Pensauken Formation — Description. 75 



directions can be made out. Thus there is a slope of 40 feet 

 from South Amboy to Metuchen, of nearly as much from En- 

 glishtown to Griggstown, of 50 feet from Perrineville to Kings- 

 ton, of 30 feet from Allentown to Trenton, and of 10 feet from 

 Philadelphia to Haddonfield. These localities, taken two by two, 

 are approximately at right angles to the axis of the valley. If 

 these slopes could be assumed to represent the original surface 

 of deposition, they would suggest either (1) that the main de- 

 posits were made from the southeast above Philadelphia, and 

 from the west below that city, an inference not borne out by 

 the constitution of the formation; or (2) that the original sur- 

 face of the formation has been warped a little since its develop- 

 ment, being tilted a little to the northwest at the north, and to 

 the southeast at the south. Along the northwest margin of the 

 plain at Metuchen, Trenton, and Philadelphia, the upper surface 

 of the Pensauken has a nearly constant level. If the formation 

 has been warped, therefore, it would seem that its northwest 

 margin has remained more nearly fixed, or has moved as a unit, 

 while the main body of the formation of the southeast has suf- 

 fered more deformation. 



At Raven Rock, 20 miles above Trenton, a bench at 200 feet, 

 140 above the river, is covered by what is probably Pensauken 

 gravel, though good exposures have not been seen. The ma- 

 terials of the gravel are largely clastic (sandstone, quartzite, 

 etc.), but bits of crystalline material occur. The gravel occurs 

 down to Wilburtha, at progressively lower levels, but in meager 

 remnants only. There is, however, enough to show its former 

 presence. 



The underlying formations. — The Pensauken sands and 

 gravels rest on various formations of older rock. In the vicinity 

 of Trenton, and thence to Princeton Junction, it rests on schists 

 in some places, and about Philadelphia, it rests on similar beds 

 over considerable areas. Here the upper formation thickens as 

 the surface of the schist declines eastward. It most places the 

 surface of the schist below the Pensauken is disintegrated to 

 depths of 6 to 10 feet. Locally the disintegrated schist is so 

 like the material of the Pensauken as to make their differentiation 



