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 I,ake, 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 



