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7 May 1369.

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THE

GEOLOGICAL MAGAZINE.

GEOLOGICAL

No. LXIV.-OCTOBER, 1869.

ORIGINAL ARTICLES.

I. ON THE FORMATION OF THE CHESIL BANK, Dorset. BY HENRY WILLIAM BRISTOW, F.R.S., F.G.S., and WILLIAM WHITAKER, B.A. (Lond.), F.G.S., both of the Geological Survey of England. [PLATES XIV. & XV.] ·

[A Paper read before the Geological Society of London, May 26, 1869.]

WE

"Aloft where Chesill, lifts

Her ridged snake-like sands, in wrecks and smouldring drifts,
Which, by the South-wind raysd, are heav'd on little hills:
Whose valleys with his flowes when foming Neptune fills,
Vpon a thousand swannes the naked Sea-Nymphes ride
Within the ouzie Pooles, replenisht euery Tide:

Which running on, the Isle of Portland pointeth out."

Poly-olbion, by Michael Drayton, Esq., Fol., Lond., 1613, p. 24.1

E do not propose to enter into the questions of the source whence the pebbles of the Chesil Beach are derived, nor of the way in which they are heaped up; these and other like matters having been almost exhaustively treated by Mr. J. Coode, to whose paper we refer the reader for a detailed account of the bank. The subject with which we propose to deal is simply the cause of the formation of a long shingle-bank, separated from the mainland by a strip of water. This has not been noticed at length by any writer, as far as we know, though three theories of the origin of the bank have been brought forward.

Sir Henry De la Beche is the author of one of these. He says: "It (the Chesil bank) protects land which has evidently never been exposed to the destructive fury of the Atlantic swell and seas, which break with fury against the bank; for the land behind is composed of soft and easily disintegrated strata, which would speedily give way before such a power. Perhaps a gradual sinking of the land

Judging from the quaint map that accompanies this description, the Chesil bank must have been much the same in Drayton's time as now. However, all the nymphs that we saw were more or less clothed. The general appearance of the coast in question is shown in Plate XV., which is from a sketch made by Mr. Bristow twenty years ago; being a view of the Chesil Bank and the "Fleet" or "Backwater," looking westward from near Fleet House.

2 Proc. Inst. Civ. Eng., vol. xii., p. 520 (1853).

VOL. VI.-NO. LXIV.

28

might produce the present appearances; for though the sea would have attacked the land when the relative levels were different, the form of the bay, and the projection of the Isle of Portland, would soon cause a beach to be formed, which would rise as the land sunk, so that finally no traces of a back cliff could be observed. Under this hypothesis Portland would not have formed an island, but merely the projecting point of a bay, which, with its exposure, would soon have accumulated the beach required. It may be remarked that this supposed gradual sinking of the land is in accordance with appearances more westward on the same coast, where the facts presented seem to require this explanation."1

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Sir C. Lyell is the author of another theory. He says; "The formation of this bar may probably be ascribed, like that of Hurst Castle, to a meeting of tides, or to a great eddy between the peninsula and the land. We may expect the slightest impediment in the course of that tidal wave, which is sweeping away annually large tracts of our coast, to give rise to banks of sand and shingle many miles in length, if the transported materials be intercepted in their passage, In later editions this is repeated, with the addition or to a submarine shoal or reef between the peninsula and the land," in which form it is supported by Mr. Coode, according to whom "the isolation of the bank is due to the existence of a level, or nearly level, bench of clay, upon which the shingle is thrown and rests, as upon a shelf." In Sir C. Lyell's last work the theory is thus stated: That part of the bar which attaches Portland to the mainland rests on Kimmeridge Clay, which is sometimes exposed to view during storms. The clay may have formed a shoal, and the set of the tides in the narrow channel may have arrested the course of the pebbles, which are always coming from the west."

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Colonel G. Greenwood, however, accounts for the occurrence of the Chesil bank on the supposition that, in consequence of a rise of the land, the sea would have a shallower shore, against which it could throw up the pebbles. His words are as follows: "Raised beaches exist along our south coast, and I think that it is the shallows caused by the rising of the land that has [have] allowed the accumulation of double beaches between Giens [East of Toulon] and the land, as well as between Portland and the land (Chesil beach and Smallmouth sands). .. Nature has no sooner divided the island from the continent than, by hoisting up the land, she sets the same workman, the sea, whom she first employed to sever them [it] from the land to join them [it] to the land again."5

These three theories may be described as the sinking theory; the standing still theory (by implication, as its advocates say nothing of rise or fall), and the rising theory; each of those conditions of the Geological Manual, Ed. 3, 8vo. Lond., 1833, p. 80. The theory is repeated in the author's later work, the Geological Observer, 8vo. Lond. (1851), p. 65, and Ed. 2 (1853), p. 56.

Principles of Geology, vol. i. p. 281 (1830), 1st edition.

3 Proc. Inst. Civ. Eng., vol. xii. p. 542 (1853).

Principles of Geology, Ed. 10, vol. i. p. 534 (1867).

• Rain and Rivers, Ed. 2, 8vo. Lond. (1866), pp. 119, 132.

shore having been invoked in turn as best explaining the origin of the bank. Our own theory will, we believe, suit all conditions.

There is one point on which all must agree; it is that the bank could not have been formed without the huge natural groyne, or breakwater, of the Isle of Portland, which bounds it on the east, and stops the shingle in its easterly course; but beyond this we venture to differ from the explanations that have been given to account for the presence of the shingle in so anomalous a position.

The above theories rest on the supposition that the form of the neighbouring land, at the time of the formation of the bank, was much the same as now; and although the theory of Sir H. De la Beche would seem to allow that the beach might have been formed against land, and separated merely by the sinking of the land (in this case the shingle ought surely to have been driven back as the land sunk), yet the other theories imply that the bank was originally formed as a detached mass, separated from the land as now by a narrow channel of water, unlike other long tracts of shingle, which are formed against the land, and which, travel as they may, touch the land. On the other hand, the theory that we suggest needs no such supposition, but starts with the reasonable assumption that the Chesil Bank may have been formed at first in the same way as the ordinary shingle-beaches of our coast, and that what was once an ordinary beach, banked up against the land, has been since separated, as a bank or bar, by the denudation of the land behind it, such denudation having taken place in a way that would hinder the backward motion of the shingle, and would leave a narrow channel (the present Fleet) between the bank and the land.

In order to make our theory more easily understood, it will be well to give a short description of the Chesil Bank. In doing this, we shall avail ourselves of Mr. Coode's account, which has made needless any measurements on our part; at the same time we ought to state that both of us can speak from personal knowledge of the coast and of the bank, the first named of us having done the Geological Survey mapping of that district, while the other spent great part of a summer holiday in an examination of the Dorsetshire

coast.

The Chesil Bank (including under that name the whole of the continuous strip of shingle from Burton Bradstock to Portland) is the largest accumulation of shingle in this country, and more than fifteen miles long. On the N.W., for five or six miles, it touches the shore, but on the S.E., from Abbotsbury, it is divided from the main

1 We are aware that at the mouths of many rivers bars of shingle stretch a long way across from one side, sometimes indeed to such an extent as to turn the rivers along the shore (between the land and the shingle), in the direction of the prevailing set of the currents, for some distance. But these are not really analogous to the Chesil Bank, where the shingle-beach is far longer, and where there is no river emptying into the sea, but only a succession of very small streams. There are also cases of shingle-banks completely damming up streams, and with a marsh or expanse of fresh-water on the land side, as at Slapton Sands, South Devon, and Cuckmere, in Sussex.

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