Marine sediment

“ It often happens that a delta is prevented from extending further seawards owing to the prevalence of some marine current that comes sweeping along the coast-line and cuts away the accumulations thrown down by the river. The sediment thus removed is often carried to great distances, and eventually settles down as a fine mud along the floor of the sea, entombing any fucoids, infusoria, shells, corals, fish-bones, or other relics that may lie at the bottom. ”

“ The occurrence of all of them in one and the same upraised marine formation by no means implies that they belong to the same era, for in the beds of all great rivers and estuaries, there are changes continually in progress brought about by the deposition, removal, and redeposition of gravel, sand, and fine sediment, and by the shifting of the channel of the main currents from year to year, and from century to century. ”

“ When the enormous transporting power of rivers is considered, when we think of the amount and variety of sediments together with terrestrial and fluviatile organic remains annually brought down to the sea by rivers there to be mixed with the marine sediments and the organisms living in the sea, we find it not so difficult to realize that the same phenomena happened in the past. ”

“ The freshwater sediments of the Paris basin, studied in connection with those of Auvergne, Velay, and Cantal, offer a very complete view of the eocene lake deposits, and lead to the conclusion that the marine and freshwater strata of that basin are to be considered as marking sometimes the independent action of the sea and land floods in one basin, and sometimes their periodical alternation; the land floods always coming from the south, and the marine sediments from the north or west. ”

“ On the other hand, the greater part of the surface of the land consists of marine sediments which must have been derived from land that has perished in the process, while all the continental surfaces, except, perhaps, some high peaks and ridges, have been many times submerged. ”

“ All marine sediments that have been carefully examined yield globules of this kind; and, as is shown by the common researches of M. Tissandier and myself, they were equally abundant in the ancient geological seas. Thus, to cite an example that has struck us very forcibly, the green sand extracted from the artesian well of Passy, at five hundred and sixty-nine metres below the surface, and which is of the Albian (lower Cretaceous, Potomac group) , is full of spherules as perfect as those which are extracted from the dust that has accumulated in the towers of the Cathedral of Paris. ”

“ There is also no end to the thickness of mud and sand which may accumulate in shallow water, provided that fresh sediment is brought down from the wasting land at a rate corresponding to that of the sinking of the bed of the sea. The latter, again, may sometimes sink so fast that the earthy matter, being intercepted in some new landward depression, may never reach its former resting-place, where, the water becoming clear may favour the growth of shells and corals, and calcareous rocks of organic origin may thus be superimposed on mechanical deposits. ”

“ On the floors of the seas and oceans we have not only the region where the greater part of the sedimentation is effected, but that in which the work assumes the greatest variety. The sea bottoms, as regards the deposits formed upon them, are naturally divided into two regions—the one in which the débris from the land forms an important part of the sediment, and the other, where the remoteness of the shores deprives the sediment of land waste, or at least of enough of that material in any such share as can affect the character of the deposits. ”

“ He points out that the deposits laid down by the ocean do not extend far out to sea, “that consequently the elevations of new mountains in the sea, by the deposition of sediment, is a process very difficult to conceive; that the transport of the sediment as far as the equator is not less improbable; and that still more difficult to accept is the suggestion that the sediment from our continent is carried into the seas of the New World. ”

“ For the most part, the deposits of mud, sand, and gravel are made under the sea or in lakes and estuaries, but they are also formed on the land in lodgment basins, in low-gradient valleys, and on base-plains. The deposits of sediment on land have received less recognition than they deserve. When formed under the sea or in other life-sustaining waters, shells and other organic material are liable to be entrapped and to form a part of the rock. These organic remains, or fossils, greatly aid in interpreting the deposits in which they occur. ”

“ Considering the enormous power of denudation which the sea possesses, as shown by numberless facts, it is not probable that a sedimentary deposit, when being upraised, could pass through the ordeal of the beach, so as to be preserved in sufficient masses to last to a distant period, without it were originally of wide extent and of considerable thickness: now it is impossible on a moderately shallow bottom, which alone is favourable to most living creatures, that a thick and widely extended covering of sediment could be spread out, without the bottom sank down to receive the successive layers. ”

“ Those who have been in the habit of examining accurate charts of sea-coasts, where sediment is accumulating, will be aware, that the surfaces of the banks thus formed, generally slope from the coast very gently towards a certain line in the offing, beyond which the depth in most cases suddenly becomes great. I may instance the great banks of sediment within the West Indian Archipelago, [3] which terminate in submarine slopes, inclined at angles of between thirty and forty degrees, and sometimes even at more than forty degrees: every one knows how steep such a slope would appear on the land. ”

“ By such changes the water becomes once more clear and fit for the growth of stony zoophytes. Calcareous sand is then formed from comminuted shell and coral, or, in some cases, arenaceous matter replaces the clay; because it commonly happens that the finer sediment, being first drifted farthest from coasts, is subsequently overspread by coarse sand, after the sea has grown shallower, or when the land, increasing in extent, whether by upheaval or by sediment filling up parts of the sea, has approached nearer to the spots first occupied by fine mud. ”

“ It is improbable in the highest degree that broad, lofty, isolated, steep-sided banks of sediment arranged in groups and lines hundreds of leagues in length, could have been deposited in the central and profoundest parts of the Pacific and Indian Ocean?, at an immense distance from any continent, and where the water is perfectly limpid. ”

“ In like manner alterations in the Earth's crust cause the ocean to be ever subjecting the organisms it contains to new combinations of conditions. Here the water is being deepened by subsidence, and there shallowed by upheaval. While the falling upon it of sediment brought down by neighbouring large rivers, is raising the sea-bottom in one place, in another the habitual rush of the tide is carrying away the sediment deposited in past times. The mineral character of the submerged surface on which sea-weeds grow and molluscs crawl, is everywhere occasionally changed ”

“ The ceaseless grinding of waves and tides erodes the coast line and adds débris to the marginal sea bottom. The finest sediment is carried out by the tides so far as to reach the ocean currents, and thus is strewn broadcast over portions of deep-sea bottom, and will also in due time be consolidated into rock. ”

“ Only the sand and the finest-grained sediments of the shore zone are carried outwards over the continental shelf by the tides or by the reaction-currents along the bottom set up by on-shore winds. The very finest sediment is kept in a state of movement until it drops into the gulleys or furrows of the shelf, where it can come to rest together with the finer fragments of the remains of littoral or bank vegetation. ”

“ Occasionally, in those deep and uninhabited parts of the ocean, never reached by any but the finest sediment in a normal state of things, the bottom would become densely overspread by gravel, mud, and boulders. ”

“ Yet in these cases there has been a supply of sediment from the waste of the coast and the interior, especially in Patagonia and Chili. Nevertheless wherever the bottom of the sea has been continually elevated, the total thickness of sedimentary matter accumulating at depths suited to the habitation of most of the species of shells can never be great, nor can the deposits be thickly covered by superincumbent matter, so as to be consolidated by pressure. ”

“ It is further to be observed, in the case of these beds, that if we begin at the west side of Europe and proceed easterly, or at the east side of America and proceed westerly, they become progressively thinner, the greater amount of material being deposited at the edges of the future continents; just as on the sides of a muddy tideway the flats are higher, and the more coarse sediment deposited near the margin of the channel, and fine mud is deposited at a greater distance and in thinner beds. ”

“ Estuary and Shore Deposits.
Rivers which discharge themselves into the ocean, where tides and currents break with a certain regularity the quiet of its waters, exhibit always at their mouths, and often along the lower part of their channels, another set of phenomena.
Where the tide enters a river's mouth, and periodically combats the freshes, these are "backed" to certain distances, their motion is nearly destroyed for a time, and the sediment, which was only suspended by the agitation of the water, is dropped in the interval of quiescence. ”

“ This opinion is, however, untenable; for the sinking down of the bed of the ocean is one of the means by which the gradual submersion of land is prevented. The depth of the sea cannot be increased at any one point without a universal fall of the waters, nor can any partial deposition of sediment occur without 564 the displacement of a quantity of water of equal volume, which will raise the sea, though in an imperceptible degree, even to the antipodes. ”

“ It is possible that the weight of the sediment let fall at the mouths of great rivers, like the Ganges, the Mississippi, and the Po, may cause the depression of the strata on which they are deposited, and hence if man promotes the erosion and transport of earthy material by rivers, he augments the weight of the sediment they convey into their estuaries, and consequently his action tends to accelerate such depression. There are, however, cases where, in spite of great deposits of sediment by rivers, the coast is rising. ”

“ The only question exciting any general distrust is that the sedimentary deposit in the lake, it is held by some, will shorten the life of the reservoir by filling the lake at such an early period as to render the scheme of doubtful expediency, and opinions differ very widely upon this subject, which is, indeed, a problematical one, and can only be determined, even approximately, by actual measurements of a great majority of the annual flow, for the quantity of sediment changes with flow and season. ”

“ Hence, when the ocean-bed was here and there raised to the surface—when its strata of sediment with their contained organic fragments were torn up and long washed about by the breakers before being re-deposited—when the re-deposits were again and again subject to this violent abrading action by subsequent small elevations, as they would mostly be; what few fragile organic remains they contained, would be in nearly all cases destroyed. Thus such of the first-formed strata as survived the repeated changes of level, would be practically "azoic;" like the Cambrian of our geologists. ”

“ Beneath the seas, except in their very greatest depths, and perhaps even there, the process of forming strata is continually going on. Next the shores, sometimes for a hundred or two miles away to seaward, the principal contribution may be the sediment worn from the lands by the waves and the rivers. Farther away it is to a large extent made up of the remains of animals and plants, which when dying give their skeletons to form the strata. ”

“ This explanation of different sediments coming to the same part of the sea from various quarters, may probably be applied to every system of stratified rocks, containing, as constituent members, limestone, sandstone, and clay; but it is necessary previously to investigate the directions in which the agencies concerned in producing each sort of sediment were most powerful ”

“ No part of the results obtained by the Challenger Expedition has a profounder interest for geologists and geographers than the proof which they furnish that the floor of the ocean-basins has no real analogy among the sedimentary formations which form most of the framework of the land. We now know by actual dredging and inspection that the ordinary sediment washed off the land sinks to the sea-bottom before it reaches the deeper abysses, and that, as a rule, only the finer particles are carried more than a few score of miles from the shore. ”

“ The coasts of continents and continental islands are strewn, as every one knows, with the wreck of the land—with gravel, sand, and mud, derived from the demolition of our rocks and soils. The coarser débris accumulates upon beaches and in shallow littoral waters, while the finer materials are swept further out to sea by tidal and other currents—the sediment being gradually sifted as it is borne outwards into deeper water, until only the — 328 — finest mud and silt remain to be swept forward. ”