POSTGLACIAL FAULTS OF EASTERN NEW YORK 



2 7 



be of the reverse type, indicating compression. The direction of 

 the throw must depend upon the attitude of the planes of struc- 

 ture — stratification or cleavage, or both — along which the move- 

 ment takes place. In stratified rocks which have been thrown 

 into anticlines and synclines, and have been subsequently 

 rather deeply base-leveled, it is probable that the continuance 

 of the lateral pressure which gave rise to the folds would con- 

 centrate the horizontal strain upon the cores of the synclines in 

 such a manner as to cause successive boat-shaped layers of 

 rock with their wedge-shaped cross-sections to rise upward. By 

 reason of the inward dip of the strata about the synclinal axes 

 the appearance of overthrust would appear in the slips which 

 marked the movement, as shown in the accompanying theoreti- 

 cal diagram [fig. 8]. Thus synclinal cores must have a tend- 



Fig. 8 Diagram showing cross-section of a normal upright syncline with a rising core due to 

 slipping of successive layers in the trough under lateral compression 



ency independently of resistant remnant beds in their troughs 

 to stand above the general level or to give rise to upfolds in the 

 horizontal newer strata which in certain districts lie unconform- 

 ably upon them. Small faults of this nature may arise in folded 

 strata without direct connection with those greater and more 

 deep seated faults which appear so abundantly in eastern North 

 America to have had their origin in the fracture and displace- 

 ment of the crystalline Precambric terrane. It is worthy of 

 note that the few instances of postglacial faults as yet reported 

 appear to be everywhere associated with highly inclined strata. 

 It is to be presumed that the same stresses are equally opera- 

 tive in the regions of horizontal strata with consequent displace- 

 ments which take the form of horizontal thrust-planes whose 

 time of movement is not so readily determined. 



The bearing of these observations and inferences upon the 

 ancient water levels of the Hudson valley in the present state of 

 our knowledge would seem to indicate that the shore lines of 

 the eastern side of the Hudson gorge north of the Highlands 



