CHAP. VII ] 



THE GLACIAL EPOCH. 



Ill 



when " till" is exposed to the action of water, it dissolves into a 

 similar soft sticky unctuous mud. The present glaciers of the 

 Alps, being confined to valleys which carry off a large quantity of 

 drainage water, lose this mud perhaps as rapidly as it is formed ; 

 but when the ice covered the whole country, there was com- 

 paratively little drainage water, and thus the mud and stones 

 collected in vast compact masses in all the hollows, and espe- 

 cially in the lower flat valleys, so that, when the ice retreated, 

 the whole country was more or less covered with it. It was 

 then, no doubt, rapidly denuded by rain and rivers, but, as we 

 have seen, great quantities remain to the present day to tell the 

 tale of its wonderful formation.^ There is good evidence that, 



1 This view of the formation of "till" is that adopted by Dr. Geikie, 

 and upheld by almost all the Scotch, Swiss, and Scandinavian geologists. 

 The objection however is made by many eminent English geologists, 

 including Mr. Searles V. Wood, Jun,, that mud ground off the rocks 

 cannot remain beneath the ice, forming sheets of great thickness, be- 

 cause the glacier cannot at the same time grind down solid rock and 

 yet pass over the surface of soft mud and loose stones. But this 

 difficulty will disappear if we consider the numerous fluctuations in the 

 glacier Avith increasing size, and the additions it must have been con- 

 stantly receiving as the ice from one valley after another joined together, 

 and at last produced an ice-sheet covering the ^vhole country. The 

 grinding power is the motion and pressure of the ice, and the pressure 

 will depend on its thickness. Now the points of maximum thickness 

 must have often changed their positions, and the result would be that the 

 matter ground out in one place would be forced into another place where 

 the pressure was less. If there were no lateral escape for the mud, it 

 would necessarily support the ice over it just as a Avater-bed supports the 

 person lying on it ; and when there was little drainage water, and the ice 

 extended, say, twenty miles in every direction from a given part of a valley 

 where the ice was of less than the average thickness, the mud would ne- 

 cessarily accumulate at this part simply because there was no escape for 

 it. Whenever the pressure all round any area was greater than the pressure 

 on that area, the debris of the surrounding parts would be forced into it, 

 and would even raise up the ice to give it room. This is a necessary 

 result of hydrostatic pressure. During this process the superfluous water 

 would no doubt escape through fissures or pores of the ice, and would 

 leave the mud and stones in that excessively compressed and tenacious 

 condition in which the "till" is found. The unequal thickness and 

 pressure of the ice above referred to would be a necessary consequence 

 of the inequalities in the valleys, now narrowing into gorges, now opening 

 out into wide plains, and again narrowed lower down ; and it is just in 



