116 LEWIS G. WESTGATE 
and weathering. Chamberlin and Salisbury’ go as far as any recent 
text in emphasizing the importance of weathering and plucking, and 
so in limiting the relative importance of abrasion in valley-cutting. 
They say that in any valley cross-section the amount removed by 
corrasion may be measured by a rectangle the width of which 
is the width of the stream, and the height of which is the depth 
of the valley. It seems, however, that even this relatively small 
proportionate amount, while allowed to the stream, must be 
denied to stream-abrasion, and divided between plucking and 
weathering. 
Theoretically stream-abrasion is less probable than abrasion by 
glaciers. The cutting particles are not held against the rock bottom 
by any overlying mass of ice; indeed, the weight of the particles is 
lessened by their immersion in water. The smaller particles are 
largely carried in suspension, striking the bottom only at intervals. 
Fragments too large for suspension move over the bottom with rolling 
and not with sliding friction. 
The form usually shown by the rock-bed over which the stream 
flows bears evidence to the inadequacy of mechanical wear of detritus 
in shaping it and in lowering the bed. The Olentangy River below 
Delaware, O., for example, is flowing over nearly horizontal beds 
of Devonian limestone. The bed of the river, which has since glacial 
time been cut a dozen feet into the hard rock, consists of a succession 
of very broad, low steps, each step being a limestone stratum, its 
down-stream limit determined by vertical joint-faces. In some 
places the edges, and in a few places the surfaces, of these steps are 
slightly rounded, as if by mechanical wear; but this in no way affects 
the large fact that the rock in the stream-bed is bounded by stratifi- 
cation- and torsion-joint planes. The agency effective in removing 
the rock from the stream-bed has taken it away in large blocks; the 
rock has not been scratched away by the mechanical rubbing of 
fragments swept down by the stream. The ordinary processes of 
weathering are believed to have loosened the jointed limestone, and 
the blocks were later swept away by the stream. As in the case of 
glacial erosion, before abrasion could reduce a joint-block, weather- 
ing processes isolated the partly worn block, and delivered it to the 
1 Chamberlin and Salisbury, Geology, Vol. I, p. 108. 
