432 ELIOT BLACKWELDER 
raised until sufficient force is accumulated to sweep out the obstruc- 
tion. Sudden rises of as much as 6 feet, which we noted at the mouth 
of the canyon, are attributed to this condition. 
Within the valley of the Alsek, above the lowest canyon, numerous 
glaciers of all sizes line the slopes on both sides. All the valleys are 
filled with huge lobes of ice. The smaller canyons have steep moun- 
tain glaciers, and scarcely a ravine in the higher mountains is with- 
out its little glacial tongue (Fig. 6). But this by itself is another 
story which is still too imperfectly known to be presented at this 
time. 
Among the glaciers which have been described in the preceding 
pages we may readily distinguish four types. In the cirques and 
ravines high up in the mountains there are many little cliff-glaciers 
which are often no longer than wide, and are usually steeply inclined. 
The majority of them lie more than 2,500 feet above sea-level. In 
the larger canyons, valley-glaciers of the alpine type are found. These 
are tongues of ice usually several miles in length, fed by ample snows 
in the gathering grounds at their heads. The Miller and Canyon 
Glaciers are good examples. To another type, exemplified by the 
Yakutat and perhaps the Beasley, Tarr has applied the name ‘through 
glaciers,’’’ meaning the great lobes which protrude outward from the 
inner ice-plateau through gaps in the mountains. These are char- 
acteristic of the Yakutat-Alsek region, and are apparently numerous 
in the upper valley of the Alsek River. They are all large. The 
fourth type includes the great plateau-like sheets of ice which Russell 
has called ‘‘piedmont glaciers.” They are fed by alpine glaciers 
from various directions and are among the largest glacial features 
of the region. ‘The Alsek Glacier exemplifies this group. 
One of the questions of interest with reference to all glaciers relates 
to their fluctuation in size. It is needless to say that, during the pre- 
vious geologic epoch when all the glaciers of North America were 
greatly expanded, those of the Brabazon Range responded in the same 
way to the general causes of increase. Since that time they have 
retreated to fractions of their former lengths and have been reduced 
in thickness by 2,000 feet or less, according to their sizes and the con- 
t Tarr and Martin, Bulletin of the American Geographical Society, XXXVIII 
(1906), p. 149. 
