HYDROLOGY OF NEW YORK 



G85 



merit of the area shown by the surface topography to be tribu- 

 tary to the proposed point of taking? 



3) Is there ai>y geologic reason to expect that the yield 

 of ground water available per square mile will be materially 

 different in quality or in quantity from that in the region already 

 developed west of Massapequa? * 



4) Is there any apparent advantage in one location over an- 

 other for tapping the subterranean waters of the yellow gravel? 



The geologic structure of Long Island consists of the follow- 

 ing formations, beginning with the lowest: (1) the primitive or 

 crystalline rocks; (2) the cretaceous formation; (3) the tertiary 

 formation; and (4) the terminal moraine of the continental glacier 

 of northeastern North America. 



The water-bearing horizons are limited to yellow gravel above 

 the blue clay, gray gravel below it and certain layers of sand in 

 the cretaceous formation. The yellow gravel, which receives and 

 holds nearly the entire rainfall of the island, ranks first in im- 

 portance, while the gray gravels and cretaceous gravels are second 

 in importance. The supply from the gray and cretaceous gravels, 

 if heavily drawn upon, is likely to become brackish, and is also 

 likely to become stale or mineralized. 



In regard to deep wells, it is stated in Professor Crosby's report 

 that while here and there a deep well may tap a supply of good, 

 potable water, sufficient for the supply of a small village or 

 factory, the geologic formation gives no hope of finding any 

 large, permanent deep-well supply sufficient in volume to form a 

 substantial increase to the water resources of Brooklyn or add 

 materially to the volume that can be obtained from shallow wells. 



Before the real facts in regard to tbe geologic formation of 

 Long Island were understood it was considered that probably a 

 large volume of the fresh water found its way over to Long Island 

 from the mainland by percolating through the deep, porous strata, 

 but in the light of more extended information Professor Crosby 

 considers that whatever yield of fresh water comes from the deep 

 wells must have its origin from rain falling upon Long Island 

 alone — there is no water coming to Long Island from the 

 mainland. 



