Dungeness and Romney Marsh

Barrier Dynamics and Marshland Evolution

Antony Long, Martyn Waller, Andrew J. Plater, Martyn Waller, Andrew J. Plater

• Verlag: Oxbow Books
• Jahr: 2007
• ISBN:
  9781782974871
• Thema: Social Sciences
• Thema: Archaeology

The Romney Marsh / Dungeness Foreland depositional complex comprises an extensive tract of marshland and associated sand and gravel barrier deposits, located in the eastern English Channel. This monograph presents the results of a programme of palaeoenvironmental investigation aimed at improving our understanding of this internationally-significant coastal landform. The focus is on the evidence for landscape change during the late Holocene, from c. 3000 BC onwards, and on identifying the local, regional and global driving mechanisms responsible for the changes observed. The research details the results from two related projects, each funded as part of English Heritage's Aggregate Levy Sustainability Fund scheme. The first project concerns the late Holocene evolution of the port of Rye, located in the southeast part of the complex, and the second the depositional history of the gravel foreland. Topics explored include the vegetation and land-use history of the study area, methodological issues relating to the collection and interpretation of radiocarbon dates from coastal lowlands, the role of compaction in influencing landscape and sea-level change, and the effects of medieval storms on coastal flooding and landscape change. This monograph is intended for students and researchers interested in Holocene coastal evolution and sea-level change, coastal vegetation history and land-use history, and the development of new techniques for reconstructing past environmental change in coastal lowlands.

1

Introduction

Antony J. Long, Martyn P. Waller and Andrew J. Plater

1.1 Introduction

At the height of the last glacial maximum, about 24000–18000 years ago, global sea-level fell up to 120 m below its present level as water from the oceans became trapped on land as ice. This lowering of sea level exposed vast areas of continental shelf around the world and, in northwest Europe, the North Sea shrank such that Britain became part of the European land mass. The subsequent melting of the ice sheets caused the re-flooding of these continental shelves, a time transgressive process that meant that the shallow Strait of Dover only became flooded relatively recently, after c. 6000 calibrated years (cal. yrs) BC, and Britain regained its island status. Most of the great ice sheets had melted by c. 4000 cal. yrs BC or were close to their present size. From this time onwards, changes in the height of the sea with respect to the land (known as “relative sea-level” (RSL)) was determined by the complex interplay of subtle on-going changes in ocean volume due to minor climate changes, as well as other processes such as land uplift or subsidence. In southern England, beyond the former margins of the British and Fennoscandanavian ice sheets, isostatic sinking of the land meant that RSL continued to rise by about another 5 m or so from 4000 cal. yrs BC to present.

The rise in global sea-level since the end of the last glacial maximum resulted in the reworking of extensive volumes of sediment that had accumulated on the floor of the continental shelves during the sea-level lowstand. The exposed floors of the North Sea and the English Channel were mantled with a variety of sediments; the former included extensive glacial deposits whilst the latter, lying almost entirely beyond the former ice limits, was covered in terrestrially-derived fluvial sediments deposited by the great rivers that once meandered across its exposed floor, including the former Thames, Rhine, Meuse and Somme. Some of these sediments infilled former river valleys incised into the floor of the Channel, others accumulated as spreads of sediment in terraces and sheets, several tens of metres thick in places, that were reworked and transported landwards as the Channel floor was flooded.

Computer modelling suggests that during the initial stages of flooding, the eastern English Channel was a shallow, relatively low energy embayment, with a low tidal range (Austin 1991). However, as water depths increased and as the Strait of Dover became flooded, so the tidal range increased, and the area became transformed into a much higher energy environment. Strong nearshore currents, moving in an easterly direction along the south coast of England from the Isle of Wight and through the Strait of Dover, initiated and sustained the nearshore drift of sediment from the west to the east. These sediments began to infill a series of small estuaries that discharge from the Hampshire, Sussex and Kent coasts into the English Channel, with small sand and gravel barriers developing locally at estuary mouths (Jennings & Smyth 1991).

Most of the sand and gravel barriers that developed around the British Isles during the current interglacial (or “Holocene”, the time period since c. 11430 ± 140 ¹⁴C years BP, or 9560 and 9300 cal. yrs BC) are relatively modest, ephemeral features. These barriers have often passed through several phases of growth, stability and destruction in response to variations in the rate of RSL, changes in storm magnitude and frequency, as well as variations in the supply of sediment (Orford et al. 1991). Despite their variability, these barriers have played a critical role in shaping the patterns of coastal and estuary evolution around many parts of the British Isles by moderating the wave and tidal energy that enters an estuary and distorting long-shore and cross-shore sediment transport pathways. Insights into the early configuration of these barriers can be seen from the preserved remnants of these early structures, especially in northern Britain where land uplift has resulted in a fall in RSL since the mid Holocene (e.g. Dawson et al. 1999). However, the upwards trend in Holocene RSL in southern England has meant that most of the early barriers here are now drowned and either lie buried or have been entirely reworked by wave and tidal processes. Most, that is, except for one exceptional stretch of coastline located in the eastern English Channel. Follow the coast from the Isle of Wight eastwards and one traces a shoreline crenulated by shallow embayments and projecting headlands. Many of these headlines are made of bedrock, most famously the 160 m-high Chalk cliffs known as Beachy Head (Figure 1.1). However, to the east of Beachy Head is an equally impressive headland, this time comprising the largest expanse of exposed and buried sand and gravel barrier in the British Isles, a vast expanse of low-lying sediment, never exceeding +7 m Ordnance Datum (OD, present sea-level), that projects some 20 km into the waters of the eastern English Channel. Known as the Romney Marsh/Dungeness Foreland depositional complex, this unique landform owes its existence to a particular interplay of RSL and sediment supply that has created one of the largest and best-preserved coastal barriers and associated hinterland marshes in the British Isles and, arguably, in northwest Europe.

The first attraction of the Romney Marsh/Dungeness Foreland depositional complex to any coastal geomorphologist is the corrugated gravel beach ridge plain that comprises Dungeness Foreland (Figures 1.2a, b). The oldest of the exposed beaches are located in the central part of the study area, where they form low, subdued landforms, partly submerged by marshland sediments that abut against and in places overlap them. These beaches have a typical surface elevation of about +1 m OD, some 6 m or so below their contemporary equivalents at the present foreland ness. The beaches rise in elevation as they become younger, tracing the rise in RSL that has occurred during the mid and late Holocene and which partly controlled their height of deposition.

Compared with the bold geomorphology of Dungeness Foreland, the lower relief of Romney and Walland Marshes (although Romney Marsh is used as a generic name, it strictly applies just to the area north of the Rhee Wall (Figure 1.1)) which lie in its protective lee may, at first glance, appear of secondary interest. But this is not so. The subtle surface relief of the marshland tells a rich story of changes in tidal flooding, land claim, as well as differential land movement due to variable compaction of the sediments that underlie the marshland surface. These marshland sediments are thick, locally reaching 30 m or more, and contain within them a rich archive of coastal and sea-level change that is intimately linked to changes in the dynamics of the barrier itself, as well as variations in processes within the Wealden catchments of the rivers (Rother, Tillingham, Brede and Pannel) that discharge into the study area from the west.

What is it, therefore, that makes this particular depositional complex so interesting to study? Firstly, the area contains an exceptionally well-preserved suite of sediments and landforms that tell a fascinating story of the interplay between physical and human processes during the last 6000 years or so. This resource contrasts with the often incomplete record preserved in smaller coastal landforms that have experienced several phases of barrier development and breakdown during the mid and late Holocene. Secondly, the interplay between barrier and marshland landforms and sediments makes the study area ideal for examining the processes that control barrier evolution over a variety of timescales. For example, here it is possible to assess the impact of an individual storm event (or cluster of storms, such as those of the 12th and 13th century AD) on the complex, or the response of the landform to century to millennial-scale changes in RSL. Thirdly, the back-barrier marshland complex contains a very extensive suite of minerogenic (clays, silts and sands) as well as organic (saltmarsh, freshwater reedswamp, fen carr, and raised bog) deposits preserved as peats. Palaeoenvironmental analyses of these deposits provide insights into past environmental changes in the study area, including shifts in climate and prehistoric and more recent land-use history. Fourthly, human occupation and resource use during historic and pre-historic times is strongly coupled with the environmental and landscape histories of the barrier and back-barrier systems, as well as their contiguous river basins and coastal waters. In this respect, unravelling the natural history of the region presents us with detailed insights into the lives of former peoples who lived and worked on the gravel foreland and marshland–perhaps being opportunistic in the first instance but exerting progressively greater imprint on the landscape during the archaeological past. These and other reasons that we hope to explain in this monograph, make the Romney Marsh/Dungeness Foreland depositional complex of more than simply local interest; indeed we believe that its sediments and landforms record the interplay of complex global, regional and local processes that combine to shape one of the most fascinating parts of the British coastline.

1.2 This monograph

This monograph reports the results of two interdisciplinary research projects conducted under English Heritage’s Aggregate Levy Sustainability Fund research programme. The projects concern the landscape history of two areas of the Romney Marsh/Dungeness Foreland depositional complex (Figure 1.1), with a particular focus on the interplay between natural and human processes during the late Holocene (from about 3000 cal. yrs BC to present). Although conducted as distinct projects, the two programmes of research are complementary and this monograph provides an opportunity to present the results of each in a single integrated volume. The field and laboratory aspects of the research presented here were completed over an 18 month period, commencing in November 2002. This research generated a large data archive, a brief public outreach report (Long et al. 2004), and two technical reports (Roberts & Plater 2005; Plater et al. 2006). There followed a 12-month publication contract that enabled the formal publication of our research findings, first in a series of specialist research papers (Long et al. 2006a; Long et al. 2006b; Roberts & Plater 2007; Schofield & Waller 2005; Stupples & Plater 2006; Waller & Schofield 2007; Waller et al. 2006). The aim of this monograph is to draw together the research conducted under the Dungeness Foreland and Rye projects, thus far published in a series of separate papers, into a single publication, and also to present aspects of the two projects not previously published.

Figure 1.1 Location map of the Romney Marsh / Dungeness Foreland depositional complex.

A large number of individuals and organisations have been involved in the Dungeness Foreland and Rye area projects. The core academic staff of Antony Long (Durham University), Andy Plater (Liverpool University) and Martyn Waller (Kingston University), were supported by research staff located at each respective institution (Damien Laidler, Paul Stupples and Ed Schofield). An Optically Stimulated Luminescence (OSL) dating programme was conducted by Helen Roberts (Aberystwyth University) in conjunction with Andy Plater, radiocarbon dating support was provided by Alex Bayliss and John Meadows of the English Heritage Scientific Dating Section, and palaeomagnetic (PSV) dating was undertaken in collaboration with John Shaw and Sigrid Hemetsberger (Geomagnetism Laboratory, University of Liverpool). Peter Wilson was project manager for English Heritage and provided us with excellent support throughout. Much of the work presented here has benefited from the support of the Romney Marsh Research Trust, both during the project itself but also in the years prior to this work when much of the groundwork for our investigations was laid.

1.3 The study area

The Romney Marsh/Dungeness Foreland depositional complex comprises three main landscape units; first a series of valleys that drain from the Weald and enter into the western margin of the st...