DAVIS: GLACIAL EROSION. 
279 
large measure of glacial erosion, one must be struck with the undue 
attention that they have given to lake basins and the relative inat¬ 
tention to valleys. This disproportion is probably to be explained 
as a result of the greater contrast that prevails between a river and 
a lake than between a river and its branch; it is perhaps for this 
reason that the attention of geologists and geographers has gener¬ 
ally been directed to the origin of lakes rather than to the relation 
of branch and trunk streams, even when the former cascade from 
their lateral valleys into the main valley. That glacial erosionists 
made so little claim for the general deepening of glaciated valleys 
while they demanded a great deepening of those parts of valleys 
which have been scoured down to form lake basins, has always 
seemed to me a difficulty in the way of accepting the demanded 
measure of lake-basin erosion ; and this difficulty was supported by 
the well-attested observation that the side slopes of glaciated valleys 
manifest no marked or persistent increase of declivity in passing 
from above to below the limit of glaciation. If glaciers had scoured 
out deep lake basins, like 
those ) of Maggiore and 
Geneva, they ought to have 
significantly deepened the 
valleys up-stream from the 
lakes; and if the valleys were 
thus significantly deepened, 
it seemed as if their slopes 
should be steeper below than 
above the limit of glacial 
action. The denial of the latter requisite seemed to me to carry 
with it the denial of the two preceding suppositions. 
Features of Strongly Glaciated Valleys. — It is true that the 
uppermost limit of glaciation, Q R, Fig. 4, in Alpine valleys is 
not attended by a persistent change in the steepness of the valley 
sides, AE, CJ; but on descending well within the glaciated val¬ 
ley, a very strong change may usually be found in the slope of 
the valley walls. The larger valleys, once occupied by heavy 
glaciers from the lofty central snow fields, are characterized by 
“basal cliffs,” E F, J II, that rise several hundred or even a thou¬ 
sand feet above their broad floors, and thus enclose what may be 
called a “ bottom trough,” E F II J, half a mile or a mile wide. 
The bottom trough of the Ticino, as seen when one looks up 
stream towards Giornico, is shown in Plate 1, Figure A. The 
