Ancient Tidal Action and Planes of Marine Denudation

Abstract

THERE is at least one question in ancient physical geology ofr which the speculations of Prof. R. S. Ball (NATURE, vol. xxv. pp. 79, 103) regarding the magnitude of Tidal Waves in times past seem to throw fresh light, namely, the origin of “planes of marine denudation.” for those readers of NATURE who may not be familiar with this term, first proposed by Prof. Sir A. Ramsay, let me endeavour briefly to describe them. If we protract to a true scale the outlines of certain tracts of the British Isles, of Europe, or of America, we shall find that the higher portions of the ridges tend to rise to a certain level, which, on being connected by an imaginary plane, form a gently-sloping surface over a considerable area, it may be of hundreds or thousands of square miles in extent. Now, if in addition to this we insert the stratification of the district crossed by the section, and taken from actual observation, it will often be found that this imaginary plane is formed of the truncated edges of highly-inclined strata, or of the denuded summits of anticlinal arches of contorted or folded strata. When such strata are of hard and tough materials it is clear that they must have been planed down by an agent of great power and of long-continued action, but the result has been to convert originally highly uneven surfaces of flexured strata into approximately horizontal surfaces, over which inequalities have been worn off. Through such planes the existing river-valleys have been cut down, but between neighbouring valleys there is to be found the intervening ridge, trending upwards to the original, now imaginary plane. The Silurian district of Central Wales offers a remarkable example, which has been used by Prof. Ramsay (“Mem. Geol. Survey,” vol. i.). Let any one on reaching the summit of one of the long ridges to the south of Cader Idris place his head on the ground, and in this position survey the tract of hilly country lying to the southwards, and he will realise the nature of the plane surface, out of which the valleys have been hollowed. But there are many more remarkable instances even than this. The central plain of Ireland is an example on a larger scale, over which the Middle and Upper Carboniferous rocks have been swept away, leaving a floor of limestone; but it would be impossible to explain the course of its great river, the Shannon, without referring its origin to a time when a sloping plain stretched from the present sources of that river amongst the Leitrim Hills to Shannon harbour below Limerick, because now its channel traverses a ridge of Old Silurian rocks at Killaloe, which could not have existed as such when the river first commenced to run over a tract formed of Carboniferous beds since denuded. But it is amongst mountainous districts that the evidence of the former existence of old planes is most remarkable, because least expected. The higher ridges of the Grampians seen at a distance, or accurately drawn from a hypothetical standpoint (as on Mr. Knipes' panoramic picture), forcibly bring home this idea to the mind. The ridges and peaks with very few exceptions tend to rise to an imaginary plane connecting the higher elevations, while several actual terraces coincide with the plane itself. Out of this old plane the existing valleys have been cut down, during the vast period of time descending from the pre-Devonian to the present. A still more ancient plane was that in which the Cambrian sandstones and conglomerates were strewn, formed of tough gneiss and hornblendic schists, with a gentle rise towards the east. The Scandinavian Promontory offers an illustration on a grand scale, and to these we might add the pre-Triassic plane formed of the denuded Devonian and Carboniferous rocks of Belgium and the Rhine highly tilted, convoluted, and contorted, through which the existing rivers have carved out their channels. But I refrain from adding additional illustrations, as I must pass to the consideration of the question, How have such ancient planes been formed? Where was the agent capable of abrading down hundreds or thousands of feet of the most obdurate rocks over hundreds or thousands of square miles, and of transporting power sufficient to carry away the débris of these vast ruins? The geologist answers, “Only give me an unlimited time, and the waves, tides, and currents of the seas acting along the coast-lines as they at present act, will effect all that you demand.” Granted that with “unlimited” time all this may be effected, but this is a demand which the astronomers will not concede, and geologists must pay some respect to astronomers and mathematicians after all. But even with the aid of (practically) “unlimited” time a serious objection meets us at the threshold. It is undeniable that the crust of the earth is always on the move, either upwards or downwards; those who are not intensely uniformitarian in their views contend that this oscillatory motion of the crust was much more rapid in past geological times than at the present day. If this be admitted, and I hold that it is a necessary consequence of the constantly decreasing rapidity with which the secular cooling of the surface has progressed downwards to the present day, how, let me ask, are you to get the coast to remain sufficiently long within range of such wave action as we see at present, to admit of the abrasion of the land to any considerable distance. The effects of wave action along our existing coasts, where formed of the more solid strata, is admittedly very slow, and in order to produce any great planing effects, the same coast-level (approximately) must be presented to it for a lengthened period; but with the required (practically) “unlimited” time, the coast-level would be placed out of reach, either by elevation or submergence. The hypothesis of approximately unlimited time seems to me, therefore, to be untenable. And what we require is not time but force, in order to account for the planing away of vast masses of obdurate strata over extensive areas. Such additional force Prof. Ball has supplied us with. He has shown that at a comparatively early stage of geological history the tides may have had a denuding effect several hundred times more powerful than the present. With such a “stupendous tidal grinding-engine” we may indeed conceive the work we have to account for accomplished, and the hypothesis of Prof. Ball approaches certainty, when it is considered that the character of the floors of the sea adjoining our coast lines gives but slight evidence that such planes of marine denudation as I have attempted to describe, are in course of formation at the present day. They are phenomena of the past, not of the present, when wave and tidal action has, happily for mankind, subsided into restricted limits as compared with that of Palaeozoic and Mesozoic times.