CHAPTER XIII 

 GEOLOGY OF THE KENSICO DAM SITE 



Kensico reservoir at Valhalla, 2 miles north of White Plains, is 

 one of the links in the Bronx river aqueduct. -It is to be greatly 

 enlarged and made a very important storage reservoir for the new 

 Catskill system. In line with this plan a new dam is to be built 

 near the old site that will rise 100 feet higher than the present 

 structure. 



Extensive investigations^ have been made to determine the charac- 

 ter of rock floor for this massive dam. Sites both above and belofw 

 the present one have been studied with the question of safety and 

 efficiency and permanence as well as that of economy of construc- 

 tion in view. Involved with this is also the source of suitable stone 

 for its construction. 



Geological surroundings 



Glacial drift covers the rock floor of this and neighboring valleys^ 

 to a depth of 10 to 20 feet. No rock is exposed in the valley bottom 

 at the Kensico site, but at the extremities of the proposed dam the 

 rock floor comes to the surface in small outcrops. The material 

 constituting the drift cover is essentially a loose, somewhat porous 

 till passing into modified types, especially gravels and sands imme- 

 diately south of the ground tested. 



The character of bed rock at the two extremities and beyond the 

 limits of the dam is easily seen from the outcrops to be Fordham 

 gt'ciss on the east and Manhattan schist on the west. Between, 

 although nothing can be seen, Inwood limestone is found by the 

 borings as was to be expected. No other formations occur, although 

 the Yonkers gneiss, an intrusive in the Fordham at a little greater 

 distance figures prominently in studies of material. 



The formations are in normal order and are of the usual petro- 

 graphic character. All dip westward at angles that vary from 45 

 to 65 degrees and have a general strike a little east of north. It is 

 evident that the whole series represents an eroded limb of a simple 

 fold. 



1 These explorations have been in direct charge of Mr Wilson Fitch 

 Smith, division engineer, whose headquarters for the Kensico division is at 

 Valhalla, N. Y. Preparations for construction have already been begun. 



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