254 
In view of the fact that the glaciers of 
Yakutat and Glacier bays have been in a 
state of rapid recession for a century or 
more, all that is necessary in explanation 
of the recession since 1899 is to consider 
it an accelerated part of this grand retreat 
which must be due to a deficiency of snow 
supply following an excess in supply, or 
an emptying of the glacier reservoirs suc- 
ceeding a filling of them. Of course, the 
rate of recession may readily have been 
temporarily modified by crevassing due to 
earthquake shaking, or locally modified 
by variation in exposure to wastage, or 
checked or increased by variations in pre- 
cipitation or temperature. These, or any 
other temporary or local causes, are but 
minor episodes in the general withdrawal 
of glaciers which a century or two ago had, 
for some reason as yet unknown, been 
made to advance farther than they could 
maintain their fronts. 
Some of the Factors Involved in the Phe- 
nomena of Advance and Retreat 
Under the simplest of circumstances the 
advance or retreat of a series of glaciers is 
a complex phenomenon in which so many 
factors are involved that a full analysis of 
them can not be undertaken here. Yet 
some of the factors stand out with such 
distinctness that I may take time to briefly 
point them out. The nature of the glacier 
terminus is of fundamental importance. 
If the end of an ice tongue is in water it 
makes a great difference in the rate both 
of advance and recession whether the water 
is salt or fresh, whether it is deep or shal- 
low, whether it is in active movement or 
is quiet, whether there is or is not a free 
escape for the icebergs, and whether the 
relative area of ice cliff is small or great. 
All these factors are effective in addition 
to the rate of supply of ice to be dis- 
charged. If, on the other hand, the ter- 
SCIENCE 
[N.S. Vou. XXXV. No. 894 
minus is on the land, there are influences 
of exposure, of elevation, and of amount of 
moraine cover, as well as the amount of ice 
supplied. 
Illustration from Yakutat Bay 
It is clear that there must be a very great 
difference, especially in recession, according 
to whether the ice front is on the land or 
in the sea, for in the latter position wastage 
is far more rapid than in the former. This 
finds clear illustration in the Yakutat Bay 
region, for during the recent great expan- 
sion of the glaciers, a century or more ago, 
not only were the tidal Nunatak, Turner 
and Hubbard glaciers caused to advance, 
but the glaciers ending on the land also 
pushed forward, presumably at about the 
same time. Along the margin of Mala- 
spina Glacier, for instance, the same phe- 
nomena of overridden gravels and buried 
forests are discovered as in the area over 
which Nunatak-Hidden Glacier advanced. 
But while the tidal glaciers have receded 
10 to 20 miles, the recession of Malaspina 
Glacier has been, at the most, but a fraction 
of a mile; and in some parts of its moraine- 
covered margin, on which forest grows, it 
has remained practically stationary for at 
least half a century. This extreme differ- 
ence may possibly be in part due to a more 
constant maintenance of the ice supply in 
the Malaspina Glacier, though of this there 
is no proof; it certainly is partly due to 
the difference in rate of recession of gla- 
ciers terminating on the land and in the 
sea. 
Modification of Local Climate as a Result 
of Advance and Retreat 
In. interpreting both the cause and the 
rate of advance or recession of glaciers it 
is evident that the mere fact of advance 
encourages advance, while recession en- 
courages continuation of recession. When 
