August i6, 1906] 



NA TURE 



593 



scepticism respecting tiie Helvetian Interglacial Epoch is 

 based, not upon any preconceived objection to the idea, 

 but upon tlie failure' of the hypothesis when I have put it 

 to the test in this and other districts ; and I find also that 

 niv experience in this particular runs parallel with that of 

 many other investigators of the so-called " middle glacial " 

 deposits of England. 



Matinn Detritus in Glacial Gravels.— Vtom certain 

 characters of the moundy gravels on Elamborough Head 

 and in Holderncss, such as their rudely linear arrangement, 

 their indifference to the contours, and their relation to the 

 middle or I'urple boulder-clays, it appears most probable 

 that they represent the material deposited along the niargin 

 of the ice-sheet by the surface-waters flowing from it and 

 from the adjacent land.' From the occurrence of more or 

 less fragmentary marine shells in them, the gravefs were, 

 however, originally supposed to be of marine origin, and 

 this view is still' upheld by some geologists. It is the 

 same question in which so many of the .so-called " middle 

 glacial " sands and gravels of the British Islands are in- 

 volved, and upon which there has been so much discussion. 

 If it be permissible for me to reiterate the well-known 

 argument by which the presence of marine shells in gravels 

 of glacial origin is explained it may be outlined as 

 follows. 



Since the basins around our islands are known to have 

 been occupied by the sea at the beginning of the Glacial 

 Period, and since these basins were afterwards filled by 

 ice-lobes, which, as we have seen, moved outward in many 

 places upon the land, dragging with them much of the 

 material of the old sea-floor, it is inevitable that a certain 

 amount of marine detritus will occur in the deposits 

 formed by the ice or derived from its melting. Just as we 

 find shells, and sometimes even transported masses of 

 marine deposits, intact in the Basement Clay, so wo find 

 marine relics likewise, though usually more scattered and 

 less perfect, in the gravels derived from the same ice- 

 sheet. This deduction is consistent with our know^ledge of 

 existing glaciers and ice-sheets ; thus. Sir Archibald Geikie 

 has recorded the presence of sea-shells in the moraine of a 

 Norwegian glacier"; Profs. E. J. Garwood and J. W. 

 Gregory have found an excellent illustration of the same 

 phenomenon in one of the Spitzbergen glaciers ' ; and Prof. 

 R. D. Salisbury, in describing the characteristic upturning 

 of the lavers, of ice at the end of one of the glacial lobes 

 which descends into a shallow bay in North Greenland, 

 gives the following instructive note on the conditions which 

 he observed : " Here the upturning of the layers brought 

 up shells from the bottom of the hay, and left them in 

 marginal belts where the upturned layers outcropped. 

 These shells were mingled with other sorts of d(5bris. In 

 one case their quantity could have been measured by some 

 such unit as the wagon-load."* 



In our islands, as Prof. P. F. Kendall has clearly shown 

 in discussing the drifts of Western England,' it is only 

 where the ice-lobes have passed over portions of the pre- 

 existing sea-floors that we find marine remains in the 

 drift deposits ; while in other places, at the same or lower 

 elevations, where there is proof that the ice-flow was from 

 the land, such remains are invariably absent. 



The occurrence of these shells in a few places at high 

 elevations, all explicable by consideration of the geo- 

 graphical circumstances, gave rise to the idea of a great 

 mid-glacial submergence, and upon this idea the hypo- 

 thesis of a mild interglacial epoch has mainly hinged. In 

 Prof. Geikie's latest scheme this supposed submergence is, 

 indeed, reduced to moderate limits, but it is still the 

 essential factor in the argument. 



The same idea of a moderate degree of submergence, 

 accompanied by temperate conditions of climate, has been 



LampluEh, "Drifts of Flamborough Head." 

 c. vol. xlvii. fi?oi). pp. 3S4-43T. 

 '■ "Geological Sketches at Home and Abroad 



Quari. /our 



(London, li 



1 Geology of Spitzbergen." 



'■> "Contributions to the Glacia 

 /ourn. Cecil. ScK.. vol. liv. (iSq8), p. 



•* "Glacial Geology of New Jersey." Hep. Geoi, Survey o/ Ne^M 

 vol. V. (1902), p. 81. (The quoted italics are in the orieinal.) 



•'• In the late Prof. H. Carvill Lewis's " Glacial Geology of Great 

 and Ireland " (London, 1894), i^ppendix A, pp. 425-431. 



■. Gcol. 

 i2), pp. 

 Quart. 

 Jersey, 

 Britain 



NO. 1920, VOL. 74] 



applied by Mr. Clement Reid ' to the shelly gravels of 

 lioldcrness. Mr. Reid has also proposed to include the 

 buried cliff-beds of Sewerby in the same interglacial stage ; 

 but as the gravels rise to nearly 100 feet above the level 

 of the old beach in northern Holderness, and are separated 

 from it by the Basement boulder-clay, 1 air sure that this 

 correlation cannot be sustained. 



These Holderness gravels are supposed to be absent 

 from the coast sections, and it is suggested that they may 

 lie below sea-level in this quarter ; but this is not very 

 probable, as they are found at an elevation of 50 feet 

 within a few miles of the coast in southern Holderness, 

 and the Basement boulder-clay rises well above sea-level 

 in the cliffs at Dimlington. It is true that the gravels of 

 the coast sections afford no support to the idea of a niild 

 interglacial submergence, and are evidently of similar 

 origin with the rest of the glacial deposits, but I can see 

 no other reason against their correlation with the gravels 

 of the neighbouring interior. Except in two or three 

 limited tracts, the shells in the Holderness gravels are as 

 fragmentary, and nearly as scanty, as in the moundy 

 gravels of 'Flamborough Head, which from their character 

 and position cannot be of marine origin. Even at the 

 exceptional places referred to, where the fossils are more 

 plentiful, there is a mixture of forms, including an 

 abundance of the freshwater shell Corbictila fliimiiialis, 

 which seems to denote their derivation from preexisting 

 local deposits ; and in the new section at Burstwick, de- 

 scribed by Mr. T. Sheppard,"" these shelly gravels revealed 

 the same close association with the boulder-clay that is so 

 frequently displayed in the glacial gravels of the coast 

 sections. 



The Kirmington Section.— There is, however, one case 

 known to me in the east of England, and only one, in 

 which an undoubtedly contemporaneous fauna occurs in 

 beds intercalated with the boulder-clay series. = .\t 

 Kirmington, in North Lincolnshire, a brickyard is worked 

 in a deposit of estuarine clay lying in the middle of a 

 broad shallow valley which cuts across the Chalk Wolds 

 about eight miles south of the Humber. Recent investi- 

 gation bv a Research Committee of the Association, in 

 which I 'took an active share, has shown, somewhat un- 

 expectedly, that the surface of the chalk at this place 

 descends 'to present sea-level, and that the estuarine warp 

 is underlain bv more than 60 feet of drift, consisting of 

 sand and chalky gravel, with two thick bands of tough 

 clay containing far-travelled stones.'' The boring in which 

 these beds were proved was insufficient to show precisely 

 whether the stony clays possessed the distinguishing featui-es 

 of true till, but there can be no doubt as to their glacial 

 character, since we know of no deposits of this kind in 

 the east of England except those of glacial age. At 

 the base of the estuarine warp, at 6,:; feet above Ordiiance 

 datum, we found a thin seam of siit and peat containing 

 a few freshwater shells and plant remains, w'hich. like 

 the very scanty fauna of the overlying warp, give no precise 

 indicati'on of 'climatal conditions, though suggesting that 

 the climate was cooler than at present. The cstuariipe 

 bed is overlain by a coarse gravel of rolled flints, and in 

 one part of the section this gravel is covered by 3 or 4 feet 

 of red clay with far-travelled stones, resembling the Upper 

 boulder-clay or Hessle Clay of Holderness. The character 

 and fauna of the warp show that it must have been laid 

 down between tide-marks, and we therefore gain an exact 

 measure of the sea-level at .the time of its accumulation, 

 and also, I think, of the highest limit of marine sub- 

 mergence in this part of England during any stage of the 

 Glacial Period. 



The position of the deposit, at the fringe of the great 

 sheet of drift which covers the lowland east of the Wolds 



1 " The Geology of Holderness." .Vem. Geo!. .Sunvy (iSSgX 



■- "On another Section in the so-called Interglacial Gravels of Holder- 

 ness." Proc. rm-ks. Geol. and Polylech. Soc., vol. xiii. (1S95). rp. 1-14. 



3 The freshwater deposit which 1 found some years ago at Bridlington, 

 and at first thought to be probably intercalated with the boiilder-claj-, 

 proved on fuller exposure to lie above the boulder-clav, with which it had 

 become entangled bv later disiurbanre. See Geol. Mas;., dec. ii.._voh vi. 

 (1S79), p. S93; and Prac. Yorks. Geol. and Polytech. Soc, vol. vii. (iSSi). 

 p. 380. 



■< Ref. British Assoc, for 1904, pp. 272-4. 



