GEOLOGY OF THE NEW YORK CITY AQUEDUCT 12/ 



sional minor fold seen in the river gorge or an escarpment where 

 the sedimentary character alone would hardly account for it [see 

 pi. 24, High Falls]. Explorations have shown that the evidence of 

 the minor structures is reliable and that disturbances occur at some 

 places even to the extreme western margin. 



Physiography. In spite of the drift cover which obscures 

 many original inequalities it is readily seen that the prevalence of 

 the gentle westerly dip over most of the area, together with the 

 succession of so many different beds of varying resistance to ero- 

 sion, have allowed the development of a succession of long dip 

 slopes and steep escarpments on a more pronounced scale than the 

 present topography shows. It is clear that the Rondout is really a 

 series of these unsym.metrical valleys. The principal large dip 

 slopes are formed by the Shawangunk conglomerate and the Onon- 

 daga limestone. In each case an original stream had adjusted its 

 course fully to the structure and was shifting slowly by the sapping 

 process to the west against the opposing edges of the overlying 

 strata which form the bordering escarpment. One of these unsym- 

 metrical valleys lies along the easterly base of the Hamilton escarp- 

 ment and is continuous with the lower course of Esopus creek 

 farther to the north. In the area under special study it is not 

 occupied by a stream now but is filled with glacial drift so com- 

 pletely that the original stream has been evicted. It is evident, 

 however, from computations based upon the average dip of the 

 slope carried to the base of the escarpment that the bed rock floor 

 ought to be from 200 to 300 feet below the present surface in the 

 deepest portion. Borings have proven this to be the case both 

 along the present line near Kripplebush and also on the first trial 

 line across the Esopus at Hurley. 



The same thing is true near High Falls in the center of the valley 

 where Shawangunk conglomerate forms the dip slope and the 

 escarpment is formed by the Helderberg limestones. In this case 

 the drift filling is very deep also, and Rondout creek flows upon it 

 quite independent of rock structure except where it has cut across 

 the margin as at High Falls. 



In the eastern half of the valley the hard Shawangunk conglom- 

 erate forms the chief rock floor and largely controls the contour by 

 its own foldings and other displacements. Thus the Coxing kill 

 tributary valley lies in a syncline of the conglomerate with occa- 

 sional remnants of overlying beds as outliers adding some variety 

 to the form. The Shawangunk mountains, as a physiographic 



