Sept. 19, 1889] 



NATURE 



489 



once more invaded Germany, ploughing through the interglacial 

 deposits, and working these up in its ground-moraine. So far as 

 I can learn, the prevalent belief among geologists in North 

 Germany is that there was only one interglacial epoch ; but, as 

 already stated, doubt has been expressed whether all the facts 

 can be thus accounted for. There must always be great difificulty 

 in the correlation of widely-separated interglacial deposits, and 

 the time does not seem to me to have yet come when we can 

 definitely assert that all those interglacial beds belong to one 

 and the same geological horizon. 



I have dwelt upon the recent work of geologists in the peri- 

 pheral areas of the drift-covered regions of Northern Europe, 

 because I think the results obtained are of great interest to 

 glacialists in this country. And for the same reason I wish next 

 to call attention to what has been done of late years in elucidat- 

 ing the glacial geology of the Alpine lands of Central Europe 

 — and more particularly of the low grounds that stretch out 

 from the foot of the mountains. Any observations that tend to 

 throw light upon the history of the complex drifts of our own 

 peripheral areaS cannot but be of service. It is quite impossible 

 to do justice in this brief sketch to the labours of the many 

 enthusiastic geologists who, within recent years, have increased 

 our knowledge of the glaciation of the Alpine lands. At pre- 

 sent, however, I am not so much concerned with the proofs of 

 general glaciation as with the evidence that goes to show how 

 the Alpine ground-moraines have been formed, and with the 

 facts which have led certain observers to conclude that the Alps 

 have endured several distinct glaciations within Pleistocene 

 times. Swiss geologists are agreed that the ground-moraines 

 which clothe the bottoms of the great Alpine valleys, and extend 

 outwards sometimes for many miles upon the low grounds beyond, 

 are of true glacial origin. Now the>e ground- moraines are 

 closely similar to the boulder-clays of this country and Northern 

 Europe. Like them, they are frequently tough and hard-p'essed, 

 but now and again somewhat looser and less firmly coherent. 

 Frequently also they contain lenticular beds, and more or less 

 thick sheets of aqueous deposits — -in some places the stony clays 

 even exhibiting a kind of stratification — and ever and anon such 

 water-assorted materials are commingled with stony clay in the 

 most complex manner. These latter appearances are, however, 

 upon the whole best developed upon the low grounds that sweep 

 out from the base of the Alps. The only question concerning 

 the ground-moraines that has recently given rise to much dis- 

 cussion is the origin of the materials themselves. It is obvious 

 that there are only three possible modes in which those materials 

 could have been introduced to the ground-moraine : either they 

 consist of superficial u orainic debris which has found its way 

 down to the bottom of the old glaciers by crevasses ; or they may 

 be made up of the rock-rubbish, shingle, gravel, &c., which 

 doubtless strewed the valleys before these were occupied by ice ; 

 or, lastly, they may have been derived in chief measure from 

 the underlying rocks themselves by the action of the ice that 

 overflowed them. The investigations of Penck, Blaas, Bohm, 

 and Bruckner appear to me to have demonstrated that the 

 ground-moraines are composed mostly of materials which have 

 been detached from the underlying rocks by the erosive action 

 of the glaciers themselves. Their observations show that the 

 regions studied by them in greit detail were almost completely 

 buried under ice, so that the accumulation of superficial moraines 

 was for the most part impossible ; and they advance a num- 

 ber of facts which prove positively that the groand-moraiies 

 were formed and accumulated uuder ice. I cannot here re- 

 capitulate the evidence, but must content myself by a reference 

 to the papers in which this is fully discussed.^ These geologists 

 do not deny that some of the material may oc:asionally have 

 come from above, nor do they doubt that pre-existing masses of 

 rock-rubbish and alluvial accumulations may have been incor- 

 porated with the ground-moraines ; but the enormous extent of 

 the latter, and the direction of transport and distribution of the 

 erratics which they contain cannot be thus accounted for, while 

 all the facts are readily explained by the action of the ice itself, 

 •which used its sub-glacial debris as tools with which to carry on 

 the work of erosion. 



Prof. Heim and others have frequently asserted that glaciers 

 have little or no eroding power, since at the lower ends of exist- 

 ing glaciers we find no evidence of such erosion being in opera- 



' Penck, "Die Vergletscheriing der deutschen Alpen." Blaas, "Zeitschr. 

 d. Ferdmande"'^s," 1885. Bohm, /a/jr. d. k. k. geol. Reichsanstalt. 1885, 

 Kd. XXXV. Heft ^. Bruckner, "Die Vereletscherung d. Salzichgebietes," 

 &c., i826 



tion. But the chief work of a glacier cannot be carried on at its 

 lower end, where motion is reduced to a minimum, and where 

 the ice is perforated by sub-glacial tunnels and arches, under- 

 neath which no glacial erosion can possibly take place ; and yet 

 it is upon observations made in just such places that the prin- 

 cipal arguments against the erosive action of glaciers have been 

 based. If all that we could ever know of glacial action were 

 confined to what we can learn from peering into the grottoes at 

 the terminal fronts of existing glaciers, we should indeed come 

 to the conclusion that glaciers do not erode their rocky beds to 

 any appreciable extent. But as we do not look for the strongest 

 evidence of fluviatile erosion at the mouth of a river, but in its 

 valley- and mountain-tracks, so if we wish to learn what glacier- 

 ice can accomplish, we must study in detail some wide region 

 from which the ice has completely disappeared. When this 

 plan h.is been followed, it has happened that some of the 

 strongest opponents of glacial erosion have been compelled by 

 the force of the evidence to go over to the other canap. Dr. 

 Blaas, for example, has been led by his observations on the 

 glacial formations of the Inn Valley to recant his foroier views, 

 and to become a formidable advocate of the very theory which 

 he formerly opposed. To his work and the memoirs by Penck, 

 Hriickner, and Bohm already cited, and especially to the admir- 

 able chapter on glacier-erosion by the last-named author, I 

 would refer those who may be anxious to know the last word on 

 this much-debated question. 



The evidence of interglacial conditions within the Alpine 

 lands continues to increase. These are represented by alluvial 

 deposits of silt, sand, gravel, conglomerate, breccia, and lignites. 

 Penck, Bohm, and Briickner find evidence of two interglacial 

 epochs, and maintain that there have been three distinct and 

 separate epochs of glaciation in the Alps. No mere temporary 

 retreat and re-advance of the glaciers, according to them, will 

 account for the phenomena presente i by the interglacial deposits 

 and associated morainic accumulations. During interglacial 

 times the glaciers disappeared from the lower valleys of the 

 Alps — the climate was temperate, and probably the snow-fields 

 and glaciers approximated in extent to those of the present day. 

 All the evidence conspires to show that an interglacial epoch 

 was of prolonged duration. Dr. Briickner has observed that 

 the moraines of the last glacial epoch rest here and there upon 

 loss, and he confirms Penck's observations in South Bavaria that 

 this remarkable formation never overlies the morainic accumula- 

 tions of the latest glacial epoch. According to Penck and 

 Briickner, therefore, the loss is of interglacial age. There can 

 be little doubt, however, that loss does not belong to any one 

 particular horizon. Wahnschaffe * and others have shown that 

 throughout wide areas in North Germany it is the equivalent 

 in age of the " upper diluvium," while Schumacher {Hygienische 

 Topographie von Strassbiirg i. E., 1885) points out that in the 

 Rhine Valley it occurs on two separate and distinct horizons. 

 Prof. Andrese has likewise shown {Abhandl. z. geol. Specialkarte 

 V. Elsass Lothriiigen, Bd. vii. Heft 2) that there is an upper and 

 lower loss in Alsace, each characterized by its own special 

 fauna. 



There is still considerable difference of opinion a« to the mode 

 of formation of this remarkable accumulation. By many it is 

 considered to be an aqueous deposit ; others, following 

 Richthofen, are of opinion that it is a wind-blown accumula- 

 tion ; while some incline to the belief that it is partly the one 

 and partly the other. Nor do the upholders of these various 

 hypotheses agree amongst themselves as to the precise manner 

 in which water or wind has worked to produce the observed 

 results. Thus, amongst the supporters of the aqueous origin of 

 the loss, we find this attributed to the action of heavy rains 

 washing over and rearranging the material of the boulder-clays 

 (Laspeyres, Erlduterungen z. geol. Specialkarte v. Prenssen, 

 SiC, Blatt Grobzig, Z'6rbtg,und Petersberg). Many, again, have 

 held it probable that the loss is simply the finest loam distributed 

 over the low grounds by the flood-waters that escaped from the 

 northern inland ice and the viers de glace of the Alpine lands of 

 Central Europe. Another suggestion is that much of the 

 material of the loss may have been derived from the denudation 

 of the boulder-clays by flood-water, during the closing stages of 

 the last cold period. It is pointed out that in some regions at 

 least the loss is underlain by a layer of erratics, which are 

 believed to be the residue of the denuded boulder-clay. 



' Abhandl. z. geol. Specialkarte 71. Prcussen, &c.. Bd. vii. Heft i; 

 Zeitschr. d. deiitsch. geol. Gesethch. 1885, p. 904; 18S6, p. 367. 



