382 GEOLOGY. 



changes is often difficult of reading. The outlines of drainage basins, 

 as well as the courses of individual streams, were often affected. 



One of the characteristics of streams which have been thus de- 

 ranged is found in the lack of harmony between different parts of their 

 valleys. Within the glaciated area a stream often flows in a capacious 

 preglacial valley, then in a narrow post-glacial gorge of wholly different 

 aspect, whence it may emerge again into another section like the first. 

 Most streams whose courses were modified by the ice or its deposits 

 afford illustrations. 



Again, preglacial valleys, even valleys of considerable length, were 

 sometimes filled completely, so that their courses are only known, so 

 far as they are known, by borings, which reveal the great depth of 

 the drift, and of the old channel. Many stream valleys, in the areas 

 of heavy drift, are wholly postglacial, showing the completeness with 

 which the old drainage lines were sometimes effaced. 



The Succession of Ice Invasions. 



It was formerly thought that there was but a single ice invasion 

 of brief duration, followed by a rapid retreat attended by great floods 

 arising from the melting of the ice; but the more careful studies of. 

 later years have revealed a series of invasions separated by very 

 considerable intervals. It is not yet known how far the ice re- 

 treated in the intervals between the advances, but there is con- 

 vincing evidence that some of the intervals were long, much longer 

 than the period which has elapsed since the last ice retreated. There 

 is also good evidence that in some of them the climatic conditions 

 became at least as mild as they are today. While there are differ 

 ences of view with reference to the entire disappearance of the ice- 

 sheet from the plains of Labrador and Keewatin ; and respecting the 



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

 XXXVIII, U. S. Ceol. Surv., Chapter XII. For changes in the Upper Ohio, see 

 Chamberlin and Leverett, Am. Jour. Sci., Vol. XLVII, 1894 (contains references to 

 earlier work of Carll, Chance, White, Stevenson, Lewis, Wright, Lesley, Spencer, 

 . Randall, and Foshay, in the same region) . For changes in the Erie and Ohio Basin, see 

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

 13, U. S. Geol. Surv., and for changes in the course of the upper Missouri and its tribu- 

 taries, 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 Xew York have been 

 summarized by Tarr, Phys. Geol. cf Xew York, 1902, with references to earlier literature. 



