276 



NATURE 



[SEPTEMIiER I, 1 910 



vertical wall of a gravel pit ; and a corrie, measured by 

 feet instead of furlonj^s, at the foot of one of the Binns 

 near Burntisland, or, on a much reduced scale, in a bank 

 <;f earth. On all these the same agent, plunging water, 

 has left its marks — runlets of rain for the smaller, streams 

 for the larger ; convergent at first, perhaps, by accident, 

 afterwards inevitably combined as the hollow widened and 

 deepened. Each of the great cirques is still a " land of 

 streams," and they are kept permanent for the greater 

 part of the year by beds of snow on the ledges above its 

 .valls. 



The " sapping and plucking " process presents another 

 difficulty — the steps already mentioned in the floors of 

 valleys. These are supposed to indicate stages at which 

 the excavating glacier transferred its operations to a higher 

 level. But, if so, the outermost one must be the oldest, 

 or the glacier must have been first formed in the lowest 

 part of the incipient valley. Yet, with a falling tempera- 

 ture, the rever.se would happen, for otherwise the snow 

 must act as a protective mantle to the mature pre-glacial 

 surface almost down to its base. However much age 

 might have smoothed away youthful angularities, it would 

 be strange if no receptacles had been left higher up to 

 initiate the process ; and even if sapping had only modified 

 the form of an older valley, it could not have cut the 

 steps unless it had begun its work on the lowest one. 

 Thus, in the case of the Creux de Champ, if we hesitate 

 to assume that the sapping process began at the mouth of 

 the valley of the Grande Eau above Aigle, we must sup- 

 pose it to have started somewhere near Ormont Dessus 

 and to have excavated that gigantic hollow, the floor of 

 which lies full 6000 feet below the culminating crags of 

 the Diablerets. 



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 .\ge had greatly deepened the valleys 

 of mountain regions. That view is adopted by Profs. 

 Penck and Bruckner in their work on the glaciation of 

 the Alps," the value of which even those who cannot accept 

 some of their conclusions will thankfully 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. With an 

 increase of speed the walls become more vertical ; with a 

 diminution the valley widens and has a flatter bed, over 

 which the river, as the base-line is approached, may at 

 last meander. Lateral streams will plough into the slopes, 

 and may be numerous enough to convert them into 

 alternating ridges and furrows. If a valley has been 

 excavated in thick horizontal beds of rock varying in 

 hardness, such as limestones and shales, its sides exhibit 

 a succession of terrace walls and shelving banks, while a 

 marked dip and other dominant structures produce their 

 own modifications. 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 becomes less rapid, changing, 

 say, from 45° to not more than 30°, 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 Son 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 

 anv one of these valleys prior to the Ice Age must often 

 have been at least 1800 feet above its present level." .^s 

 a rough estimate, we may fix the deepening of one of 

 the larger Pennine valleys, tributary to the Rhone, to have 

 been, during the Tee .'Xge, 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 



1 " Die Alpen in Eiszeitalter." ipoo. 



2 The amount varies in different valleys ; for instanre, it was fullv 2880 

 feet at Amsteg on the Reuss, just over 2000 feet at Krieg in the Rhone 

 Valley, about 1000 feet at Guttanen in the Aare Valley, about 1550 feet 

 above Zermatt, and 1100 feet above Saas Grund. 



NO. 2 13 1, VOL. 84] 



part of the larger valleys, but their lips generally are less 

 deeply notched. Whatever may have been the cause, 'this 

 rapid change in slope must indicate a corresponding change 

 of action in the erosive agent. Here and there the apex 

 of the V may be slightly flattened, but any approach to 

 a real U is extremely rare. The retention of the more 

 open form in many small, elevated recesses, from which 

 at the present day but little water descends, suggests that 

 where one of them soon became buried under snow,' but 

 was insignificant as a feeder of a glacier, erosion has been 

 for ages almost at a standstill. 



The V-like lower portion in the section of one of the 

 principal valleys, which is all that soine other observers 

 have claimed for the work of a glacier, cannot be ascribed 

 to subsequent modification by water, because ice-worn rock 

 can be seen in many places, not only high up its sides, 

 but also down to within a yard or two of the present 

 torrent. 



Thus valley after valley in the .\lps seems to leave no 

 escape from the following dilemma : Either a valley cut 

 by a glacier does not differ in form from one made by 

 running water, or one which has been e.xcavated by the 

 latter, if subsequently occupied, is but superficially 

 modified by ice. This, as we can repeatedly see in the 

 higher ."Mpine valleys, has not succeeded in obliterating the 

 physical features due to the ordinary processes of erosion. 

 Even where its effects are most striking, as in the 

 Spitallamm below the Grimsel Hospice, it has not wholly 

 effaced those features ; and wherever a glacier in a recent 

 retreat has exposed a rock surface, that demonstrates its 

 inefficiency as a plough. The evidence of such cases has 

 been pronounced inadmissible, on the ground that the 

 glaciers of the .\lps have now degenerated into senile 

 impotence ; but in valley beds over which they passed 

 when in the full tide of their strength, the flanks show 

 remnants of rocky ridges only partly smoothed away, and 

 rough rock exists on the " lee-sides " of ice-worn mounds 

 which no imaginary plucking can explain. The ice seems 

 to have flowed over rather than to have plunged into the 

 obstacles in its path, and even the huge steps of limestone 

 exposed by the last retreat of the Unter Grindelwald Glacier 

 have suffered little more than a rounding off of their 

 angles, though that glacier must have passed over them 

 when in fullest development, for it seems impossible to 

 explain these by any process of sapping. 



The comparatively level trough, which so often forms 

 the uppermost part of one of the great passes across the 

 w%atershed of the .Mps, can hardly be explained without 

 admitting that in each case the original watershed has 

 been destroyed by the more rapid recession of the head 

 of the southern valley, and this work bears every sign of 

 having been accomplished in pre-Glacial times. Sapping 

 and plucking must have operated on a gigantic scale to 

 separate the Viso from the Cottian watershed, to isolate 

 the huge pyramid of the Matterhorn, with its western 

 spur, or to make, by the recession of the \'al Macugnaga, 

 that great gap between the Strahlhorn and Monte Rosa. 

 .Some sceptics even go so far as to doubt whether the 

 dominant forn» of a non-glaciated region differ very 

 materiallv from those of one which has been half buried 

 in snowfields and glaciers. To my eyes, the general out- 

 lines of the mountains about the Lake of Gennesaret and 

 the northern part of the Dead Sea recalled those around 

 the Lake of .\nnecy and on the south-eastern shore of 

 Leman. The sandstone crags, which rise here and there 

 like ruined castles from the lower plateau of the Saxon 

 Switzerland, resembled in outlines, though on a smaller 

 scale, some of the Dolomites in the Southern Tyrol. The 

 Lofoten Islands illustrate a half-drowned mountain range 

 from which the glaciers have disapoeared. Those were 

 born among splintered peaks and ridges, which, though 

 less loftv, rival in form the .Aiguilles of Chamonix, and 

 the valleys become more and more ice-worn as they 

 descend, until the coast is fringed with skerries every one 

 of which is a rnche moutonitce. The ni've in each of these 

 valleys has been comparatively ineffective ; the ice has 

 gathered strength with the growth of the glacier. .As can 

 be seen from photographs, the scenery of the heart of the 



1 My own studies of mountain districts h.nve led me to infer that on slopes 

 of low cnde the action of snow is nres-rvative rather than destructive. 

 That conclusion was confirmed by Prof. Garwood in a communication to the 

 Royal Geographical Society on June 20 of the present year. 



