Memoirs of the Geological Survey of England and Wales
Memoirs of the Geological Survey of England and Wales is in Victorian Books.
Victorian Books, Memoirs of the Geological Survey of England and Wales 1846
Memoirs of the Geological Survey of England and Wales II
On the Denudation of South Wales and the adfacent Counties of England. By Andrew C. Ramsay, F. R. S., Director of the Geological Surrey of Great Britain.
It may be considered as an aidom in geology, that the materials of all sedimentary strata, mechanically deposited, have been derived from the wreck of pre-existing rocks. This waste and re-^formation of rocks, is partly the result of atmospheric influences and the agency of running water, and also, to a great extent, is attributable to the ordinary destructive action of the sea on certain coasts.
The subject of this paper is intimately connected with the latter part of this proposition, in so far that many of the results sought to be explained and established, are referred to the influence of this powerful geological agent.
The subject of this paper is intimately connected with the latter part of this proposition, in so far that many of the results sought to be explained and established, are referred to the influence of this powerful geological agent.
At periods within the range of authentic geological history, as exhibited in deposits which date their origin with the earliest known organic appearances, it is not impossible that the intensity of some of these operations may have differed from that which we now observe. Thus, for example, if it be supposed that at any time the atmosphere and the sea contained a greater proportion of carbonic acid than at present, then the disintegration of certain rocks nfust have been sensibly accelerated by the union of this acid with their alkaline constituents.
The dash of the sea on a line of cliff presenting the necessary conditions, would then more rapidly work its destruction than could now be effected on a similar line of coast under the same relative circumstances. But, as such a state of things cannot by possibility be allowed in more modem geological epochs, there is, therefore, no ground for supposing that during their continuance, the average effects and intensity of these causes on the general surface of the globe, exceeded those of which we now have every-day experience.
It is with these times that our argument b principally concerned.
It has been stated, that geology forms the basis of physical geography. The forms of existing coasts are readily referable to certain of its varied phenomena, such as the relative strength and durability of the materials opposed to the influence of the waves, the disposition of the rocks composing these formations, and their relative exposure to prevalent winds and occasional heavy gales.
The physical features of the interior of a country are equally the result of geological operations ; the most important of which (at least in the country we are about to consider) are referable to a combination of causes, viz., the form its rocks have assmned from disturbance and contortion subsequent to consolidation, and the effects of the action of the sea on formations of various hardness, or which, from disposition arising from these contortions and other causes, offered different degrees of resistance to that action during sundry elevations and depressions of the ancient land ; for, let it be distinctly understood that the great external features of the country under review are conddered to be the result not of the inland influences either in operation now or at any past period of its history, but rather of the very influences even now modifying the form of our existing coasts — such influences acting over an indefinite period of past time, indefinite as regards our ordinary methods of calculation, but perhaps to a great extent definable witUn certain prescribed geological epochs. It will be hereafter shown, that the action \ of the weather and the power of running water, have only slightly modified the form the land assumed as it last issued from the waves.
But, to whatever causes the existing features of any country may be assigned, it is evident that without data by means of which we may form true conceptions of that form, it is impossible to reason correctly either of the manner of action of these bygone operations, or of the magnitude of their efiects. Such data are to a certain extent supplied by d^e construction of geological sections on a true scale, vertically and horizontally, having for their base line the level of the sea. Without these, it is vain to attempt precise argument on a variety of phenomena, either as regards questions of physics or natural history. If a false scale be adopted, the form and elevations of the country, the thickness, inclinations, and general arrangement of its component strata, are all distorted, producing results equally exaggerated and irreconcilable. And, as a further consequence, it is impossible correctly to estimate the original extent of the broken and disjointed strata, and thus to attempt to form any approximate conception of the amount of matter destroyed and rearranged, during the processes that reduced them to their present fragmentary condition. During the progress of the Geological Survey of Great Britain, in South-Western England and in South Wales, a number of sections have been constructed on a true scale of six mches to a mile ; and on evidence afforded by them the following reasonings are adduced.
What first strikes the eye on examining certain of these sections, is the remarkable curvature and distortion to which the strata composing all the formations, from the top of the coal measures downwards, have been subjected. Following these breaks and curves, the same series of rocks are seen repeatedly to rise to the sur&ce, sometimes in rapid, sometimes in wide*spreading undulations. When, in accordance with the curves indicated by surfiice dips, vast masses of rock are carried in these sections deep down into the earth, far below our actual cognLsance, it is yet impossible to doubt their undergroimd continuity, when we find again and again, the same set of beds diving downwards in one district, and (perhaps somewhat modified) agaih outcropping to the surface in remote parts of the country. The abuse of this fact, now familiar to every geologist, in earlier times led to the hypothesis of the original universal continuity of all strata over the entire circumference of the globe. But if the inference now drawn be legitimate, a littie reflection will show, that in the case of curved and conformable strata, the same arguments that apply to the continuity of rocks below the surface, may often safely be employed to prove the original connexion of contorted strata, the upturned edges of which may frequently be far apart Attention being pven to the physical relations of all the rocks in any country, such restoration of masses of rock to the form they once possessed, is within the limits of safe inference. And i^ in the cases above noticed, this original continuity of distant masses, and their spreading over tracts irbere they have left not a trace, be once granted, then the vast amount of matter we shall be able to show has been removed from such tracts, may well make us cautious in disbelieving the probable or possible destruction of other masses, once resting above the rocks that compose the present surface, but of the former existence of which above that sur&ce, we have at first sight no direct evidence. Outliers, cape-like projections, and anticlinals of various strata, so common on our maps of geological England, sufficiently illustrate the first proposition ; and the frequent occurrence of vast thicknesses of strata, disposed vertically or at high angles, afford perfect evidence that such strata were not originally discontinued at their present outcrop, since such supposition would involve the necessity of asserting, that the rocks in question were deposited in successive layers, forming together, at their extreme edge, a wall or highly inclined plane, often many thousand feet in height.
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