GEOLOGY OF THE SCHROON LAKE QUADRANGLE 8l 



Lake quadrangle (plate lo). Southward, also, the topographic 

 influence is pronounced. It seems quite clear that the downthrow 

 side is on the east, this being most evident in the North Creek 

 quadrangle where, just east of the Gore mountain mass, the dis- 

 placement is at least 1500 feet. Within the Schroon Lake quad- 

 rangle _the topography does not indicate so much displacement, 

 though it is mostly at least some hundreds of feet except at the 

 north where it is much less. In the Minerva stream valley no out- 

 crops occur along the fault, but in the bed of the brook just west of 

 Washburn ridge there are several exposures of rock badly broken 

 by the faulting in zones parallel to the course of the stream. It is 

 very important to note that the strike of this fault is almost exactly 

 at right angles to the strike of the foliation of all the rocks, and 

 also at right angles to the strike of the prominent belt of Grenville 

 strata around Minerva and Olmstedville. It is difficult to con- 

 ceive how such a long, narrow valley could have developed for 12 

 miles across these structures except along, a fault zone of weakness 

 in the rocks. The lowlands in the vicinity of Minerva and Olmsted- 

 ville, and also in the vicinity of North Creek, are due to more rapid 

 erosion of the comparatively weak Grenville strata in those 

 localities. 



Hoffman notch fault. Evidence for either a fault or zone of 

 crushed or excessively jointed rock in the Hoffman notch valley is 

 twofold. In the first place, the long, straight, deep, narrow valley, 

 with north-northeast strike parallel to most of the prominent faults 

 of the eastern Adirondacks and at right angles to the rock struc- 

 tures of the immediate region, alniost certainly must have been 

 carved out along a fault zone 'of weakness. This valley is 5^ 

 miles long with a maximum depth of 1200 to 1500 feet. In the 

 second place, actual crushed to even brecciated rock zones in the 

 bottom of the valley and parallel to it were observed at a number 

 of places, the principal ones being as follows : several ledges in the 

 brook between one-half and il4 miles north of the pond in Hoff- 

 man notch and several ledges in the brook between ij^ and 2 miles 

 south of the same pond. The brook northeast of Hoffman notch 

 pond follows a short branch fault for about one-half of a mile 

 with much evidence of crushed rock. That the Hoffman notch 

 fault continues northward across the east-west Blue Ridge- 

 Boreas river road is proved by the existence of a ledge in the 

 Branch brook just north of the road where closely spaced jointing 

 with strike N 20° E is well shown. 



