DAVIS : GLACIAL EROSION. 
283 
recession in the margin of the fall” (’43,380). But however ap¬ 
propriate a discordance of branch and trunk may be in early youth, 
it cannot endure long enough to be associated with maturely opened 
main valleys. It should be noted that discordance of side and main 
valleys may also be found where a large river has lately been turned 
to a new path, as in the normal progress of the capture of the upper 
course of one river by the headward gnawing of a branch of another 
river (see reference to Russell below), or in the new arrangements 
of drainage lines in a region from which a glacial sheet has lately 
withdrawn. Furthermore, the valleys of very small wet-weather 
streams are frequently discordant with the valley of a serpentine 
river, if they enter it from the upland that is under-cut by the con¬ 
cave bank of the river. But these cases cannot find application in 
the hanging valleys of the Alps. The hanging valleys that open on 
sea cliffs, such as those of Normandy, are of course quite another 
matter. 
Overdeepened Main Valleys and Hanging Lateral Valleys. — 
Now it is characteristic of the bottom troughs of the glaciated 
Alpine valleys that they are broad-floored; they cannot be described 
as canyons in any proper sense of that word: the walls are steep 
enough, but they are too far apart. If the existing breadth of the 
troughs had been acquired in the ordinary manner by the lateral 
swinging of the main stream and by the lateral weathering of the 
walls, the long time required for such a change would have amply 
sufficed for the lateral streams to cut down their valleys to grade 
with the main valley; and their persistent failure to do so indicates 
the action of something else than normal river work in the widening 
of the main valley. This is the very kernel of the problem. 
If a main valley were excavated along a belt of weak rocks, the 
side valley might stand for some time at a considerable height above 
the main valley floor. Certain hanging valleys in the Alps seem at 
first sight to belong to this class, but such is not really the case. 
For example, where the Linth flows into the Wallen See, the well- 
defined bottom troughs of the river and of the lake both pass 
obliquely through a syncline of strong lower Cretaceous limestone, 
which forms cliffs on their walls. Side streams drain the high 
synclinal areas; one such stream cascades from the west into the 
Linth trough back of the village of Niifels; another cascades from 
the north into the Wallen See near its western end. The first 
explanation for such falls is that they are normally held up on the 
resistant limestone; but it should be noted that the bottom troughs 
