CHAP. XIII.] PLEISTOCENE EPOCH. 287 



valley opening into the Firth of Forth at Grangemouth is 

 260 feet below the same level. 



It is clear, therefore, that the difference between the 

 level of the late Pliocene and early Pleistocene land surfaces 

 amounted to several hundred feet. This upheaval was 

 doubtless sufficient to restore the land connection between 

 Scotland and Greenland which had been temporarily inter- 

 rupted during Pliocene times, and the influence of this 

 geographical change in accelerating the rate at which snow 

 and ice were accumulating in our latitudes must have been 

 very great. Changes in the direction of the Arctic currents, 

 and possibly also a deflection of the Gulf Stream (see 

 "Historical Geology," p. 562) were likewise important 

 factors in producing that intensity of Arctic cold and that 

 southerly extension of the Arctic climate which is known 

 as the Glacial Period. For if at the present time Britain 

 were united to Greenland and the Gulf Stream deflected 

 from our shores, so that both our eastern and western 

 coasts were washed by currents from the Arctic regions, it 

 is certain that the climate would return to something very 

 like the conditions of the Glacial Period. 



It was probably during this elevation of the British 

 area that the early glaciation of Scotland and northern 

 England was accomplished, and it can hardly be doubted 

 that the agents concerned in this glaciation were confluent 

 glaciers or ice-sheets. The relation of the Boulder-clays to 

 this glaciation is a matter which has already been partially 

 discussed, and as I wish to avoid asserting that all Boulder- 

 clays were formed in the same manner, I will ask the 

 reader to consider that the application of the following 

 remarks is confined to the Glacial deposits of the English 

 lowlands and the borders of the Irish Channel, and more 

 especially to the later portions of these deposits. 



The change which led to the formation of these deposits 

 appears to have been a general subsidence by which the 



