82 Quaternary Formations of Southern New Jersey. 



Geographic variations. — The Pensauken formation has its best 

 and most distinctive development in the Raritan Bay-Trenton- 

 Salem depression, a belt 10 to 20 miles wide and about 90 miles 

 long-. Throughout this valley, wherever sufficient remnants of 

 the formation are present to afford a basis for generalization, 

 it is found that the constitution of the formation changes from 

 its northwest border toward the southeast. This change affects 

 both the size and the kinds of the constituents. The sorts of 

 material which are coarse along the northwestern margin, become 

 finer to the southeast, and the decrease in coarseness from the 

 northwest may be said to continue to the line A-B of Fig. 42. 

 Considerable bowlders (2 to* 4 feet in diameter) are not rare, 

 and in places they are common near the northwest border of the 

 belt. All of them are from formations which outcrop to the 

 north. But as the line A-B is approached from the northwest, 

 large bowlders become rare, and reach a foot in diameter in ex- 

 ceptional cases only. 



A change in the lithologic character of the material accom- 

 panies the chang'e in size noted above. While pieces of granitic 

 and other igneous rocks are common along the northwest border 

 of the formation, they decrease toward the line A-B, Fig. 42. 

 Shale and sandstone derived from the Newark series have the 

 same distribution ; and so have pieces of sandstone and quartzite 

 from the Paleozoic formations north and northwest of the 

 Newark series. 



Southeast of the line A-B, bowlders of northwesterly origin 

 are essentially absent almost everywhere; but bowlders derived 

 from formations of southern New Jersey are present. Frag- 

 ments of ironstone (sand cemented by iron oxide) derived from 

 the Cretaceous and younger formations are rare along the north- 

 west border of the belt,_but abundant to the southeast (especially 

 southeast of the line A-B. Fig. 42), where, locally, they con- 

 stitute as much as 70 per cent, of the gravel. Pebbles of quartz, 

 too, are less preponderant to the northwest than to the south- 

 east. These pebbles in this formation were derived in large 

 numbers from the Bridgeton and other beds which lay to the 

 southeast. They and the ironstone make up most of the stony 



