DAYIS: GLACIAL EROSION. 
299 
process resemble those under which river diversions and adjustments 
take place; namely, a considerable initial altitude of the region, 
allowing a deep dissection; a significant difference of drainage areas 
or of slopes, whereby certain glaciers incise deeper valleys than 
others; a considerable diversity of mountain structure, permitting 
such growth and arrangement of subsequent glaciers as shall bring 
the head reservoir of a subsequent ice stream alongside of and 
somewhat beneath the banks of a consequent ice stream. Thus 
glacial systems may come to adjust their streams to the structures 
upon which they work, just as happens in river systems. 
The load transported by a glacial system may at first be supplied 
largely by waste plucked and scoured from the beds of the glacial 
channels as well as by waste detached from the enclosing slopes; 
but in time, when the graded condition of the chief channels is 
reached and their further deepening almost ceases, by far the largest 
share of load will be supplied from the subaerial valley sides, where 
weathering of the ordinary kind will ravine the slopes, thus produc¬ 
ing a topography that is strongly contrasted with the smooth walls 
of the glacial channels. If the initial glacial system should incise its 
channels so deeply beneath a lofty highland that the supply of waste 
from the valley sides continued to increase after the development of 
graded glacial channels, it is conceivable that the channel-beds 
might have to be aggraded for a time, as is believed to be the case 
with river channels under similar conditions; but owing to the 
receipt on the glacial surface of waste from the valley sides, it is also 
conceivable that this analogy may not closely obtain. Toward the 
end of the ice stream it may well happen that the diminution of its 
volume and the consequent diminution of its capacity to do work 
will result in the aggradation of its bed by waste that cannot be 
carried further forward. At the same time, the outflowing river may 
be unable to wash away all the waste that is delivered to it, and so, for 
a time through later youth and early maturity, the river may act as 
an aggrading agent and build up a broad, flat alluvial fan, such as 
fronts the terminal moraines of the Alpine glaciers that once 
descended to the plain of Lombardy. Some response to the change 
thus produced in the altitude of the end of the glacier may be 
expected far up its channel, whose bed would thus come to be 
aggraded with till. Similarly, the ice sheets that spread from the 
Scandinavian and Laurentian highlands over the lowlands on the 
south changed their behavior from degrading agents in the central 
area to aggrading agents on the peripheral area. Hence, a belief in 
