A Manual of Elementary Geology / or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments

by Sir Charles Lyell

Fig. 91.

Faults and denuded coal strata, Ashby de la Zouch. (Mammat.)

In the Newcastle coal district it is ascertained that faults occur in which the upward or downward movement could not have been less than 140 fathoms, which, had they affected equally the configuration of the surface to that amount, would produce mountains with precipitous escarpments nearly 1000 feet high, or chasms of the like depth; yet is the actual level of the country absolutely uniform—affording no trace whatever of subterranean movements.[69-B]

The ground from which these materials have been removed is usually overspread with heaps of sand and gravel, formed out of the ruins of the very rocks which have disappeared. Thus, in the districts above referred to, they consist of rounded and angular fragments of hard sandstone, limestone, and ironstone, with a small quantity of the more destructible shale, and even rounded pieces of coal.

Allusion has been already made to the shattered state and discordant position of the carboniferous strata in Coalbrook Dale ([p. 62.]). The collier cannot proceed three or four yards without meeting with small slips, and from time to time he encounters faults of considerable magnitude, which have thrown the rocks up or down several hundred feet. Yet the superficial inequalities to which these dislocated masses originally gave rise are no longer discernible, and the comparative flatness of the existing surface can only be explained, as Mr. Prestwich has observed, by supposing the fractured portions to have been removed by water. It is also clear that strata of red sandstone, more than 1000 feet thick, which once covered the coal, in the same region, have been carried away from large areas. That water has, in this case, been the denuding agent, we may infer from the fact that the rocks have yielded according to their different degrees of hardness; the hard trap of the Wrekin, for example, and other hills, having resisted more than the softer shale and sandstone, so as now to stand out in bold relief.[70-A]

Origin of valleys.—Many of the earlier geologists, and Dr. Hutton among them, taught that "rivers have in general hollowed out their valleys." This is true only of rivulets and torrents which are the feeders of the larger streams, and which, descending over rapid slopes, are most subject to temporary increase and diminution in the volume of their waters. The quantity of mud, sand, and pebbles constituting many a modern delta proves indisputably that no small part of the inequalities now existing on the earth's surface are due to fluviatile action; but the principal valleys in almost every great hydrographical basin in the world, are of a shape and magnitude which imply that they have been due to other causes besides the mere excavating power of rivers.

Some geologists have imagined that a deluge, or succession of deluges, may have been the chief denuding agency, and they have speculated on a series of enormous waves raised by the instantaneous upthrow of continents or mountain chains out of the sea. But even were we disposed to grant such sudden upheavals of the floor of the ocean, and to assume that great waves would be the consequence of each convulsion, it is not easy to explain the observed phenomena by the aid of so gratuitous an hypothesis.

On the other hand, a machinery of a totally different kind seems capable of giving rise to effects of the required magnitude. It has now been ascertained that the rising and sinking of extensive portions of the earth's crust, whether insensibly or by a repetition of sudden shocks, is part of the actual course of nature, and we may easily comprehend how the land may have been exposed during these movements to abrasion by the waves of the sea. In the same manner as a mountain mass may, in the course of ages, be formed by sedimentary deposition, layer after layer, so masses equally voluminous may in time waste away by inches; as, for example, if beds of incoherent materials are raised slowly in an open sea where a strong current prevails. It is well known that some of these oceanic currents have a breadth of 200 miles, and that they sometimes run for a thousand miles or more in one direction, retaining a considerable velocity even at the depth of several hundred feet. Under these circumstances, the flowing waters may have power to clear away each stratum of incoherent materials as it rises and approaches the surface, where the waves exert the greatest force; and in this manner a voluminous deposit may be entirely swept away, so that, in the absence of faults, no evidence may remain of the denuding operation. It may indeed be affirmed that the signs of waste will usually be least obvious where the destruction has been most complete; for the annihilation may have proceeded so far, that no ruins are left of the dilapidated rocks.

Although denudation has had a levelling influence on some countries of shattered and disturbed strata (see [fig. 87.] [p. 63.] and [fig. 91.] [p. 69.]), it has more commonly been the cause of superficial inequalities, especially in regions of horizontal stratification. The general outline of these regions is that of flat and level platforms, interrupted by valleys often of considerable depth, and ramifying in various directions. These hollows may once have formed bays and channels between islands, and the steepest slope on the sides of each valley may have been a sea-cliff, which was undermined for ages, as the land emerged gradually from the deep. We may suppose the position and course of each valley to have been originally determined by differences in the hardness of the rocks, and by rents and joints which usually occur even in horizontal strata. In mountain chains, such as the Jura before described (see [fig. 71.] [p. 55.]), we perceive at once that the principal valleys have not been due to aqueous excavation, but to those mechanical movements which have bent the rocks into their present form. Yet even in the Jura there are many valleys, such as C ([fig. 71.]), which have been hollowed out by water; and it may be stated that in every part of the globe the unevenness of the surface of the land has been due to the combined influence of subterranean movements and denudation.