could have facilitated travel over several routes from the north and west.  But there was probably no easy route to the east of the Mackenzie Mountains until long after the LGM.

Proponents of migration via the Ice Free Corridor may have failed to take full account of the Mackenzie Mountains.  Those mountains were the first highlands encountered by storms crossing the Beringia Plains, and they must have received huge amounts of snow.  A suitable passage south of the mountains is very unlikely, as ice must have been very extensive there.  The route around the north of the mountains would have been especially barren, and possibly blocked by valley glaciers coming down from the mountains.  Anyone wishing to reach the interior would have had to negotiate a passage around the south edge of the Mackenzie Mountains or along one of the valleys that lead southward toward the Peace River gap.  But the Mackenzie Mountains show evidence of many great valley glaciers that would have blocked almost every possible route during the LGM.  Even for a long time after the end of the Ice Age this region must have been extremely forbidding, and barren of the large game animals that might have provided sustenance to early humans.  Of course, once people made the 500 kilometer passage around those mountains, they might have had a relatively unobstructed way toward the south.

Conclusions: what does this all imply?