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The real explanation of the phenomenon is that the features as 

 they now stand are not due to the prolonged action of subaerial 

 forces alone, but are due to the modification of old weathered 

 surfaces by ice. The whole of the preglacially weathered rock, talus, 

 swallow holes, weathered joints, and all, has been removed by long- 

 continued glacial action, and the old preglacial configuration has 

 been replaced by another of essentially different character. The 

 ice of the glacial period, here, probably from first to last, flowed in 

 a nearly uniform direction down the dales, and did not shift about 

 and change its direction of flow as it did in many other parts. 

 Under these circumstances we can readily understand how a mass 

 of ice two thousand feet in thickness, grinding a surface formed of 

 rocks presenting very variable degrees of resistance to mechanical 

 erosion, would, in the course of the long interval representing the 

 whole of the Glacial Period, cut its way farther into soft beds like 

 shales than it would into the tougher and less-easily eroded lime- 

 stones. It is well known that such minor differences in hardness 

 as are presented by veins of barytes traversing a limestone, or even 

 such as the difference in hardness between the matrix of a limestone 

 and that of the fossils included in it, often results in corresponding 

 inequalities of the surface where the rock has been glaciated. And 

 where the difference in hardness is more marked, as it is in the 

 case under notice, a corresponding difference in the resulting effects 

 is what might have been expected. 



In connection with the subject of glacial erosion there is one 

 point that seems to have been overlooked by many of those who 

 have written about the vertical limit of the ice-sheet. It has been 

 assumed, seemingly upon insufticient grounds, that the rough and 

 craggy form of the higher parts of such districts as are well-glaciated 

 in their valleys is good proof that these higher parts were never 

 overridden by the ice. But if the view above advanced be correct 

 — that the ice removed from the low ground all the preglacially 

 weathered parts of the rock — it follows that because the stay of the 

 ice at the higher points was brief as compared with its stay lower 

 down, much less of the high lying weathered rock was removed. 

 Consequently, when the whole surface once more became exposed 



