DAVIS: GLACIAL EROSION. 
315 
transformed from the V to the U type by its action, the distal 
extremity of the tributary will be cut off and the original stream- 
formed declivity replaced by the precipitous side-wall of the normal 
glacier valley” (’ 94 , 351, 359). It is explicitly stated by McGee 
that this explanation does not demand great glacial erosion, because 
the U-canyon of glacial origin need not be much deeper, although 
significantly wider, than the preglacial V-canyon of river origin. 
But the last of the above quotations postulates a special condition — 
a region of rapid preglacial corrasion by streams, and in so far 
does not seem applicable to the case of the Ticino or of the many 
other Alpine valleys; for the well-opened slopes of the lateral 
valleys, and the still wider flare on the upper slopes of the main 
valleys in the Alps, proves that during their formation the main 
stream must have attained a graded slope which the lateral streams 
must have joined in accordant fashion; and there is nothing to 
show that the open and graded floor of the main valley was sig¬ 
nificantly trenched by river action in preglacial time. On the con¬ 
trary, the shallowness of the trenches now found in the lateral 
hanging valleys proves that even if the main valley had been 
trenched, it could not have been cut down very deep. 
Tarr 07i Cayuga Lake , 189L — A significant instance of discord¬ 
ance has been pointed out by Tarr and taken by him as direct evi¬ 
dence of the glacial erosion of a lake basin. He shows that the 
north and south trough of Cayuga Lake, New York, lying in the 
line of ice motion, is about three hundred feet deeper than the floor 
of Salmon Creek, a tributary whose course is oblique to the ice 
motion; and he ascribes the break of grade between the two val¬ 
leys to greater erosion in the deeper one. He generalizes so far as 
to refer to Lake Ontario as probably exhibiting further instances of 
discordant valleys ( ’ 94 ). 
I)e Lapparent on Hanging Valleys , 1896. — A clear and brief 
statement is made by de Lapparent in his “ Lemons de G6ograpliie 
Physique,” as if the matter were well known and undisputed. 
Under the heading, “ Caract&res. des vallees glaciaires,” he writes 
in effect as follows: “When a glacier disappears, the lateral val¬ 
leys, which had been eroded before the glacial period with relation 
to the local baselevel determined by the river that the glacier after¬ 
wards replaced, may, on the disappearance of the ice, no longer pre¬ 
sent accordant junctions with the main valley. Cascades and rapids 
will therefore occur at their mouths in greater number than in a dis¬ 
trict of the same strength of relief which has not been glaciated. 
