338 W. B. CLARK — UPPER CRETACEOUS FORMATIONS OF NEW JERSEY. 



been above shown, of unconformity, although this has not been absolutely 

 proven. 



The Rancocas formation is overlain conformably by the Manasquan 

 formation, although but few points of contact have been observed on 

 account of the very general overlapping of the Miocene deposits through- 

 out this area. The best exposures are seen in the valley of Manasquan 

 river, near Freehold, and on the southwest branch of Rancocas creek, in 

 the vicinity of Medford, where the lime-sands of the '„ Rancocas forma- 

 tion are overlain by the light colored clay at the base of the Manasquan. 



Divisions — General characteristics. — The Rancocas formation through- 

 out New Jersey admits of subdivision, upon both lithologic and paleonto- 

 logic grounds, into two members, but beyond the Delaware river in Dela- 

 ware and Maryland the distinctions, so clearly marked in the north, are 

 gradually lost. The two divisions in New Jersey are, in ascending order, 

 the Sewell marls, so named from Sewell, in Gloucester county, and the 

 Vincentown lime-sands, so called from Vincentown, in Burlington county, 

 in both of which localities the deposits are characteristically developed. 



Sewell marls. — These marls form a very constant horizon that can be 

 readily traced the entire distance across the state of New Jersey. They 

 consist typically of dark greensand marls, throughout which the glau- 

 conite is thickly disseminated, although this substance diminishes in 

 amount in passing southward. The Sewell marls have a thickness of 

 about 30 feet in northern New Jersey, but become somewhat reduced in 

 amount in the southern counties, declining to less than 20 feet in Salem 

 county. 



The Sewell marls are characterized by a highly fossiliferous band near 

 the top, although casts of molluscan shells and the bones of saurians are 

 found throughout this division. The fossiliferous zone at the top is often 

 packed with shells, the lower portion of it being made up almost exclu- 

 sively of the shells of Gryphasa vesicularis, while the upper part is often 

 composed of the shells of Terebratula harlani. These fossil layers are re- 

 markably persistent, extending as an almost continuous band across the 

 state. [ 



Vincentown lime-sands. — These lime-sands are well developed through- 

 out the central and southern counties of New Jersey, and consist of 

 highly calcareous greensands, the calcareous element being supplied by 

 the vast number of Bryozoan shells which crowd the beds. At times 

 the beds are almost purely calcareous, and at many points, especially 

 near the top of the Rancocas formation, become consolidated into firm, 

 limestone ridges. The Vincentown lime-sands have a constant thick- 

 ness of about 20 feet in northern and central New Jersey, but increase 

 gradually to the southward and in Salem county suddenly expand until 

 they attain a thickness of about 100 feet.- 



