62 



PHYSIOGRAPHY 



(4) Whenever trees on the sides of valleys are overturned, they 

 disturb more or less soil, some of which is likely to roll down. 



These and various other processes help to loosen rock or soil on 

 the slopes, and prepare it for descent, and the descent or removal 

 of matter from the slopes of a valley always increases its width. 



Fig. 48. The Colorado River flowing through the Imperial Valley of south- 

 ern California, and under-cutting its bank, 1906. (U. S. Geol. Surv.) 



All valleys, therefore, are being widened all the time. In most pro- 

 cesses of widening, the stream itself is an important factor, for it 

 sooner or later carries away most of the material which descends to 

 the channel. Along the bases of the slopes of many valleys there 

 is much debris (talus) waiting to be carried away (Fig. 49) . 



As a result of all the processes which wear back their slopes, 

 valleys may become very wide. The widening of adjacent valleys 

 may go on until the high land or divide between them becomes very 

 narrow (Fig. 50) , or even until it is worn away altogether. ' 



Valley flats. After streams have cut their valleys down to 

 low gradients, they make flats, or flood plains, in their bottoms 

 (Fig. 1, PI. XVII, p. 64 ). These flats are always below the level of 

 the surface in which the valley lies. The Mississippi River at 

 Dubuque has a flat between one and two miles wide, and about 600 

 feet above sea-level. Near St. Louis the flat is 10 miles wide, and 

 about 400 feet above sea-level. At Memphis it is about 35 miles 



