Origin of Eskcrs. — Crosby. 13: 
or localized pressures, and therefore pressures the maxima of 
which must exceed the average of the combined vertical and 
flowage pressures for the same depth. In other words, the ice 
may flow by intergranular melting and freezing without any 
excess of melting over freezing. 
Again, it is probable that during the period of glacial 
erosion the basal pressure is sufficient to prevent the develop- 
ment of both tunnels and crevasses. Nansen found neither 
crevasses nor superglacial streams nor evidence of appreciable 
surface melting ; and it is certain that surface crevasses would 
be closed at no great depth by the freezing of water in them, 
as long as the mean annual temperature remains, as now, far 
below the freezing point. We must conclude, therefore, that 
glacial erosion was, and is now, except in the case of alpine 
glaciers whose valleys have been swept free of debris, ac- 
complished in the practical absence of water, derived either 
from the basal or the superficial melting of the ice. 
We thus find that, even in the case of a waning ice-cap,, 
superglacial. and still more narrowly subglacial, drainage is 
confined to the marginal portion of the ice ; and in view of the 
observations by Nansen and others on the inland ice of a 
mountainous country, we may question whether crevasses 
could have been a prominent feature of even the marginal por- 
tion of the Pleistocene ice-sheet on the plain portions of the 
glaciated area. Or, in other words, is it not probable that on 
an approximately level but rough surface, remote from moun- 
tainous tracts, the ice would, in general, have ceased to fiow 
before it became thin enough to permit effective crevassing or 
the development of fissures traversing its entire thickness? I 
am inclined to think that the burden of proof rests upon those 
who assume the negative ; although glacial pot-holes are com- 
monly supposed to testify to the existence of at least occasional 
crevasses ; and inequality of basal melting may be cited as a 
possible cause of crevassing not dependent upon flow. 
Stone says :* "Neglecting basal melting, we divide 
the ice-sheet into a zone or area of diffused superglacial 
waters, a zone of superficial streams, and a zone of subglacial 
streams." I accept these zones, but hold farther that, in large 
part, the second zone extended to the margin of the ice. the 
third or outer zone being characterized by superglacial streams 
• U. S. G. S. Moa., 34. p. 420. 
