CHAPTER VII. CLAYS IN TERTIARY FORMATIONS. CONTENTS. The Beacon Hill and Cohansey formations. Definition of terms. Fossils. Clay deposits. Distribution. Character. The Shiloh marl. The Alloway clay. Occurrence. Character. The micaceous, talc-like clay. The fluffy sand. The Asbury clay. Stratigraphic relations. The Eocene marl. The following subdivisions of the Tertiary beds are recog nizable in New Jersey, beginning from the top downward. 8. A coarse gravel member. Beacon Hill gravel,... 7. A sand member. Cohansey sand,...... Pliocene or 6. A marl bed-Shiloh marl,. . . . 5. A thick clay bed-Alloway clay,. 4. A micaceous talc-like clayey sand (near Woods Miocene. town only),... 3. A fluffy sand,.... 2. A clay bed-Asbury clay,.. 1. A marl bed-Blue marl,. Eocene. THE BEACON HILL AND COHANSEY FORMATIONS. Definition of terms.-Under the term Beacon Hill, as used in the Annual Reports of the Survey from 1893 to 1900, there have been included two somewhat different beds; the upper, a bed of coarse gravel, chiefly quartz and chert, and the lower, a bed of coarse quartz sand, with occasional small pebbles. Locally, the sand is cemented into beds of sandstone. The lower member also contains lenses of clay, which are frequently of considerable economic importance. Whether these two members are a unit, or are really two separate formations, has been an unsettled question. Until recently they have, on the whole, been regarded as but one formation, but the other alternative has not been lost sight of. However, data now in the possession of the Survey, the result of recent field work by Mr. Knapp, apparently indicate that there is good ground for separating the two, and he proposes the term Cohansey for the sand member, thus restricting the term Beacon Hill to the upper gravel. It has been impossible to fix definitely the age of these beds. No adequate stratigraphic ground for separating them from the underlying Miocene beds has been found, but it is quite possible that such a separation should be made, particularly since the scanty paleontological data at hand favor slightly their reference to the Pliocene, rather than the Miocene. The evidence, however, is not decisive, and the exact age of these formations must remain doubtful. Fossils. In the vicinity of Bridgeton numerous well-preserved plant remains have been found in the Cohansey sandstone. Mr. Hollick, who has studied these remains, makes the following statement: "Probably about fifty species are represented in the collections which have been made-all of them angiosperms, many of them referable to living species, and some of them identical with species now growing in the vicinity of Bridgeton, such as Ilex opaca, Nyssa sylvatica, etc. "A comparison between this fossil flora and the living flora of eastern North America indicates a close identity between the former and that now in existence at about the latitude of Virginia. In many of its elements it is unique and distinct from that of any other American Tertiary horizon. The collections of Eocene and Miocene plants which have been made in the West contain different species, and those from Bridgeton are 1N. J. Geol. Surv., Report on Forests, 1899, pp. 197-198. either rare or entirely wanting in them. As a whole, the flora is more nearly comparable with that of certain European Upper Miocene localities, and we may regard it as that which immediately preceded the close of the Tertiary period, and this conclusion is emphasized by the well-recognized fact that in Europe biologic evolution was in advance of America, so that the European Eocene flora is largely comparable with the American Miocene, European Miocene with American Pliocene, and European Pliocene with the American living flora." Mr. Hollick also states1 that the leaf-bearing formation could be referred either to the late Miocene or Pliocene on the basis of these fossil plants. Some obscure casts of Molluscan shells2 have been found near Millville, but they are not decisive. The paleontological evidence, therefore, does not enable us to determine the age of the beds with any more certainty than does the stratigraphy, although it suggests their Pliocene age. Clay Deposits. Beds of clay, often of considerable extent, occur somewhat widely in the Cohansey sand. They range in thickness up to 24 feet, as reported, but in the great majority of cases they are only 8 or 10 feet thick, rarely exceeding 12, so far as known. Horizontally they vary from a few acres to several hundred in extent. In only those cases in which the deposit has been exhausted, or in which many borings have been made, is the size of these clay lenses known. Distribution. These clays occur in the southern portion of the State, for the most part southeast of the divide separating the tributaries of the Delaware from the streams which flow directly into the Atlantic. That is, they occur in Ocean and Atlantic counties, the southern portion of Burlington, Camden and Gloucester, and the central part of Cumberland. In other words, they occur in the sandy pines district of the State. This is, in a general way, the area southeast of the line on the map, Plate X, which 'Letter to the writers. 2 N. J. Geol. Surv., Ann. Rep. for 1896, p. 254. represents the northwestern limit of the Miocene deposits. At present they are dug at Bridgeton (191), Rosenhayn (185), Millville (183), Mays Landing (195), Da Costa (197), Elwood (198), Winslow Junction (201), Woodmansie (213), Whitings (212), Toms River (206) and Herbertsville (218, 219). They have also been dug at Seven Stars, White Oak Bottom, north of Whitings, Wheatland, Union Clay works, Tuckerton (210, 211), Mayetta (209) and near Cedar Grove, all in Ocean county; at Mount Misery and Chatsworth, in Burlington county; near Atco, Williamstown Junction and Blue Anchor (202, 203, 204), in Camden county; at Egg Harbor City, and near Centre Grove, in Cumberland county. At Mayetta a tract over 700 acres in extent is reported to be underlain by clay, with a thickness of 24 feet. Clay probably belonging to the same formation has been observed in the pines west of Toms River, near Davenport (208), and also north of Toms River and south of White Oak Bottom (207). It is also reported at South Park, Burlington county, and from many other places in the pines, as indicated on the map. It is entirely probable that similar lenses of clay exist at many other points within this district, but their location can be determined only by boring. In the absence of any natural exposures, such as stream bluffs, or of artificial excavations, such as road or railroad cuts, and in the presence of the widespread and continuous covering of sand and pine forest, their location is a matter of great difficulty. Character. These clay lenses vary considerably from place to place. They are usually somewhat sandy, particularly near their borders. The upper foot or two frequently contains scattered pebbles. In color they are white, yellow, chocolate and black. The latter clays contain much lignite, usually, however, in a finely broken condition. Many of these black clays, however, burn buff, Plate XVII, Fig. 1. The white clays are often more or less yellow mottled near the surface, owing to the infiltration of iron from the overlying yellow gravels. The clay beds are underlain by coarse sand, into which they pass either by thin alternating layers of sand and clay or by a gradual transition, the clay becoming more and more sandy. |