112 NEW YORK STATE MUSEUM 



more likely to occur in the deeper channels. To this class of 

 deposit belong some of the gravel beds of the " Tongore " site, 

 notably that shown in one of the deep shafts. It is probable that 

 the zones where the wash rig experienced a '' loss of water " are 

 most of them of this type. 



I 

 2 Summary of geologic history 



In preglacial time the Esopus valley was occupied by a stream 

 of similar capacity to the present Esopus creek. Its channel lay 

 to the north side of the narrow valley, having adjusted itself in 

 conformity to the slight dips of the Hamilton sandstones and its 

 principal joints. At the points under investigation this original 

 channel is buried under several kinds of glacial deposits whose 

 source of accumulation was chiefly from the north and northeast, 

 blocking the stream channel and forcing the stream to the opposite 

 (south) side. The direction of movement was favorable to the dam- 

 ming of the Esopus creek valley and the deposits indicate that 

 this occurred at several different times and at different elevations 

 and that corresponding lake conditions occasionally prevailed. It 

 is equally clear that there were intervals of retreat of the ice with 

 attendant stream action and the development of gravel beds, fol- 

 lowed by another ice advance, either obliterating the surface 

 features or covering the previous deposits with another till layer. 

 With each successive withdrawal the local streams found them- 

 selves more or less completely out of place, and consequently their 

 characteristic deposits formed in these intervals may be found in 

 unlooked for places wholly inconsistent with present surface 

 contour. 



At the final withdrawal of the ice, Esopus creek found itself 

 entrenched along the southern margin of the valley and has cut a 

 postglacial rock gorge instead of removing the compact till from the 

 original channel. But wherever only modified drift, either sand or 

 clay, was the valley filling it scooped out great bends so that a large 

 proportion of this type has been removed from the valley, and 

 only the margins remain as terraces or covered beneath other pro- 

 tecting deposits. 



3 Application to the choice of dam site 



a '* Olive Bridge " site. The trenches and shafts together 

 with surface exposures indicate that the glacial drift at the Olive 



