late Holocene basal neritic deposits of the broad bay between The Lizard and. Start Point are ..... crabs; shelly material was found in all stages of disintegration.
J. mar.
433
bioi. Ass. U.K. (1967) 47, 433-444
Printed in Great Britain
BASAL SAND AND GRAVEL PATCHES WITH SEPARATE INDICATIONS OF TIDAL CURRENT AND STORM-WAVE PATHS, NEAR PLYMOUTH By N. C.
FLEMMING AND
A. H.
STRIDE
National Institute of Oceanography, Wormley
(Text-figs. 1-8) Diving observations and more wide-spread coverage by Asdic show that the late Holocene basal neritic deposits of the broad bay between The Lizard and Start Point are represented by three main facies whose character depends on the two bed-transport paths converging there. The sheet sands laid at the end of the path with a good supply show a progressive geographical change in texture. On the other path, where sand is in short supply, there are elongated patches of somewhat coarser, well-sorted sand with' silt' balls, alternating with shell-rich gravel patches. While the sand patches are laid out along the path of the strongest tidal currents, the ripple marks of the gravel can only be constructed by long waves from the Atlantic. INTRODUCTION
Samples of the sea floor south of Plymouth were taken for zoological study as early as 1898 (Allen, 1899), but not until 1953 was the variability of the sediments really appreciated. Holme (1953) indicated that while the sediments east and south of Eddystone were almost uniform over a large area, those between Eddystone and the northern shore of the bay were made of relatively small patches of sand and gravel. Examination of a larger area of floor by means of Asdic equipment confirmed the uniformity of the one ground and the remarkable patchiness of the other ground, provided actual boundaries, and showed that the patches of sand were elongated parallel with the path of the strongest tidal currents and so were generated by them (Stride, 1959, 1963). This work and the later use of Asdic (Fig. I) showed that the sand patches are limited to the western and northern parts ofthe bay, beyond which (Fig. 2) there is first an acoustically uniform zone, where sampling by Holme (1953) showed the deposits to be of almost uniform grade, and finally a zone of sand waves, where the tidal currents reach about I knots at the sea surface (Sager, 1961). Available tidal current data suggest that in the western and northern parts of the bay there is an easterly bed-transport path extending along the trend of the patchy sediments (Fig. 3), while the southern and eastern parts of the
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The courses followed by R.R.S. 'Discovery II' and R.R.S. 'Discovery' in the vicinity of Plymouth while using Asdic (1958-66).
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Fig. 2. The location of the zone with sand and gravel patches and other late Holocene facies in the vicinity of Plymouth. Rock outcrops are not shown.
Fig. 3. Bed-transport paths in the vicinity of Plymouth with the approximate limits of the bed-load convergence.
BASAL SAND AND GRAVEL PATCHES
435
bay are affected by the west-going sand stream of the western half of the English Channel. Between the two paths there is an area of bed-load convergence, occupying much of the middle of the bay, typified by symmetrical sand waves (Fig. 2, S) and deposits which are almost uniform and may be thicker than elsewhere in the bay. The purpose of the present study was to provide further data about the bed-transport pattern and especially to obtain more information about the character of the linear sand patches and the ground between them, brief mention of which has been made already (Flemming, 1965 a). The sand patches are a depositional form, which has been differentiated from the sand ribbons (S.S.) of high-velocity zones (Belderson & Stride, 1966). While both types of sand body are elongated parallel to the dominant tidal current, the ribbons are often 40 times as long as they are wide, while the patches have a much smaller ratio. Any attempt to fix a rigid limit distinguishing the two forms by proportions alone would be arbitrary in the present state of knowledge. METHODS OF STUDY The floor was examined by a team of divers led by one author (N. C. F.) during April and September 1964, using diving techniques which have been described elsewhere (Flemming, 1965b). Such an approach allowed observations to be related unambiguously to the sediment patches to a degree that is rarely possible from a ship at the sea surface. The divers determined the nature and orientation of known sediment boundaries, took samples by hand, trowel and corer, and studied the surface form of the sediments (Fig. 4). Their operational methods are given below. In the two widely spaced areas' B' and' c' (Figs. 4-7) a bottom line of light nylon was laid between two buoys at right angles to the trend of the sand patches, previouslyrevealed by means of Asdic. A diver or pair of divers descended one buoy rope and followed the guide line to the other buoy either by swimming or by means of an electric tug. Sketches and measurements were made during the traverse, and samples taken. One buoy was then lifted and swung round the other so that the traverse could be continued on the next dive without missing any ground. Single traverses varied in length from 140 yards (swimming) to 400 yards (with electric tug) in depths of 150-17° ft. The location of the buoys was determined by horizontal sextant angles between points on land at the beginning and end of each traverse. In order to see the regional significance of these observations samples were also taken nearer to the coast and from the beaches at Chapel Point, Mevagissey, Pentewan, Charlestown, Lantic Bay, Polperro, Looe, Portrinkle and Crafthole. In the analysis use has also been made of sample data for the eastern part of the bay (Holme, 1953).
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