Page:The Journal of geology (IA journalofgeology21894univers).pdf/180
The formation has a rather constant thickness of 100 feet.
Rancocas formation (Middle Marl Bed).—The Middle Marl Bed is not as prominent a feature in Monmouth county where the type localities for the other formations are found as farther southward. The Rancocas Creek in Burlington county cuts through the Middle Marl Bed exposing a full sequence of the deposits of that formation, while in the neighboring area extensive exposures of the strata are found.
The formation is largely a greensand, although much more highly glauconitic in the lower than in the upper half of the formation. Although the lower half is largely a pure greensand, it becomes in some portions of the state very argillaceous toward the base, forming the so-called "chocolate marl," while toward the top it becomes crowded with shells, the upper two feet characterized by the presence of Terebratula Harlani, the most persistent fossiliferous zone in the state. The upper half of the formation is highly calcareous, frequently appearing as limestone ledges, known as "yellow limestone," and often containing as much as 80 per cent. of carbonate of lime. It is extremely fossiliferous, and has afforded many beautifully preserved specimens of Bryozoa, Echinodermata and Foraminifera. The fossils are, in the main, different from those in the underlying formations.
The strata reach a thickness of about 45 feet.
Manasquan formation (Lower portion of the Upper Marl Bed).—The name Manasquan Marl was in an earlier publication made to include the Yellow Sand, together with the "green sand" and "ash marl" of the Upper Marl Bed of Professor Cook. For that horizon the term Manasquan formation is retained. It is typically developed in the valley of the Manasquan River and its tributaries.
Like the preceding formation it is essentially a greensand throughout, although distinctly quartzose in the lower part, and at times argillaceous in the upper layers. The fossils so far as observed are confined exclusively to the more highly greensand member, but the number of species is not large. The high per-