DAVIS: GLACIAL EROSION. 
319 
[or of a glacier], not to its valley.The glacier moves down the 
gorge, scouring and cutting the bottom and sides as it travels. 
The ends of the mountain spurs are planed off instead of being 
trimmed to sharp, angular points, as is done by streams in gorges 
cut by them.Where the main glacier is joined by a branch, the 
bed of the branch is commonly found to be at a higher level than 
the bed of the main glacier, because being larger and heavier the 
main glacier has greater cutting power; indeed, in many cases the 
beds of small branches are hundreds, or even thousands, of feet 
higher than that of the main glacier to which they are tributary. 
The parallelism between the glacier and the river in their channels 
is further illustrated by this fact. The surface of the ice in the 
main glacier and in the branch must have been at the same level, 
although the bottoms, as stated above, differ greatly in elevation. 
So it is with a river at the point of junction of branches. The 
surface of the water must be practically at the same level in all 
cases, but the bottoms of the channels differ by the difference in 
depth of the streams at their point of junction. This fact affords 
us a measure of the minimum thickness of the ice at any place. It 
cannot have been less than the vertical distance between the bed 
of the main glacier and that of the tributary, and, indeed, must in 
all cases have been greater, owing to the thickness of the tribu¬ 
tary” (’ 98 , 417-428, especially 418-420). 
Penck on Alpine Valleys , 1899. — A no less explicit and detailed 
statement of the peculiar features of glacial channels and their rela¬ 
tions to river channels was made by Penck at the meeting of the 
International Geographical Congress in Berlin, September, 1899. 
The discordance of lateral and main valley floors was described as a 
general feature of all the larger Alpine valleys within their glaciated 
areas. The possibility of explaining the discordance by faulting, as 
suggested by Rothpletz for the Lintlithal, was considered, but rejected. 
The contrasts of the glaciated and non-glaciated Alpine valleys were 
strongly emphasized. The excess of the depth in the main valley 
beneath the floor of the hanging laterals was taken as a minimum 
measure for glacial erosion, and the term “ over-deepened,” already 
adopted on earlier pages of this essay, was applied to valleys thus 
worn to a greater depth than would have been possible to normal 
rivers. The publication of Penck’s address is awaited with interest. 
Harker on Glacial Valleys in Skye , 1899. — A brief article by 
» 
Harker on glaciation in Skye describes the valleys as eroded in 
massive gabbros, with U-shaped cross-section, especially in the upper 
