It was not until the turn of the century that the work of Albrecht Penck and Eduard Brückner established the history of the Alpine glaciations on a solid scientific foundation that has endured pretty well till today. They found four major glaciations and named them in neat alphabetical order after four Alpine valleys—Günz, Mindel, Riss, and Würm.[3] They divided the Würm glaciation at first into two periods of activity, and later into a number of smaller oscillations toward the end. There has been some controversy over the subdivisions of the Würm, and one to three Danubian glaciers have been suggested hundreds of thousands of years before the Günz; but the general hypothesis brought forward by Agassiz and the amplifications of his successors are now definitely established. With all this goes much knowledge of the ice sheets that covered Scandinavia, northern England, and Germany as far south as Dresden, and North America from ocean to ocean and down to Long Island and the Ohio and Missouri rivers.
Authorities agree that the last melting of the ice sheets and glaciers in the Alpine region began somewhere between 20,000 and 15,500 years ago. After considerable shrinkage and oscillation, the ice increased again for about 5,000 years, and then began to shrink once more. There is some disagreement as to when the Great Ice Age ended; a recent and very minor Daun glaciation has been rather rashly dated as late as only 3,500 years ago. These calculations are only for the Alpine region, and we must remember, of course, that the great ice sheets of northern Europe and North America behaved somewhat differently.
[Illustration: THE ICE FIELDS OF THE LAST GLACIATION
_At the height of the last glaciation 5,000,000 square miles of North America were covered with ice, as against 2,500,000 in Eurasia. The volume of ice was three times as great. The shore lines are those of the present rather than glacial time. (Map after Flint, 1957; Antevs, 1928; and Flint and Dorsey, 1945; estimates from Daly, 1934.)_]
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.