46 NEW YORK STATE MUSEUM 



region southwest of the Adirondacks. Most of the common Adiron- 

 dack 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 southward movement 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 southwesterlv 

 current, but that the border currents 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 

 maintained. According to this the Port Leyden quadrangle was 

 ftrst invaded by a tongue of ice which flowed southeastward up the 

 Black river valley. When the general ice sheet had here reached 

 a thickness of several thousand feet the main current was south- 

 westerly, but with a southeasterly under current in the valley bot- 

 tom. The ice first melted from the highlands and left a tongue of 

 ice in the valley which gradually melted and retreated northward. 



ICE EROSION 



Erosion of the Precambric rocks 



As the ice moved across the quadrangle, the preglacial rock sur- 

 face was more or less scratched, polished and eroded. In the case 

 of the Precambric rocks it is doubtful if the ice did any very dee]) 

 cutting. Its work of erosion involved mostly the removal of masses 

 of decayed and weathered rock material near the surface. The evi- 

 dence is conclusive that the weathered materials were rathor 

 thoroughly scraped off the Precambrics as shown by the remarkable 

 freshness of the rocks wherever exposed and by the smoothed and 

 rounded character of the outcrops [see pi. 3]. The highly jointed 

 character of these rocks no doubt greatly aided the ice in its work 

 of erosion. Mention should be made of the great number of erratics 

 of Precambric rock material strewn over much of the region, 

 especially toward the south. One of these erratics measures about 

 17 feet high and 27 feet across [see pi. 11]. The larger ones are 

 mostly of the hard, homogeneous syenite or granite. Probably 

 the greatest amount of erosion of the Precambrics occurred along 

 Black river between Lyons Falls and Lowville, but this matter will 

 be referred to below. 



' he. cit. p. 213-28. 



