August i6, 1906] 



NA TURE 



399 



<-xi>iiiij; marine deposits, occupies a similar position in 

 Miany places. From my knowledge of the conditions under 

 which the patches of marine detritus occur in the Base- 

 imni Clay of East Yorkshire, 1 think it most probable that 

 <hi- ^hrll-beds at Clava, Inverness-shire, and in Kintyro,' 

 uhii h lie at or near the base of the boulder-clay, represent 

 ihr disturbed sea bottom of early glacial times; while that 

 ai (hapelhall, near Airdrie, appears to have been a very 

 small isolated patch in the boulder-day, as no further 

 trace of it was found in the search carried out by a Com- 

 miid I- of the Association. These beds are certainly in- 

 adi<.|uate as proof of a mild interglacial submergence. 



In Eastern .\berdeenshire and the neighbouring coast- 

 lands the drifts have been indefatigably studied by that 

 hdiiuured veteran among glacialists, Mr. T. !•". Jamieson." 

 I he general succession of the drifts is remarkably similar 

 ic) I hat in liast Yorkshire, and the evidence for the mild 

 Helvetian Epoch is almost exactly that which we h.ave 

 alre.KK I'onsidered in England, Ireland, and the Isle of 

 M.in. ■ 



•• .\,H,iccti<i)i " {Third InicglacM). •• Mecklcnhurgiaii " 

 (i'otirth Glacial), " Lower Forestian " (Fourth Inter- 

 glacial), " Lower Turbarian " (Fiftli Glacial], " Upper 

 Forcstian " {Fifth Inlerglacial), and " I'pper Turbarian " 

 {Sixth Glacial) Epochs. 



.According to the terminology usually adopted by British 

 geologists, the Glacial Period came to an end with the 

 linal disappearance of the confluent ice-sheets from our low- 

 lands, and the events which followed are classed as Post- 

 .glacial. But the latter period has been sufficiently long to 

 cover some extensive changes in the relati\"e distribution 

 <if land and sea in Western Europe, accompanied by modifi- 

 caliiins of climate tending on the whole toward progressive 

 amelioration. To classify these changes into a further 

 series of three interglacial and three glacial epochs, as 

 Prof. J. Geikie has done, is, so far as the British evidence 

 is concerned, mainly a question of personal opinion as to 

 the arrangement of the sequence and the application of 

 terms. As we have already seen, the interpretation of the 

 North European sequence, on which Prof. Geikie greatly 

 depends for proof of these later epochs of glaciation, has 

 been challenged abroad even by geologists favourable to the 

 general principle of interglacial epochs ; and we are, there- 

 fore, the more fully entitled to question its application in 

 this country. 



In Scotland, Prof. Geikie claims that the " Mecklen- 

 burgian " glaciation was marked by the reappearance of 

 glaciers in the mountain valleys, and by their later 

 extension over part of the neighbouring lowlands in the 

 form of " district ice-sheets." After these had melted 

 away during the " Lower Forestian " interglacial time, 

 there is supposed to have been a regrowth of vallev-glaciers 

 that came down to sea-level during the " Lower Tur- 

 barian " stage. Then another melting away marked the 

 ■■ I'pper Forestian," followed by a fresh appearance of 

 glaci<'rs in the glens of the higher mountain groups during 

 I he " L'pper Turbarian " glacial epoch. 



But all the phenomena on which this scheme is built 

 seem explicable on the hypothesis of a graduallv waning 

 glaciation, during which there were occasional local 

 advances of the mountain-glaciers in their glens, due to 

 temporary increase of snowfall. We have already dis- 

 cussed the probability that the growth of the individual 

 ice-sheets was largely influenced by the local impact of 

 snowfall under changing meteorological conditions, and it 

 seems equally probable that similar changes, in reverse 

 order, accompanied the waning of the same sheets. 



Indeed, from the study of recent glaciers, it has been 

 shown that the presence of separate moraines need not 

 indicate separate stages of advance in the ice. In discuss- 

 ing the influence of englacial drtris on ice-flow, the late 



1 " Report on the Character of the High-level Shell-bearinK Deposits at 

 'lava, Chapelball. and other Localities." Ke^. Brit. .4ssoc. /or 1S93 

 Claval. pp. 483-514; ;'4;Vi'. for 1894 (Chapelhall), pp. 307-315; ibid, for 

 3o6 (Kintyrel, pp. ^78-390- 



■- Mr. Jamieson's latest papers: "The Glacial Period in Aberdeenshire 

 ,nd the Southern Border of the Moray Firth," Quart Joiirn. Geol. Soc. 

 ol. Ixii. (1006). pp. T3-59, and "On the Raised Beaches of the Geological 

 ;uivey of Scotland," Geol. Mag., dec. v., vol. iit. (1Q06), pp. 22-25, contain 

 ,n excellent descriptive summary and discussion of the glacial sequence. 



NO. 1920, VOL. 74] 



Prof. Israel C. Russell has the following pertinent remark : 

 " The considerations . . . lead to the suggestion that a 

 series of terminal moraines in a formerly glaciated valley, 

 or a sitiillar succession of ridges left by a continental 

 glacier, are not necessarily evidences of repeated climatic 

 oscillations, but may have been formed during a uniform 

 and continuous meteorological change favourable to glacial 

 recession. That is, a d(ibris-charged sheet may retreat for 

 a time, then halt, and again retreat, owing to its terminus 

 becoming congested with foreign material, in response to 

 a climatic change which would cause a glacier composed 

 of clear ice to recede continuously and without halts."' 



Prof. Geikie states his case for the " .Mecklenburgian 

 district ice-sheets " with intrepid but unconvincing 

 persuasiveness." He acknowledges that no interglacial de- 

 posits of the preceding Neudeckian epoch have been recog- 

 nised in Britain, and bases his argument upon the relation 

 of the hill-drift to that of the lowlands. Into the intri- 

 cacies of this argument it is impossible for me to enter, 

 but there is one point which requires particular notice. 

 The shelly boulder-clay around Loch Lomond is held to 

 represent the Mecklenburgian glaciation, and its marine 

 detritus to have been derived from a sea-floor belonging 

 to the " 100-foot raised beach," which is supposed to mark 

 an early stage of the same glacial epoch. But, as Mr. 

 T. F. Jamieson ' has shown, there is no valid reason for 

 regarding this boulder-clay as newer than the bulk of the 

 shelly boulder-clays of Scotland ; it rests directly upon the 

 solid rock, except at one place, where a wedge of blue 

 clay with shells was found beneath it ; and no older 

 boulder-clay is known in the district. Even from the 

 original description of the deposit given by Dr. R. L. 

 Jack,' quoted with approval by Prof. Geikie, we can 

 gather no other interpretation ; for although Dr. Jack 

 thought that the shells were more probablv derived from 

 an interglacial than from a preglacial bed, he still re- 

 garded the boulder-clay in which they occur as older than 

 the "great submergence" — i.e., than the Helvetian inter- 

 glacial epoch of the new classification. 



The evidence yielded by the freshwater deposits that 

 overlie the drifts in .Scotland, so far as I can judge, runs 

 parallel with that of the similar deposits in Yorkshire and 

 the Isle of Man. The researches of the late James Bennie 

 brought to light several instances in which arctic plants 

 and other remains occur in such deposits, but always at 

 or near their base, and sometimes overlain by higher beds 

 containing a temperate flora. By Mr. C. Reid, who has 

 determined most of the material, these arctic plant beds 

 are classed as " Late-Glacial," and the subsequent deposits 

 as " Neolithic." '' 



-Some evidence for changes of climate in the uplands 

 during post-glacial times has been recently obtained from 

 the study of peat mosses by Mr. F. J. Lewis; and these 

 changes have been arranged according to the scheme,'' 

 with Prof. Geikie "s approval," by supposing that only 

 certain parts of the sequence are represented in some places. 

 Thus, in the Highland mosses (and presumably also on 

 Cross Fell, in Cumberland),' where arctic plants are found 

 at the base of the peat, it is assuired that earlier beds 

 have been swept away by glaciation ; while in the Southern 

 Uplands an additional glacial and interglacial epoch are 

 supposed to be represented. But as in all cases the peats 

 lie above the glacial drifts, their suggested classification 

 into five stages, ranging from the " Mecklenburgian " to 

 the "Upper Turbarian," seems highly speculative; and it 

 has yet to be decided whether the changes indicated bv 

 the plants are so great as to fulfil the requirements of the 



1 "The Influence of Debris on the Flow of Glaciers." Journ. Geol. 

 vol. iii. (1895), p. 83T. 



- " Great Ice Age," 3rd ed., chap. xx. 



3 "Some Changes of Level in the Glacial Period." Geol. Mag., dec. v., 

 vol. ii. (1905), pp. 4S7-8. 



« " Notes on a Till or Boulder-clav with Broken Shells . . . near Loch 

 Lomond," v^c." Trans. Geol Soc. Glas^ovj, vol. v. (1874), pp. 5-26. 



5 "Origin of the British Flora," p. 53. 



•» "The History of the Scottish Peat Mosses and their Relation to the 

 Glact.-tl Period." Scottish Geo,^: Ma_^., vol. xxii. (1906), pp. 241-252 ; see 

 also Traits. Royal Soc. Edinburgh, vol. xH. (1005). part iii.. No. 28. 



^ " Late Quaternary Formations of Scotland." Zeitsciiri/t/nr GUiscker- 

 kttnde, vol. i. (1Q06), pp. 21-30. 



8 F. J. Lewis, " Interglacial and I'ostjlacial Beds of the Cross Fell 

 District." Ref. British Assoc. /or 1904. pp. 798-9. 



