290 NEOZOIC TIME. [CHAP. XIII. 



limits of their descent. The lower ground, when it rose 

 above the sea, appears to have been quite free from perma- 

 nent snow, and was tenanted by the numerous animals 

 which ha.ve left their bones in the caves and the river- 

 gravels. 



The rainfall, however, was probably greater than it is 

 now, and the country being covered with an almost uni- 

 versal mantle of Boulder-clay, no large tracts of pervious 

 rock were then exposed, such as now absorb a portion of 

 the rainfall ; consequently, nearly all the rain was shed off 

 the land in the shape of running water, all the streams and 

 rivers were larger and swifter than they are at present, and 

 the process of valley erosion was thus carried on at a rapid 

 rate. 



Since the Glacial deposits to a great extent draped the 

 pre-glacial surface of the country without completely filling 

 up the pre-existent valleys, the streams were naturally 

 directed into the same depressions ; but the channel which 

 each began to carve out of the G-lacial Beds did not every- 

 where coincide with that of the river which occupied the 

 valley in pre-glacial times, so that in many cases we have a 

 post-glacial valley system superimposed upon an antecedent 

 one, as in the case of the Mersey and of many rivers in 

 Scotland (" Physical Geology," p. 61). Cases also occur 

 where the older valleys have been so blocked up by Glacial 

 deposits at certain points that the post-glacial streams 

 were ponded back, and have made new channels for them- 

 selves through the country between the older valleys ; inte- 

 resting instances of this occur in Lincolnshire. 1 



It is not certain, however, that there was only one epoch 

 of submergence during the continuance of glacial condi- 

 tions, and there are reasons for thinking that there were 

 two such epochs. Again, it is possible that the upheaval 



1 See " Quart. Journ. Geol. Soc.," vol. xl. p. 160. 



