Englacial Drift. — Crosby . 
215 
shearing and flexing movements. The flexures are certainly 
very remarkable, considering that, strictly speaking, ice is 
neither viscous nor plastic. Chamberlin’s explanation of 
movement as the result of granulation and a continuous ad¬ 
justment of the granular structure to gravitative stresses 
through differential melting and regelation, obviates in a 
measure the difficulty of accounting for the complex move¬ 
ments observed; but the fact remains that relatively slight 
obstructions, whether of solid rock or uncompacted drift, are 
sufficient to originate sharp overthrust flexures and obliquely 
ascending shear-planes, which are marked by prominent bands 
of debris—ground moraine in process of absorption by the 
ice. To Chamberlin belongs the credit of observing and de¬ 
picting, more clearly, perhaps, than any previous writer, the 
mechanism of the transfer of drift from a subglacial to an en- 
glacial position. There can be no doubt now that in a lee the 
ice, at least under certain conditions, will drag, in conse¬ 
quence of basal friction, sufficiently to give rise to a sharp 
flexure or a thrust fault between it and the ice which passes 
over the obstruction. A part of the latter is being constantly 
curved downward and backward and added to the stagnant 
ice; and thus the thickness of the latter increases with the 
distance from the lee slope; and the axial plane of the flexure 
rises to higher levels in the ice. Or, if the conditions, of gran¬ 
ulation, velocity, etc., determine shearing as well as or instead 
of flexing, the shearing plane will likewise tend to rise in the 
ice. 
In an earlier publication,* Prof. Chamberlin has analyzed 
glacial motion and shown the importance, in a detailed study, 
of distinguishing vertical pressure, due to the thickness and 
weight of the ice, and flowage pressure, due to its horizontal 
movement. The former culminates in the basal central and 
the latter, as a rule, in the superficial peripheral tracts. These 
two pressures cooperate on stoss slopes, and hence glacial 
erosion reaches its maximum intensity there. Glacial striae 
record, almost exclusively, the movements of the ice-sheet in 
its final stages; and it is well understood that the general ab¬ 
sence of striae on lee slopes above a certain low angle of 
declivity means that the flowage pressure then so far predomi- 
*Seventh Ann. Report, U. S. Geol. Surv., 186-192. 
