﻿GEOLOGY OF THE NEW YORK CITY AQUEDUCT 273 



are now to be seen, and the strata therefore appear as narrow belts 

 that dip steeply into the ground. This is more marked in the 

 Hudson river district than in the Catskill, and is still more strikingly 

 true of the Highlands. 



There are evidently at least three different epochs of folding inter- 

 rupting the processes of sedimentation and followed by periods of 

 erosion before sedimentation was again resumed. These breaks 

 constitute so called stratigraphic unconformities and occupy the 

 relative positions indicated in the foregoing tabulated scheme [see 

 pt 1]. 



In each epoch of folding the compressive forces accomplishing 

 this work seem to have acted in a southeast-northwest direction 

 causing successive series of folds with a northeast-southwest trend. 

 The total amount of crustal shortening accompanying these move- 

 ments is not known, but that it must be many miles is indicated by 

 the fact that the strata of the older series of formations stand pre- 

 vailingly on edge. All stages between small amount of movement 

 to very great displacement are represented. 



Accompanying the folding in each epoch there has been a ten- 

 dency to rupture and displacement of the " fault " type. There are 

 multitudes of them varying from movements of too little amount to 

 be regarded in a broad way to those of several hundred feet. Most 

 of the larger and more persistent ones are strike faults and follow 

 the main ridges or valleys, sometimes governing the location of 

 escarpments or gorges. Dip faults crossing the formations also 

 occur and doubtless have guided the adjustments of many tributary 

 streams, and occasionally portions of the larger water courses. The 

 thrust fault is most common. This is especially true of the larger 

 ones and particularly those parallel to the trend of the other struc- 

 tural features. 



The chief effects of these movements may be summarized as 

 follows : 



1 Formations are cut out of their normal order and nonadjacent 

 ones are brought in contact. 



2 Cliffs (escarpments) and sharp gulches are more common. 



3 Crush zones (belts of brecciated material) are developed. 



4 The crush zones give an additional control of stream adjust- 

 ments. 



All of these effects are common. Many of those faults dating 

 back to the earlier epochs are obscure and not readily located. Many 

 of the older weaknesses of this sort have been healed by recrystalli- 



