320 



NATURE 



{Feb. 26, 1874 



country perhaps are these deposits so largely developed, 

 and nowhere have they been so elaborately worked out 

 as by the distinguished band of geologists who have 

 made Scotch glacial formations a special study. 



The first seven chapters are devoted to a description of 

 the Till, the lowest member of the Scotch Drift ; and an 

 explanation of the line of reasoning that has led geo- 

 logists to acquiesce almost unanimously in the opinion 

 that it was formed on land beneath a sheet of ice, which, 

 during a period of intense cold, overspread the whole 

 country, and pushed its way far out over the shallow bed 

 of the surrounding sea. 



So far the author lias only been repeating and en- 

 forcing the conclusions of his predecessors, but in chaps. 

 II — 14 he enters on ground which is all but his own. 

 It has been long known that layers of well-bedded sand 

 and gravel occur in the heart of the Till, and between it 

 and the older rocks. These deposits however are local 

 and of small extent, or had been detected only in borings 

 or underground workings, and had had comparatively 

 little attention paid to them. Mr. J. Geikic has for the 

 first time pointed out that in spite of their small develop- 

 ment they are full of meaning ; and that, when this 

 meaning is realised, the fragmentary nature of their 

 occurrence is only what is to be e.xpected, and that the 

 wonder is, not that there is so little of them left, but that 

 any of them should have survived to tell the tale which he 

 has so ably extracted from them. And the story they 

 tell us is this. They are evidently the products of running 

 water, alluvial or lacustrine deposits mostly ; now former 

 observers had realised in a vague sort of way that they 

 were a proof of changes of climate, which permitted water 

 to tlow over what had been before an ice-bound wastc^ 

 but we have now clearly brought before us that the abate- 

 ments of the intense cold, which these beds indicate, were 

 not Iccal and temporary, but wide-spread and of long 

 duration, and that they recurred sevei'al times during the 

 period of the first great glaciation. Thus we are led to 

 see that the first subdivision of the great ice-age was not 

 one dreary unbroken lapse of Polar winter, but that it 

 included mild intervals, when the ice shrank back, 

 possibly disappeared altogether, when vegetation reap- 

 peared, and when herds of the great mammals returned 

 from the southern retreats into which they had been 

 driven during the most intense phases of the cold. And 

 these facts enable us to realise more vividly the immense 

 lapse of time represented by one division alone of the 

 Glacial Formation. For if " we consider that the succes- 

 sion of changes happened not once only, but again and 

 again, we cannot fail to have some faint appreciation of 

 the lapse of time required for the accumulation of the Till 

 and the Interglacial Deposits.'' Lastly Mr. Geikie has 

 pointed out that these alternations of intense glaciation 

 and comparative mildness are fully in accordance with 

 the theory so ably expounded by Mr. Croll, that changes 

 in climate are due to the combined effect of the Precession 

 of the Equinoxes and variation in the eccentricity of the 

 earth's orbit, a theory which.- he has lucidly expounded in 

 chaps. 8 — 10. 



■We next come to certain deposits, the meaning of which 

 seems first to have been clearly read by the author of the 

 present work. At last the conditions which gave rise to 

 the Till began to pass away and the climate to improve 



slowly, and the great glaciers ceased to be confluent ; a 

 depression of the land ensued so that the sea followed the 

 retreating margin of the ice ; but after a while, perhaps 

 owing to an upward movement, the glaciers terminated 

 on dry land. Mountain peaks now began to rise above 

 the ice, and showered down on to its surface loads of 

 dS7-is torn from their exposed faces by frost. As the 

 burden was shot over the ends of the glaciers, it gave rise 

 to huge heaps of morainic rubbish, which at first fell into 

 the sea, and afterwards, as the ice drew back, was shed 

 upon the land. In this way were formed the subdivisions 

 of the glacial formation which the author has distin- 

 guished as Boulder Clay and Morainic Rubbish. During 

 this period the author believes that many of the erratic 

 blocks, which form so conspicuous a feature among gla- 

 cial deposits, were stranded from the ice-sheet as it drew 

 back ; and he gives good reasons for preferring this e.x- 

 planation to the older notion, which supposed these 

 travellers to have been dropped from ice-bergs during the 

 submergence which came a little later on. 



As the climate gradually improved, the melting of the 

 ice swelled the rivers and gave rise to mighty floods, 

 which thundered down the narrow mountain glens, 

 sweeping before them portions of the Till and Morainic 

 rubbish, and, when they emerged on the open valleys of 

 the lowlands, spread out the worn and rounded mate- 

 rials in 'oroad sheets of gravel. One point here we must 

 pause specially to call attention to. Geologists had long 

 been aware of the disappearance of the great ice-sheet 

 and of a gradual submergence of the land which followed 

 it, but we now learn that the first of these events had 

 made considerable progress, perhaps had been completed, 

 before the second had fairly set in. 



The first act of the drama we are looking at may 

 be said to close here ; the second opens with the 

 commencement of the submergence just mentioned. 

 The land began to sink and went down till the sea 

 reached to some 1,200 feet, perhaps in some cases to 

 nearly double that amount, above its present level ; 

 and as each of the previously formed Drifts, Till, Boul. 

 der Clay, Morainic Rubbish, and Gravel, was brought 

 under the action of the waves, they sifted and sorted 

 it, washing out the fine dirt, and rounding and redu- 

 cing in size the pebbles ; and in many cases the clean 

 gravel and sand so formed were piled up along each suc- 

 cessive coast line in mounds and long ridges, which still 

 retain the distinctive outline originally impressed on 

 them by wave- and tidal-action. These hummocky piles 

 and ridges arc known as Kames or Eskers. That many 

 Kames owe their present shape in the main to the direct 

 action of the sea alone there can scarcely be a doubt ; we 

 find them sometimes for instance enclosing hollows ti'////- 

 out any outlet, a little tarn or peat moss occupying in 

 some cases the central depression, and in such a case they 

 must have been piled up by shifting currents, for in no 

 other way could the closed basin in the middle have been 

 produced. But many so-called Kames are only the rem- 

 nants of large sheets of gravel, the greater parts of which 

 have been carried away by denudation. It would be 

 better perhaps to restrict the term to those mounds 

 which were piled up originally very much as they stand 

 now. Even with this limitation, however, Kames are 

 plentiful enough, and they are found in greatest abun- 



