1 86 
SCENERY OF SWITZERLAND. 
across the strike of several layers differing in hard- 
ness, A, C, E, being soft while B, D are tough or 
hard. In such a case the valley will widen out at 
A, C, E. Speaking generally we may say that the 
depth of the valley is mainly due to the river erosion, 
the width to weathering. Thus the Urseren Thai on 
the St. Gotthard, the broad stretches of valley at 
Liddes, and at Chable on the Dranse (Valais), are 
due to the more readily disintegrated Carboniferous 
or Jurassic strata. On the other hand, the depth of 
the valley will tend to arrive at the regular “regimen” 
(Fig. 47), and must in any case follow its normal 
course; but the width will depend on the de- 
structibility of the strata. Even however the hardest 
rocks will give way in time, so that the inclination of 
the sides will depend on the hardness of the rocks 
and the age of the valley. Other things being equal, 
the older the valley the gentler will be the slopes of 
the sides. 
Flat valley plains may be formed either by rivers 
or in a lake, and the surface view is the same in 
either case. The inner structure, however, as shown 
in a section, is very different. A river plain shows 
irregular, lenticular masses of gravel and sand. A 
stream running into a lake deposits fine mud in 
gently inclined layers, but as soon as it comes to the 
