Fig. 155. Succession of Deposits recording a Transgressing Sea
c, conglomerate; ss, sandstone; sh, shale; lm, limestone
Rate of deposition. As deposition in the sea corresponds to denudation on the land, we are able to make a general estimate of the rate at which the former process is going on. Leaving out of account the soluble matter removed, the Mississippi is lowering its basin at the rate of one foot in five thousand years, and we may assume this as the average rate at which the earth’s land surface of fifty-seven million square miles is now being denuded by the removal of its mechanical waste. But sediments from the land are spread within a zone but two or three hundred miles in width along the margin of the continents, a line one hundred thousand miles long. As the area of deposition—about twenty-five million square miles—is about one half the area of denudation, the average rate of deposition must be twice the average rate of denudation, i.e. about one foot in twenty-five hundred years. If some deposits are made much more rapidly than this, others are made much more slowly. If they were laid no faster than the present average rate, the strata of ancient sea deposits exposed in a quarry fifty feet deep represent a lapse of at least one hundred and twenty-five thousand years, and those of a formation five hundred feet thick required for their accumulation one million two hundred and fifty thousand years.
Fig. 156. Thick Offshore Deposits of Coarse Waste recording the Presence of a Young Mountain Range near Shore
The sedimentary record and the denudation cycle. We have seen that the successive stages in a cycle of denudation, such as that by which a land mass of lofty mountains is worn to low plains, are marked each by its own peculiar land forms, and that the forms of the earlier stages are more or less completely effaced as the cycle draws toward an end. Far more lasting records of each stage are left in the sedimentary deposits of the continental delta. Thus, in the youth of such a land mass as we have mentioned, torrential streams flowing down the steep mountain sides deliver to the adjacent sea their heavy loads of coarse waste, and thick offshore deposits of sand and gravel ([Fig. 156]) record the high elevation of the bordering land. As the land is worn to lower levels, the amount and coarseness of the waste brought to the sea diminishes, until the sluggish streams carry only a fine silt which settles on the ocean floor near to land in wide sheets of mud which harden into shale. At last, in the old age of the region ([Fig. 157]), its low plains contribute little to the sea except the soluble elements of the rocks, and in the clear waters near the land lime-secreting organisms flourish and their remains accumulate in beds of limestone. When long-weathered lands mantled with deep, well-oxidized waste are uplifted by a gradual movement of the earth’s crust, and the mantle is rapidly stripped off by the revived streams, the uprise is recorded in wide deposits of red and yellow clays and sands upon the adjacent ocean floor.
Where the waste brought in is more than the waves can easily distribute, as off the mouths of turbid rivers which drain highlands near the sea, deposits are little winnowed, and are laid in rapidly alternating, shaly sandstones and sandy shales.
