86 Quaternary Formations of Southern New Jersey. 



remnant of a deposit which was, at the outset, much more ex- 

 tensive. 



In the vicinity of Philadelphia, the bottom of the formation 

 runs down below its normal level in a relatively narrow pre- 

 Pensauken valley in the bottom of the broad Delaware Valley. 

 When deposition began, this narrow valley was filled with such 

 material as the drainage then afforded. Later, after its bottom 

 had been filled, deposition took place at higher levels. 



The deposits of the lower level, especially below 40 feet, are 

 somewhat unlike those of higher levels, especially above 70 feet. 

 Between these levels there is a mixture of the material character- 

 istic of the lower and the higher horizons. The Pensauken ma- 

 terial at the low levels is composed of quartzitic material largely, 

 with some gneiss, schist, and shale, but with little granitic gravel. 

 Its elements are not well rounded, and have been worn but little. 

 The gravel appears to have been brought to its present position 

 by the Schuylkill, and becomes finer with increasing distance 

 from the debouchure of that stream. Above the 70-foot level, 

 the Pensauken is commonly arkose, as generally in the area 

 where it is normally developed. This arkose material was prob- 

 ably contributed in part by the Delaware after that river began 

 to bring its load of gravel and sand down to this latitude; but 

 much of it came from the schists of the Trenton-Philadelphia 

 region. Pensauken gravel and sand likewise fill some rather 

 deep side valleys in this latitude, one near the present course of 

 Crosswicks Creek, and others farther south. In the side valleys, 

 the difference between the material of the upper and lower 

 levels is not so pronounced. 



Bearing of constitution on origin. — The persistence of the 

 arkose sand and its uniformity over great areas, has a bearing 

 on the origin of the Pensauken. At first thought, it might seem 

 to favor the hypothesis of submergence, as a deposit in a sound 

 might be more uniform than deposits made by a series of rivers. 

 This argument is, however, not very convincing, since the ma- 

 terials for the deposit, even in a sound, would have to be con- 

 tributed by rivers, to a large extent. 



There are local variations in composition corresponding, in a 

 measure, with what might be expected along shores. Thus along 



