FEBRUARY 16, 1912] 
rock, mixed in the snow out of which the 
glacier is made, and spread out over its 
surface, is concentrated by ablation in the 
dissipator until the ice surface often be- 
comes completely covered by a sheet of 
moraine, to which the name ablation mo- 
raine has been given. It is naturally upon 
the lower ends of the glaciers that the ab- 
lation moraine is most extensively devel- 
oped; but in some instances it extends far 
up the valleys, almost or quite to the snow- 
line. Then the valley glacier looks so little 
like an ice stream that it may not be recog- 
nized as one by the casual observer; and 
on some of the Alaskan maps such 
glaciers have found no place. 
Since only a portion of the Alaskan 
glaciers bear ablation moraine it is evi- 
dent that special conditions are demanded 
for its development. It is best developed 
on those ice tongues with steep walls and 
steep heads, whose width is not too great 
for avalanches to spread out well toward 
the middle, and whose valley walls are of a 
friable rock. In proportion as these con- 
ditions vary, the extent of the moraine 
sheet also varies. Normal weathering and 
the spread of the falling rock through the 
snow fields and over the ice tongues are 
undoubtedly sufficient to account for the 
formation of a sheet of ablation moraine; 
but the excessive development of such 
moraine in some portions of the Alaskan 
region may perhaps be due in part to the 
aid which earthquake shaking gives in the 
downthrow of avalanches from the glacier 
valley walls. When a glacier bearing a 
sheet of ablation moraine has melted away, 
it leaves not only a deposit of till with 
seratched stones, but overlying this a sheet 
of coarse, angular fragments and weath- 
ered materials. Such deposits are to be ex- 
pected in mountain regions of former 
glaciation. 
SCIENCE 
247 
Influences Modifying Rate of Recession of 
Glaciers 
The ablation moraine is one of the fac- 
tors influencing the position and rate of 
recession of glacier fronts; another factor 
is the position of the front, whether on the 
land or in the sea; for in the latter case 
recession is far more rapid and active than 
in glaciers ending on the land. For ex- 
ample, in the St. Elias region, while the 
Guyot, Seward, Marvine, Lucia, Yakutat 
and other glaciers that end on the land 
have spread out from one to twenty miles 
beyond~ the mountain front, the great, 
rapidly-moving tidal Hubbard Glacier, 
near by, ends at the head of Disenchant- 
ment Bay, ten miles or more back among 
the mountains. Both tidal and non-tidal 
glaciers are exposed to surface wastage by 
melting and evaporation; but the tidal 
glaciers are further exposed to the effective 
attack of the salt water which quickly re- 
moves the ice fragments that fall into it. 
Therefore, other things being equal, the 
tidal glacier will naturally terminate 
farther back among the mountains than 
non-tidal glaciers of similar character. 
Glaciers advancing into rivers are also 
actively attacked, as is illustrated by the 
Childs and Miles glaciers in the Copper 
River Valley, and by glaciers in the Alsek 
Valley. To a lesser degree the same ten- 
deney to more rapid retreat is present in 
glaciers that terminate in lakes, as the 
Yakutat Glacier does. 
Among ice tongues ending on the land 
there is great difference in the rate of 
wastage according to exposure and eleva- 
tion; but even more important is the pro- 
tective influence of the cover of ablation 
moraine. This finds best illustration in 
those glaciers which spread out fan-shaped 
at the mountain base, attaining a state of 
stagnation or semi-stagnation along their 
margins. Here, near sea level, in a rainy, 
