CHAPTER XX 

 THE GENERAL QUESTION OF POSTGLACIAL FAULTING 



WITH ITS BEARING ON THE PERMANENCE OF ENGINEERING STRUC- 

 TURES. 



Evidences of postglacial faulting and other recent movements 

 have of late attracted a good deal of attention. The experience of 

 San Francisco in the exceptionally disastrous earthquake and fire, 

 traceable directly to earth movements of the nature of faulting 

 which dislocated or injured the water conduits rendering them use- 

 less, is fresh in the minds of men everywhere who have public 

 responsibilities of this kind. If displacements are occurring at 

 the present time, or if any related movements are continuing, or if 

 there is evidence of recent disturbances of tliis sort in this region, 

 they have a decidedly important bearing upon the permanence of 

 all engineering structures that cross them. 



Xo undertaking is more vitally concerned with this question than 

 the Catskill aqueduct. Although the principal factors to be taken 

 into account have been considered in other connections \^see 

 "Faults" and "Folds," pt i] a unified statement may encourage 

 a more intelligent understanding of the bearing of these structures 

 in southeastern New York on this specific question. 



The region included in this discussion extends from the Catskill 

 mountains to New York city. It will be convenient, for the pur- 

 poses of this argument, to divide the whole area into three districts 

 whose boundaries are determined by decided differences in com- 

 plexity of geologic history. These lines necessarily follow closely 

 the boundaries of greater stratigraphic unconformities. The 

 youngest groups of strata have suffered only such changes as have 

 accompanied movements of later geologic periods. But before they 

 were formed the underlying groups of rocks Avere just as pro- 

 foundly aft'ected by earlier disturbances. In this region, at least, 

 three such groups of large importance exist. The oldest or lowest 

 has been affected by not only everything that has influenced the 

 younger strata but by disturbances of a still earlier time w^iich very 

 much increase their complexity. 



On this basis it is convenient to think of the three districts as 

 follows : 



A Catskill district. Including that portion of the region west 

 and northwest of the Shawangunk mountains and marked by the 



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