262 The Parallel Drift-Hills of Western Neio York. 



came, it moved up the shallow valley, described above, radiating 

 to the east and west as it proceeded; and in the valley it remain- 

 ed until its final retreat to the north. 



Under this ice were formed the parallel hills, in a manner 

 which, so far as I know, has only been explained by Geikie, and 

 in the following words: "In narrow and deep hollows, like the 

 npland valleys, the ice was not liable to such deflections as took 

 place over the 'debatable' grounds, and the till forming below it 

 consequently escaped being squeezed to and fro; the valleys were 

 filled with streams of ice flowing constantly in one and the same 

 direction, and the probabilities are therefore strong that the 

 debris which accumulated below would be spread out smoothly. 



"In the lowlands the effect produced by the varying direction 

 and unequal pressure of the ice-sheet is visible in the peculiar 

 outline assumed by the till. Sometimes it forms a confused 

 aggregate of softly-swelling mounds and hummocks; in other 

 places it gives rise to a series of long smoothly-rounded banks or 

 'drums' and 'sow-backs,' which run parallel to the direction taken 

 by the ice. This peculiar configuration of the till, although 

 doubtless modified to some extent by rain and streams, yet was 

 no doubt assumed under the ice-sheet, — the 'sow-backs' being 

 the glacial counterparts of those broad banks of silt and sand 

 that form here and there upon the beds of rivers. 



"Perhaps the most admirable example in Scotland of this pe- 

 culiar arrangement or configuration of the till occurs in the 

 valley of the Tweed, between the Cheviot Hills and the Lammer- 

 muirs. In this wide district, all the ridges of till run parallel 

 to each other, and in a direction approximately east and west. 

 This, too, is the prevailing trend of the rock-striations and roches 

 moutonnees in the same neighborhood."* 



If our theory be correct, the region which we are considering 

 must, indeed, have been "debatable," no less than some of the 

 localities mentioned by Geikie, for it must have been the north 

 and south line whence the ice was deflected both eastward and 

 westward, and fluctuations of lateral pressure must have been 

 both numerous and striking. Add to this the change of form 

 assumed by the ice in passing from the broad basin of Ontai'io 



* The Great Ice Age, by James Geikie, F. R. S. E., etc., p. 88. N. Y., 1874. 



