Pleistocene Geology of Moravia Quadrangle 42 1 
Hence it is concluded that the over-deepening of valleys is accom- 
plished slowly by the stone tools plowing and rasping the solid 
rock; the nature of the surface being eroded, the quantity of the 
tools, the pressure of the ice, and the time through which these 
continue to act are factors in the process. 
The conditions that govern the erosive work of an alpine glacier 
are probably very much the same in nature as operated in a conti- 
nental glacier, but the pressure of the ice in the two cases is quanti- 
tatively different. The degrading tools are held to their work of 
erosion by the weight of the ice mass above; for this reason, the 
longitudinal valleys of central New York were altered by the 
ice cap. Rock in valleys always sustained a greater pressure than 
rock of the upland; hence the valleys suffered more erosion, thus 
supplying the tools for sustained erosive-work. Furthermore 
the shoe of ice filling the valleys bore down heaviest on the valley- 
bottom, the pressure decreasing up the side walls as the thickness 
of the ice also decreased, but not proportionally with the ascent 
for the reason that the ice-shoe tends to spread laterally under 
weight. This lateral pressure combined with the vertical pres- 
sure produces the U-profile, an erosion-product never arising 
from the work of water. 
Any discussion of conditions that obtained during Pleistocene 
times must be partly theoretical. In quantity of ice, Greenland 
affords the nearest approach to a continental glacier; in the inter- 
pretation of the features that probably characterized the margin 
of the Pleistocene ice-sheet, Alaskan studies have been most help- 
ful. The alpine glaciers are strictly analagous to the valley 
dependencies of the great ice-sheet only when it fronted in moun- 
tainous topography. The gradient of the valleys of central New 
York was generally towards the ice, hence the conditions were 
quite different from what is seen today in the Alps. It is largely by 
inference based on such facts as observers have recorded in the 
above regions, and on the distribution and nature of the drift 
sheet itself, that we interpret the varying mode of its origin, and 
reconstruct the shifting outline of the ice-front. 
The most conspicuous feature of glacial Work is the stupendous 
erosion seen in some valleys. In other valleys deposition took 
33 Xarr: Zeitschrift fur Gletcherkunde, band iii (1908), ‘‘Some Phenomena of the 
Glacier Margins in the Y akutat Bay Region, Alaska/’ pp. 8 i-i 10. 
