48G PKOCEEDINGS OF BALTIMORE MEETING. 



element is in some doubt, since small fragments of this conglomerate are not 

 always to be distinguished with certainty from bits of Medina and Oneida. Thus it 

 will be seen that the Pensauken formation is lithologically very much more hetero- 

 geneous than the preceding Miocene. 



The physical condition of the constituents of the Pensauken beds is significant. 

 The granitic and gneissic material is uniformly thoroughly decomposed. Enough 

 of this material was present to render the beds, in their present decomposed condi- 

 tion, distinctly arkose. This is especially true in the northern part of the formation. 

 The trap and gabbro fragments are less completely decomposed, but are always 

 decayed to a considerable distance from the surface. Occasional gabbro bowdders, 

 as well as certain bowlders of gneissic character, are found, sources for which are 

 not known within the state of New Jersey. The bowlders sometimes reach a con- 

 siderable size, being two, three and even four feet in diameter. In no instance has 

 one of these bowlders been seen to be striated. This evidence is no more than 

 negative, since nothing except the sandstone and quartzite bowlders are sufficiently 

 well preserved to have retained striae, even if they were once present. 



DEVELOPMENT AND THICKNESS 



The Pensauken formation is best developed just south of the Triassic belt which 

 reaches from South Amboy to Trenton, but isolated remnants of it occur much 

 farther north. The most northern well preserved remnants which have been cer- 

 tainly identified occur north and northwest of Somerville and south of the outer- 

 most Watchung mountain. There are, very possibly, remnants of the same forma- 

 tion farther north. 



The original thickness of the Pensauken formation across the middle portion of 

 the state appears to have been something like 30 feet on the average. In some 

 places its thickness is considerably greater than this, but where this is true the 

 formation appears to fill the narrow valleys which were cut in the peneplain, on 

 which the Pensauken gravel as a whole was deposited. 



OROGENIC AND EROSIVE CHANGES 



Elevation and emergence followed the deposition of the Pensauken beds. How 

 far southward the emergence extended is not known, but it certainly aflected the 

 central portion of the state, including n:iost if not all the area represented on the 

 geological map of New Jersey as Cretaceous. On this Pensauken surface a drainage 

 system was developed, the valleys being comparable in depth to those which exist 

 today. They were sufficiently wide to indicate that the period necessary for their 

 development was considerable. From the Cretaceous belt of the state something 

 like half the Pensauken formation was removed. The shape and the depth of the 

 valleys cut at this time indicate that the land was not high, and that they devel- 

 oped slowly. 



Jamesbueg Formation 



ORIGIN AND COMPOSITION 



Then followed a period of subsidence and partial submergence, during which the 

 sea reached an elevation of about 130 feet from Trenton to New Brunswick. Dur- 

 ing this submergence there was deposited a mantle of gravel and loam, derived 

 principally from the Cretaceous, the Pensauken and the Miocene formations. This 

 constitutes the third of the "yellow gravel" formations, and is for the present 

 known as the Jamesburg formation. 



