DAYIS: GLACIAL EROSION. 
281 
It is not satisfactory to explain the bottom trough as having 
been worn out by normal trunk-river erosion, leaving the side 
streams as it were hanging or suspended above them, for to admit 
such an origin would be to go counter to all that has been learned 
regarding the systematic development of valleys. Here it is with 
regret that I must differ from the opinion of two eminent Swiss 
geologists who explain the deepening of the main valleys by a 
revival in the erosive power of the rivers as a result of a regional 
uplift, while they regard the hanging lateral valleys as not yet 
accordantly deepened by their smaller streams. It is true that 
narrow trenches are cut in the floors of the hanging valleys, show¬ 
ing that their streams have made some response to the erosion of 
the bottom trough in the main valley, and if the bottom trough 
were a narrow canyon, this relation of trunk and branch streams 
might be considered normal; but if the breadth as well as the 
depth of the bottom trough had been acquired by normal river 
erosion, the side valleys should now, it seems to me, have been 
trenched much deeper than they arQ, to some such slope as ST, 
Fig. 4. 
The opinions of Rtitimeyer and Heim on this question are as 
follows : — Rtitimeyer gave an excellent account of hanging lateral 
valleys thirty years ago in his description of the valley of the 
Reuss (’ 69 , 18-24). He recognized benches or Thalstufen on each 
side of the valley above the basal cliffs of the existing bottom 
trough, and regarded them as the remnants of a former, wide open 
valley floor. Side valleys of moderate fall enter the main valley 
about at the level of the Thalstufen , and their waters then cascade 
down over the basal cliffs to the Reuss. Glacial erosion is dismissed 
as incompetent to erode the bottom trough; indeed, the time of 
glacial occupation of the valley is considered a period of rest — a 
sort of U pupa stage” — in its development. The discordance of 
main and lateral valleys is ascribed entirely to the differential 
erosion of their streams. Heim’s views on this matter are to be 
found in his “ Meehanismus der Gebirgsbildung ” (’ 78 , 1, 282-301) 
and in an article “ Ueber die Erosion im Gebiete der Eeuss ” (’ 79 ). 
He recognizes that the bottom troughs have been excavated in the 
floors of pre-existing valleys, whose stream lines had been reduced 
to an even grade (profile of equilibrium, “ Gleichgevnchtslinie ”) 
and whose lateral slopes had been maturely opened. The side 
streams must at that time have eroded their valleys deep enough 
to enter the main valley at accordant grade as stated above. Since 
