220 James Geikie — On Changes of Climate. 



Yalley-gravels have been carried into their present position by float- 

 ing ice, or whether they may not have been derived from the 

 denudation of a Boulder-clay. In the former case the stones would 

 give evidence of cold conditions ; in the latter they would afford no 

 proof of anything in particular, save river- erosion. This we may be 

 sure of, that the river- drainage of England must have been fre- 

 quently interrupted during the cold periods of the Glacial epoch, by 

 deposits from land-ice, from icebergs, and from the drifting and sifting 

 action of marine currents. When a cold period had passed, and the 

 rivers once more began to find their way down the slopes of the 

 land to the sea, it is more than likely that they would not be able 

 always to get into their former channels. In some cases these would 

 be filled up, or partially so, with Till or Boulder- clay, or marine 

 gravel and sand. And such obstructions of the drainage and con- 

 sequent deflection of the streams might be, or rather must have 

 been, repeated again and again. Thus, on the supposition that some 

 at least of the older valley -gravels are interglacial and perhaps pre- 

 glacial, their abnormal position in reference to the present lines of 

 drainage would receive a simple explanation, — an explanation which 

 it seems to me is not antagonistic, but supplementary to the leading 

 ideas entertained by Mr. ]?restwich. 



The well-known fact, that the mammalian remains and flint imple- 

 ments of the older valley-gravels are most frequently found at or 

 near the bottom of such deposits, appears to favour these general 

 conclusions. Surely, if the gravels where wholly of postglacial age, 

 we might expect to meet with flint implements and bones as plenti- 

 fully in the upper as in the lower portions of the beds. But that 

 such is not the case agrees well with the supposition that the mam- 

 maliferous beds are mere patches of old interglacial river-deposits, 

 covered up by later accumulations, some of which are undoubtedly 

 fluviatile, while others may be marine or estuarine. 



Again, it is noteworthy that while the older river-gravels in the 

 south of England are usually well developed, as in the basin of the 

 Thames, those of the valleys further north occur for the most part 

 in mere patches. This circumstance appears inexplicable on the 

 supposition that the valley -gravels are wholly postglacial ; but if the 

 theory here supported have any truth in it, the apparent anomaly is 

 only what might have been expected. For, in the first place, the 

 valleys of the Thames and those of the south of England, being far 

 removed from the centres of glaciation, have not been subjected to 

 the same degree of erosion as the valleys to the north ; and, in the 

 second place, the ground south of the Thames does not appear to 

 have been submerged during the accumulation of the kames and 

 esker drift, and so has escaped the great tear and wear which then 

 overtook the superficial accumulations in Scotland, and also as I 

 believe in the drowned districts of England. 



It is thus then that I would explain the sorely wasted aspect and 

 often anomalous position of the valley-gravels of England and the 

 almost entire absence of mammal-bearing drifts in Scotland and 

 Ireland. In those countries and the hilly districts of England, the 



