4 ALBRECHT PENCK 



having been eroded in a former river basin, the side branches of which 

 suffered less lowering than the main branches. This fact is now 

 generally admitted by all who have studied the relation between the 

 high-hanging valleys and the main valley, and it is generally acknowl- 

 edged that the latter, in comparison with the former, has been over- 

 deepened Jby erosive action. But there is still a diversity of opinion 

 as to this action. Some authors, like Kilian, Garwood, and Freeh, 

 believe that it has been exercised by rivers, while the hanging valleys 

 were occupied by glaciers and protected by them against the erosive 

 action of water. 



This idea ascribes to river-action phenomena such as are usually 

 not developed by it. The trough does not bear the features of a 

 common river valley ; it has the width to which river valleys attain 

 in their maturity, but it has not the normal grade which rivers always 

 have in this phase of their development. Their slopes have the 

 steepness of youth, as is proved by innumerable landslides occurring 

 along them. It is a peculiar association of young and mature valley 

 features we meet with in our large Alpine, trough-like valleys — an 

 association which cannot be understood by the assumption of normal 

 river action ; and no attempt has been made until recently to show how 

 it was brought about by rivers. As to the hanging valleys, however, 

 there are not a few cases in which they were not occupied by glaciers. 

 Therefore glacial action protecting their bottoms cannot have caused 

 the elevation of their bottoms above those of the main valleys. The 

 hanging valley is not a feature characteristic of glaciated regions only, 

 the hanging mouth being confined to the latter. 



He who wishes to examine the Alpine valleys in the light of river- 

 work must not first take the valley floors into consideration; he must 

 observe the river channels. There the law of Playfair has no appli- 

 cation. While in the state of their maturity the surfaces of two rivers 

 unite at the same level, their bottoms will not do so; the bottom of a 

 larger, deeper stream generally lies deeper than that of its smaller 

 and shallower affluent. The bottoms of side-river channels are hang- 

 ing above those of the main rivers. We have here steps at the mouths, 

 as in the Alpine valleys. While the surfaces of rivers grade down 

 continually, their bottoms show irregularities which resemble those 

 of the floors of some Alpine valleys. The forms of mature valleys 



