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Thursday, 2 October 2014

Bristol Channel Palaeolandscapes

This is an earlier post of mine, from 2011, which is very relevant for our discussion on the sequence of events at Flat Holm:

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:
University of Birmingham:
Royal Commission on the Ancient and Historical Monuments of Wales:

There is a great deal of detail in the report, which deals with Liverpool Bay and the Bristol Channel area.  It's a pity that in the latter research area, the work was not extended eastwards to the islands of Flat Holm and Steep Holm, which are conventionally taken as being situated at the junction between the Severn Estuary and the Bristol Channel.   Here are the two key maps:

This is the reconstructed situation in the Late Upper Palaeolithic, after the Devensian Glaciation but before the sea had started its incursion into the area now known as the Bristol Channel.

During the Mesolithic (after 10,000 yrs BP) the sea was flooding eastwards, gradually converting freshwater lagoons and lakes into brackish water bodies and then extensions of the sea, forcing rapid ecological and landscape change.

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......

1 comment:

TonyH said...

Vince Gaffney is notable more recently for his involvement with the Hidden Landscapes Project highlighted of late at Stonehenge and via BBC TV.