THE WORK OF GLACIERS 165 



Switzerland, and many other of the high falls of the world are of 

 this origin. Hanging valleys of a different origin have been dis- 

 cussed elsewhere. 



The courses of valleys are straighter after glaciation than before. 

 This is due to the fact that the glaciers, because of their rigidity, cut 

 off the " spurs " on the inside of the curves. 



The line on the side of a U-shaped valley above which glacial erosion 

 was not effective is marked by a change in slope, forming a sort of 

 " shoulder." Such " shoulders " are of some economic importance in 

 Switzerland, since they usually afford good pasturage and are favorite 

 spots for hamlets, as they are not subject to the severe cold of the deep 

 valleys. The " shoulders " are usually about 1000 feet above the 

 bottom of the valleys in the Alps, but are sometimes as much as 3000 

 feet above. 



Mature Glaciated Valleys. — It is evident that valleys which were 

 formerly occupied by glaciers will not be U-shaped unless the glaciers 

 were at work for a long time, and every gradation can be seen between 

 them and V-shaped valleys, in which the inside of the curves (spurs) 

 have been little cut away and the beds are still broken by falls. In 

 valleys long subjected to glacial action the spurs are cut away, the 

 bottoms widened, and the sides smoothed. In the upper and middle 

 portions where the weight of the ice and its movement were 

 greatest, basins may have been formed in which lakes now rest. 

 Lake Chelan, Washington, is probably such a lake, as are also 

 the beautiful lakes of the Scottish Highlands. Even maturely 

 glaciated valleys may not have graded beds, since, under certain 

 conditions (p. 157), a glacier erodes one portion of its bed more 

 deeply than another. 



Destruction of Features of Glaciated Valleys. — The characteristic 

 features of glacial valleys are destroyed in process of time by the work 

 of erosion and weathering, very much as are those of cirques. Talus 

 slopes accumulate at the bases of the cliffs ; landslides sometimes cover 

 considerable areas of the bottoms with debris ; the streams from the 

 mountains build out alluvial fans and cones; and in the course of 

 time the " shoulders " are worn away by weathering and the action 

 of the rills which tumble over them. The streams from the hanging 

 valleys cut down their beds so that they enter the main valleys 

 through deep canyons. It is possible by these criteria roughly to 

 determine the length of time since the disappearance of the glacier 

 from the valley. 



