THE WORK OF GLACIERS - 163 



short distance after their union the waters of the two streams flow 

 side by side, but gradually they merge. 



Since streams are no longer overloaded after the retreat of their 

 glaciers, they begin to erode the alluvial deposits of their former 

 flood plains and in this way form the terraces which so often border 

 stream valleys in glaciated regions. As the streams deepen their beds 

 in their partially filled valleys, they occasionally fail to find their 

 former channels, and after excavating broad valleys in the recently 

 deposited gravels may cut narrow gorges into solid rock. This is 

 shown in the diagram (Fig. 147), in which a stream flows from its 

 alluvium-filled valley (in the background) into a deep, postglacial 

 gorge in the foreground. 



Landscape Modified by Glacial Action 



Characteristics of Glaciated Valleys. — A striking feature of moun- 

 tain valleys which have been subjected to the long-continued erosion 

 of thick glaciers is the flatness of the floors and the steepness of the 

 valley sides, as contrasted with the V-shaped valleys cut by streams. 

 A cross section of a valley which has been shaped by glaciers is 

 typically a gigantic U, sometimes more than 3000 feet deep and three 

 miles wide. The tributary streams of such valleys usually enter 

 them over falls. The high, tributary valleys are called hanging val- 

 leys (Fig. 148), and their occurrence is proof that the main valley has 

 been deepened by glacial action. 



This peculiar relation between the main valley and its tributaries 

 can best be understood by following the history of a valley from the 

 time it was first occupied by a glacier until it again became free from 

 ice. When a main valley is occupied by a thick glacier, it will in 

 time be deepened and broadened, especially near the bottom, and the 

 valley sides will at the same time be oversteepened. This excavation 

 is termed over deepening. Since the main valley is well filled *with 

 ice, it is evident that the glaciers of the tributary valleys will not 

 be able to lower their beds far below the surface of the main glacier. 

 Consequently, when the glaciers disappear from the valleys, the 

 side valleys will no longer enter the main valley at grade, but by 

 falls. In other words they have become hanging valleys. In this 

 way those steep-sided, picturesque valleys were formed for which 

 Switzerland and British Columbia are famous. The many falls of 

 the valleys of the Yosemite, California (Fig. 148), and Lauterbrunnen, 



