﻿Il6 NEW YORK STATE MUSEUM 



below the site. The great thickness of these laminated beds, in 

 places a hundred feet or more, together with the abundance of sand 

 in them, and the caving tendencies exhibited by them in one of the 

 large shafts, indicates poor conditions for such a piece of work. 



The behavior of one of the test shafts throws some light on con- 

 ditions within the drift deposits. At this place after sinking into 

 the underlying gravel beds there was " no water " at first, but after 

 going a few feet deeper there was an abundant flow, that did not 

 rise much in the shaft. This case seems to support the following 

 interpretation. 



The gravels encountered do not form an isolated pocket or lens, 

 else it would have carried water full from the first. It must be 

 a fairly continuous porous zone with large feeding connections else 

 it would run dry, and it must have an easy discharge else it would 

 have risen above the level of the first gravels. Therefore it must 

 be a rather well marked subterranean water passage or porous zone 

 of considerable extent. Such conditions would make an impervious 

 core wall to bed rock at this site a necessity and its construction a 

 matter of considerable difficulty. At this site also because of the 

 small cross section of the ridge, there is little chance for the inter- 

 locking of layers or the blocking of the porous ones by a till barrier 

 to check the lateral seepage, and there is no chance to move farther 

 down stream to secure such conditions. 



4 Summary 



Because of the (a) higher bed rock throughout, and (b) the 

 more uniform and impervious quality of drift deposits, and (c) 

 the more massive cross section of drift barrier for foundation, and 

 (d) the perfectly tight contacts of till and bed rock, and (e) the 

 limitation of the more porous materials to higher levels and (/) the 

 glacial history connected with the development of all these parts, 

 " Olive Bridge " is the preferable location for the proposed Asho- 

 kan dam on Esopus creek. 



