28 R. T. CHAMBERLIN AND W. Z. MILLER 
In general, so far as these principles hold, there should be a 
tendency, strong or feeble according to the quantitative factors, for 
surficial shearing over a less movable portion below. Rotational 
strains thus brought into being might conceivably in some cases be 
a primary cause of low-angle shearing, or in other instances might 
co-operate as a secondary factor with other more important causes 
in producing a similar result. 
Many glaciers, notably those in North Greenland, have devel- 
oped in places horizontal shearing planes which are often grouped 
into distinct zones.t. The englacial drift is definitely arranged along 
these planes of movement, and this.in turn influences the rate of 
melting on the steep edge of the glacier, so that these planes of shear 
have, in many instances, become very conspicuous. ‘These lines 
of débris are especially prominent in the lee of an embossment of 
rock over which the glacier has just passed. In most cases the 
shearing planes may be interpreted as due to the greater resistance 
of the rock knobs below. They seem to be further developed by 
the increased load of débris in the lower part of the glacier and by 
drag on the bottom beneath the moving mass. The rotational 
strain thus engendered causes nearly horizontal slippage of the 
upper portion over the lower. Where formed in the lee of an 
embossment of rock another factor enters to increase the rotational 
element. The rock mass protects the lower portion of the glacier 
from much of the push which the upper portion is receiving.’ 
Thus while the upper portion of the ice is free to move forward, not 
only is the resistance of the lower portion of the ice to forward 
motion increased, but at the same time the actual thrusting to 
which that portion is subjected is diminished. 
C. E. Decker has described various minor folds and small 
thrust faults, mostly of Quaternary age, which affect the strata 
close to the surface in northeastern Ohio and northwestern Penn- 
sylvania.2. The fault planes of these thrusts are commonly inclined 
at low angles (Fig. 17). If their proximity to the surface is of real 
tT. C. Chamberlin, ‘Glacial Studies in Greenland,” Bull. Geol. Soc. Amer., 
VI (1894), 203-10. 
2T. C. Chamberlin, op. cit., pp. 207-8. 
3 Charles E. Decker, unpublished manuscript. 
