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POPULAR SCIENCE MONTHLY 



polemics in opposition to the belief in former continental glaciation, 

 which almost every one now considers definitely established, though 

 after a hard fight. 



It does not seem necessary at the present time to undertake to 

 show hoiu the glaciers did this, nor to prove that they could do it 

 when the evidence is so clear that they actually did do it. Suffice it 

 to say, that if glaciers smooth, scratch and pluck the rocks over which 

 they pass, as every one knows they do (Fig. 13), it requires only a 

 sufficiently long continuation of this action to lower valleys to any 

 extent up to the time when they cease to further smooth, scratch and 

 pluck. A century ago it seemed to many observers that at the slow 

 observed rate of recession of Niagara Falls it was impossible to explain 



Fig. 14 Looking Up (East) Nunatak Fiord. The rock knoll, or Nunatak, in the middle 

 of the picture, 1,400 feet high, splits the Nunatak glacier one arm, on the left, depcending to 

 the sea through the broader valley, the other occupying a smaller U-shaped valley on the right 

 side of the Nunatak. but not upon reaching the sea. When first seen by Prof. Russell in 1S91 

 these two arms nearly enclosed the Nunatak. The site of the hanging (Fig. 7) valley is on the 

 right side of the picture. Photograph by Lawrence Martin. 



the seven miles of gorge as a result of this process. No one now doubts 

 this explanation of the Niagara gorge; and it is not doubted that the 

 Colorado Canyon has been formed by slow sawing into the strata, like 

 that which the river is now engaged in, but continued through a long 

 period of time. An application of the same principle — a slow rate 

 of erosion working for a long period of time — is all that is necessary to 

 understand profound glacial erosion, once it is granted that glaciers 



