We have some fairly exact knowledge about the retreat of the ice across Sweden. This has resulted from the theory of Baron Gerhard de Geer that the varves—layers of alternately coarse and fine clays deposited in lakes in front of the retreating glaciers—represent the summer and winter sediments released by the melting ice. (We have a somewhat similar index in the tree-ring count of wide and narrow rings originated by A. E. Douglass and improved upon by Harold S. Gladwin. Both tree rings and varves may reflect changes in solar radiation.) De Geer counted the varves and determined that the ice sheet began to retreat in southernmost Sweden some 14,000 years ago, and Ragnar Liden determined that it had disappeared by 6840 B.C. De Geer’s Swedish-American pupil Ernst Antevs applied the same system in North America, and found the ice beginning to retreat from Long Island 36,500 years ago.[4] A calculation of the time required for the wearing away of the postglacial Niagara Gorge has produced about the same result, but this has been seriously challenged by Richard F. Flint.[5] The picture of glaciation is more complicated in North America than in the Old World. Europe had two main areas of ice—a small one in the Alps, a much larger one in Scandinavia, the British Isles, northern Germany, and Poland—but they were self-contained. North America had three ice centers—the Labradoran east of Hudson Bay, the Keewatin west of the bay, and the Cordilleran in the Canadian Rockies; these three sheets of ice did not always grow or shrink at the same time or at the same rate, and they occasionally overlapped (see maps on pages 26 and 27). Incidentally, most of the ice of the glacial period was in the New World. The area of land covered was almost twice as great as in the Old World, and the bulk of ice three to five times as great.[6] River Terraces and Beach Lines There are other evidences of glaciation besides varves, erratic blocks, moraines of stones and mixed debris along the sides and fronts of the ice streams, and scratches and polish on the native rock over which the glaciers passed. Four raised terraces are found along the sides of many river valleys. Four raised beach lines, first found in the Mediterranean region, have now been noted in the Americas and Australia. Submerged beach lines and land-bridges have been found at certain places under the ocean, as well as deep channels prolonging present rivers far out to sea.