56 EDWIN W. HUMPHREYS AND ALEXIS A. JULIEN 



up to the surface of the solid rock. The last phase appears to have 

 been the plucking-up of huge thin slabs from a mass of thinly 

 foliated granite, somewhere in the valley adjoining on the west; 

 and their deposit as a ground-moraine over this inclined plane, 

 with intervening sheets of bowlder clay, in a kind of natural 

 masonry, for further protection of the underlying soft schists. 



Evidences and measure of the superincumbent pressure. — Soft as 

 this granite is now found, it is obvious that it must have possessed 

 much strength and rigidity at the time of its transport by the ice 

 in the form of slabs, mostly from a few inches up to a foot in thick- 

 ness, although commonly ten to twenty feet or more in greatest 

 extension. The pressure upon them, as well as their rigidity, is 

 shown by the frequent fractures and faulting, and the bending of 

 thin edges. Still more significant is the crushing of the rock within 

 the slabs at the contact with overlying bowlders of trap, which 

 have been pressed down into pockets in the granite. In one case 

 (Fig. 7) the bowlder appears to have been lifted subsequently some- 

 what out of its pit and the clay forced in beneath it. In another 

 (Fig. 8) the crushed granite rises around the imbedded bowlder, 

 which was eighteen inches in diameter, as if the rock was almost 

 plastic, either on account of the great pressure or of its own softened 

 condition, or both. We had almost hoped to have found here a 

 natural record of the weight of the superincumbent ice, and there- 

 fore of its thickness, by estimating the volume of the granite crushed 

 beneath the imbedded portion of the bowlder. This was found 

 impracticable, from the impossibility of determining the crushing 

 strength of the rock at the time of its penetration. However, we 

 already possess some measure of the thickness of the ice-sheet in 

 this region in the presence of glacial striae, often an inch in depth, 

 at points 250 to 300 feet in elevation; e.g., on the edges of the 

 gneiss over the summit of Inwood Heights, Manhattan Island, and 

 on the trap along the edge of the Palisade escarpment, on the west 

 side of the Hudson River. These imply a pressure which could 

 hardly have been exerted by a sheet of ice less than 1,000 feet in 

 thickness. 



