GEOLOGY OF THE BROADALBIN QUADRANGLE 



51 



era border. The smoothest surface on the southwest is just what 

 would be expected because that region was longest a land surface, 

 thus affording opportunity for cutting away nearly all irregulari- 

 ties. Within the Broadalbin quadrangle all but a few miles of this 

 ancient shore line have been lost to view due to the extensive 

 faulting and the portions not thus faulted out are most heavily 

 drift covered. However, certain phenomena are very clearly ex- 

 hibited, especially between Barkersville and North Galway, and give 

 us important additional information for the southern Adirondacks. 



ZZZI 



EZZ^ 



GRENVI LLE. 



S C A I 



O T S D A M. 



THERESA 



D R I FT. 



Fig. 9 Section passing from a point two-thirds of a mile south of Barkersville south-south- 

 westward through North Galway. The overlap of the Theresa transition beds upon 

 the Precambric hillock is clearly shown 



In the vicinity of North Galway the evidence is conclusive that 

 the Precambric surface on which the Potsdam was deposited was 

 fairly uneven. The tongue of Grenville quartzite which extends 

 out so prominently here is very significant because the same quartz- 

 ite stood out as a ridge in the shallow Potsdam sea and was never 

 covered with Potsdam sandstone or conglomerate. So far as can 

 be determined the Potsdam is wholly absent around this Grenville 

 tongue except along the east side of the southern border where it 

 is entirely represented by the coarse conglomerate. Elsewhere the 

 passage beds of the Theresa formation rest directly upon the Gren- 

 ville, thus overlapping the Potsdam. This feature is perhaps best 

 shown along the road three- fourths of a mile northeast of North 

 Galway where the passage beds are practically in contact with the 

 Grenville and show a low southwesterly dip. [See fig. 9.] On 

 the south side of the Grenville tongue the passage beds show a 

 southwesterly dip of three or four degrees. The sandstone, and 

 even limestone, layers close to the Grenville contain quartz pebbles 

 up to one-half inch in diameter which were derived from the Gren- 

 ville and deposited in the encroaching sea. The Grenville here 

 certainly projects upward as much as fifty feet into the passage 

 beds which means that, at the height of the Potsdam sea, this Gren- 

 ville rose fully fifty feet above sea level. If we add to this the 



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