DAVIS: GLACIAL EROSION. 
317 
the fjeltls — the profound erosion in the former, and the arrest of 
erosion on the plateau — admits of only one explanation. While 
rivers and rapid ice-streams, flowing in previously excavated valleys, 
were actively engaged in deepening these, the adjacent fjelds were 
buried under sheets of neve .In short, while rivers and glaciers 
were deepening the great valleys and making tlieir walls steeper, 
the intervening mountain-heights were gradually being reduced and 
levelled by denudation.It was somewhat otherwise in the Alps, 
where the hydrographic system, perfectly regular in preglacial 
times, was only slightly modified by subsequent glacial action. Yet 
even there erosion proceeded most rapidly along the chief lines of 
ice-flow. Were the great rock-basins of the principal Alpine valleys 
pumped dry we should find the mouths or openings of the side 
valleys abruptly truncated, and their waters cascading suddenly 
into the ice-deepened main-valleys. For, as Dr. Wallace has shown, 
it is the present lak Q-surface, not the lak e-bottom, that represents 
approximately the level of the preglacial valley. In a word, erosion 
proceeded most actively in the main valleys, the bottoms of which 
have been lowered for several hundred feet below the bottoms of the 
side-valleys. Precisely the same phenomena are repeated in Scot¬ 
land. Were all the water to disappear from the Highland lakes and 
sea-lochs, we should find waterfalls and cascades at the mouth of 
every lateral stream and torrent ” (’ 98 , 246-249). 
It is evident from these extracts that the deepening. of valleys is 
regarded as greatest where lake basins have been eroded beneath the 
preglacial valley floors; and this belief is explicitly expressed in the 
following extract from the latest edition of the same author’s “ Great 
Ice Age,” the standard work on that subject: “ Take the case of a 
glacier creeping down an Alpine valley and spreading itself out 
upon the low ground at the foot of the mountains. Let us suppose 
that, in the upper part of its course, the incline down which it moves 
is greater than the slope of the lower reaches of the valley. When 
the glacier attains the more gently inclined part of .its course, it is 
evident that its flow must be retarded, and there will therefore be a 
tendency in the ice to accumulate or heap up. Now we know that 
the pressure of a body in motion upon any given surface varies with 
the degree at which that surface is inclined; as the inclination 
decreases the pressure increases. It follows from this that when the 
glacier leaves the steeper part of its course, and begins to creep 
down the gentler slope beyond, it will press with greater force upon 
its rocky bed, and this increased pressure will be further intensified 
