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 wo 
have seen, great quantities remain to the present day to tell the 
tale of its wonderful formation . 1 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 with 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 whole 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 water-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 lowrnr down ; and it is just in 
