204 
SCENERY OF SWITZERLAND. 
In each river system the terraces occupy cor- 
responding levels, but in different systems they have 
no relation to one another. They afford, as we shall 
see in the next chapter, valuable evidence as regards 
the former history of rivers. 
Hitherto I have assumed that the river deepens 
its bed vertically. This is not, however, always the 
case. If the strata are inclined the action of the 
Fig. 56.— Diagram of River Valley. 
water will tend to follow the softer stratum, as for 
instance, in the following diagram, where A represents 
a harder calcareous rock overlying a softer bed B. 
The enormous amount of erosion and denudation 
which has taken place may be estimated from the 
fact that terraces can still be traced in some cases 
at a height of 3000 metres above the present river 
beds. 
As we approach their source, valleys become 
