1874. Climate of the Glacial Period. 441 
to the coasts of Norfolk, if it did not extend further south. 
At this time the southern part of the German Ocean bed 
must have been occupied by a great freshwater lake whose 
arms ran up the valleys of the Thames and other rivers. 
The commencement of the cutting out of the Straits of 
Dover was, I believe, caused by the overflow from this great 
lake finding an outlet across the neck of land, which was 
gradually worn down, and the beds of gravel mantling all 
the lower hills of the Thames valley were, I think, beaches 
formed at the successively lower levels at which the lake 
stood. The ice to the north was now gradually receding, 
and leaving great banks of moraine rubbish in the old ocean 
bed, to be ultimately levelled by the sea when it long after- 
wards returned, and which now form the Dogger and other 
great submarine banks. At the highest point at which the 
freshwater lake stood, and which marks the extreme rigour 
of the Glacial period, we have no organic remains, but many 
boulders in the beach deposits apparently transported by 
coast ice. Lower down, the ice had retired a little to the 
north ; the climate was still severe, but the mammoth, the 
woolly rhinoceros, the musk-ox, the lemming, and other 
animals fitted to live in an Arctic climate, left their remains 
in the old beaches. Still lower, we find more southern 
mammalia coming upon the scene, accompanied by fresh- 
water shells, three of which are not found so far north. I 
thought formerly that their presence merely intimated a 
lowering of the lake in autumn, or that the ice had melted 
so far back that it partly drained around Scotland; but, on 
fuller consideration, I cannot believe that the hippopotamus 
came up, or the Cyrena fluminalis permanently lived in, water 
chilled by the melting of Continental ice; and I have come 
to the conclusion that the ice must have retired so far back 
that it drained entirely to the north of Scotland, and that it 
had left a great moraine stretching across the ocean bed, 
where the Doggerbank now lies, that closed the flow of the 
southern waters northwards. The Straits of Dover, and 
probably another barrier much further to the west, had by 
this time been so far cut through that the rivers stood but 
little above their present levels when the hippopotamus came 
up them, possibly only in summer. Then, too, existed great 
river conditions similar to those under which Cyrena flumi- 
nalis now thrives in Cashmere and Africa. As some addi- 
tional evidence in favour of this theory, I may add that one 
of the three river-shells, Unio pictorum, has been dredged 
off the mouth of the British Channel in the course of the 
supposed great river. 
