THE PLEISTOCENE OR GLACIAL PERIOD 873 



ridges and uplands a few hundred feet in height (200 to 400 feet 

 in Maine ) 1 . Kame-terracee and deltas, also, are often well above 

 the bottoms of the depressions with which they are associated. 



. Changes in Drainage Effected by Glaciation 

 The unequal erosion of the ice-sheets, but especially the irregu- 

 lar disposition of the drift, produced a profound effect upon the 

 topography of the planer parts of the glaciated area. One result 

 was the derangement of the drainage. This is seen in the thou- 

 sands 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 (1) 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 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, by encroaching on the middle 

 course of a /alley, as in the case of the Ohio, forced drainage around 

 its front, and the drainage lines thus established were often 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. The 

 Mississippi, the Ohio, and the Missouri, the master streams of the 

 United States within the glaciated area, and a host of their tribu- 

 taries, suffered in this way. 2 



1 Stone, Mono. XXXIV, U. S. Geol. Surv., p. 434. 



2 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. XL VII, 1894 (contains 

 references to earlier work hi 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. 



