Fig. 47.—Map of North America showing the area buried under ice during the Great Ice Age of the Quaternary period; the three great glacial centers; and the extent of mountain glaciers in the west. (After U. S. Geological Survey.)
The accompanying map shows the area of nearly 4,000,000 square miles of North America covered by ice at the time of maximum glaciation, and also the three great centers of accumulation and dispersal of the ice. The directions of flow from these centers have been determined by the study of the directions of many thousands of glacial scratches on rock ledges. The Labradorean (or Laurentide) glacier spread out 1,600 miles to the south to Long Island and near the mouth of the Ohio River. The vast Keewatin glacier sent a great lobe of ice nearly as far south, that is into northern Missouri. "One of the most marvelous features of the ice dispersion was the great extension of the Keewatin sheet from a low flat center westward and southward over what is now a semiarid plain, rising in the direction in which the ice moved, while the mountain glaciers on the west (Cordilleran region), where now known, pushed eastward but little beyond the foot-hills." (Chamberlin and Salisbury.)
The Labradorean and Keewatin ice sheets everywhere coalesced except in two places. One of these is an area of about 10,000 square miles mostly in southwestern Wisconsin. In spite of several ice invasions during the Ice Age, this area, hundreds of miles north of the southern limit of the ice sheets, was never ice-covered. There is a total absence of records of glaciation within this area, and so we here have an excellent sample of the kind of topography which prevailed over the northern Mississippi Valley just before the advent of the ice. A much smaller, nonglaciated area occurs in northeastern Missouri near the southern limit of ice extension.
The Cordilleran ice sheet was the smallest of the three, and it was probably not such a continuous mass of ice, the higher mountains projecting above its surface. A surprising fact is that neither this ice sheet nor any other overspread northern Alaska, which is well within the Arctic Circle, during the Ice Age. More than likely the temperature was low enough, but precipitation of snow was not sufficient to permit the building up of a great glacier.
At the same time that nearly 4,000,000 square miles of North America were ice-covered, about 600,000 square miles of northern Europe were buried under ice which spread from the one great center over Scandinavia southwest, south, and southeast over most of the British Isles, well into Germany, and well into Russia.
In both North America and Europe the high mountains, well south of the great glacier limits, especially the Sierras, Rockies, Alps, Pyrenees, and Caucasus, supported many large local glaciers in valleys which now contain none at all or only relatively small ones.