430 ELIOT BLACKWELDER 
probably impossible, to cross this portion of the glacier without going 
far eastward to the foot of the mountains. 
On the northwest border the glacier descends gradually to the 
alluvium which bounds it on that side. Where the glacier reaches 
the river itself it has a vertical front 4 miles in length, which is inter- 
rupted only by the rock hill (called Gateway Knob) at the mouth 
of the canyon. To the north of this hill there appears to be 
little movement in the glacier, as bergs drop off only infrequently. 
Fic. 7.—Front of the Alsek Glacier. Dirty portion north of Gateway Knob, 
showing the horizontal stratification of the ice. 
South of the rocky knob the front shows the beautiful blue-green 
color characteristic of freshly broken clean ice. Large masses of ice, 
usually the blocks between crevasses, break off from this front with 
thunderous crashes on an average of more than once an hour during 
the summer. The front itself is nearly 200 feet high, but even these 
huge masses of ice disappear completely beneath the water into which 
they fall, and then bob up from below and float away down the 
river. ‘This indicates that the depth of water along the front is very 
great. 
