Introduction

Figure 1.

Wallasea Island on the River Crouch, Essex. RSPB LIDAR base elevations and locations of soil sampling sites: Profile 1 - Control Arable Pit 1 (CA1); Profile 2 - Control Grassland Pit 2 (CG2); Profile 3 - Flooded Grassland Pit 3 (FG3); and Profile 4 - Control Grassland Pit 4 (CG4).

Numerous coastal archaeological sites in the UK (and elsewhere) have been influenced by marine submergence, during the Pleistocene and more recently from rising early Holocene sea levels. Terrestrial archaeological sites and their associated soils were affected in different ways, along with micro- and macro-fossils that aid environmental reconstruction (Bell et al. 2000). The policy of managed coastal realignment in the UK and global likelihood of coastal flooding will continue to influence coastal archaeology and consequently there is a need for experimental data on effects on archaeological soils and sites.

This contribution reports experimental results from a 2008 pilot study designed to investigate marine inundation of archaeological sites, employing Wallasea Island, which underwent a trial inundation in 2006 ahead of total flooding in 2010.

Wallasea Island, Crouch River estuary, Essex, UK

This is an area of reclaimed land originally embanked in the thirteenth or fourteenth centuries (Heppell 2004), and which is to undergo planned inundation by the Royal Society for the Protection of Birds (RSPB) in 2010 (http://www.ucl.ac.uk/archaeology/staff/profiles/macphail.htm). An area flooded deliberately by DEFRA (the UK Government Department of Rural Affairs) in 2006 was compared to three control sites in this pilot study (Figure 1).

Figure 2.

a: Newly flooded (since 2006) areas of arable and grassland within breached sea walls. Pit 3 (FG3) is located on grassland adjacent to the 'borrow' ditch; River Crouch in background. b: Seaweed-covered estuarine muds over grassland soil profile (Pit 3). Borrow ditch, flooded arable fields and DEFRA breached sea wall in background.

Investigations

In addition to the study by the author of the soils and sediments using soil micromorphology (including microprobe), bulk grain size and chemistry (John Crowther, University of Wales, Lampeter), foraminifera and ostracods (John Whittaker, Natural History Museum), molluscs (Mike Allen, Allen Environmental Archaeology), and pollen (Gill Cruise, UCL) were investigated. Four profiles were chosen (Figure 1): 1) Control dryland arable; 2) Control dryland grassland; 3) Flooded grassland next to original 'borrow ditch' (Figures 2a and 2b); and 4) Control dryland grassland from the lowest elevation within the sea walls.

Results

The control arable (Profile 1) and grassland (Profile 2) sites produced expected results. Laboratory studies therefore focused upon Profiles 3 and 4, the flooded grassland and its unflooded control (Figures 3a and 3b) respectively. Twenty-two contexts were identified in 10 thin sections and analysed in detail. The chief characteristics of the unflooded grassland are: a grass and herb litter layer with 'peaty' organic matter, strong leaching of cations and phosphorus and instances of ferruginisation that increase down-profile; pollen of herbs, grasses and wetland plants (Cyperaceae) are present; specific conductance - a measure of 'saltiness' - is high (Na and Cl present throughout); abundant well-preserved saltmarsh and tidal flat foraminifera occur (Table 1). This profile yields insights into potential effects of saline groundwater resulting from rising sea levels. Inundation of Profile 3 led to: burial of the grassland soil by finely laminated estuarine sediments (Figure 3b), with muds infilling voids around the surface plant litter and roots; the sediment seals the Litter-mineral soil boundary. Calcareous laminae (Figure 4b) include algae which contribute to the detrital organic matter content; partial decalcification and plant material ferruginisation occur even after only 2 years of sediment 'ripening' (Figure 3b). Microprobe mapping of the soil-sediment interface (Figures 4a and 4b) confirmed the presence of salt (NaCl - Figures 4c and 4d) consistent with a very high specific conductance. Foraminifera reflect the sediment-buried topsoil sequence and have the potential to show in detail the state of preservation and species associated with the different buried soil and sediment types (Table 1). High arboreal pollen percentages (36%) recorded in the estuarine muds cannot be linked to an increase in trees locally and probably reflect the massive change in pollen catchment from local to one of regional or even extra-regional scale, consistent with palynological studies of Holocene intertidal sediments in Essex (Wilkinson & Murphy 1995). (Full results are presented at http://www.ucl.ac.uk/archaeology/staff/profiles/macphail.htm)

Table 1.Wallasea Island: assessment of organic remains, foraminifera and ostracods from Flooded (Pit 3) and Control (Pit 4) grassland (by John Whittaker).

Figure 3.

a: Control Grassland Pit 4 (CG4): ~16 cm long resin-impregnated block. Humic Ah horizon, a mixed/dumped Ahg/Bg1, a buried 'surface' (arrow), and buried bAhg/Bg1 horizon. Mottling and blackened organic matter reflecting the low elevation/wet environment. b: Flooded Grassland Pit 3 (FG3): ~11 cm-long resin-impregnated block, showing algae (a) coated estuarine clay laminae (Est) over buried Ahg and Bg horizons.

Figure 4.

Flooded Grassland Pit 3 (FG3): microprobe map of thin section M5A1 (junction of buried Ahg horizon and overlying estuarine mudflat laminae) showing distribution of a: Si; b: Ca; c: Na which occurs both as saline (NaCl) salts and as sodium carbonate; and d: Cl. Width of thin section is ~50 mm. (maps by Kevin Reeves, Institute of Archaeology, UCL).

Conclusions

The research provides insights into the following progressive effects: a) increased soil wetness and influence of saline groundwater, and b) marine flooding and burial by estuarine mudflat deposits. This pilot study helps elucidate taphonomic complications affecting archaeological sites, including the short timescales involved, and permits more accurate interpretation of previously inundated archaeological sites.

In the field of intertidal archaeology these results are already improving our comprehension of prehistoric land surfaces which were inundated during the early Holocene. Within a context of increased international awareness of the effects of global warming and sea level rise, this pilot study also serves as a useful precursor to any construction of archaeological experiments.