ABRASION BY GLACIERS, RIVERS, AND WAVES La gis 
Before glacial abrasion has been able to smooth away the inequalties 
produced by plucking, the process of plucking has produced new 
inequalities. ‘The effect of abrasion in wearing down the valleys is 
neutralized by the removal of the joint blocks when only partly 
abraded. It has been a pluck-and-heal process, with plucking 
always ahead. It is 
not a question here 
whether the blocks 
plucked are removed 
mechanically by the 
ice, or are loosened by 
subglacial weather- 
ing; the point em- 
phasized is that 
valley - deepening 
does not take place 
through scratching 
by material carried 
in the bottom of the Fic. 1.—Joint-controlled glacially eroded surface. 
ice. Direction of ice-movement was to left. The view 
shows the inability of abrasion to obliterate the control, 
i ; by jointing and plucking, of the surface form. Lake 
new in this  state- Creek, above Twin Lakes, Colo. 
ment of the process 
of glacial erosion. Plucking is recognized more and more. The 
relative incompetency of glacial abrasion is mentioned here because 
it leads up to the consideration of the inadequacy of stream-abrasion. 
The same class of facts is appealed to for evidence in both cases, and 
these facts have been recognized much more widely in the case of 
glacial erosion than in that of stream erosion. 
There is nothing 
STREAM-ABRASION 
Stream-abrasion has generally been considered an important ele- 
ment in valley-cutting. It was clearly distinguished by Gilbert? 
and recent texts usually consider it, though no attempt has ordinarily 
been made to indicate the relative importance of abrasion, plucking, 
t Gilbert, Geology of the Henry Mountains, p. tot. 
