-----------+----------------+--------------------+---------------- Alps. |Northern Europe.| North America. | Date. B.C. -----------+----------------+--------------------+---------------- I Gunz | ? | Jerseyan or | ? | | Nebraskan | II Mindel | Lower Diluvium | Kansan | 430,000-370,000 III Riss | Middle Diluvium| { Illinoian } | 130,000-100,000 | | { Iowan } | IV Wurm | Upper Diluvium | Wisconsin | 40,000- 18,000 -----------+----------------+--------------------+---------------- The correlation is based on the amount of weathering and erosion which the various deposits have undergone. The time which has elapsed since the glaciers of the last or Wurm stage were in active retreat has been estimated by comparing the growth of peat-bogs, river-deltas, etc., during historical times with that since the last retreat of the ice. But the most conclusive method is due to the Swedish geologist Baron de Geer, who has actually counted the years since the ice in its final retreat left any particular point between Ragunda and the south of Sweden. The work is based on the idea that the lamination observed in certain marine and lacustrine clays in Sweden is seasonal, the thick coarse layers being due to the floods produced by the rapid melting of the ice in summer, and the thinner and finer layers being due to the partial cessation of melting in winter. By correlating one section with another it is possible to date any particular layer with great exactness, and further to prove the existence of several great climatic fluctuations during the retreat. The topmost limit of the section is given by the surface of the old floor of Lake Ragunda in Jemtland, which received its waters from one of the permanent glaciers and was accidentally drained in 1796. De Geer finds that the edge of the last great ice-sheets lay over southern Scania about 12,000 years ago, and further estimates 8000 years for the retreat across the Baltic. These results are in general agreement with those obtained by other methods, and we may accordingly, with some confidence, put the date when the ice-sheet of the Wurm glaciation finally left the coast of Germany at about 18000 B.C.