The stony west bank of Pennard Pill

Sea Lavender on lichen covered stones of the river bank at Three Cliffs Bay Just before the Pennard Pill watercourse takes a dramatic swing to loop around the giant sand dune to get to the shore at Threecliff (Three Cliffs) Bay on the Gower Peninsula, the right hand or west bank is composed of rough stones and then transitions into a salt marsh. [The area lies on the opposite side of the river to a more substantial and higher shingle bank that can be reached via a set of concrete stepping stones].

The low shingle surface of the right bank is relatively stable. Despite regular tidal inundations of brackish water, life clings to the limestone. Last August it was particularly attractive, covered with bright patches of yellow and black lichens, and ground-hugging clumps of partially red-stemmed plants with clusters of small pink flowers. I will have to find out what these plants are the next time I visit. I didn’t take close-ups. I had thought they might be Sea Heath (Frankenia laevis) but apparently that does not grow in this area – although it likes the same kind of habitat.  I think Sea Sandwort was also present. However, the numerous flowering stems of Sea Lavender I did recognise; and these plants were found equally spread in stony ground and on the wetter salt marsh area.

Rocks on the West Side of Three Cliffs Bay

Limestone rock strata at Three Cliffs Bay

The rocks on the west side of Three Cliffs (Threecliff) Bay on Gower in South Wales are made up predominantly of Lower Carboniferous Limestone. Although there are some Devonian rocks higher up the valley, these are mostly obscured and hard to spot. The starting point for the images shown in this post is the south face of the large vegetated dune that juts out into eastwards into the bay and causes the Pennard Pill river to be diverted in a great meander loop in order to reach the sea.

At the foot of the dune (SS 535 881), the northern end of the western cliffs emerge. There is a very small exposure of Avon Group strata, comprising grey-green finely bedded shales and mudstones which used to be known as the Lower Limestone Shales. Most of this early part of the Carboniferous sequence is hidden from view by the sand deposits but it extends westwards into the area called Stonefields.

On top of the Avon Group shales lies a sequence of Pembroke Group strata starting with the Black Rock Limestone Subgroup, then the Gully Oolite Formation, Caswell Bay Mudstone Formation, High Tor Limestone Formation, and finally the Hunts Bay Oolite Subgroup. The strata, though horizontal when first laid down, have been steeply tilted by subsequent earth movements to 60 – 80 degrees south. Looking at the exposures of rock in the face of the cliffs, the rocks become increasingly younger to your left (southwards) and older to your right (northwards).

A basic description of these rocks can be found in Barclay (2011) and George (2008) and the geological map for the Swansea area (Sheet 247). The rock layers reflect not only the conditions under which they were laid down initially but also the effects of great pressures that resulted in fracturing and faulting at later dates.The first of the sequence of rock layers belongs to the Black Rock Limestone Subgroup that, if I may quote from Gareth T. George’s excellent field guide, “comprise alternating bioclastic limestones and bioturbated lime mudstones and shales, which are succeeded by thicker-bedded bioclastic packstones with graded bedding and sets of hummocky cross-stratification (HCS) ” etc.

Stylolitic seams are said to be common. These occur where pressure within the rock causes some minerals to dissolve and seems to result in specific types of irregular white lines of calcite within limestone, and also irregular crystalline textures along some flat surfaces. This phenomenon may be responsible for a number of the features shown in some of the images in the gallery below.

REFERENCES

Barclay, W. J. (2011) Geology of the Swansea District – a brief explanation of the geological map Sheet 247 Swansea. Sheet explanation of the British Geological Survey. 1:50 000 Sheet 247 Swansea (England and Wales). British Geological Survey, Keyworth, Nottingham, NERC, ISBN 978-085272581-8, pp 1 – 9.

George, G. T. (2008) The Geology of South Wales – A Field Guide. Published by gareth@geoserv.co.uk, ISBN 978-0-9559371-0-1, pp 77 – 82.

More Rayed Trough Shells at Rhossili

Living Rayed Trough Shell in a shallow tide pool on a sandy beach

Following the line of the limestone cliffs towards Kitchen Corner as the tide receded, the tide pools and beach were littered with dozens of living Rayed Trough Shells (Mactra stultorum Linnaeus) as they popped up to the surface of the sand. I don’t quite know why they chose to do this but it afforded an opportunity to see the living animal as opposed to the dead ones and empty shells that wash up more frequently on Rhossili Beach.

Two pale fleshy tubes joined together were extended between the two hinged shell valves. One inhalent siphon for sucking water with suspended nutrients inwards, and one exhalent siphon for dispelling de-oxygenated water with bodily waste products. I was afraid that these bivalved molluscs would die while gaping and exposed to the air, so I picked up a few and put them in the water of the pools but they were not very lively and did not re-bury themselves. I was surprised that no-one else seemed to notice them. Even the dog that I saw appeared more interested in splashing in the pools than snacking on the free harvest.

Tracks and Trails on Whiteford Sands

Furrowed trails made by common winkles on wet beach sand

You don’t exactly have to keep your nose to the ground to see them but you do have to be a keen observer to notice all the different tracks and trails left on the soft wet sediments of the beach at low tide. Larger marks left by people and vehicles are the first ones you see. Bird footprints are every where. The birds are feeding on all sorts of invertebrate seashore creatures like worms, small crustacea and molluscs – all of which leave holes, burrows and furrows as they move in and out of the sand and across the surface. Some of the pictures shown here simply aim to give the general context for the area of Whiteford Sands that I was walking across. If you look closely the other images, you will see not only the ripples in the sand but also the intricate network of traces left by the virtually invisible organisms that inhabit this ecosystem. The larger furrows in photos 1, 12 and 13 are made by the common winkle (Littorina littorea Linnaeus). I cannot name each animal that is responsible for each of the other types of trace. However, I am sure that there will be some specialists out there who could, especially those researchers concerned with the interpretation of trace fossils (the ichnologists).

Click images to view full size.

View looking west towards the sea at Whiteford Sands

View looking north-east towards the dunes at Whiteford Point

Wet seashore sand with marine invertebrate and other tracks and traces

Wet seashore sand with marine invertebrate tracks and traces with bird footprints

View looking north-east over wet seashore sand ripples with marine invertebrate tracks and traces at Whiteford Sands

Wet seashore sand ripples with marine invertebrate tracks and traces

Wet seashore sand ripples with marine invertebrate tracks and traces

Wet seashore sand ripples with marine invertebrate tracks and traces

Wet seashore sand ripples with marine invertebrate tracks and traces

Wet seashore sand ripples with marine invertebrate tracks and traces

Furrowed trails made by common winkles on wet beach sand

Furrowed trails made by common winkles on wet beach sand

Clustered Periwinkles at Whiteford

Large groups of common winkles clustered around the base of large stones on the beach

At low tide many thousands of common winkles or periwinkles (Littorina littorea Linnaeus) seek shelter from dessication and predation by clustering together in the few hiding places available on the beach. At Whiteford Sands these niches include the overhung bases of larger stones, crevices in ancient timbers from the rapidly emerging submerged forest, and nooks and crannies in the recently exposed ancient peat. Alternating layers of peat and clay, overlain by rocks from glacial till, provide algae-covered surfaces on which gastropods can feed, and islands of low tide refuge in the vast expanses of sand on this sea shore.

Large groups of common winkles clustered around the base of large stones on the beach

Large groups of common winkles clustered around the base of large stones on the beach

Large groups of common winkles clustered around the base of large stones on the beach

Large groups of common winkles clustered around submerged forest wood on the beach

Large groups of common winkles clustered around submerged forest wood on the beach

Large groups of common winkles clustered around submerged forest wood on the beach

Large groups of common winkles clustered in exposed ancient peat beds on the beach

Large groups of common winkles clustered around submerged forest wood on the beach

The West Side of Worm’s Head

View of the Worm's Head from the Rhossili cliffs

1. Looking at the tidal island of Worm’s Head from the cliff top of the Rhossili Headland on the Gower Peninsula, South Wales.

I had often enjoyed exploring the rocks, gullies, and pools of the wave cut platform which forms a causeway between the Rhossili Headland and the tidal island of Worm’s Head. It was not until last year, on a glorious April day with a particularly low spring tide, that I actually ventured onto the island itself for the first time. The Worm’s Head is divided into several sections. The largest and probably the highest of which is the Inner Head. A section known as The Long Neck connects this to the Middle Head with its famous Devil’s Bridge on the way to the Outer Head.

The island is composed of Carboniferous Limestone but the rock layers become younger from east to west, passing from Black Rock Limestone Group, to Gully Oolite, and then High Tor Limestone. I decided to walk along the shore on the westerly side of the island. To see what I saw, look at the pictures in the gallery below. If you click on any picture in the gallery you can view the photographs in slideshow format with their captions.

I walked along the beach as far as the Long Neck, which is a narrow stretch of jagged rocks formed by steeply sloping strata that connects the Inner Head to the Middle Head.  It looked quite difficult to negotiate the crossing although many people were having fun trying to do it. However, fearing that I might not have enough time to reach the Outer Head before the tide returned and covered the causeway again, I then turned round and followed the footpath at the base of Inner Head. The west flank of the hill was covered with bright yellow gorse flowers and patches of wild violets. It was a beautiful walk and I hope it won’t be long before I visit again to explore the rest of the island.

 

Winter Walk at Whiteford Sands

Red fishing buoy flotsam

Crisp and cold, bright and sunny, just right for blowing away the cobwebs with a walk along the strand at Whiteford Sands. On this particular winter’s day the tide had brought ashore lots of flotsam – fishing nets, buoys, floats, and crates, shoes, hard hats, and miscellaneous plastic rubbish that rested on a driftline of sand, pebbles or shells. Here are some of the things that caught my eye as I strolled the high water mark from Cwm Ivy Tor to the spit beyond Whiteford Point on Boxing Day 2013. Click on any of the images in the gallery below to view in a larger format and slideshow.