CHAPTER IV 



GEOLOGICAL FEATURES INVOLVED IN SELECTION OF 

 SITE FOR THE ASHOKAN DAM 



Topographic features of the southeastern margin of the Cats- 

 kills, where the chief water supply is available, fixes the approxi- 

 mate location and bounds of the principal reservoir. The accom- 

 panying map, a portion of the western part of Rosendale quad- 

 rangle, shows the situation. The part of the work involving the 

 chief geological problem was the choice of the principal dam sites 

 on the Esopus. This is known as the Ashokan dam. This part of 

 the Catskill system belongs to the Reservoir Department under 

 i\Ir Carlton E. Davis as department engineer. 



There were originally considered three sites : ( i ) at " Broadhead 

 bridge," (2) at ''Olive Bridge," (3) at "Cathedral gorge" or the 

 " Tongore " site. Any one of these seemed possible from a topo- 

 graphic standpoint. Later developments in regard to storage ca- 

 pacity and engineering considerations finally reduced the practicable 

 sites to two — the ''Olive Bridge" and the "Tongore." These 

 were then explored thoroughly as an aid to determining whether 

 or not there were favorable or unfavorable conditions at either 

 location. Trenches were dug, shafts were sunk, wash holes were 

 put down, and drill borings were made. The amount of such work 

 done was sufficient to show the actual conditions both of the drift 

 and bed rock and incidentally to throw some light on minor matters 

 in geologic history. 



This discussion is essentially a summary of these data and a com- 

 parison of the geologic conditions indicated by the explorations^ of 

 these two sites and a statement of some of the geologic character- 

 istics of the area. 



I General geologic conditions as shown by the explorations 



Bed rock is dark colored Devonic sandstone and shale, the 

 Sherburne formation, lying almost horizontal, strongly jointed, 

 plainly bedded, and of good quality for the foundation of the dam. 



At both locations the present Esopus flows in a postglacial gorge 



1 In this work of exploration a very efficient staff of engineers was engaged. 

 Among those having very much to do with the features here discussed are 

 Thaddeus Merriman, division engineer, J. S. Langthorn, division engineer 

 and Sidney Clapp, assistant engineer. 



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