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 (SW Britain)

J. R. L. ALLEN'32 & M. G. FULFORD2 'Postgraduate Research Institute for , 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- border demonstrates that tidal wetlands reclaimed duringthe Roman periodranged much further seawardthan the modern coastline. Vigorouslate medieval-early modern , 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 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 , 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 or emerging from the hinterland.Whereas tidal range is 14.8 m measured at on the coast river valleys partly infilled with Flandrian coastal near , so that the tidal are strongand the are well known from many parts of Britain (e.g. Hawkins waters richly charged with silt. Opening to thesouthwest, 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 tothe Severn. Although In the Severn Estuary, however, apparently undergoing apparently conservative of the fine 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 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 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, 63981in the intertidal zone on the left 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 demonstratethe 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

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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 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

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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.

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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 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. Vertical silted-up palaeochannel can be traced as an almost straight slickensides appear on some of the fault surfaces. The mean structure for about 350m northwestward across Hills Flats grain size of the silts and the percent sand increase gradually (Figs lc, 2m & 3a). In places after storms, the mainly pale with distance seaward along the channel axis, as is expected brown silts of the fill can be seen in sharp, erosional contact from general principles (Allen 1994), whereas the amount of with thepeats and pale green silts of the Wentlooge clay decreases and that of silt remains essentially unchanged Formation. The position of the palaeochannel was mainly (Fig. 7), none of the trends being statistically significant at established,however, by attemptingto securesamples by the 5% level. For about 25 m northwest of the edge of the test-pittingthough the superficial cover of contemporary peat ledge, the stone blocks (Fig. 4) contribute substantially sediment and ponded tidal water. to the channel fill along itsnortheastern margin (Fig. 3c). Where it crosses the extensiveledge defined by the They are accompanied by occasional pebbles of thesame second peat (Figs lc & 3a),the palaeochannel is a kinds as form the gravel sheets (see below), a few peat and steep-sided structure 12-25 m wide. Its position is marked turf blocks, and rare Triassic debris. Table1 summarizes by slightly elevated sheets of compacted gravel (see below) the lithologies represented in a sample of 155 of the stone and,where these are missing, by slight depressions below blocks; the largest mapped block measured 0.60 X 0.37 X the surface of the peat (the latter resists erosion more than 0.24m. Mingled in places with these blocks and other the channel fill). For about 100m beyond the clifflet at the coarsedebris embedded in the fill are clusters of angular edge of the peat ledge, the palaeochannel is exposed at the lumps of goethite iron (9-412 g in weight) and devoid of same level as the silts of the Wentlooge Formation beneath any signs of water-wear. the peat. Here its line is marked by a short series of bold, elongated mounds protected partly by compacted gravel but chiefly by an armouring lag of large, undressed stone blocks Marginal rotational slips (Fig. 4) which have undergone, or are undergoing, release fromthe channel fill. The width is nomore more than In a zone up to 10 m on each side of the palaeochannel, the 10-12 m at thiserosional level. Directevidence of the more eroded, outer surface of the peat ledge is scored by a palaeochannel is most difficult to secure over the final 140 m set of closely spaced, straight to slightly curved, subvertical, of its exposure (Fig. lc), where the structure is narrower still tensional fractures with the same general orientation as the and revealed at the level of the Mercia Mudstone Group, edge of the structure (Fig. 3c). There is little or no vertical although the lie is clearenough in air photographsas an displacementevident across thefractures, which therefore almostlinear perturbation,at a slight depression, of the resemble joints, except along the southwestern margin of the gravel dunes (Fig. 2). The palaeochannel creates agap in palaeochannel.At two places here,the fractures separate theouter rim of the rock platform,where the level to back-tiltedand rotated silt-peat slices that descend toward slightly concave-up,but in detailuneven, base is also the channel axis (Figs 3c & 8). exposed (Fig. 5). Inland, a lag of sandstone slabs and other debrisunderlies the gravel dunes, preventing the use of a probing rod, auger or corer.However, atthe gap in the Dating evidence rock rim and at four other places (Fig. lc), a short trench could be maintained free from water for long enough for the Theabundance of heavy metals (Zn, Cu, Pb)related to channel fill to bepartly sectioned and recorded tothe industrialisation and urbanisation affords useful evidence for bottom. the age of the muddy sediments in the Severn Estuary (Allen & Rae 19861987; Allen 1987b). Using Allen’s (1987b) form of plot, in which the amount of rubidium in Palaeochannel fill ppm less 16 ppm is used as a grain-size proxy, the mainly The lowermostseveral decimetres, asexposed in the pale brown silts from the palaeochannel fall along the curves excavations through the gravel dunes, are composed of pale for pre-industrial (older than about AD 1850) deposits (Fig. greento pale grey or brown,sandy silts with bands of 9). Geochemically, the silts can be seen to differ

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mapped edge of

stone blocks

Fig. 3. The Hills Flats palaeochannel. (a) Surveyed topography (contours in metres relative to approximate Ordnance Datum) and geological relationships. (b) Distribution of sediment samples and dating evidence. (c) Occurrence of back-tilted peat slices, channel-parallel fractures on peat ledge, and strew of quarried stone blocks.

significantly from the contemporary sediments of the estuary 640 f 50 years BP (Beta 61770) (i.e. thirteenth/fourteenth (Allen 19876; French 1993). century). The other artefacts are pottery sherds. At about Three artefacts were recovered from the channel fill (Fig. 70 m from the marsh, and close to the southwestern margin 3b), providing anarrow range of olderdates. A woven of the channel (Fig. 3b), pale grey to brown sediments of the structure (Fig. 10) of hazel (Corylus sp.) roundwood, either fill yielded two dispersed but joining fragments from the a hurdle fence or part of a fish trap, was found protruding base of a heavily sooted, large (diameter 0.34m), plain, from deep in the fill at about 40 m from the edge of the salt unglazed, sagging-bottom cookingpot. Where they had marsh (Fig. 3b). A composite sample from the best remained concealed within the silt, thesherds were fresh, preserved wood gave aconventional radiocarbon age of sharp-edgedand unworn. The body of the pot is ahard,

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v

a a- a-

2- . .. .-.- ,-.-.. -- . j :- -: ._- KEY : - . 4 -. .T- 5.- 4 ...... M palaeochannel Fig. 4. Quarried stone blocks. peat rafts and turf blocks undergoing V structure A erosional release from the silts infilling the palaeochannel. Spade for v structure B \ scale 0.94 m tall.

mid to dark grey and very sandy clay. The inclusions, in thin-section, are of a largely disaggregated fine- to Fig. 6. Textural properties of silts infilling the palaeochannel and coarse-grained, feldspathic-lithic quartz sandstone. The structures A and B, as determined by laser granulometry. predominance of microcline andorthoclase suggests the manufacture of the pot in the Forest of Dean,where the Upper Old Red Sandstone has this mineralogy (Allen 1965), and the form indicates a twelfth to fourteenth century date. The thirdartefact, recovered from silts enclosing adense cluster of stone blocks accompanied by goethite lumps on the opposite margin of the channel (Fig. 3b), is a large, unworn sherd from the base of a swollen jug with an apple-greenexternal glaze and traces of a crude ornamentation in dark brown slip. The body is ahard, off-white to cream clay with scattered inclusions of fine to very coarse quartz sand, a very little orthoclase in rounded grains, lumps of coarsemudstone grading to very fine sandstone, and ferruginous particles. The jug was made in thefourteenth century, judging from the form and decoration,and from the general shape and fabric could

0 (Cl 85 -

I 1 I I 0 50 100 150 200 250 seaward axial distance fromarbltrary origin, m Fig. 7. Seaward variation along the palaeochannel in the textural Fig. S. Intersection of the palaeochannel (between fipre on right properties of the infilling sediments. The distance scale begins at a and spade 0.94 m tall on left) with resistant bcds in the Trias convenient arbitrary point on the inland projection of the general forming the outer rim of Hills Flats. View landward 10 southeast. line of the palaeochannel. Sample points in Fig. 3c.

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Table 1. Lithological composition of quarried stone blocks (n = 155) sampled at four points (Fig. 3b), are texturally distinct from exposed along the northeastern margin of the palaeochannel those filling the nearbypalaeochannel and are therefore likely to be of a different age and/or facies (Fig. 6). The two Lower Old Red Sandstone sets of samples are statistically different atthe 5% Chiefly red-purple, micaceous, fine- to medium-grained 21.9%. probability level in mean grain size and in the proportions of quartz sandstones: some brown, poorly micaceous clay, silt andsand. The fill of structureA is nonetheless sandstones. estuarine.Smear slides showed an abundance of sponge Upper Old Red Sandstone (Quartz Conglomerate) spicules and mixed assemblages of open-marineand Pale brown-red,coarse to very coarse-grainedsandstones 1.3%. estuarinediatoms and foraminifera. No artefacts were with pebbles of vein quartz. encountered while trenching structure A, but the amounts of Trias heavy metal present in the sediments demonstrate that the Pale grey-green, yellow-orange weathering, gypsiferous. 68.4% structure and its fill antedatesthe mid-nineteenthcentury very fine to-medium-grained sandstones. (Fig. 9). Near the salt marsh, however, the trench was filled Lias almost exclusively by sand-striped fine gravels resembling Pale grey, fossiliferous calcisiltites. 8.4%. some of the modern pocket-beach depositsat Hills Flats. Total 100.0% Structure A is interpreted as the lowermost part of a large, silted-up field drain. In size and shape it is similar to chiefly linearfeatures exposed intertidally on peat ledges on the have originated in the Bristol area (McCarthy & Brooks coast of southeast (Allen & Fulford 1986; Allen 1988, p. 346). 1987~).These, dating from the Roman period, are aligned Hencethepalaeochannel was active during the with extant field drainsand, when traced to a mud cliff, thirteenthand fourteenth centuries and, at the erosional present the steep-sided, flat-bottomed cross-section typical levels at whichit is now exposed, contains no sediment of a drain. younger than the mid-nineteenth century. Structure B (Fig. 12), lying within theoutcrop of the channel fill and post-dating that fill (Fig. 3a), is exposed as a curved, recessed, drain-like feature marked, like structure A, by a superficial layer of mobile gravel. Its width is about Minor structures 0.75 m, but the depth is uncertain,as the fill superficially resembles that of the palaeochannel withwhich it is in The most obvious of the two minor structures associated on contact. The fill consists of pale brown (locally pale grey), theforeshore with the palaeochannel is a metre-wide, mainly structureless, clayey-sandy silts of estuarine origin, sediment-filled trench (Fig. 3a, structure A) which curves with abundant sponge spicules and rare open-seadiatoms smoothly across the peat ledge some 30m to the northeast and foraminifera. Texturally, the deposit differs from the (Fig. 11). The trench, briefly noted by Allen & Fulford palaeochannel fill and again could be of a different age (1987), hasa flat bottomand sharp, almost vertical sides, and/or facies (Figs 3b & 6). There is a statistically cuttingup to 0.6m below the second peat. It is infilled significant difference at the 5% probability level between the chiefly by pale brown, clayey-sandy silts with occasional two sets of samples in terms of the silt and clay percentages. bands of quartz sand and granule to fine pebble gravel Geochemically, the fill of structure B antedatesthe mid-nineteenthcentury (Fig. 9). Proxy size-for-size, the dominated by moderatelyrounded Triassic debris.These deposits, less resistant to erosionthan the adjoining peat, sediments from structure B are noticeably poorer in copper and lead than eitherthe palaeochannel or structure A, are capped by several centimetres of mobile gravel, giving pointing to a difference in facies (?more organic-rich thestructure its distinctive surface expression. The silts, deposit) but not necessarily any great difference in age. We also interpret structure B as the lower part of a silted-up field drain, but recognize that its poorer definition in comparison with structureintroducesA note aof uncertainty.

The sheet gravels Emerging from beneath the mud cliff andramp of contemporary sediments (mobile beach gravels) at the edge of the salt marsh are a series of sheets of crudely bedded, compacted, immobile gravel which sharply overlie the palaeochannel fill (including structure B) and lap on to the peat ledge (Figs 3a & 12). They represent a concentration of debris which otherwise is dispersed widely but very thinly in the intertidal zone of the estuary. The gravels consist of a variety of mainly well roundedpebbles and some small cobbles set in a matrix of muddy sand. Typically, the pebbles exposed on the surface are blackened by an organic Fig. 8. Fracture-bound. back-tilted (channel axis to left) peat rafts coating, and some provide holdfasts for weed, although on the southwestern margin of the palaeochannel. Spade for scale is storm waves cause occasional losses and have gradually 0.94 m tall. exposed the original internal bedding. The lithologies

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A A A 400 A t AA A~ ga 300 t 0 .: .: 200 lc

100

A

50

f 25 n n 8 KEY A structure A v structure B 0 0 palaeochannel fill A Contemporary estuarine silts 0 latepre-industrial estuarine silts

I

A A A A A

Q5 50 0 -0

0 I I I I l I I 25 50 0 25 75 100 175 125 150 200

( Rbppm- 16) ppm

Fig. 9. Variation in the amount of (a) zinc, (b) copper and (c) lead in the sediments infilling the palaeochannel and structures A and B, as determined by XRF analysis, compared to suites of late pre-industrial and contemporary intertidal silts from the Severn Estuary (data of Allen 1987b). The quantity shown on the abscissa (a grain-size proxy increasing with the clay-mineral content) is the amount of rubidium in ppm in a sample less 16 ppm.

present(Table 2) can all be matched in the Pleistocene centuryform. There is also less-abraded, black-glazed gravels of the middle and inner estuary (Wills Welsh Borders earthenware of the eighteenth century, but 1938). Werecovered a small number of water-worn nothing of later date.We conclude that thesheet gravels artefactsfrom the gravels, including Romano-British may not be a contemporary deposit and may date from the (Severn Valley Ware, Black Burnished Ware Category l), eighteenth or nineteenth century. late medieval (fourteenth century jug/pitcher), andearly modern (including late seventeenth-eighteenthcentury Land-claim Devon Gravel-temperedWare) pottery, together with The salt to occasionally brackish marsh which constituted in unstamped clay tobacco pipes of mid- to late-seventeenth its natural state the main outcrop of Flandrianestuarine

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Fig. 10. Part of the medieval woven structure of hazel (?hurdle. fish Fig. 12. View of structure B looking eastward. Spade for scale is trap) embedded in silts infilling the palaeochannel. Scale 0.15 m 0.94 m tall. long.

alluvium in theneighbourhood of HillPill has in recent Coastal change and marsh development millennia been the subject of extensive land-claim, when sea defences and sluiceswere constructed and anetwork of The coast of the Severn Estuaryat Hills Flats has large drainage ditches dug. experienced a long and complex history of change, in which The earliest defences appear on circumstantial grounds retreat, necessitating the set-back of sea defences on at least to be Romano-British, for the ploughed fields established on one occasion, has occurred during certain periods, whereas theoutcrop of alluvium reveal two spread of occupation others have seen the build-up ofnew salt marshes and the debris of this date (Fig. lc), one to the northeast and the advance of the strand. other (also with abundant medieval sherds) to the southwest We propose that the palaeochannel we record (Figs lc & of HillPill, supporting Allen & Fulford's (1987) general 3)once carried HillPill througha thick sequence of conclusions. No seadefence survives as such today, but estuarine alluvium to a position close to the edgeof the rock what we take to be its line can be traced as a ramp through platform. Thepalaeochannel has thesame orientation as the fields for about 1 km to either side of the pill (see Allen the modern Hill Pill and is aligned on that structure. There 1993b for discussion of criteria). A sluice may be presumed is a substantial surviving outcrop of silts and peats of the to have existed where the line of the defence crosses the Wentlooge Formation on the inner part of Hills Flats, and stream. By 1659 the sluice had been moved 700m further the lowermost fill of the palaeochannel,as far out as the downstream, adding a wide green drove to Longpool Lane limit of thedune field, includes abundantpeat clasts. (Fig. lc), and by 1835 a further shift of 160m was effected Rotational slip onthe margins of the activechannel, (GloucestershireRecord OfficeD908, D272/9/2).The evidence for which we recorded (Figs 3c & 8), provides a present sluice, further seaward still, dates from 1990. It is is plausible process for the start of formation of these clasts. That process is ubiquitous in thecontemporary tidal beyonddoubt that by medieval times thearea was fully defended from marine flooding. Two occupationsites of channels(pills) of theSevern wetlands (Allen 1985), affecting unsupported slopes underlain by estuarine silts that thisperiod are known (Fig. lc) and classical ridge-and- are more than about 5 m high (Fig. 13). The sides of the furrow-medieval arable fields (Hall 1981,1982; Astill modernchannels slope at 30-40" on the average. As the 1988)-abounds in the enclosed area. palaeochannel is about 15 m wide at today's deep erosional level (Fig. 3a),its width at the surface of the wetland through which it formerly flowedwould have been of the

.. Table 2. Lirhologies represented by pebbles (n = 318) in rhe sheer

'i gravels

Vein quartz 27.1 % Flint (subrounded) 3.1 Chertified oobiosparite Chertified 0.3 Pale brown quartzite 24.2 Purple quartzite 10.1 Micaceous sandstone (Old Red Sandstone) 0.6 Gypsiferous sandstone (Trias) sandstoneGypsiferous 5.0 Other quartzites and sandstones 24.5 Fossiliferous calcisiltite (Lias) 3.8 Metamorphic rocks (slates, schist, gneiss) 1.3 Fig. 11. View of structure A looking southwestward across the peat Total 100.0 ledge from near the salt-marsh cliff. Spade for scale is 0.94 m tall.

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tidal channel achieved its presentstatus as a redundant structure. A major phase of salt-marsh growth and advance followed by retreat of the coast occured some time between the mid-seventeenth and mid-nineteenth century (Fig. 14c). The salt marshes recorded in 1835 (GRO D272/9/2) range everywhere to seaward of the coast asrecorded in1659 (GRO D908). By 1835 the mouth of Hill Pill had shifted slightly from its seventeenth-century position and a distinct bend to the right had formed just inland. Deep excavations for the 1990 sluice (Fig. lc) showed thatthe spacethus swept by the meanderingchannel had been filled by pale brown, clayey-sandy silts which, like erosively-based deposits of the same character exposed on the marsh cliff between ChapelHouse and WhiteHouse (Allen & Rae 1987), we confidently ascribe to the Rumney Formation. If Fig. 13. Marginal rotational slips affecting the left bank of Berkeley onthe basis of theirgeneral appearance, textural Pill. Gloucestershire. a contemporary tidal channel. Spade for scale distinctness from the palaeochannel fill (Fig. 6), and is 0.94 m tall. heavy-metal content (Fig. 9), the deposits filling structure A are also attributableto the RumneyFormation, then the marshes and high tidal mudflats of the accretionary phase order of 25-35m, assuming the presence of an additional could have extended seaward at least as far as the general lie 5 m of alluvium. In Fig. 14a we suggest a possible minimum of the inferred earliest seabank (Fig. 14a, c). outer position for the edge of the early wetland. Possibly structure B (Fig. 3a) was associated with the If the early wetland extended so far to the northwest, is later stages in the formation of the new marshes. It occupies thecoastal seabank seen today of Romano-British date? a position on thepalaeochannel destroyed by 1659, The evidence points to a substantial erosional retreat of the stratigraphically post-dates the infilling of that structure, is coast, necessitating the setting-back of these defences at filled with texturally and chemically distinct, pale brown silts least once. StructureA (Figs 3a & ll), interpretedas the generally similar to the RumneyFormation (Figs 6 & 9), bottom of a field drain, is arguably part of a collecting ditch and resembles in plan the lower part of a large field drain. associated with a seabank that ran parallel with an extended The structure possibly records anattempt ata small Hill Pill, placing the earliest coastal defence at least 200m land-claim, acontemporaneous parallel for which can be further seawardthan the presentstructure (Fig. 14a). The found at Pilsale Long Warth at the mouth of Oldbury Pill presence of medieval artefacts nearby in the palaeochannel, 5 km tothe southwest,where anarea of Rumney including freshly broken and sooted pottery, suggest that as Formation, concealing medieval ridge-and-furrow (Allen late as the fourteenth century erosion had not yet reached 1988), was eventually reclaimed. that position and that a substantial tidal channel remained in By1879-80 ( Gloucestershire 55NW, operation. The stone blocks clusteredalong the northeast 1st ed.), the edge of the marsh (RumneyFormation) had margin of the palaeochannel (Fig. 3c) could all have been retreated inland almost to the position of the clifflet marking quarried locally and probablyrepresent a simple landing the outer limit of the high marsh today (Figs lc & 14d). place that had eventually collapsed intothe pill, inviting Rock and sand then formed the foreshore. These had been comparison with the more elaborate twelfth and thirteenth replaced with mud by 1901 (OS Gloucestershire 55 NW, 2nd century wood and stone quay, also constructed using local ed.), which had grown further against the salt-marsh cliff by rocks, at Woolaston Grange on the oppositeshore of the 1919-20 (OS Gloucestershire 55NW-NE, 3rd ed.). These estuary(Fulford et al. 1992). The lumps of unabraded deposits occupy the position of the intermediate marsh goethite ore associated with the stone blocks closely (Awre Formation) of today (Figs lc & 14d). The deposits of resemble present in the Forest of Dean,and could the low marsh (Northwick Formation), arising after a short represent imports by boat from there. episode of coastal retreat and cliff-cutting, belong mainly to By 1659 (GRO D908) the coast had retreated to an the post-war period (Figs lc, 14d). Given the ceramic extreme inland position only a little outside the line of the evidence we secured for their age, the gravel sheets can be presentseabank (Fig. 14b), which representedcon-a attributed to the nineteenth century phase of erosion, when siderable setting back and the renewal of tidal sedimentation the shore was mapped as sandy and rocky. on the abandoned part of the original land-claim. In the Our work makes Hills Flats only the third locality in the earlyseventeenth century, the Commissioners of Sewers Severn Estuary at which it is possible to describe recent (and (GRO D272/1/2-4) ordered new seabanks at Hills Flats and dramatic) coastal change in any detail. At the Wentlooge nearby Oldbury Flats, together with the revetting of some of Level onthe coast of southeast.Wales (Allen & Fulford the surviving ones. Physical evidence for set-back is 1986; Allen 1987c, 1990b), land-claim of Romano-British provided by thetruncation of medieval arable fields date was followed in late medieval times by the setting back (ridge-and-furrow) and latest medieval hedged fields (Allen of sea defences over a frontage of 12 km in response to the 1992, 1993h) over a frontage of 2.25 km northeast of White vigorous erosion of the Flandrian wetland sequence. House (Fig. 14b). It was probablyduring this retreat that Broadly asat Hills Flats, new mudflats and marshes copious Romano-British occupation debris (Allen & Fulford (Rumney Formation) were built upward and outward from 1987) reached the foreshore, eroded from the northernmost not earlier than the late-seventeenth century, although the settlement (Fig. lc). It was also during this retreat that the coast in many places is today retreating again. Much closer

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Fig. 14. Summary of coastal change at Hills Flats over the last two millennia. (a) Romano-British to earlier medieval times: (b) The mid seventeenth century. (c) The early nineteenth century. (d) Accretionary phases represented by the Awre Formation (inception late nineteenth century) and Northwick Formation (inception mid-twentieth century).

to Hills Flats lies the intertidal platform of Oldbury Flats, intricate networks of tidal creeks (e.g. Pethick 1992). dissected by a complex of silted-up, Flandrian palaeochan- The Severn Estuary is not alone in Britain in having an nels open to the tide in the Iron Age and Roman periods eroding alluvial coastline, and further good examples can be (Allen & Fulford 1992). Surviving Flandrianoutcrops, seen in the Kentand Essex marshlandsandthe: together with archaeological and documentary evidence, Lincolnshire shore.Work todate in Kent (Spurrell 1885, show that here, as at Hills Flats and the Wentlooge Level, 1889; Evans 1953; Kirby 1990) and Essex (Wilkinson & reclaimed (Romano-British) marshes ranged much further Murphy 1995) amply demonstrates the potential in such seaward than their present limits, and that coastal erosion areas forpalaeogeographic studies based on intertidal extending intothe seventeenthcentury and necessitating exposures of Flandrian coastal sedimentsand using an set-back was followed by the accretion-erosion cycles eclectic approach. Every available means should be used to expressed by the Rumney,Awre and Northwick Forma- secure a better understanding of the response of the lowland tions. Pervasive erosion of late medieval to seventeenth coastal zone to sea-level movements and storminess, factors century date in the Severn Estuary, coincident with the cool that are expected to be sensitive to global warming. and disturbed conditions of the Little Ice Age (Grove 1988; Lamb 1988, 1991), is strongly suggested by these three scattered sites. Because the Little Ice Age was an at least Conclusions Europe-wide event, stratigraphical evidence for the coastal (1) Asilted-up palaeochannel, exposed ata deep conditions which accompanied it may be expected to occur, erosional level, ranges for about 350m across the intertidal and should be sought, elsewhere in Britain. rock platform known as Hills Flats onthe Avon- Although the SevernEstuary offers exceptional oppor- Gloucestershireborder. Parallel with it overpart of its tunitiesfor palaeogeographic work, on account of its course is the trace of a substantial field drain. underlying erosional regime, there remains one particularly (2) The palaeochannelextends the line of the modern pressing research challenge. It is the problem of the Hill Pill, an active tidal channel with a small freshwater extension of palaeochannels visible in the intertidalzone , and is cut into estuarine silts andpeats dating inland into the wide alluvial land-claims on the margins of from mainly the second half of the Flandrian. Locally, the the estuary, many of which date from the Romanperiod palaeochannel cuts down slightly into the Mercia Mudstone (Allen & Fulford 1986,1987, 1992). In these agricultural Group, lying unconformably beneath the Flandrian deposits. landscapes, heavily modified from theirnatural state by (3) Artefacts of the twelfth tofourteenth century, repeated shifts between arable farming and pastoralism, together with stone blocks interpreted as a collapsed landing little or no surface trace now remains of what may in terms place, are present in the estuarine, clayey-sandy silts that of general experience be expected to have been large and dominate the channel fill.

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Received 13 January 1995; revised typescript accepted 13 June 1995 Scientific editing by tan Fairchild

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