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 Rise and Fall of Whiteford Sands

View looking towards the lighthouse at Whiteford on the Gower Peninsula showing rock strewn beach with patches of sand

The photographs in this post illustrate the way that vast quantities of wind- and wave-borne sand at Whiteford Sands on the Gower Peninsula move around the shore over time. I have taken one fixed object, a piece of ancient timber with an unmistakable shape that projects from the early to post Holocene deposits of peat and clay, and taken shots of it on every visit to the beach over the past ten years or so. The following images show how the sand level changes periodically to reveal or conceal the underlying layers with the surface scattering of rocks that were dumped by the melting ice during the last glacial event. Beaches like Whiteford are incredibly dynamic. Click on any image in the gallery below to view as a slideshow in chronological order.

 

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.

Pebbles at Albecq

Colourful and patterned pebbles of igneous rocks

The colours and patterns of the pebbles on the beach at Albecq on the north coast of Guernsey in the Channel Islands reflect the varied geology of the north of the island. The northern part of Guernsey is made up of igneous rocks such as granite, diorite and gabbro and formed about 400 – 500 million years ago (de Pomerai & Robinson 1994 pp 5 & 7). Most of the pebbles in these photographs would have originated in nearby bedrock that would include Cobo Granite, Icart Gneiss, Perelle Gneiss Group, and Bordeaux Diorite Complex. These rocks are constituents of what is known as the Northern Igneous Complex. The patterns in the rocks are due to the coarse mineral crystal composition. Potassium-rich orthoclase feldspar is responsible for the recurring pink-orange colour, with light grey plagioclase feldspar; quartz grains are transparent; and biotite mica is seen as small black shiny crystals (de Pomerai & Robinson 1994 p 49).

REFERENCE

de Pomerai, M. and Robinson, A (1994) The Rocks and Scenery of Guernsey. La Societe Guernesiaise, St Peter Port, Guernsey. ISBN 0 9518075 2 8.