280 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
basal cliffs are comparatively straight-walled; they have no sharp 
spurs advancing into the valley door. The rock floor, G, Fig. 4, 
is buried by gravels, F IT, to an unknown depth. It is only from 
the benches above the basal cliffs that the valley sides flare open 
with maturely inclined slopes; and it is at a moderate depth 
beneath the level of the benches at the top of these basal cliffs 
that the lateral valleys, D K, open on the walls of the main valley. 
The bottom trough within the basal cliffs and beneath the lateral 
valleys seems to be of glacial origin. It is in the first place a 
characteristic feature of all the larger glaciated Alpine valleys, as 
I am assured by Professors Penck, Bruckner and Richter, with 
whom the matter was carefully discussed last summer. The non- 
glaciated valleys manifest no such peculiar form. It is not simply 
that the terminal portion, J B K, of a lateral valley has been cut 
off by the glacial widening of the main valley floor; the main 
valley has been strongly deepened, as is assured by the relation of 
its floor, F H, to the prolongation of the floor of the lateral valley, 
KB. The first may be several hundred feet — indeed in some 
valleys, a good thousand feet — below the second. The lateral 
valleys must have once entered the main valley at grade, for the 
flaring sides of the main valley indicate maturity; the side slopes, 
A E, C J, must have once met at B. Even the lateral valleys 
have an open V section, proving that their streams had cut down 
to a graded slope, D B, that must have led them to an accordant 
junction with the main river. Nothing seems so competent as 
glacial erosion to explain the strong discordance of the existing 
valleys. 
The lateral as well as the main valleys have been glaciated, but 
the former do not exhibit changes of form so distinctly as the 
latter: in the Ticino system the lateral valleys did not, as far as I 
saw them, seem to have been much affected by glaciation, a fact 
that may be attributed to the small size of their branch glaciers in 
contrast with the great volume of the trunk glacier. There is no 
sufficient evidence that the valley floor between the basal cliffs has 
been faulted down, after the fashion of a graben ;• for although this 
origin is advocated by Rothpletz (’98, 237) for the Linththal, the 
evidence that he adduces for the limiting faults is not agreed to 
by Alpine geologists in general, and the persistent association of 
the bottom troughs with the crooked course of pre-existent, 
maturely open valleys involves special conditions of faulting that 
cannot be accepted without the strongest evidence. 
