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Page:Popular Science Monthly Volume 49.djvu/385

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CAUSES, STAGES, AND TIME OF THE ICE AGE.

ice fields by the abundant waters of their melting and of rains, was spread on the lower lands and along valleys in front of the departing ice as the loess of the Missouri, the Mississippi, and the Rhine. Marine beds reaching a maximum height of about three hundred and seventy-five feet at Neudeck, in western Prussia, give the name of this Neudeckian stage.

A moderate re-elevation of the land, to approximately its present height, advanced in the northern United States and Canada as a permanent wave from south to north and northeast, keeping nearly equal pace with the continued retreat of the ice along most of its extent. Throughout all the distance from the Atlantic to the Rocky Mountains the mainly retreating but often fluctuating ice margin formed many belts of knolly and hilly drift, called marginal moraines. It is also to be noted that the river basins which slope northward or northeastward were obstructed by the waning ice sheet, so that they were temporarily filled by great glacial lakes, as Lake Agassiz, in the basin of the Red River of the North and of Lake Winnipeg, and a very remarkable series of lakes in the basin of the St. Lawrence, the glacial precursors of the present five great lakes from Superior to Ontario. The very grand development of the marginal moraines in Wisconsin (scarcely, however, surpassing Minnesota) led to the application of the name Wisconsin to this stage of the Ice age and to its drift. In Europe this is named by Geikie the Mecklenburgian stage. Conspicuous moraine accumulations were formed in Sweden, Denmark, Germany, and Finland, on the southern and eastern margins of the great Baltic glacier.

During the maximum extent of the glacial Lake Warren, held on its northeast side by the retreating ice border, one expanse of water, as mapped by Spencer, Lawson, Taylor, Gilbert, and others, appears to have reached from Lake Superior over Lakes Michigan, Huron, and Erie, to the southwestern part of Lake Ontario. Its latest southern beach, traced east by Gilbert to Crittenden, New York, is correlated by Leverett with the Lockport moraine. This and later American stages, all of minor importance and duration in comparison with the preceding, can not probably be shown to be equivalent with Geikie's European divisions belonging to the same time.

In the next ensuing Toronto stage, slight glacial oscillations, with temperate climate nearly as now at Toronto and Scarborough, Ontario, are indicated by interbedded deposits of till and fossiliferous stratified gravel, sand, and clay. Although the waning ice sheet still occupied a vast area on the northeast, and twice readvanced with deposition of much till during the formation of the Scarborough fossiliferous drift series, the climate then, determined by the Champlain low altitude of the land, by the proximity of