128 PROFESSOR JAMES GEIKIE ON THE 



The interglacial beds, which in many places separate the lower from the upper till, 

 show that after the retreat of the earlier mer de glace the climate became progressively 

 more temperate, until eventually the country was clothed with a flora essentially the 

 same as the present. Wild oxen, the great Irish deer, and the horse, elephant, rhinoceros, 

 and other mammals then lived in Britain. From the presence of such a flora and fauna 

 we may reasonably infer that the climate during the climax of interglacial times was as 

 genial as now. The occurrence of marine deposits associated with some of the inter- 

 glacial peaty beds shows that eventually submergence ensued ; and as the shells in some 

 of the marine beds are boreal and Arctic forms, they prove that cold climatic conditions 

 accompanied the depression of the land. To what extent the land sank under water we 

 cannot tell. It may have been 500 feet or not so much, for the evidence is somewhat 

 unsatisfactory. 



The upper boulder-clay of our islands is the product of another mer de glace, which 

 in Scotland would seem to have been hardly less thick and extensive than its predecessor. 

 Like the latter, it covered the whole country, overflowed the Outer Hebrides, and became 

 confluent with the Scandinavian inland ice on the bed of the North Sea. But it did not 

 flow so far to the south as the earlier ice-sheet. 



It is well known that this later mer de glace was succeeded in our mountain regions 

 by a series of large local glaciers, which geologists generally believe were its direct 

 descendants. It is supposed, in short, that the inland ice, after retreating from the low 

 grounds, persisted for a time in the form of local glaciers in mountain valleys. This 

 view I also formerly held, although there were certain appearances which seemed to 

 indicate that, after the ice-sheet had melted away from the lowlands and shrunk far into 

 the mountains, a general advance of great valley-glaciers had taken place. I had 

 observed, for example, that the upper boulder-clay is often well developed in the lower 

 reaches of our mountain valleys — that, in fact, it may be met with more or less 

 abundantly up to the point at which large terminal moraines are encountered. More 

 than this, I had noticed that upland valleys, in which no local or terminal moraines 

 occur, are usually clothed and paved with boulder-clay throughout. Again, the aspect 

 of valleys which have been occupied by large local glaciers is very suggestive. Above 

 the point at which terminal moraines occur only meagre patches of till are met with on 

 the bottoms of the valleys. The adjacent hill-slopes up to a certain line may show bare 

 rock, sprinkled perchance with erratics and superficial morainic detritus ; but above this 

 line, if the acclivity be not too great, boulder-clay often comes on again. These appear- 

 ances are most conspicuously displayed in the Southern Uplands of Scotland, particularly 

 in South Ayrshire and Galloway, and long ago led me to suspect that the local glaciers 

 into which our latest mer de glace was resolved, after retreating continuously towards 

 the heads of their valleys, so as to leave the boulder-clay in a comparatively unmodified 

 condition, had again advanced and ploughed this out, down to the point at which they 

 dropped their terminal moraines. Subsequent observations in the Highlands and the Inner 

 and Outer Hebrides confirmed me in my suspicion, for in all those regions we meet with 



