CHAPTER XXII
THE CONTINENTAL GLACIERS OF THE “ICE AGE”
Earlier cycles of glaciation.—Our study of the rocks composing the outermost shell of the lithosphere tells us that in at least three widely separated periods of its history the earth has passed through cycles of glaciation during which considerable portions of its surface have been submerged beneath continental glaciers. The latest of these occurred in the yesterday of geology and has often been referred to as the “ice age”, because until quite recently it was supposed to be the only one of which a record was preserved.
Fig. 322.—Map of the globe showing the areas which were covered by the continental glaciers of the so-called “ice-age” of the Pleistocene period. The arrows show the directions of the centrifugal air currents in the fixed anticyclones above the glaciers.
Fig. 323.—Glaciated granite bowlder which has weathered out of a moraine of Permo-Carboniferous age upon which it rests. South Australia (after Howchin).
This latest ice age represents four complete cycles of glaciation, for it is believed that the continental ice developed and then completely disappeared during a period of mild climate before the next glacier had formed in its place, and that this alternation of climates was no less than three times repeated, making four cycles in all. At nearly or quite the same time ice masses developed in northern North America and in northern Europe, the embossments of the ice domes being located in Canada and in Scandinavia respectively ([Fig. 322]). There appears to have been at this time no extensive glaciation of the southern hemisphere, though in the next earlier of the known great periods of glaciation—the so-called Permo-Carboniferous—it was the southern hemisphere, and not the northern, that was affected ([Fig. 323] and [Fig. 304], [p.276]). From the still earlier glacial period our data are naturally much more meager, but it seems probable that it was characterized by glaciated areas within both the northern and the southern hemispheres.
Fig. 324.—Map to show the glaciated and nonglaciated regions of North America (after Salisbury and Atwood).