106 Quaternary Formations of Southern New Jersey. 



30 to 50 feet deep, which interrupted the continuity . of the 

 scarp. The arkose phase of the formation did not extend be- 

 yond the Swedesboro Plain. 



The Mount Laurel fiats. — In contrast with the area southwest 

 of Coopers Creek, a broad area about Mount Laurel and Marl- 

 ton is low and free from any surface material which can be 

 referred to the Pensauken formation (Fig. 47, p. 136). It is 

 probable that the formation was once here, and that it has been 

 eroded away. 



The altitude of the surface in this area is below the level 

 of the Pensauken to the west, much of it considerably below. 

 Various elevations in the region at heights of 80 to 90 feet 

 have a scattering of surface pebbles which suggest the former 

 presence of a gravelly formation. Such suggestions of the pre- 

 Pensauken plain as the surroundings afford would place it at 

 80 to 90 feet. If this is correct, the present lowland about Mount 

 Laurel, at 30 to 60 feet, is of post-Pensauken origin. This is 

 a singular feature topographically, because the area is distinctly 

 below the general level of the land between it and the Delaware, 

 through which the degrading streams must have passed. 



The explanation of the Mount Laurel lowland is probably to 

 be found in two principal facts : (1) The Cretaceous beds which 

 outcrop here are more easily eroded than those which outcrop 

 to the west where the surface, except for the valleys of Pen- 

 sauken and Rancocas creek, is higher; and (2) the Pensauken 

 once deposited in the area of the present flats was possibly finer 

 and thinner than that deposited by the main stream. 



Southwest of Coopers Creek, the area was not equally de- 

 graded in pre-Pensauken time, perhaps because the corresponding 

 bed of the Cretaceous is much thicker there than to the north- 

 east. Further, Rancocas Creek seems to have been the largest 

 of the streams from the east, and for that reason to have been 

 most efficient in erosion. 



Vicinity of Philadelphia. — In much of Philadelphia the base 

 of the Pensauken is below 40 feet, and in places as low as sea 

 level, and the low-lying part of the f ormation is somewhat differ- 

 ent from the part at higher levels. In the city, the Pensauken 



