Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

Between the tides muds may dry out and crack in intersecting lines like the walls of a honeycomb, and when the cracks have been filled at high tide, a structure is produced which may later be recognized and is usually referred to as “mud-crack” structure. This structure is of special service in distinguishing marine deposits from the subaërial or continental deposits.

A variation in the direction of winds of successive storms may be responsible for the piling up of the beach sand in a peculiar “plunge and flow” or “cross-bedded” structure, a structure which is extremely common in littoral deposits, though simulated in rocks of eolian origin.

The order of deposition during a transgression of the sea.—Many shore lines of the continents are almost constantly migrating either landward or seaward. When the shore line advances over the land, the coast is sinking, and marine deposits will be formed directly above what was recently the “dry land.” Such an invasion of the land by the sea, due to a subsidence of the coast, is called a transgression of the sea, or simply a transgression. Though at any moment the littoral, shoal water, and offshore deposits are each being laid down in a particular zone, it is evident that each must advance in turn in the direction of the shore and so be deposited above the zones nearer shore. Thus there comes to be a definite series of continuous beds, one above the other, provided only that the process is continued ([Fig. 18]). At the very bottom of this series there will usually be found a thin bed of pebbly beach materials, which later will harden into the so-called basal conglomerate. If the size of the pebbles is such as to make possible an identification, it will generally be found that these represent the ruins of the rock over which the sea has advanced upon the land.

Fig. 18.—Diagram to show the order of the sediments laid down during a transgression of the sea.

Next in order above the basal conglomerate, will follow the coarser and then the finer sands, upon which in turn will be laid down the offshore sediments—the muds and the lime deposits. Later, when cemented together, these become in order, coarser and finer sandstones, shales, and limestones. The order of superposition, reading from the bottom to the top, thus gives the order of decreasing age of the formations.

A subsequent uplift of the coast will be accompanied by a recession of the sea, and when later dissected by nature for our inspection, the order of superposition and the individual character of each of the deposits may be studied at leisure. From such studies it has been found that along with the inorganic deposits there are often found the remains of life in the hard parts of such invertebrate animals as the mollusks and the crustacea. These so-called fossils represent animals which were gradually developed from simpler to more and more complex forms; and they thus serve the purpose of successive page numbers in arranging the order of disturbed strata, at the same time that they supply the most secure foundation upon which rests the great doctrine of evolution.

The basins of earlier ages.—It was the great Viennese geologist, Professor Suess, who first pointed out that in mountain regions there are found the thickest and the most complete series of the marine deposits; whereas outside these provinces the formations are separated by wide gaps representing periods when no deposits were laid down because the sea had retired from the region. The completeness of the series of deposits in the mountain districts can only be interpreted to mean that where these but lately formed mountains rise to-day, were for long preceding ages the basins for deposition of terrigenous sediments. It would seem that the lithosphere in its adjustment had selected these earlier sea basins with their heavy layers of sediment for zones of special uplift.

The deposits of the deep sea.—Outside the continental slope, whose base marks the limit of the terrigenous deposits, lies the deeper sea, for the most part a series of broad plains, but varied by more profound steep-walled basins, the so-called “deeps” of the ocean. As shown by the dredgings of the Challenger expedition and others of more recent date, the deposits upon the ocean floor are of a wholly different character from those which are derived from the continents. Except in the great deeps, or between depths of five hundred and fifteen hundred fathoms, these deposits are the so-called “ooze”, composed of the calcareous or chitinous parts of algæ and of minute animal organisms. The pelagic or surface waters of the ocean are, as it were, a great meadow of these plant forms, upon which the minute crustacea, such as globigerina, foraminifera, and the pteropods, feed in countless myriads. The hard parts of both plant and animal organisms descend to the bottom and there form the ooze in which are sometimes found the ear bones of whales and the teeth of sharks.

In the deeps of the ocean, none of these vegetable or animal deposits are being laid down, but only the so-called “red clay”, which is believed to represent decomposed volcanic material deposited by the winds as fine dust on the surface of the ocean, or the product of submarine volcanic eruption. From the absence of the ooze in these profound depths, the conclusion is forced upon us that the hard parts of the minute organisms are dissolved while falling through three or four miles of the ocean water.