Romney Marsh: Environmental Change and Human Occupation in a Coastal Lowland (ed. J. Eddison, M. Gardiner and A. Long), OUCA Monograph 46, 1998, 13-29 2. Holocene Barrier Estuary Evolution: The Sedimentary Record of the Walland Marsh Region Christopher Spencec Andrew Plater and Antony Long This paper presents the results of a study concerned with the Holocene barrier estuary evolution of Walland Marsh. The focus of investigation is the immediate back-barrier environment of Scotney Marsh, where sedimentation under high-frequency low-magnitude tidal and wave processes is likely to have been punctuated aperiodically by abrupt high-energy storm events. The local palaeoenvironrnental record at Scotney Marsh is established using 3400 boreholes, pollen, diatom and radiocarbon dating. This detailed local record is then linked to the regional stratigraphic sequences by two lengthy (c. 10 km) transects of cores running west to east and south-east to north-west across Walland Marsh. Gravel underlies much of Scotney Marsh, and radiocarbon dates on peat directly above these gravels indicate their deposition sometime before c. 4000 cal. yrs BP. Subsequent tidal sedimentation in the resulting back-barrier estuary was followed, under the influence of decelerating relative sea-level rise, by the extensive development of peat between c. 3900 and 2400 cal. yrs BP. Correlation with other studies on Walland and Romney Marsh indicates that a subsequent inundation of the peat by marine conditions is part of a marsh- wide flooding phase linked to renewed relative sea-level rise and the expansion of a tidal creek system in the barrier estuary. Even in such close proximity to the barrier, the influence of low-frequency high-magnitude storms on the Holocene evolution of Scotney Marsh occurred only during initial barrier formation and again, much later, after eventual breaching of the south coast during the 13th century AD. Introduction 1988; Vollans 1988, 1995; Gardiner 1995; Hipkin 1995; The sediments of Walland Marsh preserve a palaeo- Reeves 1995; Allen 1996). In this paper, we present new environmental record of the barrier estuary system since palaeoenvironmental data from the Scotney Marsh region approximately 7000 cal. yrs BP (Long et al. 1996). In which provide evidence relating to the evolution of the addition to the detailed palaeoenvironmental evidence that fore-marsh, that area in which the marsh sediments abut has emerged from the geomorphological and geological the barrier complex of Dungeness Foreland. This local study of this sedimentary record at a number of sites across record is then linked with previously published data from the marsh (Burrin 1988; Tooley and Switsur 1988; Waller mid- and back-marsh sites using the results from extensive et al. 1988; Greensmith and Gutmanis 1990; Tooley 1990; stratigraphic surveys in a review of the regional history of Plater 1992; Long and Innes 1993; Waller 1994; Long barrier estuary evolution in Walland Marsh. and Hughes 1995; Long and Innes 1995a, 1995b; Plater The focus on the Scotney Marsh site enables the study and Long 1995; Plater et al. 1995; Wass 1995; Long et al. of Holocene barrier and barrier estuary sedimentation in 1996; Spencer 1997; Spencer et al. 1998; Plater et al. in an environmental setting where high-frequency low- press), archaeological and historical information provide magnitude tidal and wave processes are aperiodically important detail on the sequence of barrier estuary interrupted and modified by low-frequency high-magni- sedimentation, drainage, inundation and land claim (Ward tude events (especially storms), which reshape the coastline 1931; Cunliffe 1980, 1988; Eddison 1983b, 1988, 1998; and establish new boundary conditions. A further aim Brooks 1988; Green 1988; Robinson 1988; Tatton-Brown of this paper is, therefore, to assess the geomorphic 14 Christopher Spencer, Andrew Plater and Antony Long significance of coastal processes which operate over these peat and other fine-grained sediments formed. More different timescales. Of especial interest is the interdepend- recently, Tooley (1995) has also noted that the Midley ence and relative importance of long-term (centuries to Sand, as mapped by Green (1968), outcrops between millenia) sea-level rise, medium-term (decades to cent- gravel ridges in the Broomhill area and must, therefore, uries) changes in sediment supply and short-term (hours to post-date gravel deposition here as well. decades) geomorphic events on barrier estuary evolution. An alternative hypothesis for the early development of the barrier was presented by Long et al. (1996) as part of their investigation of the deep sedimentary record in the region of Rye. These authors argued that a strong Barrier estuary formation coarsening upwards trend between approximately -25 m The information relating to the early-Holocene evolution and -12 m OD may be associated with the landward of Walland Marsh has focused on the origins of the barrier migration of a coastal barrier system under the influence and, critically, its relationship to the lower-energy barrier of sea-level rise (Forbes et al. 1991 ; Orford et al. 199 1). estuary environments (e.g. Long and Innes 1995a). Indeed, Other possible explanations for this coarsening upwards it seems unlikely that the fine-grained inorganic and trend included the onset of the west-to-east movement of organic sediments that make up Walland Marsh would sand in the nearshore zone associated with a change in ever have been deposited and preserved were it not for the tidal range following the opening of the Strait of Dover protection afforded by the sand and gravel barriers of (Austin 1991), and also an increase in tidal energy caused Dungeness Foreland (Gulliver 1897; Lewis and Balchin by the rise in Holocene relative sea level. 1940). The nature of this initial barrier has been the source The exact dimensions of this early-Holocene barrier of much debate, especially in terms of its initial formation, are far from certain, largely because its western portions location and composition. have been destroyed by coastal erosion and reworked to Drew (1 864) suggested that the initial sand and gravel feed the more recent easterly expansion of Dungeness barrier owed its origin to the slack water arising from the Foreland. However, this has not stopped many authors meeting of two tides, one from the English Channel and from speculating on the possible form of the barrier at the other from the North Sea. Lewis (1932, 1937) refuted this time, one example of which is provided in Fig. 2.1 this, arguing that this phenomenon occurred over a wide from Lewis (1932). The presence of peat in front of the area from Dungeness to the Goodwins, off the coast of cliffs at the western limits of Rye Bay suggests that the Deal in the Strait of Dover. The basis for many more barrier was not anchored to the present coast (in this area recent models of barrier initiation and barrier estuary at least) until after the end of peat formation in the late- evolution is provided by the Soil Survey of Green (1968). Holocene (Eddison 1983a). In the absence of more In Green's (1968) study, and those of Cunliffe (1980) and conclusive offshore data (see Dix et al. 1998) these Greensmith and Gutmanis (1990), a surface outcrop of reconstructions must be interpreted as speculative at best. Midley Sand was interpreted as a remnant of the early Once established, the barrier provided a period of barrier system, forming a series of low-relief offshore bars prolonged protection to the back-barrier estuary of behind which the fine-grained back-barrier sediments Walland Marsh. Between c. 6000 and 3000 to 2000 cal. accumulated. Greensmith and Gutmanis (1990) suggested yrs BP, peat-forming communities spread and flourished that periodic breaching of the Midley Sand barrier by across much of the area, supporting a range of habitats floodwaters from the Wealden catchment flushed minero- ranging from saltmarsh and transitional freshwater com- genic sediment across a subtidal apron on which the gravel munities to fen can, scrub and raised bog (Waller et al. in beaches of Dungeness Foreland subsequently developed. press). Inundation of these peat-forming communities Radiocarbon dates from the base of this apron place the occurred over several hundreds or perhaps thousands of development of the Midley Sand barrier between 5742- years (Spencer et al. 1998), probably due to the flooding 571 8 and 2765-2741 cal. yrs BP. of a large estuary in the eastern portion of Romney Marsh The importance of the Midley Sand barrier has been proper (Long et al. 1998). questioned recently by a number of workers. For example, Innes and Long (1992) and Long and Innes (1993,1995b) completed three stratigraphic transects across the outcrop of Midley Sand at its typesite, the Midley Church bank. Study area and methodology These authors found that the surface sands there overlie a The interface between the barrier and barrier estuary laterally extensive peat bed, which is similar in age and sedimentary environments is of central importance in the depositional origin to the main deposit of peat recorded understanding controls on marsh-wide patterns of coastal across much of Walland Marsh. This showed that the evolution during the Holocene. Towards the western limit surface outcrop of sand at the Midley Church bank is, in of the gravel outcrop at Broomhill (Fig. 2.2), Tooley and fact, one of the youngest elements of the Walland Marsh Switsur (1988) studied a sequence of peats and intertidal stratigraphy, and that the Midley Sand (at this site at least) silts, clays and sands adjacent to the gravel barrier. These cannot have formed the initial barrier behind which the sediments illustrate both marine and freshwater influences Holocene Barrier Estuary Evolution ROMNEY MARSH WALLAND MARSH Upland 0 km 10 I.S,111~,,J Fig. 2.1. Proposed evolution of the gravel barrier conzplex, after Lewis (1932).