i54 
ALASKA GLACIERS 
gation never extends to the water, and usually does not 
affect the slope within one or two thousand feet of the 
water. Their relation is such as might arise if the topog¬ 
raphy of the upland had formerly extended farther in the 
direction of the main trough and afterward been truncated 
by the development of the trough. 
The branch troughs which unite to form the canal 
have similar walls, which rise higher before meeting the 
varied topography of the upland. They are also narrower, 
and at least two of them contract in their upper parts so 
FIG. 77. NORTHEAST WALL OF THE CHILKOOT TROUGH. 
Below is Chilkoot Bake, and beyond are Chilkoot Inlet and I*ynn Canal. The lower part 
of the mountain was shaped by a great Pleistocene glacier, the upper by small tributaries, 
which survive. The summits are ice-rounded nearly or quite to 5,000 feet. The floors of hang¬ 
ing valleys are at 3,500 to 4,000 feet. 
that the cross-profile is more nearly a V than a U. Skag- 
way Canyon, a tributary to the Taiya trough, is narrow at 
bottom, except where occupied by alluvium. Glaciation 
has smoothed its walls on a grand scale, and has degraded 
its bottom enough to render the floor of a tributary dis¬ 
cordant to the extent of 50 or 100 feet, but the type of 
cross-section acquired from pre-glacial stream erosion has 
not been destroyed. Photographs show that the upper 
