Geology . n of the ice-sheets, and especially thegreat and unequal deposition of the drift, produced a profound effectupon the topography of the planer parts of the area affected by glacia-tion. One of the conspicuous results of this alteration of the topogra-phy was the derangement of the drainage. One of the results is seen 1 Stone, Mono. XXXIV. U. S. Geol Surv., p. 434. ;no GEOLOGY. in the thousands of lakefl which affect the surface of the later drift,and to a less extent, the surface of the older. The basins of theselakes or ponds arose in various ways. There are (1) rock basins pro-duced
Geology . n of the ice-sheets, and especially thegreat and unequal deposition of the drift, produced a profound effectupon the topography of the planer parts of the area affected by glacia-tion. One of the conspicuous results of this alteration of the topogra-phy was the derangement of the drainage. One of the results is seen 1 Stone, Mono. XXXIV. U. S. Geol Surv., p. 434. ;no GEOLOGY. in the thousands of lakefl which affect the surface of the later drift,and to a less extent, the surface of the older. The basins of theselakes or ponds arose in various ways. There are (1) rock basins pro-duced by glacial erosion; (2) basins produced by the obstruction o<river valleys by means of the drift; (3) depressions in the surface olthe drift itself; and (4) basins produced by a combination of two ormore of the foregoing. The third class, as above, may be subdividedinto depressions in the surface of (a) the terminal moraine, (6) theground moraine, and (c) stratified drift. Since the stratified drift in. Fig. 508.—Diagram illustrating normal drainage in the driftless area of Wisconsin and Illinois. wThich the lakes of this last sub-class lie is largely in valleys, it wouldnot be altogether inappropriate to class some of them with group (2). In addition to the lakes and ponds now in existence, there havebeen others of a more temporary character. Some of them havealready become extinct by reason of filling or by the lowering of theiroutlets since the ice melted; others depended for their existence on thepresence of the ice, which often obstructed valleys, giving rise to ice also developed basins outside of valleys, wThen the surface slopewas favorable. For examples of such lakes, see Glacial Geology of N. J., pp. 151-159, and Fair-child, Bull. Geol. Soc. Am., Vol. X, pp. 27-68, and Sta^e XII following. THE PLEISTOCENE OR GLACIAL PERIOD. 381 Another result is to be seen in the changes in the courses of thestreams. In many cases, pre-existing valleys were filled wi
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