CHAP. XVI THE BRITISH ISLES 337 



mouth the old. channel is 260 feet below the sea-level 

 The watershed at Kilsith is now 160 feet above the sea, 

 the old valley bottom being 120 feet deep or forty feet 

 above the sea. In some places the old valley was a 

 ravine with precipitous rocky walls, which have been 

 found in mining excavations. Sir A. Geikie, who has him- 

 self discovered many similar buried valleys, is of opinion 

 that " they unquestionably belong to the period of the 

 boulder clay." 



We have here a clear proof that, when these rivers 

 were formed, the land must have stood in relation to the 

 sea at least 260 feet higher than it does now, and probably 

 much more ; and this is sufficient to join England to the 

 continent. Supporting this evidence, we have freshwater 

 or littoral shells found at great depths off our coasts. Mr. 

 Godwin Austen records the dredging up of a freshwater 

 shell {Unio pictorum) off the mouth of the English 

 Channel between the fifty fathom and 100 fathom lines, 

 while in the same locality gravel banks with littoral shells 

 now lie under sixty or seventy fathoms water.^ More 

 recently Mr. Gwyn Jeffreys has recorded the discovery of 

 eight species of fossil arctic shells off the Shetland Isles 

 in about ninety fathoms water, all being characteristic 

 shallow water species, so that their association at this 

 great depth is a distinct indication of considerable sub- 

 sidence.^ 



Time of Zast Union with the Continent. — The period 

 when this last union with the continent took place was 

 comparatively recent, as shown by the identity of the 

 shells with living species, and the fact that the buried 

 river channels are all covered with clays and gravels of 

 the glacial period, of such a character as to indicate that 

 most of them were deposited above the sea-level. From 

 these and various other indications geologists are all 

 agreed that the last continental period, as it is called, was 

 subsequent to the greatest development of the ice, but 

 probably before the cold epoch had wholly passed away. 

 But if so recent, we should naturally expect our land still 



^ Quarterly Journal of Geological Society, 1850, p. 96. 

 2 British Association Report y Dundee, 1867, p. 431. 



z 2 



