DAVIS: GLACIAL EROSION. 
295 
fading in the milder climate of the low levels appropriate to the 
close of the cycle of denudation. 
Imagine an initial land surface raised to a height of several thou¬ 
sand feet, with a moderate variety of relief due to deformation. 
Let the snow line stand at a height of two hundred feet. As ele¬ 
vation progresses, snow accumulates on all the upland and highland 
surfaces. Glaciers are developed in every basin and trough ; they 
creep slowly forward to lower ground, where they enter a milder 
climate (or the sea) and gradually melt away. At some point 
between its upper heads and its lower end, each glacier will have a 
maximum volume. Down stream from this point, the glacier will 
diminish in size, partly by evaporation but more by melting; and 
the ice water thus provided will flow away from the end of the 
glacier in the form of an ordinary stream, carving its valley in 
normal fashion. Some erosion may be accomplished under the 
upper fields of snow and nev6, but it is believed that more destruc¬ 
tive work is done beneath the ice. The erosion is accomplished by 
weathering, scouring, plucking and corrading. Weathering occurs 
where variations of external temperature penetrate to the bed-rock, 
as is particularly the case between the seracs of glacial cascades, 
and again along the line of deep crevasses or bergschrunds that are 
usually formed around the base of reservoir walls, which are thus 
transformed into corries (cirques, karen, botner) as has been sug¬ 
gested by several observers; scouring is the work of rock waste 
dragged along beneath the glacier, by which the bed-rock is ground 
down, striated and smoothed; plucking results from friction under 
long-lasting heavy pressure, by which blocks of rock are removed 
bodily from the glacier bed and banks; corrading is the work of 
subglacial streams, which must be well charged with tools, large and 
small, and which must often flow under heavy pressure and with 
great energy. All these processes are here taken together as “ gla¬ 
cial erosion.’’ 
Let it be assumed that at first the slope of a glacier’s path was 
steep enough to cause it to erode for the greater part or for the 
whole of its length. Each young glacier will then proceed to cut 
down its consequent valley 1 at a rate dependent on various fac¬ 
tors, such as depth and velocity of ice stream, character of rock 
bed, quantity of ice-dragged waste, and so on ; and the eroded 
1 A valley is understood to include the channel that is eroded along its floor. The 
channel, with its bed and banks, is therefore that part of a valley which is occupied by 
the stream. 
