GEOLOGY OF THE NEW YORK CITY AQUEDUCT 3 1 



of massive and foliated types, metamorphosed sediments in part 

 with large masses of igneous intrusions and bosses. 



c The Wallkill-Newburgh district lying immediately north 

 of the Highlands and extending to the Shawangunk range is a 

 region of gently rolling contour. Most of tlie area along the pro- 

 posed lines lies between 200 and 500 feet above the sea. There are 

 only occasional rugged hills or short ridges, such as Snake hill and 

 Skunnemunk. The valleys are broad and smooth and the divides 

 are simply broad, hilly uplands. Bed rock is chiefly Hudson River 

 slates with occasional belts of Wappinger limestone. The larger 

 features, the trend of divides and valleys, are northeast and south- 

 west, although this regularity is not so marked as in the preceding 

 two districts. But the chief streams flow either northeast or south- 

 west to the Hudson along these general lines. 



d The Shawangunk range and Rondout valley form a 

 transitional unit from the complicated structural and tectonic con- 

 ditions of the southerly districts to the uniform and almost undis- 

 turbed strata of the Catskills. Its southeasterly half is a mountain 

 ridge partaking of extensive faulting and folding and represented 

 by the Hudson River slates overlain unconformably by the thick 

 and very resistant Shawangunk conglomerate forming high east- 

 ward-facing cliffs. Toward the northwest these disturbances dimin- 

 ish, the strata gradually pass deeper beneath a great succession of 

 shales, limestones, and sandstones of the Helderbergian series, and 

 a broad valley is eroded in the softer portions. It is limited on the 

 northwest by the prominent and very persistent escarpment border- 

 ing the Hamilton series and forming the outer margin of the Cats- 

 kill mountains. 



e The Catskill area is of simple structure. The strata are 

 v/ell bedded and lie almost flat with a gentle dip northwest. The 

 surface features form a series of irregularly distributed escarp- 

 ments, hills, valleys, cliffs, gorges and mountains, rising rapidly 

 toward the west, with moderate to strong relief and reaching ele- 

 vations of 2500 feet. The failure of the northeast-southwest trend 

 of feature that is so common in all of the other districts is a marked 

 difference. It is directly due to the flatness of the strata. 



2 Stratigraphy- 

 There are no strata of prominence in association with the main 

 aqueduct younger than Devonic age except the glacial drift. Imme- 

 diately adjacent areas, however, some of which are covered by the 

 accompanying maps, and Long Island have later formations ex- 



