164 NEW YORK STATE MUSEUM 



Explorations 



Foundry brook therefore contains structures that could produce 

 considerable eitect upon the quality and condition of rock floor. 

 The rock floor is covered with heavy bouldery drift — thicker on the 

 Bull mountain flank than in the valley bottom proper. Where the 

 aqueduct line crosses the floor is at an elevation of 200 feet to 350 

 feet A. T. Hydraulic grade of the aqueduct is about 400 feet. 



The lowest bed rock found along the line is 182.3 feet and the 

 channel of the present stream coincides with the preglacial one in 

 that portion of its course. There are two secondars' channels — 

 probably tributary stream channels on the west side. One of these 

 lies under 70-80 feet of drift. 



Borings were made for the purpose of determining the rock floor 

 profile and the condition of bed rock. In most of them the ordinary 

 gneisses and granites were penetrated in normal condition. 



But in a few a very unusual condition was found. Hole no. 2 at 

 el. 34/ feet near the west or Bull mountain margin penetrated 49 

 feet of drift to el. 298. Then the drill passed into gneiss which was 

 at the top, the first 30 feet, of a fair quality. This is shown by the 

 core recovered — the first 12 feet recovering over 50''/. But the 

 percentage of recovery rapidly fell off — amounting to only 36'^r in 

 the first 50 feet. Only i foot of core was recovered in the next 30 

 feet, or only yt. AVhile from that point el. 220 feet to the bottom 

 of the hole el. 51.8, at a depth of 295.7 feet from the surface, 

 nothing but fine decomposed matter was washed up. There was no 

 core at all. This was at first reported as sand by the drillmen, and, 

 coming at a time when exploration of deep buried gorges was the 

 rule at other points of the aqueduct, there were many questions 

 about the interpretation of this new hole, the first assumption of 

 the drillers being that an overhanging ledge of a very deep gorge 

 had been penetrated passing through it into river sands below. A 

 little study of the material proved that this view is untenable. The 

 sandy wash from the drill is true disintegrated gneiss much decayed 

 and dislodged by the drill. 



But the meaning of it and the extent of it are after all important 

 additional questions. 



Interpretation and further explorations 

 It is certain that the soft material and the " sand " reported from 

 this boring represent rock decay induced by underground water 

 circulation. Water circulation is rather free as is shown by the 



