VALLEYS. 187 
water’s edge the coarser gravel rolls downwards form- 
ing a steeper slope. 
The great Swiss valleys are of immense antiquity ; 
the main ones were coeval with the mountains, and 
date back to the formation of the Alps themselves. 
Many indeed were even deeper in glacial times, hav- 
ing been to a great extent filled up by glacial de- 
posits. Penck states that longitudinal are generally 
older than cross valleys. It seems to me, on the 
contrary, that they would as a rule have begun 
simultaneously. No doubt, however, there were many 
exceptions. The Dranse was probably an older 
river than the Upper Rhone. The Rhine below 
Basle runs in a comparatively recent depression. 
The greater number of the upper Swiss valleys must 
however date back to Miocene and some even to 
Eocene times, when rapid rivers were bringing down 
immense quantities of gravel from the slopes of the 
slowly rising Alps. 
