192 NEW YORK STATE MUSEUM 



The Inwood limestone occupies about 800 feet of the bottom and 

 eastern margin of the valley, lapping well up on the Fordham 

 gneiss. The drill cores from this formation are unusually sound. 



The Manhattan schist shows much broken material. There are 

 many crush zones. This condition increases still farther west along 

 the railway near \'alhalla station. 



The Fordham gneiss appears to be sound where it can be seen 

 at the surface. 



Results of exploration. Many borings have been made. They 

 prove the general structure and succession of formations, making 

 the boundaries definite. They increase the evidences of a rather 

 wide prevalence of weak zones — some of them in the gneisses. 

 And they also indicate a more extensive surface decay than w^as 

 formerly believed to prevail. 



The chief problems from the geologic standpoint are connected 

 with the following features : 



1 Extent of surface disintegration 



2 Extent and distribution of weak zones 



3 Depth of decay and perviousness of rock 



Surface disintegration. Several borings on groimd underlain 

 by Fordham gneiss penetrated material beneath the drift and above 

 bed rock that was interpreted as residuary matter from rock decay. 

 All of this material is of local origin. Later exploration in the 

 form of a deep trench to bed rock has proven that there is an 

 extensive residuary mantle of this sort at the eastern side of the 

 valley below the present dam. In places as much as 30 feet exists. 

 Undoubtedly this material is a remnant of preglacial soil mantle 

 that was in some way protected from removal by the ice. Few 

 places are to be seen in all southeastern New York where there is 

 so much left in place. In most of it the gneissic structure is still 

 preserved, but the decay is so complete that it can be cut and 

 handled like an impure clay. 



Weak zones. It has been proven that there are weak zones 

 in the gneisses as well as in the other rock formations. In some 

 places the rock is so poor that no core is recovered for distances 

 of 5 to ro feet, and in one hole a seam of this kind 20 feet wide 

 appears. In every case, however, the drill passes through the rot- 

 ten material into the opposite wall — indicating a zone of consider- 

 a;ble dip instead of vertical position. This favors the theory that 

 the weaknesses follow the bedding largely and are perhaps due to 



