388 



NA TURE 



[August i6, 1906 



The Interglacial Problem. 



One of the most important points on which there has 

 been, and still is, wide difference of opinion among glacial 

 geologists, both in this country and abroad, is with regard 

 to the value of the evidence for interglacial periods ; and 

 it will be my aim, in bringing before you some general 

 conclusions regarding the drifts, to concentrate attention 

 principally upon this evidence. 



To keep the discussion within practicable limits I must 

 perforce assume the former extension of ice-fields over the 

 glaciated areas ; for although I know that there are still 

 dissentients from this fundamental proposition, the cumu- 

 lative evidence in its favour has been so frequently re- 

 capitulated that it would not be justifiable for me to detain 

 3'ou by repeating the arguments. 



It is now, I think, agreed by all who accept this pro- 

 position that the ice-sheets of the Glacial Period, though 

 of vast extent, had their northern as well as their southern 

 limits ; the original idea, that they represented the outer 

 portion of a polar ice-cap, having been disproved by more 

 extended researches in the more northerly part of our 

 hemisphere. Moreover, it has been found that these ice- 

 sheets had their origin in the coalescence of masses which 

 spread outward from separate areas of accumulation, acting 

 more or less independently, so that the individual sheets 

 did not all attain their farthest bounds at the same time. 

 But this recognition of independent centres of glaciation 

 has given sharper prominence to the question whether the 

 glacial deposits are to be regarded as the product of a single 

 epoch of glaciation, or whether they represent successive 

 epochs of this kind, separated by intervals during which 

 the great ice-sheets temporarily vanished. 



As opinion stands at present, probably most geologists 

 lean to the idea that the glaciation was interrupted by at 

 least one interglacial epoch, during which the climate of 

 any particular latitude became not less warm, and perhaps 

 warmer, than it now is. This is the Interglacial hypo- 

 thesis in its simplest form. But it has been frequently 

 pointed out that the criteria depended upon in the recog- 

 nition of warm interglacial conditions cannot be all assigned 

 to the same horizon, since they recur at different positions 

 in the drift series. Hence it has been claimed that two, 

 three, four, or even five interglacial epochs, with a corre- 

 sponding number of separate epochs of glaciation, mav be 

 recognised in the glacial sequence. In respect to' the 

 number, relative importance, and correlation of these 

 epochs 01 stages in different countries, or in different parts 

 of the same country, there has been, however, no pretence 

 to agreement among the upholders of the Interglacial 

 idea. 



In opposition to these views of every degree, a smaller 

 number of glaciallsts have urged that there is no proof 

 of even a single absolute interruption of the glacial con- 

 ditions from the beginning to the end of the period ; and 

 that the evidence indicates only one great glaciation, during 

 which there were wide oscillations of the margins of the 

 ice-sheets in different places, due probably to more or less 

 local circumstances. 



This radical difference of interpretation respecting the 

 constitution' of the Glacial Period assumes the greater 

 consequence in that it bears directly upon many questions 

 other than those which are strictly geological. ' Thus, the 

 antecedents and distribution of our present fauna and flora, 

 and the time and conditions of that momentous event, the 

 appearance of man in Northern Europe, are deeply involved 

 in the issue. 



Moreover, until we can tell whether it is one or several 

 periods of glaciation that we require, how can we approach 

 the other sciences for aid in our search for the cause of 

 the Ice Age? It is, indeed, essential that, before seeking 

 counsel's opinion of this kind, the geologist should have 

 all his evidence at command and well marshalled, so that 

 he can say such and such are the facts, and this the order 

 of them. Otherwise he may receive, not the desired inter- 

 pretation, but advice as to what he ought to have found 

 and instructions to go and find it. And'that such instruc- 

 tions may be detrimental rather than helpful to our progress 

 is, I think, shown by the historv of the Interglacial hypo- 

 thesis. In this matter the glacial geologists, having some 

 NO. 1920, VOL. 74"! 



evidence for the alternate extension and recession of ancient 

 glaciers, fell readily under the influence of the fascinating 

 theory brought forward by James CroU to explain the 

 Great Ice Age, whose interpretation, however, reached far 

 beyond the facts that were placed before him. 



I need hardly remind you that, according to Croll, a 

 sufficient explanation of the Glacial Period could be found 

 in certain astronomical conditions, which were shown by 

 his calculations to have recurred at definite intervals, and 

 were supposed to have produced repeated alternations of 

 cold and warm climate at the opposite hemispheres during 

 the course of the period. It is not my purpose to discuss 

 this or any other theory regarding the cause of the Great 

 Ice Age, but only to direct your attention to the influence 

 of CroU's views upon the work of observation. If the 

 theory could have been sustained, it would have given 

 into the hands of the geologist a first instalment of that 

 absolute measure of geological time which he so ardently 

 desires; and with this allurement it is no wonder that the 

 theory was welcomed and hopefully put to the test. Fore- 

 most among its exponents was Prof. James Geikie ; and we 

 must all recognise that its main importance to the field- 

 geologist arose from his powerful support and masterly 

 arrangement of the evidence favourable to the hypothesis. 



It is not surprising that, amid the complicated mass of 

 facts confronting us in the glacial deposits and among the 

 voluminous literature wherein these facts are more or less 

 skilfully enwrapped, there should have been found some 

 material to support the idea of a recurrent succession of 

 glacial and interglacial stages. But the glamour of the 

 astronomical hypothesis has waned, and it is recognised 

 that there are flaws in the physical aspect of the theory 

 and in its geological application that render it untrust- 

 worthy. I think, therefore, that the time has come when 

 we should reconsider the matter in critical mood, un- 

 influenced by the early glow of the theory, after the wise 

 example of that ancient people who debated all matters of 

 import in two opposite frames of mind. 



On the present occasion it would be impossible adequately 

 to discuss the whole subject, and I propose to deal prin- 

 cipally with my own experience in attempting to apply 

 the Interglacial hypothesis to my field-work. I hope also 

 to be able briefly to review the evidence from other parts 

 of our islands in the light of this experience. 



And here I may remind you of the important part which 

 this Section of the British Association has taken in the 

 study of the subject by organising Committees of Research, 

 provided with funds for carrying out excavation and other 

 necessary work. During the twenty-five years since we 

 last met at York I find that, including the work in certain 

 bone-caves, there have been fourteen such committees ; 

 and in many cases their operations have extended over 

 several years, so that more than thirty separate reports 

 have been published in the Annual Reports of the Associ- 

 ation.' The precise information" embodied in these reports 

 is of high scientific value, and I am sure that these results 

 are very creditable to the Section. 



Classification of the Drifts. 



I have mentioned the influence of Prof. J. Geikie in the 

 establishment of the Interglacial hypothesis ; and before 

 proceeding further it is necessary that we should re- 

 capitulate the scheme of classification which he has pro- 

 posed for the drifts on the basis of this hypothesis. This 

 elaborate scheme has been built up by a skilful combination 

 of evidence gleaned from various parts of Europe, and 

 represents the hypothesis in its extreme form. Stated in 

 downward succession it stands, in its latest development,^ 

 as follows : — ■ 



I Viz., Reports on " Ravgill Fissure" (18S3-1886); " Manure Gravels of 

 Wexford," &c. (1887-1890); "Welsh Caves" (18S6 and 1898); " Sewerby 

 ■Raised Beach" (iS38-i8go); " Elbolton Cave" (1801-1B94); "Scottish 

 Marine Drifts" (1893-1806); "Calf Hole, Skipton " (1894): " Hoxne Plant 

 Beds" (1896); "Irish Elk in the Isle of Man" (1B97-1900): "Pleisto- 

 cene Beds near Toronto" (tSgS-tgoo); " Moel Tryfaen 'Drift" (1898); 

 "Uphill Cave" (1S09-1901) ; "Irish Caves" (igoi-1904): " Kirmin^ton 

 and other Fossiliferous Drifts" (1003-1905). During the same period there 

 have also been twenty three reports of the "Erratic Blocks" Committee, 

 which bear indirectly upon the problem. 



*- " The Classification of European Glacial Deposits," Journ. Gecl. 

 (Chicago), vol. iii. (1S95), pp. 241-269. 



