Glacial History of Western Europe. 465 
moving ice would become more effective than sub-glacial streams in 
deepening its bed; but since the névé-flow is almost imperceptible 
near the head, another agency must be invoked, that of ‘ plucking’. 
The ice grips, like a forceps, any loose or projecting fragment in its 
rocky bed, wrenches that from its place, and carries it away. The 
extraction of one tooth weakens the hold of its neighbours, and thus 
the glen is deepened by ‘plucking’, while it is carried back by 
‘sapping’. Streams from melting snows on the slopes above the 
amphitheatre might have been expected to co-operate vigorously in 
making it, but of them little account seems to be taken, and we are 
even told that in some cases the winds probably prevented snow from 
resting on the rounded surface between two cirque-heads.’ As these 
receded, only a narrow neck would be left between them, which 
would be ultimately cut down into a gap or ‘col’. Thus a region of 
deep valleys with precipitous sides and heads, of sharp ridges, and of 
more or less isolated peaks is substituted for a rather monotonous, if 
lofty, highland. 
The hypothesis is ingenious, but some students of Alpine scenery 
think more proof desirable before they can accept it as an axiom. 
But even if ‘sapping and plucking’ were assigned a comparatively 
unimportant position in the cutting out of cirques and corries, it might 
still be maintained that the glaciers of the Ice Age had greatly deepened 
the valleys of mountain regions. That view is adopted by Professors 
Penck and Briickner in their work on the glaciation of the Alps,’ the 
value of which even those who cannot accept some of their conclusions 
will thankfuily admit. On one point all parties agree—that a valley 
cut by a fairly rapid stream in a durable rock is V-like in section. 
It is also agreed that a valley excavated or greatly enlarged 
by a glacier should be U-like in section. But an Alpine valley, 
especially as we approach its head, very commonly takes the following 
form. For some hundreds of feet up from the torrent it is a distinct 
V; above this the slopes become less rapid, changing, say, from 45° 
to not more than 80°, and that rather suddenly. Still higher comes 
a region of stone-strewn upland valleys and rugged crags, terminating 
in ridges and peaks of splintered rock, projecting from a mantle of ice 
and snow. The V-like part is often from 800 to 1000 feet in depth, 
and the above-named authors maintain that this, with perhaps as 
much of the more open trough above, was excavated during the 
Glacial Epoch. Thus the floor of any one of these valleys prior to 
the Ice Age must often have been at least 1800 feet above its present 
level. As a rough estimate we may fix the deepening of one of the 
larger Pennine valleys, tributary to the Rhone, to have been, during 
the Ice Age, at least 1600 feet in their lower parts. Most of them 
are now hanging valleys; the stream issuing, on the level of the main 
river, from a deep gorge. Their tributaries are rather variable in 
form ; the larger as a rule being more or less V-shaped ; the shorter, 
and especially the smaller, corresponding more with the upper part 
1 This does not appear to have occurred in the Alps. 
2 Die Alpen in Hiszeitalter, 1909. 
DECADE Y.—YOL. VII.—NO. X. 30 
