VALLEYS. 
167 
CHAPTER VI. 
VALLEYS. 
Valleys and rivers are so closely associated with 
one another, that we generally think of them as in- 
separably connected; and indeed there are but few 
valleys which have not been deepened and profoundly 
modified by the action of water. 
Nevertheless many valleys are “tectonic, that is 
to say, they are due, or stand in a definite relation, 
to geological structure; and there are some details of 
valley modelling, which are independent of water 
action, and which it may be convenient to consider 
separately. 
As already mentioned the plain of Lombardy is 
a valley of subsidence, the lower limb, as it were, of 
the great arch of the Alps. It has not been ex- 
cavated by the Po; on the contrary, that river has 
been for ages occupied in filling it up, and at Milan 
a boring was sunk 162 metres without reaching the 
bottom of the river deposits.* 
* Penck, Morphologic tier Erde, vol. II. 
