576 FRANK CARNEY 
influence of topography upon the detailed outline of the ice-front. 
Climatic control evidently occasioned the pulsations of halt and 
retreat marked by the irregularly spaced belts of thickened drift; while 
the distribution of drift within the belts themselves is due both to 
local topography and to the topography of the areas passed over, in so 
far as these areas have contributed to the load of the ice. _ Further- 
more, the broader outlines of these irregularly spaced belts reflect the 
reaction of the larger topographic features and the general direction 
of ice-movement from the dispersion centers; in consequence of this 
we have the moraines of ice-lobes. It follows, then, that no satis- 
factory control can at present be announced for the spacing of these 
belts. 
Nevertheless, the influence of topography upon the detailed expres- 
sion of the drift within the belt admits of closer definition. We would 
refer particularly to the following three conditions: (1) In a uniformly 
level area the ice-front would be without pronounced re-entrant 
angles; the drift would have a correspondingly even front, while it 
might have a very irregular surface. This type of topography is apt 
also to impose its characteristics upon the drift itself, as may be seen 
in the prairie regions. (2) In a section where the major valleys 
approach a position transverse to the general direction of ice-move- 
ment, the drift is found massed in these valleys, especially on their 
iceward sides; while in the tributaries of these major valleys are 
moraine loops or dams. (3) If, however, the chief valleys approach 
a position parallel to the general direction of ice-movement, we find 
in them lateral moraines‘ blending into loops of drift in the bottoms 
of the valleys; while the secondary valleys may be partially clogged 
or buried with drift. 
Erosion oj drijt.—With this distribution of drift there must have 
been differential erosional effects produced by a second invasion of 
ice. Rather slight modifications would be effected under condition 
(1). The work of another ice-sheet passing over such an area is com- 
pressive quite as much as erosive; the more evenly the original drift 
is distributed, the less obstruction it offers to the progress of later 
ice; whereas the weight of the overriding ice tends to compact this 
drift. 
tR.S. Tarr, Bulletin of the Geological Society of America, Vol. XVI, pp. 218, 219. 
