﻿110 NEW YORK STATE MUSEUM 



and there is a somewhat deeper buried channel a short distance to 

 the north side. In each case this old channel bed rock is probably 

 less fresh and substantial, due to former weathering, than the pres- 

 ent exposed surfaces. 



In each case glacial deposits reach a thickness of more than 200 

 feet within the narrow valley or gorge, especially along the north 

 valley wall within the limits of the proposed dam. 



Special geological conditions. The factors in which there is 

 most variation and which are of most significance in a comparative 

 study are those belonging to the glacial drift deposits. In order to 

 properly estimate the influence of some of these features it will 

 be necessary to briefly consider the types of material represented 

 at different places and the conditions under which they were 

 formed. 



Types of material. Till. Heavy bouldery till, mixed clay, 

 sand, gravel, and boulders, is the most abundant type of material. 

 It forms especially the chief surface material throughout the region, 

 and is the surface type at both sites. It becomes at places quite 

 sandy, but is almost everywhere good, impervious material because 

 of its mixed character. 



Laminated till. At a few places, notably in the Beaver kill near 

 its mouth, and in a trench above Olive Bridge, and in the " big 

 dngway " above West Shokan, strong lamination appears in heavy 

 stony till as if laid down rapidly in comparatively quiet water such 

 as the margin of a lake. This material is especially impervious. 



Gravel hillocks. A few small hillocks with morainic contour, 

 indicating a dumping ground for some glacier on a small scale, 

 occupy the flat immediately west of Browns Station. They were, 

 at a very late stage, piled into the course of a former glacial stream 

 whose delta deposits occupy the sandy bench above the 500 foot 

 contour just north of Olive Bridge. 



Assorted gravel and sand. This material is abundantly developed 

 just north of Olive Bridge. It seems to have formed a delta de- 

 posit at the mouth of a glacial stream that emptied into the main 

 valley at this point. The running water washed almost all of the 

 clay and extremely fine material farther out, where they settled in 

 the bottom of a small glacial lake that was at that time held in this 

 upper portion of the Esopus valley. The dam that held in this body 

 of water which reached above the 520 foot line stood near the 

 proposed " Olive Bridge " dam site. The materials forming the 

 clam were in part the glacial till that is now found on that site and 



