Deposition (geology)

“ Mechanical erosion of preëxisting mineral deposits or rocks and their transportation, sorting, and deposition are responsible for the placers of gold, tin, tungsten, platinum, and various precious stones, and for certain iron sands and conglomerates. Sands, sandstones, shales, and certain clays and bauxites also belong in this group. These deposits may be formed under air or under water, and under various climatic and topographic conditions. During the process of formation the minerals of differing density are more or less sorted out and tend to become segregated in layers. ”

“ Sometimes bedding seems to have been determined by strong currents which temporarily not only prevented deposition over a given area, but even cut away the loose surface of deposits already made, giving a firm surface from which succeeding deposits are distinct. This sequence of events is sometimes shown by the truncation of laminæ, and by other signs of erosion. The commoner sorts of bedded rock are limestones, shales, sandstones, and conglomerates. ”

“ They were the immediate agencies in locating and controlling the deposition of stratified rocks, though they rest back on the great movements for their working conditions. Very slow sinkings of sea-borders have permitted deposition to go on in shallow water for long periods without being interrupted by the local filling of the sea. ”

“ The making of an alluvial plain usually involves both erosion and deposition. When a stream has cut its channel to grade, downward erosion ceases, or more exactly, downward cutting is, on the average, counterbalanced by deposition. So long as a stream is cutting downward rapidly, it carries away whatever débris descends the side slopes. When it approaches grade, the débris which descends the side slopes tends to accumulate at their bases, and the V-shaped cross-section of the valley becomes U-shaped (see Fig. 172) . ”

“ For, since the process of deposition is always going on, the matter which is deposited to-day will to-morrow be covered with a new layer, and in the course of ages it may lie beneath an immense pile of mineral matter, hundreds or even thousands of feet in thickness. ”

“ The deposition of the Cretaceous. 2. The folding and erosion of the Cretaceous. 3. The deposition of the Tertiary. 4. The desiccation of the Tertiary lake basin. 5. The erosion which is still in progress. It is evident that the laccolites were not formed until the Cretaceous strata had been deposited; for their uplifts have bent and tilted all Cretaceous rocks up to and including the Masuk sandstone. ”

“ On closer inspection, we find that these terraces are stratified in the ordinary manner of alluvial or littoral deposits, as may be seen at those points where ravines have been excavated by torrents. The parallel shelves, therefore, have not been caused by denudation, but by the deposition of detritus, precisely similar to that which is dispersed in smaller quantities over the declivities of the hills above. These hills consist of clay-slate, mica-schist, and granite, which rocks have been worn away and laid bare at a few points only, in a line just above the parallel roads. ”

“ When the fissures are deep and wide and traverse different formations, conditions are afforded for very complex deposits, and for the concentration of rare and valuable material originally dispersed through a great mass of rock. Ore deposition in such veins is usually treated as a theme by itself, but it is really but a declared expression of the work which the percolating waters are doing throughout all the rocks which they penetrate. Most of the fine crystals that grace mineralogical collections were formed in cavities and fissures by deposition from circulating mineralized waters. ”

“ That the structure of fossiliferous strata has often been modified by some general cause since their original deposition, and even subsequently to their consolidation and dislocation, is undeniable. These facts prepare us to believe that still greater changes may have been worked out by a greater intensity, or more prolonged development of the same agency, combined, perhaps, with other causes. Now we have seen that, near the immediate contact of granitic veins and volcanic dikes, very extraordinary alterations in rocks have taken place, more especially in the neighbourhood of granite. ”

“ As no actual unconformity is known from the top of the Upper to the bottom of the Lower Lias, and as there is a marked uniformity in the mineral character of almost all the strata, it is somewhat difficult to account even for such partial breaks as have been alluded to in the succession of species, if we reject the hypothesis that the old species were in each case destroyed at the close of the deposition of the rocks containing them, and replaced by the creation of new forms when the succeeding formation began. ”

“ Deposition of sediment is an important natural consequence of erosion. The destination of most streams is the sea, and where tides are relatively slight the sediments discharged mostly accumulate relatively near the mouths of the rivers in the form of flat, fan-shaped delta deposits. Some rivers, like the Ganges, which carry such unusual quantities of sediment, are able to construct deltas in spite of considerable tides. ”

“ Its outcrop was carefully examined for many miles along the mountain sides with the hope of finding a locality where conditions had been favorable for (a) the presence of life during deposition of sediments, (b) the subsequent changing of sediments into rock, and (c) the preservation of the rock during the vicissitudes consequent on mountain building, so that the preservation of the life of the epoch would be as complete as possible. ”

“ The special forms which ores assume in deposition, as beds, veins, lodes, stockworks, disseminations, segregations, etc., are chiefly incidental to the local situation in which the essential chemical or physical change takes place. ”

“ The deposition of the finer parts of volcanic discharges will be considered in connection with the deposits of the deep sea.
Gradation.—The gradational processes of the land and the sea are in striking contrast. On the land, degradation predominates, and aggradation is subordinate. In the sea, aggradation predominates, and degradation is subordinate. On the land, degradation is, on the whole, greatest where the land is highest, while aggradation is of consequence only where the land is low, or where steep slopes give place334 to gentle ones. ”

“ But in particular instances this conformity of the strata is departed from, and certain (older) rocks appear inclined at various, often steep, angles, or standing even vertical, while the more recent deposits lie level or nearly so, upon them. What renders this case of disturbed stratification more impressive, with respect to the lapse of time, is the occurrence of positive circumstantial proof of the intervention of mechanical, chemical, or vital agencies of considerable duration between the elevation of the older, and the deposition of the newer strata, Thus in diag. ”

“ We may indeed accept the implements as being truly characteristic fossils of a certain class of deposits. The character of the beds, consisting as they do, of gravel, sand, and fine silt, brick-earth or loess, and their manner of deposition, are also absolutely in accordance with the river-hypothesis.
On the higher levels above but near the valleys, we frequently find these beds at a considerable distance from the existing stream; we find them at all levels on the flanks of the valleys, and occasionally almost at their bottom, or even below it. ”

“ When we see, as is so often the case, a formation composed of beds of different mineralogical composition, we may reasonably suspect that the process of deposition has been much interrupted, as a change in the currents of the sea and a supply of sediment of a different nature will generally have been due to geographical changes requiring much time. Nor will the closest inspection of a formation give any idea of the time which its deposition has consumed. ”

“ Geologically considered, the results which they accomplish, may be summed up under the two heads of deposition and erosion—the formation of new rocks in the sea, and the destruction of old rocks on the land. In the rôle of rock-makers they produce the very important original structures of the stratified rocks; while as agents of erosion they develop the most salient of the subsequent structures of the earth’s crust—the infinitely varied relief of its surface. ”

“ The deteriorating influence of high deposition may be judged from the closing of an embankment of considerable height, as 165it is often a tedious and somewhat anxious undertaking, as might be expected from excavated, shaken, and loosened earth being deposited upon soil in a similar condition, instead of upon the solid ground like the other portions of an embankment. ”

“ Furthermore, the rate of deposition along the coast, near the mouth of a large and active river, is much higher than on a coastline where no rivers empty their sediments into the sea. The rate is also largely controlled by the character of the sediments deposited. Sandstone and shale are more rapidly deposited than limestone. ”

“ If clay could be kept in a moist state fissures would seldom occur. The constant alternation of wetness and dryness creates the fissures, and water completes the disintegration. There are a few clays which are stable when kept in solid masses, as then only a small surface is affected by air and water, but if they are loosened and broken up, as in the process of excavation and deposition, they readily become in a muddy condition. ”

“ 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. ”

“ Deposits like those just described as accumulating on the bottom of the shallow sea, comparatively near the lands, are of great significance to the geologist because just such marine deposits now consolidated into sandstone, conglomerate, shale, and limestone, are so widely exposed over the various continents. A knowledge of the conditions under which shallow sea deposits are now forming, is, therefore, of great value in interpreting events of earth history as they are recorded in similar rocks which have been accumulating through millions of years of time. ”

“ Such I conceive to have been the origin of the sand, gravel, and imbedded plants of our boulder. It may be well, however, in going into the details of the subject, to take a wider view of this interesting branch of geology, and look for a little at the forms and modes of the decomposition of rocks, and the varied manner in which new sedimentary accumulations are formed.
All over the world, wherever a land surface spreads out beneath the sky, there goes on a process of degradation and decay. ”

“ When, also, we remember the fact, that no river is absolutely free from mineral matter held in suspension, but that, on the contrary, all running water is more or less discoloured with sediment, which is merely the material derived from the disintegration of rocks, it will appear to us difficult to overestimate the power of watery erosion. ”

“ No sooner are residual weathered mantles formed or sedimentary rocks deposited, whether under air or water, than processes of consolidation begin. Settling, infiltration of cementing materials, and new growths, or recrystallization, of the original minerals of the rock all play a part in the process. The mud or clay becomes a shale, the sand becomes sandstone or quartzite, the marl becomes limestone or marble. ”

“ During the process of decomposition or alteration, large quantities of mineral matter were removed, and, as this process continues, are still continuously being removed towards and into the water-collecting depressions of the various hydrographic basins. As the rocks have retained their original texture, their storage capacity for water has necessarily largely increased. Decomposition reaches the greatest depths along fissures and lodes. ”

“ In either case, deposition of the mineral matter follows the pattern of addition or “growth” from inside out. This growth, of course, does not involve a life process like that of a plant or animal. If two or more centers of deposition occur close together, the several growing concretions may touch, intergrow, and develop some weird forms, suggesting organic growth. ”

“ Everywhere throughout the world we read the same tale of subsidence and accumulation, of upheaval and denudation. The ancient sedimentary deposits which form the major portion of our land-surfaces, are the — 61 — waste materials derived from the demolition of plains, plateaux, and mountains of elevation. In some mountain-regions we read the evidence of successive epochs of uplift, separated by long intervening periods of erosion, followed by depression and accumulation of newer sediments over the denuded surface. Thus the Alps began to be elevated towards the close of Palæozoic times. ”

“ The time occupied in the deposition of each of these systems is a period. These systems of rocks are mainly of sedimentary origin, and the materials of which they are composed were derived from the land-areas which existed when these systems were being deposited. These materials were washed down from the land to the sea, and there deposited. ”