About 10,000 years ago at the beginning of the Holocene Period, sea levels began to rise and drown coastal areas. It didn’t happen all in one go but episodically over time. At the maximum extent of the Devensian ice sheet around 21,000 BP the sea level was about 100 m below present levels (Howells 2007). The melting of the ice sheet and isostatic rebound caused the relative sea level changes. Present sea level was achieved about 5000 years ago. The deposits laid down on the coast during the period of sea rise are made up of layers of soft blue-grey marine clays and silts inter-bedded with freshwater peat. This reflects the way that during the overall time of sea level rise the level rose and fell many times, depositing marine clays and silts when it flooded inland, and allowing salt marsh and peat to develop when the sea receded.
Many coastal areas have these ancient peat beds and boreholes in the UK have shown that they can exist in some places at depths up to 18 m below Ordnance Datum which provides the evidence that sea levels were once lower and the sea level has since risen. However, in the coastal zone near Swansea the peat more typically occupies depths between 2 m above and 2 m below OD, deepening southwards. Offshore the peat lies 16 – 20 m below OD (Barclay 2011).
Here at Threecliff Bay (also known as Three Cliffs Bay) on Gower, South Wales, a bank of storm beach stones and pebbles, now isolated from the shore by a large intervening dune of wind-blown sand, has covered and protected the old peat beds until recent times. The storm beach deposit and underlying peat lie in the final meander loop of the Pennard Pill before it skirts the dune and flows over the shore to sea. Now exposed, the layers of peat and clay are eroding fast. Large lumps are detached around the margins of the bed.
All over the surface of the peat there are random branching patterns of dotted lines. Each “dot” is the cross-section of a stem of salt marsh vegetation preserved in situ. At the edges of the bed, the constituent layers of soft clay and peat are revealed in cross-section. Perfectly preserved remains of plant stems and roots can been penetrating through the strata in their original life position. Rusty staining and deposits in the peat bed are caused by decomposition of the organic matter.
Barclay, W. J. (2011) Geology of the Swansea district – a brief explanation of the geological map Sheet 247 Swansea, British Geological Survey, Keyworth, Nottingham, NERC, ISBN 978-085272581-8, 24-25.
Howells, M. F. (2007) British Regional Geology: Wales, British Geological Survey, NERC, ISBN 978-085272584-9, 195-196.