The Mya arenaria Bed of the RedCrag of Essex

by JOHN McMANUS

Received 8 February 1963; taken as read 3 April 1964

ABSTRACT: A quiet water deposit containing a life assemblage of Mya arenaria isdescribed from four localities. This is a distinctive facies in the Red Crag of Essex, andmay be an important stratigraphical marker horizon.

WHITAKER (1877) gave the first comprehensive account of the Red Cragin Essex. He recorded exposures at Walton-on-Naze, Beaumont, Oakleyand Dovercourt. No detailed faunal lists at these exposures were given byWhitaker (1877), but Harmer (1900) published accounts of the faunas forexposures near Foulton Hall, Little Oakley (225292)1 and for the long­since destroyed outlier at Harwich, from figures by Dale (1732).

Harmer (1899) considered that the Red Crag of Oakley constituted adivision quite separate from that of Walton. Within his Oakley Horizon,typified by Mactra obtruncata S. Wood, Harmer (1900) included thedeposits of Red Crag from Beaumont, Oakley, Dovercourt and Harwich.Although he later published more detailed faunal lists (1914-25) from thesections at Oakley, Harmer did not revise any of his earlier suggestions.

In this paper no consideration is given to the Red Crag of Walton orBeaumont, but other exposures to the north, at Oakley, and Dovercourt,have been re-examined in temporary sections which have become availableduring the past few years. Their locations are given in Fig. 1.

Over a period of six years a series of archaeological investigations havebeen carried out, under the direction of Mr. R. H. Farrands, on a Romano­British occupation site which lies on the Red Crag of Oakley, at the easternend of Bay View Crescent, near Foulton Hall (223291). Drainage ditcheswhich cut into the surface of the Red Crag have been unearthed, and withtheir aid the sands containing the extensive fauna of this Oakley Horizonhave once more been available for examination. The frequent occurrenceof such fossils as Mactra obtruncata S. Wood, and Nucella lapillus (Linne),has again been observed. The Foulton Hall exposures are in the highesthorizon at Oakley, and clearly represent a shell bank.

One mile to the north-east of these temporary sections at Foulton Hall,small exposures may be seen in the grounds of South Hall Farm (227301).The lowermost Red Crag sands show the characteristic fauna of the

1 All National Grid references lie within the 100 km, square 62 (TM).

61

62 JOHN McMANUS

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Oakley Horizon, but here a higher sand layer, rich in fragments of Myaarenaria Linne, is present in the topmost bed exposed. Large valves, up toseven centimetres across, are common, usually partly broken. The fauna ofthis highest zone is restricted in diversity, and is dominated by Myaarenaria.

During the summer of 1958 the author carried out excavations at aposition near the junction of Fronks Road and Elmhurst Road, Dover­court (252310). This site lies within the confines of the outcrop of the RedCrag indicated by Whitaker (1877) and is one of the few places along thisoutcrop at which buildings have not subsequently been erected.

Beneath a cover of soil fifteen centimetres thick, and glacial loam sixtycentimetres thick, is a layer of Red Crag containing comminuted shellfragments. At a depth of one and a half metres from the ground surface thered sands become distinctly fossiliferous, with a sparse fauna which is dom­inated by Mya arenaria. Other forms include Astarte triangularis (Mont­ague); Astarte basterotii (Lajonkaire); Astarte obliquata (J. Sowerby);Glycimeris glycimeris Linne; Mactra arcuata (J. Sowerby); Mactra truncata(Montague); Cardiumparkinsoni (J . Sowerby); Cardium venustum (S.Wood);Pholas sp.; Nucella lapillus (Linne); Turritella communis (Risso).

No specimens of Mya arenaria have been found with the two valvesassociated, but there are many large unbroken or almost complete valves.This Mya arenaria horizon is forty-five centimetres in thickness, and belowit are sands very rich in shells of an assemblage similar to that of both

MYA ARENARIA BED, RED CRAG, ESSEX 63

South Hall Farm, and Foulton Hall, Oakley. Thus the succession of theOakley Horizon is again recognised with an upper bed of Mya arenaria.

A temporary excavation for the installation of petrol storage tanks atThe Vines Garage, Main Road, Dovercourt (246313), has revealed apreviously unrecorded exposure of Red Crag. The succession is shown inFig. 2.

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Blue-grey clays, believed to be the uppermost horizons of the LondonClay below the sub-Pleistocene unconformity in this area, occupy thelowest twenty centimetres of a section three and a half metres deep. Uponthe surface of these clays and in the base of the overlying red sands lies athin conglomerate of black-hearted flints, which show a white patina andhave an irregular shape similar to the flints of the 'Bull Head Bed'.

Above this basal boulder-bearing layer the sands are twenty to thirtycentimetres in thickness, and contain a fauna typical of the Red Crag:Astarte triangularis (Montague); Glycimeris glycimeris Linne; Mactraovalis (J. Sowerby); Mya arenaria Linne; Mytilus sp.; Tellina sp.; Neptuneacontraria (Linne); Nucella lapillus (Linne); Nucella tetragona (1. Sowerby);Turritella communis (Risso).

The lamellibranch Mya arenaria is especially abundant in this section.It is present as individual valves, from six up to a maximum of twelvecentimetres in length, many lying parallel to the bedding. About 75per centof these valves have their convex side uppermost although some have theconcave side in this position: 5-10 per cent of the Mya arenaria valves,however, appear in their life position, in pairs not separated by reworkingof the sediments. In these examples the hinge-line lies on the upper part ofthe valves, and is inclined with the posterior portion uppermost. Thisorientation is essentially transgressive to the bedding (Fig. 3) and is similarto that observed for Mya in recent sediments off the coast of Lincolnshire(Evans, 1960).

64 JOHN McMANUS

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Fig. 3, Mya arenaria in The Vines section

In the recent sediments of the Wash, and again near Portmadoc, thehinge-line in the living Mya arenaria has been observed to be nearly verticalin orientation. In the specimens observed from the Red Crag the inclin­ation of the hinge-line is at a lower angle, generally plunging at about 60°from the horizontal, while the extreme examples observed plunge at 40°and 75°. The inclination of the hinge-line in the specimens from the RedCrag may be partly due to post-depositional compaction of the sediments.

The preservation of this assemblage in which some of the Mya arenariaare in their living position indicates that virtually no post-depositionalreworking of the sediments occurred. The occurrence of valves in bothconcave and convex upward orientation, together with the frequency ofoccurrence of large unbroken shells, suggests the lack of wave action orpowerful currents during deposition. The total absence of false bedding is afurther indication of the relatively quiet waters of deposition.

In the extreme south-western part of the outcrop of the Red Crag to thenorth of the Stour-Orwell estuary some small exposures of the Red Craghave been examined (284342). The fauna was extremely sparse, andvirtually the only fossils found in some of these temporary exposures werebroken fragments of valves of Mya sp, Here the Mya bed appears to lie ina position basal to the Newbournian zone of the Red Crag. Mr. H. E. P.Spencer, of Ipswich Museum (personal communication), has also observeda bed rich in Mya arenaria in sections at Felixstowe, in the Council yardsituated between the railway-cuttings (295349).

Correlation of the Mya arenaria bed across the Stour-Orwell estuary istentative. The fauna of the Red Crag is notorious in its inconsistencies.Concentrations of shells of a single species occur at different levels indifferent localities. However, the significance of the Mya arenaria bed ofthe Dovercourt area is that it represents in part a life assemblage in which

MYA ARENARIA BED, RED CRAG, ESSEX 65

the conditions existing during deposition, or immediately after deposition,may be inferred from the present habitat or the species. The bed rich inMya arenaria may be demonstrated from points over three kilometresapart to the south of the Stour. The extension of this same facies of depos­ition to the north of the estuary is probable.

The stratigraphical succession in the Red Crag in a south-to-northdirection on the evidence of faunal changes was indicated by Wood (1872).Funnel (1962) has recently cast doubt on the validity of this stratigraphicalsuccession farther to the north. The Mya arenaria bed, which is easilyrecognised when present, may extend over a lateral distance of eightkilometres, and as such may be important as a stratigraphical markerhorizon in the part of the succession where the Newbournian and Wal­tonian subdivisions of the Red Crag meet. The Mya arenaria bed representsone single facies of relatively quiet water deposition and is easily dis­tinguished from the facies of coarse-bedded sand, which occurs below it atSouth Hall Farm, and above it at Felixstowe. It may represent depositionin conditions in which the on-shore winds were greatly reduced, or, moreprobably, deposition in temporarily sheltered waters behind the pro­tective rampart of a sand-bank. Thus at The Vines Garage, Dovercourt,the only facies present is one of a 'lagoonal' environment in which well­aerated sea-water was available in sufficient quantity to support the Myaassemblage, but probably without violent movement. The existence ofPleistocene intertidal zone in this area is not improbable.

The Mya arenaria bed has been recognised in the vicinity of South HallFarm, followed north-eastwards, and probably again located on the north­ern side of the Stour-Orwell estuary near the Walton Ferry, a total lateraldistance of approximately eight kilometres, in which the bed has fallen inaltitude from twenty-four to fifteen metres above O.D. This may bepartly due to a gentle decrease in the height of the sea-floor. A variation ofnine metres in depth from high-water mark is sufficient to produce consid­erable variations both of facies of deposition and of the life assemblagesassociated with them.

Schafer (1962) records that Mya arenaria belongs largely to an inter­tidal biofacies. Stamp (1936) and others have suggested that the Straitsof Dover were not opened until after deposition of the Red Crag and thatthe tidal range in the enclosed sea would have been higher than at present.Within the recent sediments of the Wash, Evans (personal communica­tion) has observed the occurrence of Mya arenaria in zones of five metresvertical variation. With a larger tidal range than at present it is possible thatthe lamellibranch had a correspondingly greater variation in distribution.

It is improbable that a constant facies of sedimentation existed through­out nine metres of variation of depth relative to sea-level in the shallowwater. Post-depositional warping of the surface may be partly responsible

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66 JOHN McMANUS

for the present variation in altitude. Similar warping has already beenpostulated for the interglacial deposits at Clacton (Stevens, 1960).

REFERENCESDALE, S. 1732. The History and Antiquities of Harwich and Dovercourt, pp. 18 and 19.

2nd. ed.EVANS, G. 1960. Recent Sedimentation in the Wash. Thesis-University of London.FUNNEL, B. & R. G. WEST. 1962. The Early Pleistocene of Easton Bavents, Suffolk.

Quart. J. geol. Soc. Lond., 118, 125-42.HARMER, F. W. 1899. Report ofthe British Association. Section C, 751-3.---,. 1900. The Pliocene Deposits of the East of England. Part III: The Crag of

Essex (Waltonian) and its relation to that of Suffolk and Norfolk. Quart. J.geol. Soc. Lond., 56, 705-44.

---. 1914-25. The Pliocene Mollusca of Great Britain. Palaeontogr, Soc.(Monogr.).

SCHAFER, W. 1962. Aktuo-Paldontologie nach Studien in der Nordsee, pp. 550-62.Frankfurt am Main.

STAMP, L. D. 1936. The Geographical Evolution of the North Sea Basin. J. Cons. intoExplor, Mer., 11.

STEVENS, L. A. 1960. The Interglacial of the Nar Valley, Norfolk. Quart. J. geol. Soc.Lond., 116, 309-10.

WHITAKER, W. 1877. The Geology of the Eastern End of Essex (Sheet 48 S.E.).Mem. geol. Surv. U.K.

WOOD, S. V. 1848-74. A Monograph of the Crag Mollusca. Palaeontogr. Soc. (Monogr.).

J. McManusDepartment of GeologyQueen's CollegeDundee