CHAP. XIV.] SUMMARY OF GEOGRAPHICAL EVOLUTION. 319 



mantle of soil over other parts of the land. Such was the 

 state of Britain at the beginning of that remarkable epi- 

 sode which is known as the Glacial Period ; when snow 

 began to accumulate on the mountains and to form 

 glaciers which crept farther and farther down the valleys. 

 By these advancing ice-ploughs the higher parts of the 

 country were swept clean of all surface accumulations, the 

 materials being carried down to lower levels and ulti- 

 mately worked up into the Glacial Boulder- clays, gravels, 

 and loams. 



The uncertainty which exists with regard to the real 

 succession, and to the precise mode of formation of these 

 Glacial deposits, makes it unsafe to attempt a geographi- 

 cal restoration of any of the several phases of the Glacial 

 epoch. It would be easy to follow Professor J. Geikie and 

 Professor Hull, and to give a pictorial representation of a 

 huge ice- sheet covering nearly the whole of Britain and 

 Ireland, but the accuracy of such a picture depends on the 

 assumption that every district where ice-marks and Boulder- 

 clays occur was simultaneously covered by one continuous 

 mass of ice. 



There are, however, indisputable proofs that the whole 

 country sank from a position of considerable height above 

 the sea to one of 1,800 or 2,000 feet below its present 

 level, and it has been thought that these conditions could 

 be represented by a map showing the archipelago of 

 islands that would remain above water if the British Isles 

 were now submerged to that extent. But to omit ice from 

 such a map is to omit one of its most important features, 

 for there is good reason to suppose that the extent and 

 thickness of the Scottish ice was such as to keep the sea 

 from ever being in contact with Scottish ground above a 

 level of 550 feet, so that the greater part of Scotland was 

 practically unsubmerged when the contour of 2,000 feet 

 was the coast-line of Wales. 



