DAVIS: GLACIAL EROSION. 
301 
Glacial Distributaries . — If a maturely dissected mountain range 
were occupied by snow-fields and glaciers of large size, certain 
peculiar results might be expected near the mountain base. Under 
normal preglacial conditions, a small low ridge suffices for the com¬ 
plete separation of two river systems, because the channels of rivers 
are so small in comparison to their valleys. But glacial channels are 
a large part of their valleys, and when great glaciers from the lofty 
mountain centres descend by the master valleys to the mountain 
flanks or even to the piedmont plains, distributary ice streams or 
outflowing branches may naturally enough be given off wherever 
the ice surface rises high enough to overtop the ridges by which the 
master valleys are separated from adjacent minor valleys. If a dis¬ 
tributary branch has sufficient strength and endurance, it may wear 
down the ridge that it crosses and thus increase and perpetuate its 
lateral discharge; but it cannot usually be expected to erode a 
channel as deep as that of the main glacier from which it departs. 
On the disappearance of the ice, a hanging valley will be left above 
the floor of the master valley; but in this case, the drainage of the 
hanging valley will be away from, not toward, the master. Here 
we probably have the explanation of those broad hanging valleys 
which lead from the valley of Lake Maggiore on the west and, less 
distinctly, from that of Lake Como on the east to the compound 
basin of the intermediate Lake Lugano. On going southward by 
rail from Bellinzona to Lugano, along a stretch of the St. Gotthard 
route between the great tunnel and Milan, the railway obliquely 
ascends the southeastern wall of the trough-like valley of the Ticino 
just above the head of Lake Maggiore; and at a height of several 
hundred feet over the delta flood-plain the line turns off to a well- 
marked hanging valley in which the stream runs away from the 
Ticino to Lake Lugano. The notch made by this supposed glacial 
distributary is^ a conspicuous feature in the view from Bellinzona 
and thereabouts. 
The anomalous forking of Lake Como and the open branch from 
the main valley of the Rhine at Sargans through the trough of 
Wallen See to Lake Zurich appear to be the paths of large glacial 
distributaries which eroded their channels deeply across divides that 
presumably existed in preglacial time. The west wall of the main 
valley of tire Is&re in the Alps of Dauphiny, southeastern France, is 
deeply breached by passes that lead northwest to the troughs of 
Lakes Annecy and Bourget, through which the distributaries of the 
Is&re glacial system must have flowed. Lug6on (’ 97 , 62-70) has 
