. W. J. Miller — Ice Movement and Erosion. 293 



Leyden sheet), while other good ones occur at an elevation of 

 1700 feet, two miles northwest of Reed's Mill (Pemsen sheet). 

 Another strong evidence favoring the southwesterly current is 

 the distribution of glacial bowlders over the region southwest 

 of the Adirondacks. Most of the common Adirondack rock 

 types are strewn over the region and they gradually diminish 

 in number as the distance from the mountains, becomes greater. 

 This subject has been discussed in a paper by Brigham.* 



Thus, bearing in mind all the facts, the writer is strongly of 

 the opinion that when the ice in its movement (during the 

 last invasion) struck the Adirondacks, it was divided into two 

 currents flowing around the mountains and meeting in the 

 Mohawk valley ; that during the time of maximum glaciation 

 there was a strong general current, but that the border cur- 

 rents continued as under currents (more or less checked in 

 velocity); and that, after the disappearance of the ice sheet 

 from the central Adirondacks, border currents were main- 

 tained. 



Ice Erosion - . 



It seems to be generally agreed upon by those who are 

 familiar with the region that the topography of northern New 

 York was not profoundly affected by ice erosion. Certainly 

 no valleys of any consequence have been formed by ice action. 

 Prof. Fairchild gave the testimony of Gushing, Smyth, and 

 Gilbert to this effect in a paper read before the Geological 

 Society in 1905. f The writer is in agreement with this general 

 view, but he could not consent to the idea that the ice did 

 practically no work of erosion in northern New York. 



1. Erosion of the Precambrian Pocks. 



As the ice moved across the region discussed in this paper, 

 the preglacial rock surface was more or less scratched, polished, 

 and eroded. In the case of the Precambrian rocks, it is doubt- 

 ful if the ice did any very deep cutting. The work of erosion 

 involved mostly the removal of joint blocks of the decayed and 

 weathered rock materials near the surface. The evidence is 

 conclusive that the weathered materials were rather thoroughly 

 scraped off the Precambrians, as shown by the remarkable 

 freshness of the rocks wherever exposed and by the smoothed 

 and rounded character of the outcrops. Wherever the streams 

 have cut through the mantle of drift and into the underlying 

 Precambrians the surfaces of the latter are very hard and fresh. 

 The highly jointed character of these rocks no doubt greatly 

 aided the ice in its work of erosion. In this connection men- 



* This Journal, vol. xlix, pp. 213-228, 1895. 



f Bull. Geol. Soc. Amer., vol. xvi, pp. 50-5, 1905. 



