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

 

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.

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.

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.

 

Calcite Veins in Threecliff Rocks – Part 1

View of Threecliff Bay on the Gower Peninsula

Threecliff Bay on the south coast of the Gower peninsula in South Wales is one of the most beautiful and interesting locations. The scenery is spectacular and the three rocky peaks that give the bay its name are clear to see. The Pennard Pill river follows great meandering loops as it approaches the sea and it flows down a valley created by weaknesses along a tear fault that skewed the alignment of the rock strata. The rocks on the east side of the valley do not line up with those on the west side. The strata in the east have been moved northwards.

The pictures in this post were taken where those displaced rocks outcrop in cliffs on the east side of the bay. They are composed of Carboniferous Limestone. I think they are from the Black Rock Limestone Subgroup – the only available geology map has out of date nomenclature for the various rocks types and calls this section of strata Penmaen Burrows Limestone (d1b). What fascinated me was the wonderful red tinge in the rocks due to the iron content and the intricate natural patterns of discontinuous white veins of calcite. I wonder if these veins are something to do with the pressures and heat resulting from the northwest to southeast tear fault that defines the valley. It looks as if a first set of cracks was infilled with calcite before a second set, cross-wise to the first, was formed in a subsequent event that generated yet more pressure and heat.