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IV.—The Sea Against Rain and Frost; or the Origin of Escarpments

Published online by Cambridge University Press:  01 May 2009

D. Mackintosh
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Extract

As every part of the crust of the earth has at one time been the surface, it follows that all questions connected with the origin of the present “form of the ground” must be very impotant, and that on their issue the progress of Geology must in a great measure depend. But on this subject a very wide difference of opinion at present exists. According to one party, consisting of Professor Ramsay, Mr. Jukes, Mr. Geikie, Colonel Greenwood, Dr. Foster, and others, the more abrupt inequalities of the earth's surface have been produced by subaërial or atmospheric causes. According to the other school, embracing Sir Charles Lyell, Sir Roderick Murchison, Professors Sedgwick and Phillips, Mr.Edward Hull, etc., the sea has been the principal denuding or excavating agent.

Type
Original Articles
Information
Geological Magazine , Volume 3 , Issue 20 , February 1866 , pp. 63 - 70
Copyright
Copyright © Cambridge University Press 1866

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References

page 63 note 1 There is a very extensive series of table-shaped mountains under the North Atlantic Ocean. Their comparatively level surface may be partly owing to their being covered with sand. Along their western and north-western edges the depth suddenly descends to 600 feet, and along the southern edge to 900 feet. The descent is nearly perpendicular. Besides these inequalities in the Atlantic there are deep furrows which run north and south (see article “Sea” in Penny Cyclopaedia, and “Section of the Bed of the Atlantic” in Maury's Physical Geography of the Sea). The bed of the German Ocean, though very favourably situated for becoming a level surface, would likewise appear to be far from uniform in depth (see Lyell's Principles of Geology)

page 64 note 1 Admitting that the inequalities of sinking archipelagos were partly formed by ancient subaërial causes, the sea would have an undoubted claim in the case of archipelagos which are gradually rising. But the advocates of atmospheric denudation cannot consistently appeal to submarine inequalities, as according to their theory the sea ought to reduce these inequalities previously to the area they occupy becoming dry land.

page 64 note 2 Quart. Journ. Geol. Soc., vol. xviii., p. 391.Google Scholar Among the Shetland Islands the sea is now giving rise to a series of the most abrupt inequalities. The coasts even of the mainland are singularly irregular and broken, being indented with innumerable arms of the sea, called voes, which penetrate into and intersect the interior parts of the island in such a manner that, in traversing it, a traveller cannot find himself, at any one point, farther than three miles from one of these voes, or from the open sea (Penny Cyclopædia, ”Shetland“). Among the Faroe Islands, the land close to the sea consists in general of perpendicular rocks, from 1200 to 1800 feet in height (Ibid.., “Faroe”).

page 64 note 3 Fac-siiciles of the highest and steepest inland precipices may be seen on our present sea-coasts.

page 65 note 1 The chalk escarpments above the terraces of upper greensand, in West Sussex, Hampshire, etc., furnish striking examples of the effects of an unconditional undermining agent.

page 66 note 1 Scenery of Scotland in connection with its Physical Geology. The descriptive powers of this accomplished geologist render him as worthy to be an illustrator of Playfair, as Playfair was of Hutton.

page 66 note 2 Geol. Mag., vol. ii., No. 4. After the reading of the paper at Birmingham, the President of the Sectzion, Sir R. I. Murchison, expressed his entire concurrence with the author's views in reference to the sea-shore origin of the Brimham Rocks, and the resistance they have offered to the atmosphere. He likewise referred to the preservation of glacial-markings on rocks as a proof of limited atmospheric denudation.Google Scholar

page 67 note 1 Though I have not lately had an opportunity of visiting the spot, I have no doubt that the rocky projections (including the Toadstone) and cliffs, near Tunbridge Wells, could he shown to be the work of the sea.

page 68 note 1 Rounded pebbles are not a necessary indication of the former presence of the sea. The degree of roundness, or angularity, will depend on the nature of the stones, the distance they have rolled, and the length of time the area they occupy remains at a stationary level. In the Midland Counties, drift composed of rounded pebbles, and drift composed of angular flints, graduate into each other on the same horizon. There, also, drift, interstratified with beds of sand containing sea-shells, may he seen on the same horizon with and graduating into drift in which no sea-shells have yet beeu discovered.

page 68 note 2 Buckman's Straits of Malvern.

page 68 note 3 See Lyell's Elements of Geology. In the two lines of cliff (Tnferior Oolite) which have been more or less quarried above and beyond Leckhampton, one may see a very striking illustration of the way in which atmospheric action tends gradually to obliterate a precipice. The weathering, so far as it affects the compact rocks, is chiefly limited to parts that overhang, which are gradually falling down, one block or fragment from under another, thus tending to convert an overhanging into a vertical cliff. The next stage of the process is not connected with the cliff itself, but consists of streams of oolitic fragments from the loose incoherent bed on its upper surface. These streams give rise to ridge like buttressen front of the cliff, the tendency being to conceal the cliff under a slope of débris. In this way, in all probability, a great part of the ancient sea-wall of the Cotswolds has been converted into a grass-covered slope, and quarrying operations have in some places been equivalent to irrespectively laying bare the original sea-worn rocks. But in all this we see a tendency the reverse of that ascribed to the atmosphere by subaërialists; for, instead of rendering more abrupt the inequalities of the earth's surface, it operates in the contrary direction. Among the Cotswold valleys, springs have given rise to landslips (Mr., Witchdll, Quart. Journ. Geol. Soc., vol. xx., Nov., 1864); but these landslips can never tend to increase, and far less to originate, the continuous and smooth regularity of a line of escarpment.Google Scholar

1 Sometimes the re-deposited Chalk-rubble becomes compact enough to to be mistaken for Chalk itself. See Whitaker's, W. Mr. paper in Quart. Journ. Geol. Soc., vol. xxi., 1865.Google Scholar

2 See Sir Murchision's, R. I. elaborate paper, “On the Drifts of the South-East of England,” in Quart. Journ. Geol. Soc., vol. vii., 1851.Google Scholar

1 I think it is difficult to conceive of an explanation more perfect, simple, harmonious, and convincing than Sir Charles Lyell's marine theory of the denudation of The Weald. That the same cannot be said of the atmospheric theory will, I think, appear from the following extracts:— “The rate at which the escarpment will be worn back will depend on the rate at which the river deepens its valley. ⃜ The excavating power of the, stream in the longitudinal valley will depend on that of the transverse valley, and if the sea-level remains constant, the transverse stream will go on deepening its bed and lessening its excavating power, until at last it ceases to have any at all. A slight elevation of the land would once more give the transverse stream an excavating power, which in time would be communicated to the longitudinal streams.⃜. It must not be inferred, however, that we consider escarpments to be river-clifs. The longitudinal streams, though running parallel to these escarpments, do not run immediately below them, but often, as with the Medway itself, at a considerable distance. No river-gravel in this area is ever found on the face of the escarpment, nor can we discover thereon any traces whatever of river-action” (Foster, and Topley, , Quart. Journ. Geol. Soc., vol. xxi., No. 84, 1865, pp. 471, 473). I do not think that Dr. Foster and Mr. Topley have succeeded in showing that the gravel- terraces in the basin of the Medway (containing Tertiary pebbles and Greyweather fragments, and dipping lower than the inclination of the river) are of fluviatile origin, or that a vertical extent of 300 feet has been sconed out by rain and rivers, but admitting a considerable amount of denudation alo the courses of rivers, the bays and combes of the great Chalk escarpments would still form a distinct class of phenomena, and require a separate explanation.Google Scholar