Aug. X896, 27. on SUB~ROED LAND
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Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on July 15, 2016 474 suB-~Re~.n T.A~D-SVRFACESA~ ~ARR~. [Aug. X896, 27. On SUB~ROED LAND-SVRFACES at BARRY, G~AMORGANSHIRE. By A. STRA~, Esq., M.A., F.G.S. ~th NOTES On the FAV~A and Fr.ORA by CLEmEnT REIn, Esq., F.L.S., F.G.S. ; and an APPE~vix on the MicRozoA by Prof. T. R. Jo~s, F.R.S., and F. C~AP~AN, Esq., A.L.S., F.R.~I.S. (Communicated by per- mission of the Director-General of H.M. Geological Survey. Read March 25th, 1896.) BA~Rr Is~.a~'v lies off the northern coast of the Bristol Channel, about 7 miles south-west of Cardiff. Until the year 1884 it was separated from the mainland by ~he tidal estuary of the Cadoxton River on its northern side, and on the east by a tract covered at high water, but in which the solid rock cropped up through the tidal ~ deposits at frequent intervals, the most conspicuous prominences being known as the Coston, Mark, Bendrick, and Black Rocks. In the absence of evidence to the contrary we may assume that the course followed by the river at this time was its original course, for it falls into the general south-westerly direction of the rest of the valley. The numerous outcrops of rock, moreover, in the tidal area east of the island make it unlikely that there was an outlet here of sufficient depth to drain the marshes behind it. This tidal area seems to have been one of three low co/s in the water-parting on the southern side of the Cadoxton Valley. One of these would separate the western par~, or the Litble Island, from the main part~ of Barry Island were it not for a ridge of blown sand; another occurs east of Hayes Farm, where, as shown by the Ordnance level 19 given on the map facing this page, the alluvial marsh of the Cadoxton River closely approaches the present coast. The third, presumably a trifle lower than either of the others, was submerged during the subsidence of the land, of which proof will be given in the following paper. It was therefore as a direct consequence of this subsidence that Barry was separated from Hayes Farm and became an island, while a slight increase in the extent of the movement would have given us three islands instead of one as the result of the submer- gence of the old water-parting. As to the da~e of the insulation of BarD', it will be seen subse- quently that it had not taken place before Neolithic times ; on the other hand the island is, I believe, referred to as such in the earliest historical records. In 1884 the Barry Docks were commenced. The river was diverted from its ancient course and carried to the sea by an artificial cut east of the gap referred to. Its former valley on the northern side of the island was partly filled up and partly excavated to form a deck, the entrances to which were made in the tidal area east of the island, the sea being excluded by a wall. Subsequently a second dock (shown on the map as 'Barry Docks Extension ') was commenced in this reclaimed area, and it was the excavation made Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on July 15, 2016 ,d, %, ": ,..:- "'"~ ~ f~176176176~ - :..~. ": i ~~ ~ ........... o. : ..... ., | li~ "..,........~,~:.Z,'.:........ "~. i ".. "... d'.. :~ i.~ ...... t ".. "..W,o"..:: 9-,,_ ~\~, ~:. ;',.,,,,,"t ..........ii~/ N'f~ I ....... .~..,~....... !o t~,. -*"~"~I -2.,,,,,,,,~, ..... ~-~" .~,..~ Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on July 15, 2016 476 xR. A. 8TRAHAN ON SUBMERGED LAND-BURFACI~IS [Aug. I896, for this purpose that I had the opportunity of examining in the course of my duties on the Geological Survey in 1895. The history of the invasion of the river-valley by the sea through the gap mentioned clearly reveals itself in the sequence of deposits ; but, morethan this, the river-alluvium presents a series of freshwater beds and land-surfaces, whose position in relation to the present sea-level proves that a great change in the level of the land took place during and after Neolithic: times. On some of my visits to the Dock I had the advantage of the assistance of Mr. F.T. Howard, F.G[8., and of Mr. J. Storrie, to the~ latter of whom I am indebted ibr much information respecting the sections exposed in the earlier dock, and also for the identification of some of the specimens from the present excavation. To my colleague Mr. Clement Reid, however, I am indebted for a thorougk examination of the whole of my specimens, and for the identification of both plants and shells, together with critical remarks thereon.. Prof. Rupert Jones, F.R.S., has kindly furnished me with the list of ostracoda, etc., given in the Appendix. The natural topography of the tract is preserved on the 6-inck Ordnance maps, Glamorgan L. and LI. (published 1885, surveyed in 1878), from which the accompanying map (p. 475) has been prepared. The high-water mark of ordinary tides being shown on these maps. in the usual manner, we are able to see that the whole of the new dock is being excavated in ground that was covered at high tide, : though it pretty closely approaches the eastern shore at a point known as Warren Tump (now levelled). The extreme southern point of the dock lies near what was formerly a Shoal known as Coston Rock. In both these parts the excavation is principally in the solid rock, but elsewhere it generally fails to reach the bottom of the alluvial deposits. These may be enumerated as follows :--- 1. Blown Sand. 1 2. ~brob/cu/ar~z,clay. ~ Recent subaerial and tidal deposits. 3. Sand and gravel. Strong line of erosion. 4. Blue silt, with many sedges. 5. The Upper Peat Bed ; about 4 feet below Ordnance-datum. 6. Blue silty clay, with many sedges. 7. The Second Peat, a thin layer only. 8. Blue silty clay, as no. 6. 9. The Third Peat, many large legs and stools, and roots in place underneath; about 20 feet below Ordnance-datum. 10. Blue silty clay, with reeds, willow-leaves, and freshwater shells. 11. The Fourth Peat, with large trees and roots in place. Land-shella numerous. 12. An old soil with roots and land-shells ; about 35 feet below Ordnance- datum. Rock in place. By the kindness of Mr. James Bell, En~neer to the Barry Dock Company, I have been furnished with the following levels :-- Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on July 15, 2016 Vol. 5~.] A~ ~RRI, aT.~OR~S~IS~. 477 Above Below Ordnance- Ord~ance~ datum. datum. feet. feet. Quay-level, coping round Dock, Basin and Deep Lock ""l 26"50 High-water ordinary spring tides ............................. I 19"70 High-water ordinary neap tides ................................. 1 11"30 Mean sea-level at Barry .......................................... 1 1"60 Low-water ordinary neap tides ................................ :1 8"3 Low-water ordinary spring tides ................................. I 16"4 Bottom of Dock ...................................................... I 20"0 1. The blown sand occurs only on the eastern side of the tidal area, whence it was evidently drifted by the south-west winds. The supply was never great, and is now of course entirely cut off. 2. The Scrobicularia-clay occurs in considerable force towards the north-eastern end of the excavation. It is a stiff, brownish clay, jointing vertically as it dries. It contains an abundance of ~crobicularla plans, with the valves united, and in this and other respec~;s resembies the mud which is now being deposited in the more sheltered part~ of the Bristol Channel In the present case it found no rest on the shoals east of the island, but accumulated in some force from near Warren Tump upwards along theriver-valley, as indicated on the map. The greatest thickness seen in the dock was 9 feet. 3. This sand and shingle forms the base of the Scrobicularia-clay, into which it passes insensibly upward. It ranges from 2 to 8 feet in thickness~ according to local circumstances, and is full everywhere of recent shore-shells, all more or less:rolled. Among these the following species have been identified by Mr. Clement Reid:--Scrob~laria Tlana , Tellina balthica (the thin-shelled estuarine form), Gardium edule, Patella vulgata, I, ittorina littorea, L. rudis, and L. obtusata, Southward tMs sandy and gravelly tidal wash extends in a patchy manner among the rocky shoals towards the present foreshore, where it is still in course of formation. The deposits described above were all in process of being laid down, until the area was taken in hand by the Barry Dock Company in 1884. They rest upon a conspicuously eroded surface of the strata about to be described, with which also they contrast :strongly in their contents and character. The erosion is attributable to the scour of the tide when the sea first gained access to the estuary round the eastern end of the island. The series of deposits upon the description of which we n~w :enter forms a continuous sequence from top to bottom. It may gene- rally be described as a mass of fine clayey silt with abundant remains of sedges in the position of growth throughout, and with four or more bands of peat, only three, of whleh, however, are of any importance.