46 NEW YORK STATE MUSEUM 



some probably occurred at the time of the Taconic Revolution (close 

 of the Ordovicic) or the Appalachian Revolution (close of the 

 Paleozoic) or both ; while extensive faulting certainly occurred 

 toward the close of the ^lesozoic or even later. Any important 

 topographic effects produced by Paleozoic faulting must have been 

 largely obliterated during the long erosion interval which resulted 

 in the production of the Cretacic peneplain of the Atlantic coast. 

 Most of the existing major topographic features of the quadrangle 

 have been produced by faulting along either old or new lines since 

 the development of the Cretacic peneplain as proved by the steep 

 scarps even in homogeneous rocks and by the frequent distinct tilt 

 of the fault blocks which are only moderately affected by erosion. 

 The following criteria have been used in recognizing the faults 

 of the quadrangle: (i) long, straight, distinct ridges irrespective of 

 rock character and with steep side or sides ; (2) the strike of most 

 of these ridges at high angles across the pronounced foliation of all 

 the rocks and the belts of comparatively weaker Grenville and mixed 

 gneisses; (3) very steep to actually vertical scarps even in perfectly 

 homogeneous rock masses; (4) occasional distinct downtilt of earth- 

 blocks away from the scarps; (5) distinct fault-breccia or crushed 

 ■rock zones of common occurrence ; and (6) updrag effect in the 

 black shales in the case of the fault along the western side of the 

 Wells outlier. 



Wells outlier faults 



Elbow-Three Ponds Mountain fault. Considering definiteness. 

 length, influence upon topography and relation to the Paleozoic rock 

 outlier at Wells, this is the most important fault of the quadrangle. 

 It has a north-northeast strike, length of 15VS miles across the 

 eastern side of the quadrangle, and continues northeastward along 

 the East Branch Sacandaga river for over 12 miles to beyond 

 Oregon (Thirteenth Lake sheet). Its total length is no less than 

 28 miles with very distinct influence upon the topography along the 

 whole line as shown on the contour maps. The downthrow side is 

 on the east. 



Shear zones, parallel to the fault, are finely developed in the bed 

 of Devorse creek i mile south of Blackbridge and in '' The Notch " 

 at the base of Three Ponds mountain. At the latter locality big 

 ledges exhibit a brecciated zone several feet wide with angular 

 fragments of Grenville up to i^ feet across. The Paleozoic rock 

 outlier at Wells is sharply downfaulted against the high mountain 



