II4 LEWIS G. WESTGATE 
applied to stream-action. The analysis of erosion by waves, glaciers, 
and streams, would be arranged as follows: 
Rock- | Weathering 
Corrasion } Abrasion 
Disintegration : 
= ( Plucking 
Erosion | 
Solution 
Transportation 
In geological literature abrasion is ordinarily considered an important 
factor in land sculpture. It is the purpose of this article to suggest 
that abrasion by glaciers, streams, and waves is in most cases a 
negligible factor in erosion, and to emphasize the importance of 
weathering in the work of erosion by streams and waves. Wind- 
erosion is not considered. 
GLACIAL ABRASION 
The common understanding of glacial erosion has been that it 
is accomplished by the wear of solid particles held in the bottom of 
the ice against the rock surface over which the ice moved. This 
process would be favored by the weight of the glacier and by the fact 
that particles so held are often in continuous contact with the bed- 
rock for long distances. If this process were the only or the chief 
factor involved in glacial erosion, the rasplike action of the broad 
glacier bottom should produce a smoothed, sub-even surface. Within 
the glaciated area of North America there.are many nearly level, 
glacially smoothed surfaces, but these are in regions which were level 
in preglacial times, and are areas which there is reason to believe 
were not deeply eroded. In the hilly regions of glaciated North 
America and in glaciated alpine valleys the detail of such surfaces 
is controlled by rock jointing, and glacial abrasion is limited to 
smoothing the surfaces and rounding the corners of the joint blocks. 
This hackly character of the topographic detail of surfaces covered 
by the Pleistocene ice-sheet may be. in part an inheritance from pre- 
Pleistocene time, but in glaciated alpine valleys (see Fig. 1), having 
lateral hanging valleys, the rounded-hackly surface of the lower part 
of the main valley has been produced by the normal action of glacial 
erosion in live rock, scores and perhaps hundreds of feet below the 
original surface. Here plucking, or the removal of large blocks 
bounded by joint planes, has been the important element in erosion. 
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