About winderjssc

Jessica Winder has a background in ecological studies in both the museum and the research laboratory. She is passionate about the natural world right on our doorsteps. She is enthusiastic about capturing what she sees through photography and wants to open the eyes of everyone to the beauty and fascination of nature. She is author of 'Jessica's Nature Blog' at https://natureinfocus.wordpress.com. Jessica has also extensively researched macroscopic variations in oyster and other edible marine mollusc shells from archaeological excavations as a means of understanding past exploitation of marine shellfish resources. She is an archaeo-malacological consultant through Oysters etc. and is publishing summaries of her shell research work on the WordPress Blog called 'Oysters etc.' at http://oystersetcetera.wordpress.com 'Photographic Salmagundi' at http://photosalmagundi.wordpress.com is a showcase of photographs and digital art on all sorts of subjects - not just natural history.

Rock Textures at Langland Bay 1

Texture and natural patterns in sedimentary rocks from the Carboniferous Period

Langland Bay on the Gower Peninsula was new to me. In all the years that I have been travelling to Gower I had not previously paid it a visit – perhaps because it is one of those popular beaches close to Swansea where visitors tend to throng – and I like to have the beach to myself! On this particular dismal August day the visitors were few and far between but it was clear what a delight the beach must be on better days.

A wide expanse of sand is bordered by low cliffs and rocky outcrops of Carboniferous High Tor Limestone and Hunts Bay Oolite. The bay has been eroded into the seaward-dipping rock layers of the Mumbles anticline and there are many fault lines crossing the strata. Consequently, veins of red haematite and white calcite abound along with patches of brecciated rock. Langland is best known to geologists for the sequences of glacio-fluvial and later, mostly unconsolidated, deposits that lie above the limestone. However, for myself, it was the wonderful maze of small pebble-floored coves, arches, and caves that held the interest with their wide variety of patterns and textures in the rocks, and the natural fracture patterns dissecting weathered surfaces.

Texture and natural pattern in Texture and natural pattern in sedimentary rock

Natural fracture patterns in Carboniferous sedimentary rock

Veins of white calcite and red haematite in Carboniferous limestone

Texture and natural pattern in sedimentary rock

Texture and natural pattern in Climestone

Texture and natural pattern in sedimentary rock

Texture and natural pattern in sedimentary rock

Texture and natural pattern in sedimentary rock

Texture and natural pattern in sedimentary rock

Texture and natural pattern in sedimentary rock

Texture and natural pattern in sedimentary rock

Texture and natural pattern in sedimentary rock

 

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.

Pink Aplite Veins in L’Eree Granite

Broad pink aplitic vein in L'Eree Granite

Numerous pink veins ranging in width from 10 cm to 1 metre in thickness pass through the northern part of the outcrop of L’Erée Granite in the Channel Island of Guernsey (De Pomerai & Robinson 1994). These are made of aplite. When the hot magma had nearly completely cooled and crystallised to form what we know today as the L’Eree Granite, “residual fluids escaped along cracks in the rock, depositing their dissolved load as they cooled down”.

Aplite is defined as:

a light coloured, fine-grained, equigranular igneous rock composed of subhedral to anhedral grains of quartz and alkali feldspar, and found as late-stage veins in granite bodies. The quartz-alkali feldspar composition corresponds to the lowest temperature melts in granite magma systems, suggesting that they are residual melts formed by the differentiation of granite magma. The lack of any hydrous minerals and the fine grain size points to the aplites crystallising from dry residual melts.

(Oxford Dictionary of Earth Sciences)

REFERENCES

Allaby, M. (2008) A Dictionary of Earth Sciences, Oxford Paperback Reference, Oxford University Press, 3rd Edition, ISBM 978-0-19-921194-4.

British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernsaise, ISBN 0 9518075 2 8, page 32.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 9 – Jerbourg Peninsula, . Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, pages 11 & 75.

L’Eree Granite

Close-up of L'Eree Granite with pink megacrysts of feldspar

The special thing about the L’Erée Granite is the presence of distinct large pink/orange crystals. I mean big. Some of them are a up to 4 centimetres across. They are magacrysts of feldspar which are thought to have grown very slowly deep within the earth’s crust when the bulk of the granite had already crystallised (Pomerai & Robinson 1994). The action of hot gases fed these feldspar crystals that sometimes have concentric rings of dark inclusions (as you can see in some of the photographs) marking stages in their growth between 646 + 25 million years ago in the early Cadomian Age.

REFERENCES

British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernsaise, ISBN 0 9518075 2 8, pages 30 – 32.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 9 – Jerbourg Peninsula, . Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, pages 11 – 12, & 75 – 78.

An albite dolerite dyke at L’Eree

A dolerite dyke crossing L'Eree granite on Guernsey in the Channel Islands

As you look across the granite shore between the L’Erée Headland and the island of Lihou on Guernsey, a small outcrop stands out. If you rock-hop over the boulders to this landmark, you will discover an amazing dyke on the far side. A dyke is an intrusive igneous feature. The three metre wide dark grey-brown dyke crosses the shore in a line roughly trending east north east to west south west – like a path through the rocks – but then seems to climb in a series of regular steps up and over the L’Erée Granite outcrop. The steps are in fact an example of columnar jointing – but whereas they would have originally formed in a vertical position like the hexagonal basalt columns of the Giant’s Causeway in Ireland, here they are more or less horizontal because subsequent earth movements have resulted in them having a steeply dipping to almost vertical orientation that gives rise to the staircase effect on the exposed cross-section.

The composition of the dyke is very interesting. It is dolerite and of relatively recent origin geologically – probably Palaeozoic in age. In addition, it is an unusual Perelle-type albite dolerite dyke which has a limited distribution on the island of Guernsey. This is the only albite dolerite dyke in the Northern Igneous Complex of the island. Typically this type of dolerite is grey and fine-grained containing prominent bands of white prehnite and pink-stained plagioclase feldspar phenocrysts, however, none of my photographs have captured these features. Lees et al. (1989) have shown that the albite dolerites are rocks of alkali basalt affinity.

I particularly like the way that, up close, the weathered surfaces of the dyke have the most interesting patterns and texture reminding me of low relief sculptures of quasi-geometric form.

REFERENCES

British Geological Survey Classical areas of British geology: Guernsey, Channel Islands Sheet, 1 (Solid and Drift) Scale 1:25,000. NERC, Crown Copyright 1986.

De Pomerai, M. and Robinson A. 1994 The Rocks and Scenery of Guernsey, illustrated by Nicola Tomlins, Guernsey: La Société Guernsaise, ISBN 0 9518075 2 8, 30 – 32.

Lees, G. J., Rowbotham, G. and Roach, R. A., 1989. The albite dolerites of Guernsey, Channel Isalnds. Proceedings of the Ussher Society, 7, 158 – 164.

Roach, R. A., Topley, C. G., Brown, M., Bland, A. M. and D’Lemos, R. S. 1991. Outline and Guide to the Geology of Guernsey, Itinerary 9 – Jerbourg Peninsula, pp 21 – 22, & 75 – 78. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, 22.

An Oyster Shell with a Black Pearl

Oyster shell with attached pearl

I found this shell on Rhossili Beach. Oyster shells often wash ashore there. The European Flat Oyster used to grow in abundance around the Gower Peninsula in South Wales and was commercially fished until about the 1940’s when stocks declined to such an extent that it no longer remained a viable proposition. They are presently trying to re-introduce the oyster fishery.

Fresh shells brought up by the tide would seem to indicate that Ostrea edulis still lives and breeds in the locality. The older shells have evidence that they have been around for a long time, possibly decades. Many are very thick showing that they lived for a long time. Commercially fished or cultivated oysters are usually cropped at three or four years before the shell has achieved its maximum growth and are therefore relatively small and thin. Left undisturbed, O. edulis can live for fifteen years or more. However, after a certain time, the diameter of the shell more or less ceases to increase and the animal’s energy is concentrated on thickening rather than widening the shell.

The longer the oyster lives, the greater the possibility of its shell assuming unusual shapes and abberations. Some of the mis-shapes result from the animal’s defensive reaction to infesting or encrusting organisms on or in the protective shell. Occasionally, irritation of the fleshy interior by foreign objects causes changes in the way the shell is laid down by the internal nacreous layer. This is the way pearls are formed. You may be surprised to learn that commercially fished pearls, and cultivated pearls, do not actually come from oysters. The Pearl “Oyster” – is a mis-nomer. It is in fact a Pearl Mussel. The Latin name for the Pearl Oyster species (of which there are several) is Pinctada. and the species belongs to the Family Pteriidae a close relative of the true oysters – the Ostreiidae. When Julius Ceasar came to Britain with the invasion and extolled the beauty of British pearls, which he then exploited and exported back to Rome, he was referring to pearls from freshwater mussels Margaritifera margaritifera (Linnaeus).

It is not common to find pearls in true oysters like Ostrea edulis but they do occur. They are not considered to be as valuable as those from mussels and in some cases are prone to disintegrate with time. I have seen good examples in the museum at Colchester, Essex, which is an area reknowned for its oyster fishing industry dating back to at least Roman times.

Pearls as we commonly know them usually form as distinct separate bodies within the fleshy mantle of the oyster. Occasionally, the pearls are attached to the inner nacreous layer of the shell. They can be attached by a short stalk. That is what we have here in this beach-combed oyster shell. The “pearl” is attached to the inner surface of the right valve of the shell next to the pale kidney-shaped adductor muscle scar. [The strong adductor muscle joins the two valves in life and is used to close the shell when necessary. The default position of the oyster is to have the valves open and apart and it is automatically kept in this position by the ligament at the umbonal or hinge end of the shell.] The black colour of the pearl and the shell itself is the result of spending a considerable time buried deep down in anaerobic sediments. Black oyster shells are common on Gower beaches.

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.