306 THE BEDSHIEL KAIMS 



deep grooves, large enough to be almost entitled to be 

 regarded as valleys, have been ploughed out of even the 

 hai'dest rock masses of the lowlands. 



The consideration of these facts can lead us to only one 

 conclusion, and that is that the sculpturing in question is 

 due to some agent whose operation was of a purely 

 mechanical nature, and which affected the whole face of the 

 lowlands more or less. For that agent to produce such 

 results, it must have moved over the face of the rock under 

 very great pressure ; it must have been armed with a 

 considerable quantity of grinding and abrading material, 

 and it must have continued to move in the same direction 

 for a very long period of time. 



We know of no other agent that is capable of producing 

 such effects than moving land-ice, charged in its lower portions 

 with rock fragments, and kept in contact with the rock 

 without any pad or cushion of clay between the stone-shod 

 sole of the ice and the surface of the rock. So far as the 

 pressure is concerned, it is well known that a column of ice 

 a foot square and a thousand feet high weighs about twenty- 

 five and a half tons. Twice that height would, of course, be 

 double that weight. There are reasons for believing that, at the 

 climax of the Glacial Period, the thickness of the ice in many 

 places in North Britain may well have been four thousand 

 feet : and it must have exceeded that thickness in Norway. 



There is no difficulty in accounting for the grit and 

 sand and rock fragments in the lower parts of the ice ; 

 they simply represent, as I pointed out nearly twenty- 

 five years ago, the effects of pre-glaeial weathering of the 

 rock surface removed by, and incorporated with, the ice. 

 The only point about which many are not clear is connected 

 with the causes which can make the lower parts of a mass 

 of land-ice move in contact with the rock. Many persons 

 still believe that ice flows much as treacle, or pitch, or sealing 

 wax flow down a slope — a kind of movement which, in the 

 case of a glacier of small size, may be likened to the passage 

 of a road roller over the macadam of the streets. Yet, to 

 produce the observed results, the agent in the case under 

 consideration must have been impelled forward under great 

 pressure, and its basal portion must have continued to grind its 



