Lion Business Park, Denton Marshes, Gravesend, Kent: Geoarchaeological Interim Assessment Report

In July 2006 Oxford Archaeology (OA) carried out a purposive
geoarchaeological borehole survey at Denton Marshes, Gravesend, Kent.
The investigation was commissioned by Ardent Consulting Engineers Limited in advance of the development of the site as a business park. The borehole survey forms the first stage in a two stage archaeological evaluation strategy, with the intention of providing baseline stratigraphic
data prior to a programme of trial trenching.
Following a brief review of existing geotechnical records and previous archaeological work within the area, five boreholes were drilled at selected locations. Correlation of key stratigraphic units has allowed the creation of a preliminary subsurface deposit model for the site.
Extensive Pleistocene deposits, probably deposited in a cold environment during the Devensian glaciation by either fluvial or solifluction processes, exist as units of coarse sand, gravel and stiff clay, directly overlying chalk
bedrock. Similarities, however, of some these deposits with Thanet Sands and London Clay may suggest that a more complex sequence of bedrock exists in this area than that indicated by the British Geological Survey maps. Examination of the elevations of the surface of the Pleistocene deposits revealed a significant area of high ground just to the southeast of the site that probably represents the edge of the terrace. A possible area of
higher ground was also identified near to the centre of the site, which could have persisted as an island of drier ground within a predominantly wetland environment from the late Mesolithic period onwards. As such this area, as well as the terrace edge, may have provided a focus for
prehistoric activity. Based on current regional models of sea-level change and estuarine inundation on the Lower Thames floodplain, the elevations suggest that this area may not have been submerged until after the middle
to late Bronze Age.
Overlying the Pleistocene deposits are significant thicknesses of undisturbed Holocene alluvial and peat deposits that exist throughout the
site, sealed beneath variable thicknesses of made-ground. Superficially the Holocene sequence is similar to numerous other alluvial sites investigated on the Lower Thames floodplain. Closer examination, however, reveals
significant variation representative of a range of different sedimentary environments that could have existed at any one time. This is probably a reflection of the location of the site at the edge of the gravel terrace as
opposed to a mid floodplain situation, where local factors of topography and hydrology will have effected sedimentation patterns.
A major, though discontinuous, peat body exists sealed either within minerogenic silt-clays or, at higher elevations, resting directly on Pleistocene deposits between approximately -2.0m and -5.0.m OD. The surface of the peat varies between 2.4m and 7.0m below current ground
level. The most deeply buried deposits exist in the northern and eastern parts of the site A broad Neolithic or Bronze Age date may be assigned to these deposits on the basis of elevation, and it is possible they are
equivalent to Devoy’s Tilbury III and IV peats. This would, however, require confirmation by radiocarbon dating. Further localised peat deposits were noted at higher elevations in the geotechnical logs. These Oxford Archaeology Lion Business Park, GRLION06 Interim Geoarchaeological Assessment Report © Oxford Archaeological Unit Ltd. August 2006 2may be considered equivalent to Devoys Tilbury V peat of Roman date. All records however derive from cone penetration tests carried out during
geotechnical investigations and were not recorded in either geotechnical or purposive archaeological boreholes.
The presence of extensive organic deposits preserved at depth across the site area, has clear potential for the preservation of waterlogged timbers and material for paleoenvironemntal reconstruction (pollen, diatoms,
plant remains, insects). These deposits have the potential to address aspects of floodplain development, sea-level change and vegetation patterns during the prehistoric and later periods. Suitable samples have been retrieved during the borehole survey for future analysis if deemed
appropriate. This potential, however, would be greatly enhanced if found in association with archaeological remains. Although neither the peats or minerogenic deposits examined from the purposive archaeological boreholes contained visible anthropogenic inclusions, a number of areas across the site have been identified that may have acted as a focus for activity in the past.
Examination of the thicknesses of made/ground across the site suggests that open trenched evaluation of the upper minerogenic clay/silts is achievable to the levels at which Roman archaeology was discovered in 1998 at the Water Treatment Works to the north of the site. Given the
depth of the underlying peat deposits, it is recommended that a programme of rapid test pitting during the trial trenching be carried out on targeted areas.