DAVIS: GLACIAL EROSION. 
303 
fled by measurements of the Rhone glacier, where the mean annual 
movement is 110 met. in the heavy trunk above the cascade, 27 
met. just below the cascade, and only 5 met. close to the melting 
front (Forel, ' 97 , 203). Evidently then, the erosion of the glacial 
bed, in so far as it is determined by the pressure and motion of the 
ice stream, will have its maximum some distance up-stream from the 
end of the glacier (J. Geikie, ’ 98 , 236). The glacial channel must 
therefore become narrower and shallower as its end is neared, as has 
already been stated. If the glacier ends some distance inland from 
the sea, its action will be conditioned by the grade and length 
of the river that carries away the water and waste from its lower 
end. The deepening of the distal part of the channel accomplished 
in youth might be followed by a shallowing for a time during matu¬ 
rity, when the accumulation of morainal and washed materials in 
front of the glacier compelled its end to rise. Now it may well be 
conceived that the surface slope of such a glacier near its end is less 
than the angle between the surface and the bottom of the glacier; 
and in this case, the glacial floor must become lower and lower for 
a certain distance up-stream. If such a glacier should melt away, 
the distal part of its channel would be occupied by a lake, although 
even the head of the lake may not reach to the locus of maximum 
glacial erosion. Lakes Maggiore, Como and Garda seem to occupy 
basins whose distal enclosure by heavy moraines and sheets of over- 
washed gravels has added to the depth produced by erosion further 
up-stream. It would seem, however, that a lake basin thus situated 
must be only a subordinate incident in the general erosion of the 
whole length of the glacial channel. Too much attention has, as a 
rule, been given to lakes of this kind, and not enough to the other 
effects of glacial action; it seems especially out of proportion to sup¬ 
pose that the maximum erosion by a glacier takes place near its end, 
as has been done by some authors, on account of the prevalent 
occurrence of lakes in this situation. 
If a glacier advances into the sea and ends in an ice cliff, from 
which ice blocks break off and float away, something of a basin-like 
form of its lower channel may be produced; but the dimensions of 
this basin will be determined by the climate at the termination of 
of the glacier. If the climate is such as to allow the glacier to enter 
the sea in maximum volume, then a basin is not to be expected. The 
more the glacier diminishes towards its end, the less erosion and the 
more deposition may occur beneath it, and the more of a basin may 
be developed inland from its end. 
