374 NEW YORK STATE MUSEUM 



as one follows the beds northeastward along the strike is an 

 occasional offset of the whole ridge to the northwest. This pro- 

 duces embayments in the gneiss ridges occupied by later forma- 

 tions, such as those along the eastern margin of the principal 

 southern ridge at Eastview and Pleasantville ; or it produces pitching 

 anticlines appearing in the areal geology as lenselike outcrops of 

 gneiss, such as that east of Sherman Park or those between 

 Ossining and the Croton valley; or it changes the courses of 

 streams, as at the great bend in the Hudson river from Iona 

 island to Roye hook. 



Faults are probably as numerous and complex as the folds, 

 but much more difficult to detect. Estimates of the extent of 

 displacement are valueless except where upper formations are 

 involved in the movements. It is clear, however, that the greater 

 faults follow the general strikes of the folds. Some of the 

 fault zones are so nicely healed by recrystallization that they 

 are not at all apparent by the commoner criteria, and often 

 these zones are not lines of present weakness. But where a 

 limestone formation 500 to 800 feet thick is sheared down to 

 less than 100, or entirely out, as sometimes happens, there is 

 no mistaking the essential fault nature of the displacement. 

 This is characteristic of the older movements. Where the two 

 walls are not so unlike, most of them no doubt escape observa- 

 tion. They occur not only in the valleys but across the higher 

 ridges as well. There would appear to be some cross faulting 

 of this early period also but for this the evidence is not so clear. 



A later set of faults is more easily followed. They also are 

 chiefly in line with the northeast and southwest structure, with 

 smaller cross faults, but they have developed prominent shat- 

 tered zones, slickensides and weaknesses that make detection 

 easier. The displacements noted in the cross faults of this set 

 are not in any case great and their strike does not vary greatly 

 from east and west in the clearest cases. 



The principal northeast and southwest faults of this later set, 

 however, occasionally exhibit a great throw. Among them ar,e 

 two that will serve the present purpose and that deserve special 

 attention because of their bearing on other issues. One of these, 

 not in any strict sense a single line, but rather a succession 

 of them, follows closely the northern border of the Highlands 

 ranges, each separate fault line striking out toward the north- 

 east into the bounding slates and its place taken by another 



