206 XEW YORK STATE MUSEUM 



material except one sample, which is a '" dry sample " and is still 

 more strongly micaceous. 



Borings nos. 40, 45 and 46 are all within the zone that was con- 

 sidered, from surface indications, to be likely to carrs- the deepest 

 gorge and to show the weakest rock. Because of the heavy drift 

 cover (more than a hundred feet) it is manifestly impossible to 

 locate the weakest zone more closely or judge of its exact condi- 

 tion except by borings. 



Hole no. 42 at station 634 - 28, penetrates 82.4 feet of drift and 

 reaches bed rock at about elevation 21 feet A. T. The rock i> good, 

 substantial, coarsely crs'stalline limestone. It shows as soimd con- 

 dition as can be expected in this formation even under the most 

 favorable situations. 



Hole no. 46 at station 644-77.4 is just south of the brook. It 

 penetrates J2 feet of drift and reaches bed rock at elevation 14 

 feet A.T. The rock is Fordham gneiss of typical sort and in per- 

 fectly good condition. There is no question about the soundness of 

 the rock from this point southward. 



Hole no. 45 at station 643-52.5, 125 feet north of hole no. 46 

 penetrates drift for about 150 feet (possibly a few feet less. 145 

 feet). This drift cover is interpreted as mostly sand (modified 

 drift; to 115 feet and a boulder bed from. 115 to 143 feet. After 

 the true ledge is reached it is sound and shows no unusual or ques- 

 tionable conditions. It is Fordham gneiss. 



Interpretation 



1 Weak zone. There is little doubt that this last 100 feet of 

 hole no. 40 is in the decayed weak zone that was expected to de- 

 velop in the vicinit}- of the contact between the gneiss and the Hme- 

 stone. It would be expected to pitch nortliward along the floor of 

 gneiss and extend beneath the southerly extremit}- of limestone at 

 this point [sec fig. 36]. 



2 Contact. Hole no. 40 cuts limestone, hole no. 45 cuts only 

 gneiss, therefore the formational contact lies somewhere in this 

 177-foot space. 



3 Position of old channel. Bed rock surface is lowest at hole 

 no. 45. But since the rock itself is sound gnei-s, it is not believed 

 to represent the lowest possible point. This is still more certain 

 because of the fact that the pitch is northward so that this becomes 

 a dip slope on which the preglacial stream could glide against the 

 edges of the limestone beds [sec diagram], and because the condi- 



