Correspondence — T. Crook. 47 



THE HIGH-LEVEL GLACIAL DRIFT AND THE LAND-ICE 

 HYPOTHESIS. 



Sir, — The prevailing conception with regard to the manner in 

 which the famous high-level shelly gravels have been transported, 

 according to the land-ice hypothesis, may be fairly indicated by the 

 following quotations : — 



"It is difficult to understand how the ice could climb out of such 

 a basin as that of the Irish Sea, and ascend such steep slopes as those 

 of the Welsh hills up to a height of at least 1 ,350 feet." (Sir Archibald 

 Geikie, Text-Boole of Geology, vol. ii, p. 1319.) 



"Even if the thickness of the ice cap over the Dumfries and 

 Kirkcudbright hills had been about 2,500 feet, that, with every 

 allowance for viscosity, would hardly give us a head sufficient to force 

 a layer of ice from the level of the sea -bed to a height of nearly 

 1,400 feet above it, and at a distance of more than 100 miles." 

 (Professor Bonney, Presidential Address to British Association, 

 Sheffield Meeting, 1910.) 



The advocates of the land-ice hypothesis apparently do not resent 

 this rendering of their views. On the other hand, as far as one can 

 judge from their utterances, they seem to regard it as the inevitable 

 statement of their case. 



Though in favour of the land-ice view, I find it impossible to believe 

 that a thick ice-sheet can plough up beach material with its front, 

 and climb or rather be pushed bodily up steep and lofty hill slopes to 

 heights exceeding 1,000 feet. On a small scale, with comparatively 

 thin masses of ice and low hummocks, this may be a feasible operation; 

 but if not physically impossible, it is an improbability of a very high 

 order with a thick sheet of ice, when heights of from 500 to 1,500 feet 

 have to be scaled. But is such a view necessary ? 



Thrusting and shearing are well-recognized factors in ice movement. 

 Contortions accompanied by upward overthrusts have been observed 

 by Chamberlin in the Bowdoin Glacier of Greenland ; and in a photo- 

 graphic illustration which he has given of these, some of the fracture- 

 planes are seen to be very oblique (" Glacial Studies in Greenland" : 

 Journal of Geology, 1897, p. 235). In this upward overthrust action 

 we have a rational explanation of the manner in which beach material 

 may be raised by land-ice to fairly lofty altitudes. 



The tendency of this upward thrust-action would be to translate 

 the bottom ice with its infrozen ground moraine to higher and higher 

 levels as the ice moved onwards ; and this tendency would become 

 more marked where the ice-sheet encountered a hill, or had to force 

 its way through a narrowing valley. 



Is it not already admitted that the ground moraine can traverse an 

 ice-sheet obliquely and finally reach the surface by this up-thrusting 

 action in the ice ; and why should not shells be permitted to reach 

 the surface in this way ? Is it not, moreover, by this means, rather 

 than by a simple and bodily lifting of its sole, that a thick ice-sheet 

 heaps itself to some extent against ' nunataks ' which oppose its 

 movement ? Beyond such comparatively slight heaping of the ice, 

 there appears to be no need to assume, on behalf of the land-ice 



