[Illustration: 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.] [Illustration: 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. [Illustration: FIG. 324.—Map to show the glaciated and nonglaciated regions of North America (after Salisbury and Atwood).] =Contrast of the glaciated and nonglaciated regions.=—Since we have now studied in brief outline the characteristics of the existing continental glaciers, we are in a position to review the evidences of former glaciers, the records of which exist in their carvings, their gravings, and their deposits. [Illustration: FIG. 325.—Map of the glaciated and nonglaciated areas of northern Europe. The strongly marked morainal belts respectively south and north of the Baltic depression represent halting places in the retreat of the latest continental glacier (compiled from maps by Penck and Leverett).] An observant person familiar with the aspects of Nature in both the northern and southern portions of the central and eastern United States must have noticed that the general courses of the Ohio and Missouri rivers define a somewhat marked common border of areas which in most respects are sharply contrasted (Fig. 324). Hardly less striking is the contrast between the glaciated and the nonglaciated regions upon the continent of Europe (Fig. 325).