Another Pennant’s Swimming Crab

Close up of the small Pennant's Swimming Crab at Rhossili, Gower, South Wales June 2009.

A small sand-covered Pennant’s Swimming Crab, about 2 cm across, emerging from its burrow in the wet sediments at low tide on Rhossili Beach one June.

For more details of this little seashore creature, Latin name Portumnus latipes (Pennant), see the earlier post.

A Pennant's Swimming Crab emerging from the wet sand at low tide on Rhossili beach, Gower, South Wales.

 

More Ancient Wood in Peat

Ancient tree stump from a submerged forest in life position on the beach at Whiteford Sands

Yesterday I posted pictures of a massive tree trunk that is being washed out of the peat on the beach at Whiteford Sands. It is lying on its side. Next to it are the remains of a clump of smaller trees. [I have indicated their positions with arrows in a context shot below]. The trees were growing close together. Only the stumps survive but they are preserved upright in their life position, complete with root systems penetrating downwards through the peat. I noticed for the first time in preparing these photographs for the post that someone has very neatly and professionally cut off one of the tree roots; this may substantiate my assumption that someone is in fact carrying out research on the newly emerging trees of this submerged forest.

Ancient Wood in Peat

Close-up of the intricate pattern of woodgrain in ancient wood preserved by peat.

Some of the ancient wood that has long been buried in peat and clay deposited after the last ice age has wonderful textures and woodgrain patterns. Whole recumbent tree trunks have been emerging from the peat as a result of recent beach erosion at Whiteford on the Gower Peninsula in South Wales. This wood is not fossilised but is preserved in its original state by the anaerobic conditions in which it was buried – in the same way that the bodies of the so-called Bog People were preserved.

Carboniferous Limestone Textures at Threecliff

Rock pattern and texture in Carboniferous Limestone

A further assortment of interesting rock patterns and textures in Carboniferous Limestone strata of the cliffs on the east and west sides of Threecliff Bay on the Gower Peninsula in South Wales. Of particular interest is the honey-comb texture of erosion along the bedding planes of some of the dipping layers, and the red colouring due to iron bearing minerals of other areas,  on the east side of the bay. On the west side of the bay the rocks seem less weathered and are often encrusted with patches of black lichen.

Peat at Threecliff Bay

Ancient peat bed at Threecliff Bay in Gower, South Wales

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.

REFERENCES

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.

Pebbles at Langland Bay

Pebbles from Carboniferous Period rocks at Langland Bay

The pebbles at Langland Bay are all sedimentary rock as far as I can see but they include many different rock types from shores further away. Red and green Devonian sandstones, siltstones and conglomerates; light and dark bluish-grey Lower and Upper Carboniferous Period limestones (some with fossils), and shales; Namurian sandstones, grits, shales and coal measures with black and iron-bearing deposits; and no doubt the occasional erratic brought in by the ice sheets in periods of glaciation.  Consequently there is a great variety of colours, textures and patterns. Pebbles with holes made by sea creatures such as piddocks or other boring bivalved molluscs, or by marine polychaete worms and sponges are also a frequent occurrence. The pictures show the pebbles mostly in the the positions where they were found although I may have moved the odd one or two.

Rock Formations at Langland Bay

Carboniferous limestone rock formation at Langland Bay

This post provides a context for the earlier post of mostly close-up images in Rock Textures at Langland Bay 1. Langland Bay is a popular beach near Swansea in South Wales. It is located on the south coast of the Gower Peninsula. It has a wide stretch of lower sandy shore, and pebbles landward in the central part. There are also wide expanses of low-lying rock platforms with hundreds of shallow pools in which to hunt for seashore creatures. To each side of the bay low cliffs of Carboniferous period sedimentary rock are overlain with much more recent glacio-fluvial and later poorly consolidated deposits.

The distinct layers of Hunts Bay Oolite, High Tor Limestone, and Penmaen Burrows Limestone form the southwest limb of the Mumbles anticline that extends from east to west. Here at Langland the sea has breached the rocks to create the embayment. The strata are riven by numerous joints and minor faults that have allowed the sea to penetrate, eroding away the rock to form small coves, undercuts, caves, tunnels, and passages to explore. The photographs shown here probably do not do justice to the site, as it was a very dull and overcast day when I visited, but I hope they serve to illustrate that the geology of Langland Bay is interesting from many points of view.

REFERENCE

Bridges, E. M. (1997) Classic Landforms of the Gower Coast, Series Editors R. Castleden and C. Green, The Geographical Association and The British Geomorphological Research Group, page 17. ISBN1-899085-50-5.