GEOLOGY OF THE SCHROON LAKE QUADRANGLE 79 



valley for 12 miles across the quadrangle and beyond (northward) 

 for fully 8 miles more. As regards both length and topographic 

 influence, this fracture takes rank as one of the most prominent 

 faults in the eastern Adirondacks. It probably does not extend 

 south of the quadrangle limit. The topographic evidence for the 

 fault is very strong (plate 3). The Potsdam sandstone east of 

 Grove Point and the sandstone and dolomite in the valley 7 miles 

 north of Schroon Lake village also both lie against the base of the 

 scarp and hence furnish strong evidence for faulting with down- 

 throw side on the east. Several ledges in the bed of Horseshoe 

 pond brook are badly broken parallel to the course of the stream 

 and these fvirnish still more positive evidence for the existence of 

 the fault. Nowhere else along the immediate base of the scarp 

 were outcrops of any kind observed, so that further evidence such 

 as slickensides and crushed or brecciated zones, is lacking. From 

 Thurman pond southward the trace of this fault is much less cer- 

 tain. Some idea of the minimum displacement along this fault 

 ma}'' be gained not only from the height of the scarp but also from 

 the positions of the outliers of Paleozoic strata. Thus the sand- 

 stone and dolomite 7 miles north of Schroon Lake village lie at an 

 altitude of 900 feet, while the summit of the mountain just west is 

 nearly 2300 feet, thus indicating a minimum downthrow of about 

 1300 feet on the east side of the fault. The position of the Pots- 

 dam sandstone west of Grove Point indicates a minimum displace- 

 ment of at least 400 feet, and probably 600 feet. 



Schroon lake faults. The topographic evidence for a fault, or 

 at least a zone of excessive jointing, along the western side of 

 Schroon lake is strong, as shown on the map. A very conspicuous 

 zone of excessive jointing accompanied by moderate faulting (see 

 plate 9) occurs in the road metal quarry one-half of a mile east- 

 northeast of South Schroon. The strike of this jointed zone is 

 parallel to the strike of the fracture as mapped but at a high angle 

 to the strike of the foliation of the rocks. Probably the jointing 

 exhibited in the quarry lies a little to the west of a real fault, 

 because the topography strongly points to a downthrow of fully 

 200 feet on the east side. 



From Adirondack village southward close to the lake shore there 

 is a fault which is but the northern extension of a rather prominent 

 fault several miles long which has been mapped and described in 

 the writer's report on the North Creek quadrangle. 



Along the eastern lake shore at the base of Ouackenbush hill, 

 and extending several miles into the Paradox Lake quadrangle, a 



