Late Flandrian Coastal Change and Tidal Palaeochannel Development at Hills Flats, Severn Estuary (SW Britain)
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Journal of the Geological Society, London, Vol. 153, 1996, pp. 151-162, 14 figs, 2 tables. Printed in Northern Ireland Late Flandrian coastal change and tidal palaeochannel development at Hills Flats, Severn Estuary (SW Britain) J. R. L. ALLEN'32 & M. G. FULFORD2 'Postgraduate Research Institute for Sedimentology, The University of Reading, PO Box 227, Whiteknights, Reading KG6 6AB, UK 2Department of Archaeology, The University, PO Box 218, Whiteknights, Reading RG6 6AA, UK Abstract: An eclectic range of stratigraphical. sedimentological, geochemical, archaeological and historical evidence relating to a tidal palaeochannel exposed at a deep erosional level in the modern intertidalzone onthe Avon-Gloucestershire border demonstrates that tidal wetlands reclaimed duringthe Roman periodranged much further seawardthan the modern coastline. Vigorouslate medieval-early modern erosion, linked to the cool, disturbed conditions of the Little Ice Age, forced the coastline by themid-seventeenth century to a location inland of its present line. A possible medieval landing place was destroyed and it was necessary to set back the flood defences. As the result of renewed mudflat-marsh growth, the coast at Hills Flats has built outward during modern times, but in three distinct stages and by no means as far seaward as its likely Roman position. Keywords: Severn Estuary, Flandrian, channels, salt marshes, coastal erosion. Networks of tidal channels and creeks, the larger of which evidence, we conclude, has a significant contribution to reach deep into the tidal frame, are a ubiquitous feature of make to an understanding of recent coastal change and the contemporary high tidal flats and salt marshes in British development of models for future behaviour. Our estuaries, tidal embayments, and on barrier and some open methodology is applicable not only to other palaeochannels coasts (e.g. Allen & Pye 1992; Pethick 1992; Pye 1992). At exposed in the Severn Estuary, but also toother tidal different times and places, some of the bigger channels have systems of a similar character in Britain and mainland been used for navigation and served to locate wharfage and Europe. settlements,for they reach far back intothe Flandrian The Severn Estuary (Allen 1990a) is one of the largest (Holocene) wetland outcrops and, in some cases, extend a inlets on the west coast of Britain (Fig. la, b). The extreme stream or river emerging from the hinterland.Whereas tidal range is 14.8 m measured at Avonmouth on the coast river valleys partly infilled with Flandrian coastal sediments near Bristol, so that the tidal streams are strongand the are well known from many parts of Britain (e.g. Hawkins waters richly charged with silt. Opening to the southwest, 1962; Anderson & Blundell 1965; Anderson 1968; Williams the estuary is exposed to the prevailing winds and is the site 1968; Gilbertson & Hawkins 1977; Whittaker & Green 1983; of vigorous wave action. The coastline is muddy, however, Lake et al. 1986; Eddison & Green 1988; Berridge & except locally at the mouth of the estuary, where sand or Pattison 1994), tidal palaeochannels embedded within the gravel beaches are found. On the margins of the estuary lie Flandriansequence have attracted little attention (Evans disjointed outcrops of Flandrian estuarine alluvium amount- 1953; Silvester 1988; Funnel1 & Pearson 1989; Hall & Coles ing to some 400 kmz in total area and 4 km3 in volume. The 1994; Wilkinson & Murphy 1995), perhaps because they are Flandriansequence averages about lOm in thickness, smaller than 'drowned' valleys and heavy reliance is placed swelling toward the axis of the estuary and into the buried on borehole data in Flandrian palaeogeographic work. valleys of the rivers tributary tothe Severn. Although In the Severn Estuary, however, apparently undergoing apparently conservative of the fine sediment supplied to it an erosional retreat inland (Allen 1990a), theFlandrian chiefly by the rivers, theestuary, influenced by the sequence is extensively exposed intertidally and visibly underlying regime of upward-moving relative sea level, includes many silted-up palaeochannels of a range of sizes seems in the long term to be retreating up the Severn Valley anddates. Although now exposedat a relatively deep rather than fillingup. The coastal changes we document erosional level, anumber of thesepresent a variety of from Hills Flats express a general, long-term retreat of the artefacts ranging from ceramics to boats within the fill, shore. which assist with dating and also provide evidence of the context when the channel was active, for example, a landing place or a settlement on a reclaimed salt marsh. Our aim in Setting this paper is to describea comparatively young tidal Hills Flats (Fig. la-c) crops out as a long (3 km) but narrow palaeochannel from the Flandrian sequence at Hills Flats on (650 m) rock platform [British National Grid Reference SO the Severn Esiualy. Because of its character, andthe 6297, 6397, 63981 in the intertidal zone on the left bank of archaeological features associated with it, we are able to the Severn Estuary in southwest Britain. Its uneven surface, establish a chronology and a context for the evolution of this at an elevation from about Ordnance Datum to a few metres channel, and can demonstrate the complex movements of above, is underlain by the Flandrian Wentlooge Formation the shorelineover the last two millennia. Archaeological (Allen & Rae 1987) in unconformablecontact with red 151 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/151/4888647/gsjgs.153.1.0151.pdf by guest on 01 October 2021 152 ALLEN L. J. R. & M. G. FULFORD Fig. 1. The location and general geological context of the late Flandrian palaeochannel at Hills Flats. (a) The Severn Estuary in the British Isles. (b) The Severn Estuary and its marginal alluvium. (c) Outline geology of Hills Flats and its surroundings. (d) Schematic composite geological section at Hills Flats. mudrocks and muddy sandstones of the Triassic Mercia 5300 f 60 years BP (Beta 61769). Probably Devensian Mudstone Group (Welch & Trotter 1961). The Wentlooge ice-wedge casts are visible in several places in the Mercia Formation consists of mainly green estuarine silts and Mudstone Group at or just below its contact with the brackish-freshwaterpeats which fill and bridge three overlying Flandrian beds (Allen 1987a). The topmost of the unequal, shallow depressions on the shore (Fig. lc, d). The Triassic rocks are deeply frost-shattered and weathered, and peat beds resist erosion more than the intercalated silts and merge upward into red-grey,a sandy-pebbly palaeosol. weather as extensive ledges, most notably in the case of the Within the depressions, this ancient soil becomes organic- second peat (locally in two leaves) upward in Fig. Id. Well rich and locally merges upward into a basal peat (Fig. Id). A preserved wood from a young oak (Quercus sp.) embedded variable cover of contemporary sediments largely conceals in this peat gave conventionala radiocarbon age of the Mercia Mudstone Group and Wentlooge Formation on Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/151/4888647/gsjgs.153.1.0151.pdf by guest on 01 October 2021 C OA STA L CHANGES AND TIDAL PALAEOCHANNELSTIDAL COASTALCHANGES AND 153 Hills Flats. Fields of active gravel dunes (see Allen 1993a) above it, lies a narrow belt of active salt marshes underlain permanentlyobscure theouter part of the rock platform by clayey-sandy silts (Figs lc, d & 2). The highest marsh, of (Figs lc & 2). The inner part is masked by a semipermanent which little now survives, is assigned to Allen & Rae's cover up to 0.4m thick of muddy-sandy gravel grading to (1987) Rumney Formation (see below), and the intermedi- mud. ateand lower marsh deposits respectively to theirAwre To the southeast of Hills Flats, and rising several metresFormation and Northwick Formation. Thelatter, j l I 4 . Fig. 2. Air photograph (900 X 1000 m) showing the coast in 1969 at Hills Flats and the extension of Hill Pill as a palaeochannel in the intertidal zone. D. trace of field drain (structure A): DF. field of gravel dunes: M. active salt marshes: P. palaeochannel: S, seabank. Crown copyright reserved. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/1/151/4888647/gsjgs.153.1.0151.pdf by guest on 01 October 2021 154 J. R. L. ALLEN & M. G. FULFORD distinguished by significantly elevated levels of heavy metals fine-grained quartz sand and streaks of comminuted organic andother contaminants, iswell exposed onthe mud cliff debris,accompanied locally by slightly to well rounded which in many places bounds the marshes to seaward. The pebbles,cobbles and small boulders of peat and silty, AwreFormation, unconformable Rumneyon and root-boundturf.Ill-rounded fragments of Triassic Wentlooge beds, appears widely on the cliff between Chapel mudstonesand sandstones in places are mingled with the House and White House (Allen & Rae 1987) (Fig. lc). peat clasts. The main outcrop of Flandrian estuarine alluvium Significantly higher beds in the fill are seen to landward. (Wentlooge Formation) ranges inland for several kilometres Theseare chiefly pale brown clayey-sandy silts which, from a locally revetted, earthen seabank at the inneredge of toward the margins of the structure, grade to a pale grey or the activemarshes (Fig. lc). It is drained by Hill Pill, a pale green colour (Figs 3b & 6). Smear slides show them to deeply cut, partly tidal (as far inland as the contemporary be of estuarine origin. All samples contain abundant sponge sluice) channel that divides into a number of inconspicuous spicules, usually accompanied by mixed open-seaand tributaries about 1 km inland from the mouth. estuarine foraminifera and diatoms. A number yield shreds of organic matter and pyrite framboids, suggesting reduction The palaeochannel in an anoxic environment. Typically, the silts are structureless but here and there include sandy streaks and laminae. The bedding thus revealed is either contorted and Relationships and size accompanied by small-scale faults or dips ata low to Emerging onto the shore close tothe marshedge, the moderate angle toward the channel axis.