78 NEW YORK STATE MUSEUM 



scarps, often miles long, in hard, homogeneous rock; (3) actual 

 presence of crushed, sheared, slickensided or brecciated rock zones ; 

 and (4) Paleozoic strata lying at the base of steep hills of Pre- 

 cambrian rocks. 



Age of the faulting. That some Adirondack faulting took place 

 in Precambrian time has been pretty well established, but, so far 

 as definitely known, such fractures are of minor importance. 

 There is no positive evidence for such faulting in the Schroon 

 Lake quadrangle. It seems quite likely as Cushing has suggested,^ 

 that considerable faulting took place during, or toward the close 

 of, the Paleozoic era. 



Any fault scarps, ridges or valleys which may have been pro- 

 duced by the close of the Paleozoic must have been nearly or 

 quite obliterated by the long subsequent time of erosion. If so, 

 how do we account for the present Adirondack ridges and valleys 

 which follow the fault lines or zones? Accompanying the uplift 

 of the late Mesozoic or early Cenozoic peneplain of the Atlantic 

 coast region, or following it, there was either new faulting, or 

 renewed movement along old faults, or old fault zones, including 

 zones of excessive jointing with little displacement, were not 

 affected by new movements. How much is new faulting, and how 

 much renewed faulting along old lines or zones of fracture is not 

 known, but it is quite certain that considerable faulting in the 

 eastern Adirondacks must date from the uplift of the peneplain 

 just mentioned as shown by fault scarps in homogeneous rocks and 

 by the existence of tilted fault blocks which have been little modi- 

 fied by erosion. Some relatively long, deep, narrow^ valleys of the 

 Schroon Lake quadrangle, like that which follows the Minerva 

 stream fault or the Hoffman notch fault (see below), are due 

 essentially to erosion along the fault or broken-rock zones of weak- 

 ness irrespective of when they originated, while others which are 

 broader, like the Schroon Lake valley and the lowland between 

 Green hill and Oliver hill, are due either to comparatively recent 

 sinking of fault blocks, or removal of weaker rocks by erosion 

 whereby old fault scarps are renewed, or both. Distinctly tilted 

 fault blocks, like some in the North Creek quadrangle just to the 

 south, are not certainly present in the Schroon Lake quadrangle. 



Schroon valley fault. As represented on the geologic map, 

 this is one of the two longest and most conspicuous faults of the 

 quadrangle. Its scarp marks the western boundary of the Schroon 



' N. Y. State Mus. Bui. 95, p. 405. 



