James Geikie — On Changes of Climate. 25 



"wliicli eventually buried the wliole country underneath a mass of ice 

 some 2000 feet in thickness. Nor can we have any proper conception 

 of how long a time was needed to bring about that other change of 

 climate under the influence of which slowly and imperceptibly this 

 immense sheet of frost melted away from the Lowlands and retired 

 to the mountain recesses. We must allow that long ages elapsed 

 before the warmth became such as to induce plants and animals to 

 clothe and people the land. How vast a time, also, must have passed 

 away ere the warmth reached its climax, and the temperature again 

 began to cool down. How slowly, step by step, the ice must have 

 crept out from the mountain fastnesses, chilling the air, and forcing 

 fauna and flora to retire before it ; and what a long succession of 

 years must have come and gone before the ice-sheet once more 

 wrapped up the hills, obliterated the valleys, and, streaming out 

 from the shore, usurped the bed of the shallow seas that flowed 

 around our island. Finally, when we consider that such a 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 re- 

 quired for the accumulation of the Till and the inter-Glacial deposits. 



The deposition of the Till was succeeded by a movement of de- 

 pression, during which the overlying series of gravel, sand, and 

 brick-clays was thrown down. These beds have been so often 

 described, and their general appearance is so well known, that any 

 account of them here would be quite unnecessary. It need only be 

 remarked that the sand and gravel beds appear for the most part to be 

 derived from the wreck of the Till and also from that of the terminal 

 moraines that marked the gradual retreat of the ice-sheet. But of 

 this more anon. The marine drifts invariably recline either upon a 

 sorely eroded surface of Till or upon bare rock. In places where 

 the Kame-series is wanting the Till has usually a much less eroded 

 appearance. 



I have elsewhere^ pointed out the remarkable distribution of the 

 Karnes in Scotland, and shown that none of these peculiar mounds 

 occur in valleys which at the period of greatest submergence would 

 become deep quiet fiords. In such places the Till forms broad flat 

 terraces, whose slope coincides with the general inclination of the 

 valleys. But this deposit has either vanished or appears only in 

 meagre patches in valleys which, during the period of submergence, 

 must have existed as sounds or straits connecting opener spaces of 

 sea. It is precisely in such valleys, however, where we meet with 

 the most typical assemblages of Karnes. I have inferred from these 

 facts that the last-mentioned valleys were swept by currents which 

 denuded the Till, and heaped up banks of sand and gravel ; while 

 in the quiet " fiord valley s " no currents moved, and consequently 

 the Till was left undisturbed, and no Kames were formed. 



An occasional boulder found in the heart of a Kame shows that 



during the accumulation of the Kames ice must have been floating 



about. But I have often remarked that while such included boulders 



are of somewhat rare occurrence, numbers of erratics, some of con- 



^ Trans. Glasgow Geol. Soc. vol. iii. part i. p. 54. 



