Classification of Sediments.
| Mechanical |
| Subaqueous Deposited by water. | Conglomerate, sandstone and shale. |
| Subaërial or Eolian Deposited by wind. | Sandstone and loess. | ||
| Glacial Deposited by ice. | Till and tillite. | ||
| Fluvio-glacial Glacier-water deposits. | Sands and gravels. | ||
| Chemical |
| Calcareous tufa | Deposited in springs and rivers. |
| Oölitic limestone | Deposited at the mouths of riversbetween high and low tide. | ||
| Organic |
| Formed of plant remains. | Peats and coals. |
| Formed of animal remains. | Limestones and marls. |
Winds are under favorable conditions capable of transporting both dust and sand, but not the larger rock fragments. The dust deposits are found accumulating outside the borders of deserts as the so-called loess ([Fig. 216]), though the sand is never carried beyond the desert border, near which it collects in wide belts of ridges described as dunes. When this sand has been cemented into a coherent mass, it is known as eolian sandstone. A section of the appendix (B) is devoted to an outline description of some of the commoner rock types.
The different deposits of ocean, lake, and river.—Of the subaqueous sediments, there are three distinct types resulting: (1) from sedimentation in rivers, the fluviatile deposits; (2) from sedimentation in lakes, the lacustrine deposits; and (3) from sedimentation in the ocean, marine deposits. Again, the widest range of character is displayed by the deposits which are laid down in the different parts of the course of a stream. Near the source of a river, coarse river gravels may be found; in the middle course the finer silts; and in the mouth or delta region, where the deposits enter the sea or a lake, there is found an assortment of silts and clays. Except within the delta region, where the area of deposition begins to broaden, the deposits of rivers are stretched out in long and relatively narrow zones, and are so distinguished from the far more important lacustrine and marine deposits.
Lakes and oceans have this in common that both are bodies of standing as contrasted with flowing water; and both are subject to the periodical rhythmic motions and alongshore currents due to the waves raised by the wind. About their margins, the deposits of lake and ocean are thus in large part wrested by the waves from the neighboring land. Their distribution is always such that the coarsest materials are laid down nearest to the shore, and the deposits become ever finer in the direction of deeper water. Relatively far from shore may be found the finest sands and muds or calcareous deposits, while near the shore are sands, and, finally, along the beach, beds of beach pebbles or shingle. When cemented into coherent rocks, these deposits become shales or limestones, sandstones, and conglomerates, respectively.
As regards the limestones, their origin is involved in considerable uncertainty. Some, like the shell limestone or coquina of the Florida coast, are an aggregation of remains of mollusks which live near the border of the sea. Other limestones are deposited directly from carbonate of lime in solution in the water. A deposit of this nature is forming in southern Florida, both as a flocculent calcareous mud and as crystals of lime carbonate upon a limestone surface. Again, there is the reef limestone which is built up of the stony parts of the coral animal, and, lastly, the calcareous ooze of the deep-sea deposits.
The marine sediments which are derived from the continents, the so-called terrigenous deposits, are found only upon the continental shelf and upon the continental slope just outside it. Of these terrigenous deposits, it is customary to distinguish: (1) littoral or alongshore deposits, which are laid down between high and low tide levels; (2) shoal water deposits, which are found between low-water mark and the edge of the continental shelf; and (3) aktian or offshore deposits, which are found upon the continental slope. The littoral and shoal water deposits are mainly gravels and sands, while the offshore deposits are principally muds or lime deposits.
Special marks of littoral deposits.—The marks of ripples are often left in the sand of a beach, and may be preserved in the sandstone which results from the cementation of such deposits (pl. 11 A). Very similar markings are, however, quite characteristic of the surface of wind-blown sand. For the reason that deposits are subject to many vicissitudes in their subsequent history, so that they sometimes stand at steep angles or are even overturned, it is important to observe the curves of sand ripples so as to distinguish the upper from the lower surface.
In the finer sands and muds of sheltered tidal flats may be preserved the impressions from raindrops or of the feet of animals which have wandered over the flat during an ebb tide. When the tide is at flood, new material is laid down upon the surface and the impressions are filled, but though hardened into rock, these surfaces are those upon which the rock is easily parted, and so the impressions are preserved. In the sandstones of the Connecticut valley there has been preserved a quite remarkable record in the footprints of animals belonging to extinct species, which at the time these deposits were laid down must have been abundant upon the neighboring shores.