By Simon Fitch and Vince Gaffney With Contributions from Eleanor Ramsey and Emma Kitchen
(Visual and Spatial Technology Centre Institute of Archaeology and Antiquity, University of Birmingham, Birmingham, B15 2TT)
The West Coast Palaeolandscapes Project partner’s WWW pages can be accessed at:
Dyfed Archaeological Trust: http://www.cambria.org.uk/lostlandscapes/index.html
University of Birmingham: http://archaeologydataservice.ac.uk/archives/view/wcpp_eh_2011/
Royal Commission on the Ancient and Historical Monuments of Wales: http://www.rcahmw.gov.uk/HI/ENG/Heritage+of+Wales/Maritime+Archaeology+In+Wales/
As far as sediment types are concerned, the following two maps are instructive:
DECC -- SEVERN TIDAL POWER - SCOPING TOPIC PAPER Hydraulics and Geomorphology
Note from the upper map of this pair that the only area where glacial till is dominant on the sea bed is a strip c 5 kms offshore of the Somerset coast near Minehead and Porlock. It's not known whether this strip extends all the way to Lundy Island. Perhaps surprisingly, there are no sea-floor expanses of glacial till off the Glamorgan coast in those areas that might have been affected by piedmont glacier lobes at the time the maximum Devensian glaciation. That is not to say that such material is absent -- it might of course be buried beneath finer sediments. Closer to Flat Holm there are patches of sandy and gravelly materials on the sea bed, and extensive areas of exposed bedrock. Sediments in and around the deep channel which runs between Flat Holm and Steep Holm are generally less than 10m thick.
So if there are glacial erratics on Flat Holm, what is their source? Watch this space......