CHAP. VII THE GLACIAL EPOCH 115 



There is good evidence that, when the ice was at its maxi- 

 mum, it extended not only over the land, but far out to 

 sea, covering all the Scottish islands, and stretching in one 

 connected sheet to Ireland and Wales, where all the 

 evidences of glaciation are as well marked as in Scotland, 

 though the ice did not of course attain quite so great a 

 thickness.^ 



ing power is tlie 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 mile? in every direction from a given part of a valley 

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

 necessarily 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 

 these openings in the valleys that the *' till " is said to be found, and also 

 in the lowlands where an ice-sheet must have extended for many miles in 

 every direction. In these lowland valleys the *'till " is both thickest and 

 most wide-spread, and this is what we might expect. At first, when the 

 glaciers from the mountains pushed out into these valleys, they would 

 grind out the surface beneath them into hollows, and the drainage-water 

 would carry away the debris. But when they spread all over the surface 

 from sea to sea, and there was little or no drainage water compared to the 

 enormous area covered with ice, the great bulk of the debris must have 

 gathered under the ice wherever the pressure was least, and the ice would 

 necessarily rise as it accumulated. Some of the mud would no doubt be 

 forced out along lines of least resistance to the sea, but the friction of the 

 stone-charged "till" would be so enormous that it would be impossible for 

 any large part of it to be disposed of in this way. 



^ That the ice-sheet was continuous from Scotland to Ireland is proved 

 by the glacial phenomena in the Isle of Man, where "till " similar to that 

 in Scotland abounds, and rocks are found in it which must have come from 

 Cumberland and Scotland, as well as from the north of Ireland. Tliis 

 would show that glaciers from each of these districts reached the Isle of 

 Man, where they met and flowed southwards down the Irish Sea. Ice- 

 marks are traced over the tops of the mountains which are nearly 2, 000 feet 

 high. (See A Sketch of the Geology of the Isle of Man, bv John Home, 

 F.G.S. Trans, of the Min. Gcol. Soc. Vol. II. pt. 3, 1874.) 



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