392 R. M. Deeley — Erosion hy Rivers and Glaciers. 



valleys in Colorado. There earth-movements of such a character 

 have taken place that mountain ranges have been slowly thrown up 

 right in the track of the rivers. But their erosive power has been 

 sufficiently great to enable them to maintain their courses un- 

 affected, cutting canons six or more thousand feet deep, and 

 preventing an3fthing like the formation of lake basins in their 

 main valleys. Indeed, the whole tendency of river action is to 

 fill up depressions and bring large areas of country to an almost 

 dead level. But it is notorious that the valleys in many moun- 

 tainous districts present characteristics which cannot be traced to 

 river action. To explain such phenomena as are there met with 

 we must turn to water working under different conditions, i.e. in 

 the solid form. 



Ice as well as running water, there is no doubt, has had a con- 

 siderable share in the excavation of many valleys. In them we 

 often have smoothed, scratched, and polished rock-surfaces, and 

 heaps and ridges of morainic matter on the slopes or forming low 

 hills stretching across their bottoms. Some of these consist of 

 angular debris carried down on glaciers as medial and lateral 

 moi'aines, whilst others are crescent-shaped ridges of rough gravel. 

 In the lower grounds especially these give place to wide sheets of 

 Boulder-clay which have often choked up the old valleys throughout 

 the whole or portions of their courses, and in some cases have 

 dammed up the rivers and formed immense lakes. 



One interesting feature in particular of glacier-eroded valleys 

 may be mentioned. It is the peculiar step-like way in which they 

 descend from the mountains. First we have the consolidated snow 

 on the mountain-sides descending into wide basins, along which it 

 moves with only a very small fall. It then plunges down a more 

 or less steep slope into a second long basin, and so on. Such 

 a descent is that down which the Rhone Glacier descends into 

 the gravel-covered plain beyond. But a ride down the valley will 

 reveal several such rapid descents separated by nearly horizontal 

 U-shaped stretches. Here and there, however, the cross-section 



simulates the I ) section of a river-eroded valley, but there is 



reason to believe that the level plain is merely a deep accumulation 

 of the material ejected from the cave at the glacier end. Down the 

 steep slopes separating such level stretches, the road plunges in 

 a series of zigzags, the river passing from the higher level to the 

 lower level through deep V-shaped gorges which it has cut. 



Indeed, just as a river tends to make its course through its 

 alluvial plain more and more sinuous by attacking the outside of 

 each curve, so does a glacier exert its main erosive power at or near 

 the bottom of a slope where its direction changes, and where the 

 thrust and pressure upon its bed are at a maximum. In this way is 

 produced a valley which descends to the lowlands in a series of 

 giant steps, the upper ends of which are often occupied by lakes. 



The extent to which valleys have been excavated, and their out- 

 lines altered by glacier action, is, indeed, very striking, V-shaped 

 valley's having been converted into U-shaped ones, and their 



