[ 279 ]

a. Local debris. b. . c. Blown sand-hills. d. Silurian rocks.

XXIII.—Description of a Raised Beach in or Bay, on the North-West Coast of Devonshire,

By THE REV. PROFESSOR SEDGWICK, V.P.R.S., F.G.S., AND RODERICK IMPEY MURCHISON, ESQ., F.R.S., F.G.S.

[Read December 14, 1836.]

DURING a recent examination of the county of , undertaken in the hopes of ascertaining the geological succession of the more ancient rocks of which it is composed, we were unexpectedly gratified by the sight of the finest raised beach which has come within our observation. It extends, at intervals, along the shore and cliffs, for a distance of about three miles from the northern side of the mouth of the Taw, or Barnstaple river, to the bold headland called . After toiling through the hillocks of blown sand which encum­ ber the mouth of this estuary, we were turning the rocky headland of Down, when, to our great surprise, a succession of yellow-coloured, low cliffs broke upon our view, running close along the shore, and rising to a height of about 50 feet, as an undercliff, upon the sides of the ancient rocks which form this part of the coast. It was evident (even at the considerable distance from which we first saw it) that this undercliff was composed of horizontal layers; 280 Prof. SEDGWICK and Mr. MURCHISON and as, in colour and aspect, it differed entirely from any rocks by which we were surrounded, we first imagined it to be an outlying mass of oolite or green sand; nor were we undeceived, until we actually descended to the shore, so precisely did it resemble the sea-worn cliffs of many of our younger secondary formations. We then found that the top of this undercliff was occupied, to a depth of eight or ten feet, by a mixed detritus of angular frag­ ments of the adjacent rocks, contained in a matrix of sandy loam. Beneath this drift, the yellow cliffs, which had attracted our attention, consisted, at their upper part, of finely laminated sand, with here and there an imbedded sea-shell, such as a common cardium or oyster, but generally in too fragile a condition to bear extraction. These beds of sand are about 25 feet thick ; and, as they pass downwards, they become harder, and are occasionally inter­ rupted by a course of small water-worn pebbles, perfectly identical with com­ mon beach shingles. Though usually arranged in horizontal laminae, these sands presented, at intervals, those appearances of false bedding so common in our tertiary and secondary strata (see vignette), putting on the form of wedges, the laminae of which run diagonally across the principal layers of the deposit. In descending to the shore, we found the lower part of the cliff passing into solid , and, finally, into a hard bed of shingle, or con­ glomerate, resting upon the highly inclined and broken edges of rocks, which we refer to the " Silurian System/' This portion of the cliff, consisting of hard sandstone and associated shingle beds, contains several marine shells, of existing species, among which are Cardium edule, Patella vulgaris, Mytilus edulis} Solen, Donax trunculus}.* On examining this portion (the lowest part being generally about three feet above high-water mark, at spring tides), we were forcibly struck with the analogy which the strata presented to many rocks formed before the pre­ sent aera. The grains of sands are agglutinated into a compact mass, which, on fracture, exhibits a shining, chatoyant lustre, common to calcareous grits of the secondary rocks, with an occasional tendency to run into concretionary forms; and these regularly bedded, stony masses, pass into grit and shingle on the one hand, and into soft, incoherent sand, on the other. The maximum thickness of these lower consolidated strata, at the point where we first met with them, is about 11 feet, of which the upper seven feet are sandstone, and

* We believe that all the shells in this raised beach belong to existing species. We should not, however, have been surprised had it contained some organic spoils resembling those of a warmer climate: for seed vessels of Mimosae, inhabitants of the Mexican Gulf and the West Indies, are annually washed on these shores, by currents probably connected with the great Gulf stream. We met with several examples of this kind. on the Raised Beach in Barnstaple or Bideford Bay. 281

the remaining- four coarse and hard shingle. Occasionally, however, a small band of fine shingle, or a pebble of considerable size, may be met with in the upper portion. There is no exaggeration in stating, that much of this modern deposit is quite as difficult to break as the ancient Silurian rock on which it rests. By its texture, it will resist the ordinary action of weather; but the surge, in vio­ lent storms, beating against the lower portions of it, has worn them into caverns (see vignette), exposing, in their interior, the edges of the shelly beds; whilst over them is seen the successive beds of sandstone and sand, above noticed. At the point where these appearances are presented in greatest perfection, the total thickness of the beach amounted to about 36 feet; which, with the overlying drift, and the inferior ledge of Silurian rocks, gave a heiglit of 40 or 50 feet. By reference to the vignette, it will be perceived that the bottom of the beach accommodates itself to all the irregularities of the subja­ cent rock, just as sand and gravel are arranged upon the broken edges of the chalk in the south-eastern parts of England. The deposit we have been describing, is continuous from Saunton Burrows, where it disappears under the blown sand, to the south end of Bay, on approaching" which the cliff gradually subsides. Throughout the area of that sandy bay, the rocky and high ground recedes from the shore, and there are no traces of the beach for nearly a mile; but on the northern -side of the bay it is again met with near some lime kilns, where the lower shingle beds are perhaps fuller of shells than in any other part of their course. Here the coarse shingles, in parts expanded to the thickness of 19 feet, rise rapidly to the north, from a point three or four feet above the sea-level, to a height of 60 or 70 feet above high-water mark, resting on the rugged and abrupt face of Baggy Point—one of the boldest and loftiest headlands on this coast. It is worthy of remark, that it is near the point of greatest elevation, that the materials composing the beach are most coarse and brecciated, and in the greatest quantity. Such is a brief sketch of the nature and arrangement of this deposit, the position of which naturally leads us to offer a few remarks on the causes by which it has been produced. To this end, we shall endeavour to show, 1st, that the configuration of the neighbouring coast harmonizes with the supposition that this beach has been raised (within the modern sera) from beneath the sea to the height at which we now find it; 2dly, that similar phenomena, on the coasts of Devonshire and Cornwall, and in other parts of England, agree with and confirm this hypothesis. 282 Prof. SEDGWICK and Mr. MURCHISON

1. Phenomena on the Coast and in the Neighbourhood. We have stated that the southern end of the raised beach is lost under the blown sand of Saunton. This blown sand occupies a wide area of undulating hillocks (partially covered with bent-grass), which choak the northern side of the bay; the united rivers, Taw and Torridge, finding their course through them to the sea over bars of sand, which render the access to the ports of Bideford and Barnstaple very difficult. On the southern side of the bay these sand-hills disappear; but, between the town of Appledore and the sea-shore, is a very remarkable and elevated straight ledge of rounded bowlders, called the te Popple/' or pebble bank; which, although quite analogous to the well- known " Chisel Bank," has not been noticed in the map of the Ordnance Survey. It rises to a height of about five feet above the high-water mark of spring tides, is about one mile and a quarter long, and about 15 to 20 paces wide upon its summit, with shelving slopes on each side, giving it a broad and firm base. It rises from 10 to 12 feet above the sands. The large rolled and rounded blocks, mixed with smaller gravel and pebbles, consist exclusively of a hard, grey and slightly mottled sandstone, of which there is avast abundance on the reefs and headlands between this spot and CloveUy, and which, with the other strata around Bideford, we consider to belong to rocks of the age of the coal measures. This pebble bank, stretching from Rocks Nose northward towards the mouth of the estuary, shelters the marsh land behind it from the elements ; and hence it is, that a broad and low alluvial flat, interposed between it and the town of Appledore, is covered with grass, and exempted from the de­ structive sweep of the blown sand, which has overspread the opposite side of the bay. Although it at first seemed difficult to account for the origin of this remarkable ridge of gravel, it appeared to be explained by the raised beach on the northern side of the bay : for we could not avoid inferring, that this pebble bank was one of the indications of a considerable rise of the land. In a former condition of things, when the neighbouring coast was at a lower level, the materials of the present shingle bank might have existed as a natural marine accumulation, in the form of a reef; or they may have gradually assumed their present form, while the sea and land were changing their rela­ tive level. Such a change would inevitably bring successive parts of the coast within the destructive operations of the surf, and naturally produce accumulations of water-worn materials. At all events, the " Popple Bank" appears to us not accounted for by the actual configuration of the coast and the ordinary action of the sea. on the Raised Beach in Barnstaple or Bideford Bay. 283

In addition to what has been said, we may remark, that the interior of the country near Barnstaple, has the configuration which we might expect on our hypothesis: for we may follow, both on the north and south side of the low tract near the mouth of the Taw, ridges of hills extending far into the inte­ rior, and having the exact form of an ancient coast-line. 2. Similar Phenomena on the Coasts of Devonshire and Cornwall. Were the appearances above stated without any parallel on the English coast, they would be comparatively of small interest. In fact, however, they form but one of a long series of phenomena, all tending to the same con­ clusion, viz. the changes of sea level during the modern period. We have no reason to look for frequent examples of raised beaches on the eastern shore of England: for, supposing them to have once existed, most of them would have been long since swept away by the encroachments of the sea line. But on the iron-bound coasts of Devonshire and Cornwall, this line is nearly stationary; and, where the forms of the land have favoured the manifestations of changes of sea-level, raised beaches may be traced in so many places, as to form an almost connected series of phenomena. The raised beach of Hope's Nose has already been described by Mr. Austen; and after an exami­ nation of the spot, we think his description exact, and his conclusions true*. We there find shingles and shell beds alternating in thin layers, utterly unlike any thing drifted by wind, or placed in their present position by the hand of man, and admitting of no rational explanation, except that which we have given of the similar beach in Barnstaple Bay. The Rev. Mr. Hennah long since pointed out to one of us, a shingle beach at Plymouth. Similar phenomena were observed by us, in 1828, on both sides of the western extremity of Cornwall: and in the elaborate work of Dr. Boasef; not only are numerous examples cited, of horizontal deposits of shingle and shelly marine sands at various levels above high-Water, but the true cause is assigned for some of them—a change of sea level. Unfortunately, however, his descriptions are obscured by language derived from the then prevalent diluvian theory; and on that account do not produce their deserved im­ pression. The accurate survey of Cornwall, by Mr. de la Beche, cannot fail to give us all the information we want on this interesting subject; and we may expect from him a detailed account of all the phenomena of elevation and depres­ sion (for there are proofs of both) exhibited on that coast. We may, how*

* Proc. Geolog. Soc, vol. ii. p. 102. f Trans. Roy.Geol. Soc. of Cornwall, vol. iv. pp. 259, 270, 273, 320, 466, VOL. V.—SECOND SERIES. 2 P 284 Prof. SEPGWICR and Mr. MVRCHJSQN

ever, state generally, that the Cornish raised beaches, are not all at one level, nor of one character; and though they are all of the modern aera, they do not belong to one epoch of elevation. They may be divided into 1st. High shingle beaches, resembling the large rounded blocks so con­ stantly seen at the highest water level on all rocky shores. 2dly. Mid water beaches, composed of rounded pebbles, broken shells, &c. exactly resembling materials washed up between high, and low water mark. These accumulations are seldom regularly bedded, but have more or less of a confused aspect. 3dly. Low-water beaches. These have originally extended from a few feet above low water, to an indefinite depth; and, consequently, such masses are regularly stratified, being made up of small gravel, alternating with sand, single shells, and beds of shells, all similar to living species of the present coast. Such appears to us a natural division of these beach deposits; and the Cor­ nish coast offers fine examples of each class. The phenomena, described by Mr. Carne, near the Land's End* (viz. large rounded blocks, 30 or 40 feet high, in the successive bays and headlands, and nearly all at the same level), were observed by one of the authors in 1828 ; but, misled by the diluvian theory, he then misinterpreted their meaning. But we can now perfectly understand how these high-water shingles should appear by themselves, the other parts of the beach having been washed away. The shingle beach at Plymouth may be mentioned as an example of the second class. But the third class is incomparably the most interesting; be­ cause it conducts us to the old water-worn rocks, once washed by the sea, and forming the actual ledge on which these low-water beaches rest; and also because, having been partially deposited under the sea, it is charged with abundance of marine spoils, and sometimes with regular beds of sea-shells. Of course, the shells, as in beaches of the present day, were for the most part dead, when entangled in the layers of sand; but in some cases, we have a proof, that the animals were not dead at the time they were first entangled in the deposit where we find them • though now raised many feet above the highest tidal level. Such examples may be seen north of St. Ives Bay, and south of New Quay, and at several other parts of the north coast of Cornwall f. Now in most cases of raised beaches, we may infer, a priori, that the xioast was shelving and shal­ low before their elevation ; for such beaches could adhere but imperfectly to an abrupt shore; and if adherent for a short time, would soonjbe washed away. Hence it is, that where we find raised beaches, we may look for extensive * Trans. Roy, Geol. Soc. of Cornwall, vol. iii. p. 229, et seq. + Ibid., vol. iv; p. 259. on the Raised Beach in Barnstaple or Bideford Bay. £85

low-water sands. The two sets of phenomena generally occur together, so that one becomes an indication of the other. On the coast of Cornwall, wherever we meet with low-water sands of any extent, they are drifted by the fierce western gales over all the neighbouring cliffs, and, in course of time, have, in some places, converted whole parishes into deserts. After what has been said, we need not be surprised that these drifted sands should chiefly occur along those parts of the coast, where raised beaches most prevail: in short, the two things are generally found together* This fact misled some of the early writers in the Transactions of the Royal Geological Society of Cornwall*, who attributed the raised beaches themselves to the blown sand, under which they, as well as the older rocks, were partially buried—a very great mistake, though quite natural at the time when it was made. Dr. Boase, we believe, had the merit of first pointing out this error f. The conclusions from these facts are obvious, and in perfect harmony with our observations on the beach and shingle of Barnstaple Bay; both as respects their structure and their occurrence in the mouth and seaward flanks of an estuary, so much covered by blown sands. The elevation of the northern side of Barnstaple Bay, makes it almost cer­ tain that the south side must also have been affected by a similar movement ; and in our reasoning respecting the (< Popple-Bank," and the configuration of the neighbouring country, we have assumed that both sides have been raised above a former level, by a similar movement. But,, at the time of our rapid passage along the clififs south of Rocks Nose, we were not aware of the existence of any phenomena on the northern shore of the bay, or, indeed, on any portion of the east coast of Devon, such as we have now described; and hence we omitted to look out for any direct proofs of elevation. Again ; the great elevation of the coast line, extending to Baggy Point, could hardly have taken place without simultaneously affecting the portions of North Devon and West Somerset, which form the southern shore of the . There, however, we found no traces of any raised beaches, though our atten­ tion had been previously drawn to them. Their absence is, however, easily accounted for, by the very abrupt form of these bold cliffs, and the deep seas by which they are washed ; which are so adverse to the formation, and permanent elevation, of extensive shelly and sandy deposits, like those above described. The raised beaches to which we have alluded, in South Devon and Corn­ wall, have been stated to indicate an elevation of the land, varying from 10 to 40 feet; but our case, in Barnstaple Bay, gives a maximum rise of 60 or 70 feet, indicating a greater intensity of elevatory movement as we proceed to the north. If we ascend the estuary of the Severn, and follow that river up

* Trans. Roy. Geo]. Society of Cornwall, vol. u p. 4. f Ibid., vol, iv. p. 260. 286 Prof. SEDGWICK and Mr. MURCHISON on the Raised Beach, $c.

to the highest navigable points in Shropshire, we find (as has been shown by one of the authors*) that gravel and sand, with sea-shells of existing species, occur in many places, at elevations of from 300 to 600 feet above the sea j and still further to the north, the elevation of similar deposits is maintained through Lancashire, till they approach the mountains constituting the southern fringe of the Cumbrian chain. Hence we have no hesitation in admitting, that in a portion of our island which ranges from Cornwall and Devon to the southern limits of the Cumbrian chain, there have been great movements of elevation subsequent to the creation of existing mollusca; these movements apparently increasing in intensity as we proceed from south to north, or rather from south-west to north-east. No doubt there are many portions of the coast of our island, where we not merely have no proofs of any change in the relative level of land and sea, produced by elevation during the modern period, but where we appear to have direct evidence of a partial depression of the land. It is not, however, our in­ tention to enter any further on these subjects. We have described a local phe­ nomenon of some interest, and endeavoured to show its connexion with similar phenomena on a more extended scale. But any consideration of the effects produced by such movements as we have described, on the general configura­ tion of our island, or on the drifted materials which are accumulated on its sur­ face, would lead us into details quite beyond the purpose of this short memoir.

* Proceedings Geol. Soc, vol. ii. p. 383.

d Sectional view of the Raised Beach.—a, Sand Banks. b. Raised Beach. d. Silurian Rock.