246 
by ice that both the valley bottom topog- 
raphy and the valley head divides are so 
smoothed out as to give rise to a continu- 
ous, connected glacier system with drain- 
age in different directions from flattish 
divides; but both the divides and the 
elacier distributaries from them are walled 
in by steeply rising mountains, each por- 
tion of the system having the characteris- 
tics of the valley glacier. For such a com- 
plex I have proposed the name—through 
glacier system. 
The through glacier condition is ren- 
dered possible by presence of low divides, 
and it is believed that, in general, these 
have originated during an earlier period of 
more intense glaciation when the snow and 
ice accumulated to much greater depths 
than now and flowed across the divides, 
lowering them by glacial erosion. 
In its main essential characteristics, 
even the through glacier system belongs in 
the class of valley glaciers; and the valley 
glacier phenomena in Alaska are in the 
main the same as those with which we are 
already thoroughly familiar from the 
studies of glaciers in the Alps, Himalayas 
and other mountain regions. As compared 
with those of the Alps, the larger valley 
glaciers of Alaska are far greater, and this 
naturally introduces corresponding differ- 
ences in form and behavior; but these are 
differences in detail rather than in under- 
lying principles, and may therefore be dis- 
missed in the present discussion. 
At their termini some of the Alaskan 
glaciers present features not found in the 
Alps, notably the termination in tidal cliffs 
from which icebergs are discharged, and 
expansion on the land to form piedmont 
bulbs and piedmont glaciers. At a period 
of former expansion of glaciers, the pied- 
mont condition was present in the Alps 
also; and the present Alaskan glaciers are 
more comparable with those expanded 
SCIENCE 
[N.S. Vou. XXXV. No. 894 
Alpine glaciers than with their shrunken 
descendants of to-day. 
Development of the Cascading Glacier 
As in other mountain regions, the pres- 
ent-day Alaskan glaciers, though very 
large, are mere remnants of a former far 
greater system, occupying the lower levels 
of valleys which were profoundly deep- 
ened by erosion when the former greater 
ice masses occupied them. Accordingly, 
the surface of the present-day glaciers, in 
the main valleys, is very often well below 
the level of the surface of the tributaries, 
which therefore descend with steep slope 
at their lower ends. There is every grada- 
tion, from the accordant junction of tribu- 
tary and main glaciers, to the ice step, or 
““bench,’’ where the two join; to the cas- 
eading descent of the tributary as it joins 
the main ice stream; and to the former 
tributary, now cut oft from junction with 
the main glacier, but cascading toward 
it in its lower portion where it passes out 
of its hanging valley and descends the 
steepened valley slopes in a series of 
broken steps like a great frozen waterfall. 
This condition is so well developed in 
Alaska, and is so widespread and so char- 
acteristic, both in form and cause, that the 
descriptive name cascading glacier has been 
proposed for it. 
Development of the Ablation Moraine 
Glacial erosion, which has produced 
extraordinary topographic change in the 
Alaskan mountains, has, among other re- 
sults, given rise to very steep valley walls. © 
Such steep slopes, produced by ice erosion 
during the higher stage of the glaciers, are 
now, on exposure to the air, in a state of 
instability under the attacks of the agents 
of subaerial denudation. Therefore, they 
weather rapidly, and from them rock falls 
and avalanches frequently descend. This 
