GEOLOGY OF THE NEW YORK CITY AQUEDUCT 



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Bridge site, at one stage in the glacial history, served as a natural 

 dam and that water was successfully held above it to an elevation 

 of 530 feet and perhaps more. 



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BOARD OFWATeRSUf^Ly 

 ASHOHAN ffES£/^[/'0/fl 

 OUVa BRID&E DAM SITE 



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Ccrrtttrnd ft> ^on l»09. 



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Section of Site on CenferL/ne 



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Fig 16 Location of the Ashokan dam at Olive Bridge site and a geologic 

 cross section. The small dots in the plan indicate exploratory borings. The 

 section shows the rock profile indicating a preglacial channel of the Esopus. 

 The present Esopus flows in a new postglacial::channel at a higher elevation. 



The lowest materials in contact with bed rock are heavy stony 

 till, laminated till and stony laminated clays — all good impervious 

 material wherever -exposed and tight upon bed rock. Sands and 

 laminated clays are extensively developed immediately northward 

 of the site and streaks of these deposits interlock to a limited extent 

 with the till materials of the site itself, but they do not extend far 

 and die out in wedges among the heavy deposits that characterize 

 the southern slopes of the hill forming the northern terminus of 

 the dam. These pervious streaks do not extend at any point con- 

 tinuously through this hill and consequently as a whole the present 

 barrier as it stands is practically impervious. The poorer materials 

 (assorted gravels and sands) characterize the upstream side, and 

 the better, more impervious materials (till and laminated boulder 

 clays) characterize the downstream siide of the proposed Olive 



