Septhmber I, 1910] 



NATURE 



2"5 



for a survey of our present position it is enougli to refer 

 to the suggestive and comprehensive volume pulilished last 

 year by -Mr. A. Marker;' partly, also, because the dis- 

 cussion of any branch of petrology would involve so many 

 technicalities that I fear it would be found tedious by a 

 large majority of my audience. So I have preferred to 

 Jiscuss some questions relating to the effects of ice 

 which had engaged my attention a dozen years before I 

 .attempted the study of rock slices. As much of my petro- 

 logical work has been cormected n'ith mountain districts, 

 it has been possible for me to carry on the latter without 

 neglecting the former, and my study of ice-work gradually 

 led me from the highlands into the lowlands." I purpose, 

 then, to ask your attention this evening to some aspects 

 of the glacial history of Western Europe. 



-At no very distant geological epoch the climate in the 

 northern part of the earth was much colder than it is at 

 present. So it was also in the southern ; but whether the 

 two were contemporaneous is less certain. Still more 

 doubtful are the extent and the work of the ice which 

 was a consequence, and the origin of certain deposits on 

 some northern lowlands, including those of our own 

 islands, namely, whether they are the direct leavings of 

 glaciers or were laid down beneath the sea by floating 

 shore-ice and bergs. Much light will be thrown on this 

 comple.K problem by endeavouring to ascertain what snow 

 and ice have done in some region which, during the Glacial 

 Kpoch, was never submerged, and none better can be 

 found for this purpose than the European .\Ips. 



.At the present day one school of geologists, which of 

 late years has rapidly increased in number, claims for 

 jjlaciers a very large share in the sculpture of that chain, 

 asserting that they have not only scooped out the marginal 

 lakes, as Sir A. Ramsay maintained full half a century 

 ago, but have also quarried lofty clifTs, excavated great 

 cirques, and deepened parts of the larger Alpine valleys by 

 something like two thousand feet. The other school, while 

 admitting that a glacier, in special circumstances, may 

 hollow out a tarn or small lake and modify the features 

 of rock scenery, declares that its action is abrasive rather 

 than erosive, and that the sculpture of ridges, crags, and 

 valleys was mainly accomplished in pre-Glacial times by 

 rimning water and the ordinary atmospheric agencies. 



In all controversies, as time goes on, hypotheses are 

 apt to masquerade as facts, so that I shall endeavour this 

 evening to disentangle the two, and direct attention to 

 those which may be safely used in drawing a conclusion. 



In certain mountain regions, especially those where 

 strong limestones, granites, and other massive rocks are 

 dominant, the valleys are often trench-like, with pre- 

 cipitous sides, having cirques or corries at their heads, 

 and with rather wide and gently sloping floors, which 

 occasionally descend in steps, the distance between these 

 increasing with that from the watershed. Glaciers have 

 imquestionably occupied many of these valleys, but of late 

 years they have been supposed to have taken a large share 

 in excavating them. In order to appreciate their action, 

 we must imagine the glens to be filled up and the district 

 restored to its former condition of a more or less undulating 

 upland. .As the mean temperature ^ declined, snow would 

 begin to accumulate in inequalities on the upper slopes. 

 This, by melting and freezing, would soften and corrode 

 the underlying material, which w^ould then be removed bv 

 rain and T^-ind, gravitation and avalanche. In course of 

 time the hollow thus formed would assume more and more 

 the outlines of a corrie or a cirque by eating into the 

 hillside. With an increasing diameter it would be occupied, 

 as the temperature fell, first by a permanent snowfield, 

 then by the nevi of a glacier. Another process now 

 becomes important, that called " sapping." While ordinary 

 glacier-scour tends, as we are told, to produce " sweeping 

 curves and eventually a graded slope," " sapping " pro- 

 duces " benches and cliffs, its action being horizontal and 

 backwards," and often dominant over scour. The author 

 of this hypothesis ■" convinced himself of its truth in the 

 Sierra Nevada by descending a herg^schruni 150 feet in 



t ''The Natural History of Igneous Rocks," igoc. 

 ■ '.2 May I add that hereafter a statement of facts without mention of an 

 authority means that I am speakin? from personal knowledge. 



5 In the remainder of this addre-s " temoerature" is to be understood as 

 mean temperature. The Fahrenheit scale is used. 



4 W. D. Johnson, Science. N.S.. iv. |-.'!qo\, pp. 106, ii?. 



-NO. 2131, VOL. 84] 



depth, which opened out, as is so common, beneath the 

 walls of a cirque. Beginning in the neve, it ultimately 

 reached the cliff, so that for the last 30 feet the bold 

 investigator found rock on the one hand and ice on the 

 other. The former was traversed by fracture planes, and 

 was in all stages of displacement and dislodgment ; some 

 blocks having fallen to the bottom, others bridging the 

 n.'UTow chasm, and otiiers frozen into the nevv. Clear 

 ice had formed in the fissures of the cliff ; it hung down 

 in great stalactites ; it had accumulated in stalagmitic 

 masses on the floor. Beneath the neve the teinperature 

 would be uniform, so its action would be protective, except 

 where it set-up another kind of erosion, presently to be 

 noticed ; but in the chasm, we are informed, there would 

 be, at any rate for a considerable part of the year, a daily 

 alternation of freezing and thawing. Thus the cliff would 

 ■je rapiQly undermined and be carried back into the moun- 

 tain slope, so that before long the glacier would nestle 

 in a shelter of its own making. Farther down the valley 

 the moving ice would become more effective than sub- 

 gl.acia! streams in deepening its bed; but since the nevi- 

 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 a-yvay. 

 ihe extraction of one tooth weakens the hold of its neigh- 

 bours, and thus the glen is deepened by " plucking," while 

 i* is carried back by " sapping." Streams from meltiag 

 snows on the slopes above the amphitheatre might have 

 been expected to cooperate 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, vifhich 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 mono- 

 tonous, 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. For instance, continuous observations are 

 necessary to justify the assumption of diurnal variations of 

 iemperature sufficient to produce any sensible effect on 

 rock at the bottom of a narrow chasm nearly fifty yards 

 deep and almost enclosed by ice. Here the conditions 

 would more probably resemble those in a glaciere, or 

 natural ice cave. In one of these, during the summer, 

 curtains and festoons of ice depend from the walls ; from 

 them and from the roof water drips slowly, to be frozen 

 into stalagmitic mounds on the floor, which is itself some- 

 times a thick bed of ice. On this the quantity of fallen 

 rock debris is not greater than is usual in a cave, nor 

 are the walls notably shattered, even though a gap some 

 four yards deep may separate them from the ice. The 

 floors of cirques, from which the n^w' has vanished, can- 

 not as a rule be examined, because they are masked by 

 debris which is brought down by the numerous cascades, 

 little and big, which seam their walls; but glimpses of 

 them may sometimes be obtained in the smaller corries 

 (which would be cirques if they could), and these show 

 no signs of either "sapping" or "plucking," but some 

 little of abrasion by moving ice. Cirques and corries also 

 not infrequently occur on the sides aswell as at the heads 

 of valleys, such, for instance, as the two rn the massif 

 of the Uri Rothstock on the way to the Surenen Pass and 

 the Fer i Cheval above Sixt. The Lago di Ritom lies 

 between the mouth of a hanging valley and a well-defined 

 step, and just above that is the Lago di Cadagno in a 

 large, steep-walled corrie, which opens laterally into the 

 Val Piora, as that of the Lago di Tremorgio does into 

 the southern side of the Val Bcdretto. Cirques may also 

 be found where glaciers have had a comparatively brief 

 existence, as the Creux des Vents on the Jura; or have 

 never been formed, as on the slopes of Salina, one of the 

 Lipari Islands, or in the limestone desert of Lower Egypt." 

 I have seen a miniature stepped valley carved by a rain- 

 storm on a slope of Hampstead Heath ; a cirque, about a 

 yard in height and breadth, similarly excavated in the 



1 This di-es not appe.ar 'o have occurred in the .\]ps. 

 •- A. J. Jukes-Browne, Geo!. Afag., 1877, p. 477- 



