632 THE PLEISTOCENE PERIOD 



Changes in Drainage Effected by Glaciation 



One result of the unequal erosion and unequal deposition by the 

 ice-sheets was the derangement of drainage. This is seen in the 

 thousands of lakes, ponds, and marshes which affect the surface of 

 the drift. The basins of the lakes or ponds arose in various ways. 

 There are (i) rock basins produced by glacial erosion; (2) basins 

 due to the obstruction of river valleys by drift; (3) depressions in 

 the surface of the drift itself; and (4) basins produced by a combi- 

 nation of two or more of the foregoing. Besides the lakes and ponds 

 now in existence, others have become extinct by the filling of their 

 basins or by the lowering of their outlets. 



Glaciation also changed the courses of many streams. In many 

 cases, pre-existing valleys were filled with drift in some places, so 

 that when the ice melted, the drainage followed courses which were 

 partly new. In other cases, the ice forced streams to flow around 

 its edge, and some of the drainage channels thus established were 

 held after the ice melted. There are few streams of great length in 

 the area covered by the ice which were not turned from their old 

 courses for greater or less distances by the ice or the drift which the 

 ice left. The Mississippi, the Ohio, and the Missouri, the master 

 streams of the United States within the glaciated area, and a host 

 of their tributaries, suffered in this way. 1 



Succession of Ice Invasions 



The glaciation of North America was accomplished by a se- 

 ries of ice-sheets separated from one another by long intervals of 

 time. Some of the interglacial intervals were much longer than the 

 time since the last ice-sheet disappeared, and there is also good 

 evidence that in some of them the climate was at least as mild as 

 to-day. 



The proofs of the interglacial intervals and the evidences of 



1 For changes in the Mississippi and in the rivers of Illinois, see Leverett, 

 Monogr. XXXIII, U. S. Geol. Surv., p. 120. For changes in the Upper Ohio, 

 see Chamberlin and Leverett, Am. Jour. Sci., Vol. XLVII, 1894 (contains refer- 

 ences to earlier work in this region). For changes in the Erie and Ohio Basin, 

 see Leverett, Monogr. XLI, U. S. Geol. Surv., Chap. Ill, and Tight, Professional 

 Paper No. 13, U. S. Geol. Surv. For changes in the course of the Upper Missouri 

 and its tributaries, see Todd, Science, Vol. XIX, p. 148 (1892), Geol. of S. Dak., 

 pp. 128 and 130 (1899), and Bull. 144, U. S. Geol. Surv. Changes in drainage 

 in New York have been summarized by Tarr, Phys. Geog. of New York, 1902, 

 with references to earlier literature. 



