East English Channel

MV Ocean SeekerThe Eastern English Channel was chosen as a study area due to its abundance of marine aggregates and lack of research with regard to prehistoric seabed deposits. A palaeovalley feature was chosen as a study area, situated between the licensed aggregate extraction areas 464 West and 464 East, both of which are operated by United Marine Aggregates. Palaeovalleys relating to offshore continuations of French and English rivers during periods of lowered sea level are known from the English Channel. Some of these palaeogeographic features are up to 50m in depth and several kilometres wide. Sediments within these channels, particularly those at depth can often be overlooked as they do not contain deposits economically exploited by the aggregate extraction industry. The survey was conducted aboard the MV Ocean Seeker between the 14th and 24th September 2005. The survey area was targeted for survey using geophysical (seismic and sidescan survey) and geotechnical (vibrocore and grab samples) methods.

Geophysical survey

Sidescan survey

The review of the sidescan sonar data showed a seabed comprising sandy gravel with trawl scars caused by fishing activity. During the data interpretation, the presence of irregular objects with shadows that could possibly be anthropogenic debris was noted.

Seismic survey

Palaeochannel seen on sub-bottom profiler dataThe seismic data was acquired using a boomer sub-bottom profiler. A palaeovalley feature was identified in the seismic data. It extended throughout the study area, a distance of approximately four kilometres. The valley ranged in width from between 1.5 and 2 kilometres, with a depth of 40 – 45 metres. Evidence of several phases of cut and fill events were identified in the palaeovalley feature, some of which could not be traced in all the seismic profiles. This suggests that certain phases in the development of the valley are not fully represented. It is therefore difficult to postulate on the continuous development of the valley. An example of the boomer seismic data and a modelled surface are shown.

 

Geotechnical survey

Vibrocore survey

16 vibrocores were collected from eight sites identified from the geophysical data. Two cores were collected at each site. One core was taken for Optically Stimulated Luminescence (OSL) dating which was kept in a blackened sleeve. The other core was taken for geoarchaeological description and for retrieval of environmental and radiocarbon dating samples.

Grab sampling survey

Grab SamplingThe grab samples were acquired using a Hamon grab. Each sample was approximately 8-10 litres in volume and retrieved from the upper 0.15 metres of the seabed. The samples were transferred to the environmental department at Wessex Archaeology and wet sieved through a nest of sieves in accordance with standard artefactual recovery procedures. None of the finds recovered were of prehistoric date.

Geoarchaeological analysis

The geoarchaeological analysis showed sediments relating to fluvial estuarine and marine processes. One important alluvial sequence featuring silts and clayey silts with interbedded fine sands was targeted for environmental sampling. One of the vibrocores featured gravels which had been oxidised. This is clear evidence that this deposit had been exposed to the air at some stage. This deposit was stratigraphically the oldest sediment (above bedrock) identified in this area and it is possible that it has been exposed on more than one occasion.The types of sediment units identified in the lower stratigraphy reflect high and low energy environments possibly relating to fluvial/estuarine sedimentation. Sub-units identified within these major sedimentary units revealed repeated layers of fine sands and silty clays which could indicate repeated, possibly seasonal, flooding. A brown sandy gravel with occasional mussel shell occurring in one of the vibrocores was considered to be indicative of a high energy environment with a possible marine contact inferred from the molluscs. It is possible that this may reflect rising sea levels represented by a transition between fluvial/estuarine conditions and marine conditions.

Vibrocore VC-3Vibrocore VC-7

 

 

 

 

 

 

Environmental analysis

The pollen spectrum from the silts and clays in the analysed sample is dominated by pine and birch pollen. Although both of these species are often over represented in pollen spectra, the substantial numbers recorded here do suggest local growth and dominance on drier ground. The absence of any other tree and shrub reflects and environment likely to have existed either in the very early Holocene, Flandrian Chronozone I (c. 10,000 BP) or possibly the Windermere/Allerød late glacial interstadial (c. 13,500 BP). Foraminifera and ostracods were retrieved proving sub-littoral, estuarine and freshwater environments during the early Holocene period. The general trend shows estuarine and freshwater environments replaced by sub-littoral and shallow marine deposits. The most interesting ostracod assemblage shows silts and clays which appear to have been deposited in slow moving and still freshwater environments as demonstrated by the dominance of the taxa Ilyocypris monstrifica. This is the same deposit identified from the pollen data as being a freshwater fen possibly of very early Holocene or Windemere/Allerod date.

Dating

No stasis horizons (soils) were encountered in the vibrocores suitable for radiocarbon dating. Shells were abundant in some of the deposits. Due to the well preserved nature and probable rapid deposition of some of these deposits molluscan material from suitable deposits were chosen for radiocarbon dating. The latest episode of deposition is sub-littoral gravels and sands which have been dated to 8442±35 BP. This is a clear indication that the study area was being submerged at this time. Beneath this deposit samples from series of channel features thought to form part of a braided channel system are dated to 9811±35BP and 9663±35 BP. Five samples have been chosen for OSL dating and these have been sent to OSL Laboratory, Royal Holloway where they are currently being processed.

Conclusions

The area revealed a complex sedimentary sequence relating to fluvial activity during the Pleistocene period. Terrestrial, fluvial, freshwater, estuarine and shallow marine sediments were identified proving both the preservation of non-marine sediments and that landscapes with environments favourable for occupation existed in this area. The original formation of the main valley feature is difficult to date although they are probably pre-Anglian (c. 478,000 BP). OSL dating is currently being undertaken on some of the sediments in order to test this hypothesis. By counting back the major cut and fill events observed within the seismic data it is possible to generate nine possible rises and falls of sea level in the study area. This suggests that the earliest deposit might be of Cromerian date (c. 700,000BP). This is well beyond the range of radiocarbon and OSL dating. OSL dating of some of the sediments is hoped to generate a terminus ante quem (earlier than) date for some of the earlier deposits.