88 Quaternary Formations of Southern New Jersey. 



House, but little above sea level, fresh-water shells are found 

 in clay which is probably to be correlated with the Pensauken. 

 On the whole, the belief is entertained tentatively, that rivers 

 were the agents chiefly concerned in the depositions of the for- 

 mations, and that such part (if any) of it as is marine is very 

 subordinate. 



THICKNESS. 



The formation was, on the whole, thickest along the axis of 

 the. Amboy-Bordentown-Salem belt, and thinned both to the 

 northwest and southeast. The greater thickness near the axis 

 of the belt seems to be because the bottom of the valley was 

 lowest there, when deposition began. 



It is probable that the original maximum thickness of the 

 formation in the vicinity of Philadelphia was not less than 120 

 feet. This is the elevation of its surface in Philadelphia, and 

 at Haddonfield on the east side of the river. In the vicinity of 

 the city hall in Philadelphia excavations down virtually to sea 

 level have been seen, showing Pensauken gravel down to that 

 level. 



In the vicinity of South Amboy, the surface of the Pensauken 

 reaches an altitude of 170 feet. The level of its base is about 

 90 to 100, but in two spots in that vicinity its base is known to 

 go down to within 20 feet of tide level. If the Pensauken ma- 

 terial at these low levels is not the result of slumping, it indicates 

 great maximum depth of the formation here. 



The average thickness of the formation, as it now exists, is 

 much less than that suggested by the figures given above. In 

 the vicinity of Philadelphia, it is commonly not more than 10 

 to 20 feet outside the deep valleys. About Bordentown it has 

 a depth of 20 to 40 feet; about Jamesburg, 40 to 60 feet; and 

 at South Amboy, as much as 70. Its average original thickness 

 was doubtless more than the smallest of these figures, and less 

 than the largest. 



