of the deposit we can read much regarding the conditions under which it was formed. If, therefore, in the same bank alternating layers of sand, clay and gravel are found, it indicates a change from disturbed to quiet water, and still later rapid currents over the spot in which these materials were deposited. The commonest evidence of current deposition is seen in the cross-bedded structure of some sand beds, where the layers dip in many different directions, due to shifting currents, which have deposited the sand in inclined layers (Pl. I, Fig. 2). The beds of thinly stratified or laminated sands and clays, found in many cases over the Woodbridge fire-clay bed, are another example of rapid changes in the conditions of deposition. Sedimentary clays can be distinguished from residual clays chiefly by their stratification, and also by the fact that they com Generalized section showing how beds of clay may vary horizontally and vertically. monly bear no direct relation to the underlying rock on which they may rest. All sedimentary clays resemble each other in being stratified, but aside from this they may show marked irregularities in structure. Thus, any one bed, if followed from point to point, may show variations in thickness, pinching or narrowing in one place and thickening or swelling in others, as shown in Fig. 2. In digging clay the miner often finds streaks of sand extending through the deposit and cutting through several different layers, these having been caused by the filling of channels cut in the clay deposits by streams after the elevation of the former to dry. land. Occasionally a bed of clay may be extensively worn away or corraded by currents subsequent to its deposition, leaving its upper surface very uneven, and on this an entirely different kind of material may be deposited, covering the earlier bed, and filling the depressions in its surface. If the erosion has been deep, adjoining pits dug at the same level may find clay in one case and sand in the other (Fig. 3). While in many instances the changes in the deposit are clearly visible to the naked eye, variations may also occur, due to the same cause, which would only show on burning. Thus, for example, the so-called retort clay, found in the Woodbridge region, is similar in its plastic qualities wherever found, but the Fig. 3. Section showing uneven boundary between two clay beds, due to erosion of one before the deposition of the other. shrinkage of that found in the different pits is not always the same, because it varies in fineness from place to place. It may also vary in color. Similarly, the Woodbridge fire-clay member of the Raritan (Chapter VIII) contains a layer known as the topsandy in many pits of the Woodbridge district, but around Perth Amboy it is wanting, either because it was not deposited there, or, if it was, subsequent scouring action by ocean currents in that portion of the area may have worn it away. In strong contrast to this we may take the black, micaceous, laminated sands and clays, overlying the refractory members of the Woodbridge fire-clay bed. These are found at Woodbridge, Perth Amboy, South River and intermediate points, and were widely developed, although, even here, the individual beds are very changeable. The general character of sedimentary clays is more or less influenced by the locality and conditions of deposition, which enables us, therefore, to divide them into the following classes:1 Marine clays.-This class includes those sedimentary clays deposited on the ocean bottom, where the water is quiet. They have, therefore, been laid down at some distance from the shore, since nearer the land, where the water is shallower and disturbed, only coarser materials can be deposited. Beds of clay of this type may be of vast extent and great thickness, but will naturally show much variation, horizontally at least, because the different rivers flowing into the sea usually bring down different classes of material. Thus, one stream may carry the wash from an area of ironstained clay, and another the drainage from an area of white or light-colored clay. As the sediment spread out over the bottom, the areas of deposition might overlap, and there would thus be formed an intermediate zone made up of a mixture of the two sediments. This would show itself later as a horizontal transition from one kind of clay to another. These changes may occur gradually or at other times within the distance of a few feet (Pl. II, Fig. 1). The laminations produced by vertical changes are shown in Plate XLVI, Fig. 2. The most persistent beds of this nature are found in the rocks of the Silurian and Devonian systems, but beds of considerable horizontal extent are found in New Jersey in the Clay Marl series of the Upper Cretaceous. Estuarine clays.-These form a second type of some importance in New Jersey. They represent bodies of clay laid down in shallow arms of the sea, and are consequently found in areas that are comparatively long and narrow, with the deposits showing a tendency towards basin shapes. If strong currents enter the estuary from its upper end, the settling of the clay mud may be prevented, except in areas of quiet water in recesses of the bay shore. Or, if the estuary is supplied by one stream at its head, 1A person lacking geological training will not always be able to tell to which one of the following classes a deposit belongs. 1 and this of low velocity, the finer clays will be found at a point most distant from the mouth of the river. In such cases, we should anticipate an increase in coarseness of the clay bed or series of beds as they are followed from what was formerly the old shore line up to the mouth of the former river that brought I down the sediment. Estuarine clays often show sandy laminations, and are not infrequently associated with shore marshes, due to the gradual filling up of the estuary and the growth of plants on the mud flats thus formed. The clays of the Hackensack region are probably estuarine deposits, formed at the close of the Glacial period, when the region west of the Palisades stood somewhat lower in respect to sea level than at present.1 Swamp and lake clays.-Swamp and lake clays constitute a third class of deposits, which have been formed in basin-shaped depressions occupied by lakes or swamps. They represent a common type, of variable extent and thickness, but all agree in being more or less basin-shaped. They not infrequently show alternating beds of clay and sand, the latter in such thin laminæ. as to be readily overlooked, but causing the clay layers to split apart easily. Many of the lake clays are directly or indirectly of glacial origin, having been laid down in basins or hollows along the margin of the continental ice sheet, or else in valleys that have been dammed up by the accumulation of a mass of drift across them. This wall of drift serves to obstruct the drainage in the valley, thus giving rise to a lake, in which the clay has been deposited. Clay beds of this type are extremely abundant in all glaciated regions. They are usually surface deposits, often highly plastic, and more or less impure. Their chief use is for common brick and earthenware, and none of the New Jersey ones have been found to be refractory. Flood plain and terrace clays.-Many rivers, especially in broad valleys, are bordered by a terrace or plain, there being sometimes two or more, extending like a series of shelves or steps up the valley side. The lowest of these is often covered by 1 1 N. J. Geol. Survey, Vol. V. Report on Glacial Geology, p. 196. Not necessarily thin. |