GEOLOGY OF THE NEW YORK CITY AQUEDUCT 275 



graphs are intended as a brief summary and comment on the facts 

 as there given : 



Localities where some postglacial displacement has been 

 detected. 



1 Copake, N. Y., on the eastern border of the State near the 

 southwest corner of Massachusetts 



2 Rensselaer, N. Y. 



3 South Troy, N. Y. 



4 Defreestville, N. Y. (near Troy) 



5 Pumpkin Hollow, N. Y. (near Copake) 



6 Kilburn Crag, N. H. 



7 Port Kent, N. Y. (uncertain) 



8 Attleboro, Mass. 



In addition to these there is reference to similar occurrences at 

 St John, N. B. and in the province of Quebec. All of the known 

 localities lie a considerable distance beyond, north and northeast, of 

 the Catskill aqueduct line. 



Causes of displacement. In southern New York all of the 

 cases of postglacial faulting yet discovered lie in the area of slates 

 belonging to the Hudson River series. Whether the belt now occu- 

 pied by this formation is therefore to be considered the most un- 

 stable zone, or whether there is some tendency to slight readjust- 

 ment inherent in the slates themselves causing these movements, is 

 not clear. It would seem consistent with known recent geologic 

 history to connect these displacements with the general elevation 

 and subsidences accompanying and following the glacial occupation. 

 It is perfectly clear that the whole continental border in this region 

 suffered considerable subsidence during glacial time. Also the ter- 

 races and deposits along the Hudson prove beyond question that 

 during the ice retreat, at the very close of the glacial occupation, 

 the land surface stood from 80 to 150 feet lower than now. There- 

 fore an elevation of this amount has occurred in postglacial time, 

 and probably, judging from the condition of the terraces themselves, 

 took place soon after the glacial ice withdrew. 



The stresses and inevitable warpings accompanying these mass 

 movements seem to be sufficient to account for all displacements 

 known to be of this age. There is nothing in them that necessarily 

 promises a renewal of mountain folding. But it appears that the 

 movements liave almost all been of the thrust charaiciter and in this 

 respect they differ not at all from the commoner type of the region. 



