HYDROLOGY OF NEW YORK 



713 



be covered by the terminal moraine of the continental glacier, 

 with, surface formation very impervious. The conditions, there- 

 fore, are quite different here from Long Island, where extensive 

 areas of coarse sand permit a large proportion of the rainfall 

 to sink into the earth. Moreover, the geologic structure is 

 such as to render it improbable that any such large amount of 

 ground water can be obtained as in the adjacent area of Xew 

 Jersey. 



The geology of Staten Island has been reported upon by Pro- 

 fessor W. O. Crosby of the Massachusetts Institute of Technology. 

 In this report it is stated that borings in the vicinity of Wood- 

 bridge and Perth Amboy show that the traprock and the ridge 

 of crystalline rocks are continued only at moderate depths below 

 the surface, and that in consequence the cretaceous strata under- 

 lying the entire lowlands south of the serpentine ridge, and which 

 in Not Jersey embrace several good water horizons, are cut off 

 from the catchment areas either on the mainland or on the 

 northern part of Staten Island. Moreover, the feet that the wells 

 penetrating the cretaceous strata are practical failures indicates 

 that the water-bearing strata outcrop on this island only to a 

 very limited extent or not at all. 



The gray gravel and blue gray formations of Long Island are. 

 so far as can be observed, wholly wanting on Staten Island, and 

 the yellow gravel, which is an important reservoir of ground 

 water on Long Island, has only a limited development on Staten 

 Island, being practically confined to the cretaceous lowlands, 

 which are the source of such wells as are not failures. 



Moreover, the greater portion of the area of Stateu Island 

 is covered by bowlder clay, which is of an exceptionally imper- 

 vious character, as proved by numerous ponds more or less 

 effectually sealing the catchment areas of strata which might 

 otherwise be water-bearing. The bowlder clay is in many places 

 from 50 feet to 200 feet thick. 



To summarize, the geologic conditions affecting the storage 

 and flow of ground water are generally unfavorable, because all 

 the formations, excepting the yellow gravel, are naturally of little 

 value as sources of ground water. Unfortunately, the yellow 

 gravel has not only a limited catchment area, but the small areas 



