306 O. W. Bulman — Formation of the Boulder-clay. 



where the ice has retreated within recent times. With regard to 

 this point Prof. Bonney writes as follows : 



"The Glaciers of the Swiss and Savoy Alps have been retreating 

 for several years, hence, if anything like ground moraine existed, 

 this would he a very favourable time for observing it. In no case 

 have I been able to find signs of any deposit resembling Till or 

 Boulder-clay ; the detrital matter which is scattered, generally 

 sparsely, over the slope left bare by the retreating glacier has 

 fallen from its surface, like ordinary terminal moraine. Further, 

 by availing myself of crevasses, etc., I have made ni}' way occasion- 

 ally for some little distance beneath the ice. Nothing has been seen 

 but bare rock, with now and then a film of mud or a passing stone. 

 In short, the result of an experience of some years has convinced 

 me, that if anything like the Till or Ground Moraine of recent 

 glacialists exists in the Alps, it is a very local and exceptional 

 phenomenon." ^ 



And so we find a tendency among those who believe the Boulder- 

 clay was accumulated beneath the ice, to push back the question of 

 its origin from the comparatively known regions of Alpine glaciers 

 to the unknown sub-glacial area of Central Greenland. 



Thus Sir A. Geikie refers to what must be taking place beneath 

 a vast continental ice-sheet : 



(1) "In such comparatively small and narrow ice-sheets as the 

 present glaciers of Switzerland, the rock bottom on which the ice 

 moves is usually, as far as can be examined, swept clean by the 

 trickle or rush of water over it from the melting ice. But when 

 the ice does not flow in a mere big drain (which after all, the 

 largest Alpine valley really is), but overspreads a wide area of 

 uneven ground, there cannot fail to be a great accumulation of 

 rubbish here and there underneath it. The sheet of ice that once 

 filled the broad central plain of Switzerland between the Alps and 

 the Jura certainly pushed a vast deal of mud, sand, and stones over 

 the floor of the valley. The material is known to Swiss geologists 

 as the Moraine profonde or Grundmorane ( = Boulder-clay, Till, or 

 bottom-moraine) " (Text-Book of Geology, p. 411). 



(2) "We know as yet very little regarding its (the Grundmorane) 

 formation in Greenland. Most of our knowledge regarding it is 

 derived from a study of the Till or Boulder-claj^ in more southern 

 latitudes, which is believed to represent the bottom moraine of an 

 ancient ice-sheet (ibid. p. 417). 



The weakness of the argument is further illustrated by the 

 following passage from the Physical Geology of Prof. Green, in 

 which I have italicised certain parts : 



" We have already seen that under such a sheet [continental 

 ice-sheet] there is probably formed an accumulation of clay and 

 stones known as Moraine profonde or Grund-moriine, and Till 

 resembles exactly what we picture to ourselves that this deposit imist 

 be like. There would be weight enough to give rise to the intense 

 toughness and the close and irregular packing of the stones, and the 

 1 Geol. Mag. Vol. XIII. p. 197. 



