Cuckmere Haven

At the end of December 2010, Oxford Archaeology undertook a borehole survey
within the Cuckmere Haven, East Sussex, on behalf of East Sussex County Council,
to help ground truth two recent phases of geophysical sediment mapping (Bates
2010a and 2010b). The fieldwork aimed to identify the base of the bedrock surface
and provide samples for lithological and palaeoenvironmental assessment. This
work was part of a wider heritage study of the Haven that was designed to help
inform local communities about the heritage resources of the valley and the
potential impacts of future coastal change.
The fieldwork successfully sampled one sequence to a depth of 30m within the
valley to help ground truth the geo-electric sections. Unfortunately ground conditions
prevented a second sample from reaching full depth and this was abandoned in
order to concentrate on further assessment of the borehole samples. The base of
the alluvium was encountered at a depth of 24m below ground surface and solid
chalk was encountered at 27.5m in depth. The Holocene sequence comprised basal
silty clays and peats overlain by thick laminated clayey sands. Inter-stratified sand
and clay deposits were identified between 11.40m and 5.60m in depth, and these
were sealed by overlying homogeneous clays and silty clays.
Preliminary assessment of the ostracod and foraminifera assemblages suggest a
transition from freshwater to brackish conditions at the base of the sequence. The
marine incursion of the valley is dated to 8030±30 yr BP. Thick deposits of overlying
laminated sands appear to represent brackish conditions within tidal mudflats. There
is a gradual transition into mid/high salt marsh conditions further up the sequence,
with an increasing marine influence around 8.50m, possibly reflecting tidal surges.
Brackish tidal mudflat conditions return with the deposition of the upper silty clays.
The present-day predominantly freshwater environment of the Haven is therefore a
relatively recent development.
A similar sand dominated sequence has been recorded within the Lower Ouse, but
this sequence appears to lack the thick freshwater organic and peat deposits
present within the valleys recorded to the east, such as those found in the Combe
Haven. However further dating and palaeoenvironmental assessment is required
before more comprehensive comparisons between sequences can be made.
Only a single tie point for ground truthing the geophysics currently exists, but when
considered with the other available data, the work was able to identify the base of
the bedrock and pick-up subsurface features within the geo-electric sections. The
relationship between the geophysical profile and the drill log at the site indicates
that the base of the Holocene alluvial surface coincides with the 6.38ohm/m contour
(light to dark green) and consequently we have used this to infer the shape of the
topographical template along all four transects. The revealed valley profile shows
abrupt steep valley sides on to a moderately smooth slightly concave base, possible
as the result of erosion by continuous migrating channels.
The works has confirmed the presence of significant lateral and vertical variation
within the sedimentary sequence and palaeotopography across the valley. The true
significance of this is currently unclear and highlights the need for further detailed
study and sampling.