Sept. 19, 1889] 



NATURE 



487 



vou Waltershausen ^ set forth this view io an elaborate and well- 

 illustrated paper. Unfortunately for his hypothesis, no trace of 

 the supposed great lakes or inlanrl sea has ever been detected ; 

 on the contrary, the character of the rmrainic accumulations, 

 and the symmetrical grouping and radiation of the erratics and 

 perched blocks over the foot-hills and low ground«, show that 

 these last have been invaded and overflowed by the glaciers 

 themselves. Even the most strenuous upholders of the efficacy 

 of icebergs as originators of some boulder-clays admit that the 

 boulder-clay or till, of what we may call the inner or central 

 region of a glaciated tract, is the product of land-ice. Under 

 this category comes the boulder-clay of Norway, Sweden, and 

 Finland, and of the Alpine lands of Central Europe, not to 

 speak of the hilly parts of our own islands. 



When we come to study the drifts of the peripheral areas it is 

 not difficult to see why these should be considered to have had a 

 different origin. They present certain features which, although 

 not absent from the glacial deposits of the inner region, are not 

 nearly so characteristic of such upland tracts. I refer especially 

 to the frequent interstratification of boulder clays with well- 

 bedded deposits of clay, sand, and gravel ; and to the fact that 

 these b^ulder-clays are often less compressed than those of the 

 inner region, and have even occasionally a silt-like character. 

 Such appearances do seem at first to be readily explained on the 

 assumption that the deposits have been accumulated in water 

 opposite the margin of a continental glacier or ice-sheet ; and 

 this was the view which several able investigators in Germany 

 were for some time inclined to adopt. 



But when the phenomena came to be studied in greater detail, 

 and over a wider area, this preliminary hypothesis did not 

 prove satisfactory. It wa-; discovered, for example, that 

 "giants' kettles "'-^ were more or less commonly distributed 

 under the glacial deposits, and such "kettles" could only have 

 originated at the bottom of a glacier. Again, it was found that 

 preglacial accumulations were plentifully developed in certain 

 places below the drift, and were often involved with the latter in 

 a remarkable way. The " brown-coal formation " in like man- 

 ner was violently disturbed and displaced, to such a degree that 

 frequently the boulder-clay is found to underlie it. Similar 

 phenomena were encountered in regions where the drift over- 

 lies the Chalk — the latter presenting the appearance of having 

 been smashed and shattered^the fragments having often been 

 dragged some distance, so as to form a kind of friction-breccia 

 underlying the drift, while large masses are ofien included in the 

 clay itself. All the facts p )inted to the conclusion that these 

 disturbances were due to tangential thrusting or crushing, and 

 were not the result of vertical displacements, such as are pro- 

 duced by normal faulting, for the disturbances in question die 

 out from above downwards. Evidence of similar thrusting or 

 crushing is seen in the re iiarkable faults and contortions that so 

 often characterize the clays and sands that occur in the boulder- 

 clay itself. The only agent that could produce the appearances 

 now briefly referred to is land-ice, and we must therefore agree 

 with German geologists that glacier- ice has overflowed all the 

 drift-covered regions of the peripheral area. No evidence of 

 marine action in the formation of the stony clays is forthcoming 

 — not a trace of any sea beach has been detected. And yet, if 

 these clays had been laid down in the sea during the retreat of 

 the ice-sheet from Germany, surely such evidence as I have 

 indicated ought to be met with. To the best of my knowledge 

 the only particular facts which have been appealed to, as proofs 

 of marine action, are the appearance of bedded deposits in the 

 boulder-clays, and the occasional occurrence in the clays them- 

 selves of a sea-shell. Hut other organic remains are also met 

 with now and again in similar positions, such as mammalian 

 bones and fresh-water shells. All these, however, have been 

 shown to be derivative in their origin — they are just as much 

 erratics as the stones and boulders with which they are associated. 

 The only phenomena, therefore, that the glacialist has to account 

 for are the bedded deposits which occur so frequently in the 

 boulder-clays of the peripher d regions, and the occasional silty 

 and uncompressed character of the clays themselves. 



The intercalated beds are, after all, not hard to explain. If 

 we consider for a moment the geographical distribution of the 

 boulder-clays, and their associated aqueous deposits, we shall 



' " Untersuchungen fiber die Klirna'e der Gegenwart und der Vorwelt," 

 &c. {Natntirkundige i^erhanielingenv.d. HoUaiuL Maa.tsch.d. iVetensch. 

 te Haarlem, 1865). 



^_ These appear to have been fir^t detected by Prof. Berendt and Prof. E. 

 Geinitz. 



find a clue to their origin. Speaking in general terms, the stony 

 clays thicken out as they are followed from the mountainous and 

 high-lying tracts to the low ground. Thus they are of incon- 

 siderable thickness in Norway, the higher parts of Sweden, and 

 in Finland, just as we find is the case in Scotland, Northern 

 England, Wales, and the hilly parts of Ireland. Traced south 

 from the uplands of Scandinavia and Finland, they gradually 

 thicken out as the low grounds are approached. Thus in 

 Southern Sweden they reach a thickness of 43 metres or there- 

 about, and of 80 metres in the northern parts of Prussia, while over 

 the wide low-lying regions to the south they attain a much greater 

 thickness — reaching in Holstein, Mecklenburg, Pomerania, and 

 West Prussia a depth of 120 to 140 metres, and still greater 

 depths in Hanover, Mark Brandenburg, and Saxony. In those 

 regions, however, a considerable portion of the "diluvium" 

 consists, as we shall see presently, of water-formed beds. 



The geographical distribution of the aqueous deposits which 

 are associated with the stony clays is somewhat similar. They 

 are very sparingly developed in districts where the boulder-clays 

 are thin. Thus they are either wanting, or only occur spo- 

 radically in thin irregular beds, in the high grounds of Northern 

 Europe generally. Further south, however, they gradually 

 acquire more importance until in the peripheral regions of the 

 drift-covered tracts they come to equal and eventually to surpass 

 the boulder-clays in prominence. These latter, in fact, at last 

 cease to appear, and the whole bulk of the "diluvium" along 

 the southern margin of the drift area appears to consist of 

 aqueous accumulations alone. 



The explanations of these facts advanced by German 

 geologists are quite in accordance with the views which have 

 long been held by glacialists elsewhere, and have been tersely 

 summed up by Dr. Jentzsch (fnM>. d. klinigl. preiiss. 

 gcologischen Landcsamtalt fiir 1884, p. 438). The northern 

 regions, he says, were the feeding-grounds of the inland ice. 

 In those regions melting was at a minimum, while the grinding 

 action of the ice was most effective. Here, therefore, erosion 

 reached its maximum — ground-moraine or boulder-clay being 

 unable to accumulate to any thickness. Further south melting 

 greatly increased, while ground-moraine at the same time tended 

 to accumulate — ^the conjoint action of glacier-ice and sab-glacial 

 water resulting in the complex drifts of the peripheral area. In 

 the disposition and appearance of the aqueous deposits of the 

 "diluvium " we have evidence of an extensive sub-glacial water- 

 circulation — glacier-mills that gave rise to "giants' kettles"—- 

 chains of sub-glacial lakes in which fine clays gathered — streams 

 and rivers that flowed in tunnels under the ice, and whose 

 courses were paved with sand and gravel. Nowhere do German 

 geologists find any evidence of marine action. On the contrary, 

 the dovetailing and interosculation of boulder-clay with aqueous 

 deposits are explained by the relation of the ice to the surface 

 over which it flowed. Throughout the peripheral area it did 

 not rest so continuously upon the ground as was the case in the 

 inner region of maximum erosion. In many places it was 

 tunnelled by rapid streams and rivers, and here and there it 

 arched over sub-glacial lakes, so that accumulation of ground- 

 moraine proceeded side by side wth the formation of aqueous 

 sediments. Much of that ground-moraine is of the usual tough 

 and hard-pressed character, but here and there it is somewhat 

 less coherent and even silt-like. Now a study of the ground- 

 moraines of modern glaciers affords us a reasonable explanati >n 

 of such differences. Dr. Bruckner ^ has shown that in many 

 places the ground- moraine of Alpine glaciers is included in the 

 bottom of the ice itself. The ground-moraine, he says, frequently 

 appears as an ice-stratum abundantly impregnated with silt and 

 rock-fragments — it is like a conglomerate or breccia which has 

 ice for its binding material. When this ground-moraine mells 

 out of the ice — no running water being present — it forms a 

 layer of unstratified silt or clay, with stones scattered irregularly 

 through it. Such being the case in modern glaciers, we can 

 hardly doubt that over the peripheral areas occupied by the old 

 northern ice-sheet boulder-clay must frequently have been 

 accumulated in the same way. Nay, when the ground- moraine 

 melted out and dropped here and there into quietly- flowing water 

 it might even acquire in part a bedded character. 



The limits reached by the inland ice during its greatest exten- 

 sion are becoming more and more clearly defined, although its 

 southern margin will probably never be so accurately determined 



' " Die Vergletscherung des Salzachgebietes, ^c," d'Offra/i'i 'she Abhand- 

 Itngen heraiisgegeben v. A. Penck, Band i. Heft i. 



