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.

Seashore Rocks at Church Doors

View looking west towards Skrinkle Haven with the Church Doors and Horseback Limestone ridges projecting seawards.

The South Pembrokeshire coast continues its spectacular scenic way as you travel eastwards from Manorbier to the middle cove known as Church Doors at Skrinkle Haven. The intervening mile or two sees a transition from the older red Devonian rock to younger grey Carboniferous limestone higher in  the geological succession. Looking down on the bay you can see that it is divided into three parts by two seaward projecting ridges of rock known as Church Doors and Horseback.

The rocks around the bay are now stacked in remarkable vertical layers after earth movements have altered their position from the original horizontal strata. To the right of the steep metal staircase as you descend to the beach, Avon Group Limestones, formerly known as the Lower Limestone Shales, are made from thousands of very fine layers alternating with narrow hard bands that underlie the pebbles and boulders of the shore, and are exposed in the cliffs and a narrow promontory. These strata are collectively referred to as the Church Doors Limestone.

A small natural tunnel, accessible only at very low tides, passes through the promontory from the middle cove to the west cove of Skrinkle Haven. At the moment that tunnel seems to be the only way of getting to Skrinkle Haven proper but it use is not recommended. It is only open and available for a very short time and it is easy to get stuck on the other side with no way up the cliff as the tide rises. Also, the seabed level on the far side is a lot lower that the Church Door side and water rises comparatively much faster than in the middle cove making it additionally dangerous.

The staircase down to the beach marks the point at which the Avon Group limestones are replaced by Pembroke Group limestones. Behind and to the left of the steps as you reach the beach, there is a transitional zone  marked by an increase in the proportion of limestone and chert beds. The limestone is faulted and has many white calcite veins. The promontory known as the Horseback is composed of Black Rock Limestone and has an incredible natural arch where blocks of stone have fallen. It is interesting to note the texture of the barnacle encrusted rock at the waterline with its marked pitting caused by bioerosion.

References

Allaby, M. 2008, Oxford Dictionary of Earth Sciences, Oxford University Press, 3rd Edition, 978-0-19-921194-4.

George, G. T. 2008, The Geology of South Wales: A Field Guide, gareth@geoserv.co.uk, 978-0-9559371-0-1, pp 22 and 137-141.

Howells, M. F. 2007, Wales, British Regional Geology, British Geological Survey, Nottingham, Natural Environment Research Council, 978-085272584-9, pp 112-120.

Looking down at the sparkling water in the cove at Church Doors in South Pembrokeshire

Warning sign at Church Doors Cove

Metal steps leading down to Church Doors, the middle Cove at Skrinkle Haven on the South Pembrokeshire Coast in Wales

Carboniferous Avon Group limestone rock strata with shales on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Vertical Carboniferous Avon Group limestone rock strata with shales on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of vertical Carboniferous Avon Group limestone rock strata with shales on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of vertical Carboniferous Avon Group limestone rock strata with shales strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Vertical Carboniferous Avon Group limestone rock strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of vertical Carboniferous Avon Group limestone rock strata strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of Carboniferous Avon Group limestone rock strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Natural arch in the Horseback Carboniferous Black Rock Limestone promontory on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Vertical Carboniferous Avon Group rock strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of Carboniferous Avon Group limestone rock strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of Carboniferous Avon Group limestone rock strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of Carboniferous Avon Group limestone rock strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of Carboniferous Avon Group limestone rock strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of Carboniferous Avon Group limestone rock strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of Carboniferous Avon Group limestone rock strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of Carboniferous Avon Group limestone rock strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of Carboniferous Avon Group limestone rock strata on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Natural arch in Carboniferous Black Rock Limestone in the Horseback ridge on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Carboniferous Black Rock Limestone on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Carboniferous Black Rock Limestone with bioerosion on the waterline around the natural Horseback arch on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Carboniferous Black Rock Limestone cliff around the natural Horseback arch on the seashore at Church Doors on the South Pembrokeshire Coast in Wales

Close-up detail of rock strata of the Carboniferous Avon Group (Lower Limestone Shales) in the cliff by the metal stairs at Church Doors on the South Pembrokeshire Coast in Wales

Lamprophyre Dyke at Moulin Huet

Strange natural textures in the weathering top surface of a lamprophyre dyke cutting through Icart Gneiss

Lamprophyre dykes, like the dolerite dykes previously described, are features that occur when cracks that open up beneath the earth’s surface, cutting across the pre-existing matrix, become filled with new molten rock forming flat sheets of new rock. These sheets frequently weather out on the surface as narrow linear structures of contrasting colour and texture to the surrounding bedrock. The mineral composition of lamprophyre dykes is very variable and different from that of the dolerite dykes although both are intrusive igneous rocks. It takes a real expert to determine the make up of each lamprophyre dyke.

At Moulin Huet Bay in the Channel Island of Guernsey’s Southern Metamorphic Complex, a particular lamprophyre dyke has been described by de Pomerai and Robinson (1994) and depicted by a line drawing. I was able to located the exact same feature because of the accuracy of the illustration and description, and checked the location against a geological map. The photographs in this post show the dyke with its most characteristic appearance and texture, including a honeycomb-like weathering or surface erosion. [Click on any image to enlarge and see caption].

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, pp 16-21.

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, 87- 90. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, p 22.

Rocks at Marble Bay 1

There is no marble at Marble Bay in the Channel Island of Guernsey! It looks as if there is but really there is none. The name is thought to be due to the massive vein of white quartz that crosses the beach. Equally, the name may have arisen from the phenomenon of encrusting bio-films of various types (algae, bacteria and lichens) that coat the rocks with vivid coloured patches of red, orange, yellow, and black.

The main bedrock in the bay is in fact Icart Gneiss with its large squashed pink-orange feldspar crystals (as found in the nearby Moulin Huet Bay on the other side of the Jerbourg Peninsula). This metamorphosed type of granite is riven by a single massive 2-3m thick vein of quartz in a fault zone that extends right across the peninsula so that the same vein reappears at Petit Port adjacent to Moulin Huet. Smaller branching veins of quartz also appear in the Icart Gneiss. What seems to be a large dolerite dyke with grey fine-grained texture and smooth surface additionally crosses the beach. The true appearance of each of the rock types is mainly masked by the bio-films and larger seaweeds attached to the rocks. Inter-tidally, however, some outcrops remain clear of growth, and the location of the wave-cut notch at the base of the cliffs is especially good for viewing the Icart Gneiss natural pattern and texture.

REFERENCE

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.

Seashore Rocks at Manorbier

The red-rock wave-cut platform on Manorbier beach in South Pembrokeshire is pretty spectacular. Row after row of vertically aligned strata stretch from the shore across the bay and up the cliffs in the distance. They form an incredible backdrop to the expanse of sandy beach topped by colourful pebbles at high water mark. They make an ideal place for clambering around and exploring rock pools.

Up close, each layer of rock looks slightly different – darker or lighter; harder or softer; coarse-grained or fine-grained; smooth, rough or sculpted; homogenous or with inclusions. They are different colours – red, cream, orange, purple, and green. They sometimes have internal patterns. Every layer of rock represents a particular phase of activity during the deposition of the sediments in what is known as the Freshwater West Formation between approximately 416 and 410 million years ago in the Lower Devonian Period of the Upper Palaeozoic Era. These rocks are also referred to as the Lower Old Red Sandstone.

The 580 metre thick rocks of the Freshwater West Formation are made up of clastic sediments. Clasts are particles of broken-down rock, and the size of the fragments may vary in size from boulders to silt-sized grains and are invariably the products of erosion followed by deposition in a new setting. So clastic rock is consolidated sediment composed of fragments of pre-existing rocks (Allaby 2008). Examples of clastic rocks would include conglomerate, sandstone, siltstone and mudstone. Sediments for the rocks at Manorbier were derived from the newly uplifted Caledonian Mountains and subsequently deposited in a variety or arid to semi-arid continental environments, including estuaries, broad alluvial plains, ephemeral braided-meandering rivers and alluvial fans (George 2008).

The sediment size in the rock layers depends a lot on the speed and volume of the water transporting the fragments from the mountains and across the land. The greater the speed and volume of water, the larger the particle size that can be carried – but the greater weight means that the fragments are deposited sooner than the finer particles which can travel further even when the water velocity is decreasing. The Manorbier rocks illustrate the cycles of deposition during this period, following greater and lesser flows. Each phase is marked first by a layer of the coarsest sediments usually with cross-bedded sandstones which can be red-brown, purple or even green resting on an erosion surface (Howells 2007) followed by layers of increasingly finer sediments like mudstone in a phenomenon known as upwardly-fining. The fining-upwards cycles are interpreted as the fills of ephemeral fluvial (river) channels with sinuous profiles (George 2008).

Pale-coloured nodules often present in the red mudstones and calcareous siltstones are calcrete. Calcrete is a concretionary carbonate horizon formed in the soil profile in arid to semi-arid environments. Calcretes are a feature of a palaeosol or fossil soil in which calcium carbonate is precipitated as root encrustations (rhizocretions) and as small nodules (glaebules) from water flowing through the soil profile. The glaebules grow and coalesce to form a calcrete or dense layer of calcium carbonate near to the surface (Nichols 2009).

References

Allaby, M. 2008, Oxford Dictionary of Earth Sciences, Oxford University Press, 3rd Edition, 978-0-19-921194-4.

George, G. T. 2008, The Geology of South Wales: A Field Guide, gareth@geoserv.co.uk, 978-0-9559371-0-1, pp 22 and 132-135.

Howells, M. F. 2007, Wales, British Regional Geology, British Geological Survey, Nottingham, Natural Environment Research Council, 978-085272584-9, pp 99-108.

Nichols, G. 2009, Sedimentology and Stratigraphy, Wiley-Blackwell 2nd edition, Sussex, England, p148.

All sorts of colourful & interesting rocks

This gallery displays a selection of the most colourful and interesting rocks that have been featured in posts here at Jessica’s Nature Blog over the past couple of years. While I am out walking on beaches, I am always drawn to the colours of the rocks, sometimes bright and other times more subtle, and the many different patterns and textures. Initially it is the way that the rocks look that is so appealing. So much of what I see seems like amazing natural abstract art. I try to frame the composition so that it stands alone as an attractive image in its own right. But then I get curious and lots of questions come into my mind. I always want to know what kind of rock is it? What is it called? How old is it? What is it made of? How did it get to look like that? What happened while the rock was buried? What are the elements doing to it now that it is exposed?

As an amateur with a keen interest in geology, I start by looking at maps. I try to pinpoint the exact location where I photographed the rock. Then I try to get hold of the correct geology map. Geology maps have a lot of information about the age of the rock, the type, the period in which it was laid down or developed, as well as the distribution of the different rock types in the locality. Often there are references to special papers, memoirs and so forth that discuss the geology of the area. Sometimes these publications are available on-line. I do a lot of Googling. Sometimes a visit to the library is needed. Libraries and the internet don’t always have the information I am seeking so I buy books too. Sometimes books about a specific place, and sometimes more general textbooks. I need those too because it is quite difficult to understand everything. Geology is a complex subject with a great deal of specialist terminology.

Once I am fairly certain what the rocks are, I try to write a bit about them in a straightforward way so that anyone else who is truly interested will be able to understand. It is fascinating. Slowly I learn more about the rocks and can fit the pieces together into the bigger picture. Walking along shorelines becomes a whole new experience when you are able to visualise the former environments in which the bedrock originated, or the drift geology was created, when you begin to understand what has happened to the strata over the millions of years since they came into being, and when you first begin to grasp what processes are affecting them once they are exposed to air. I love it when I can recognise strata belonging to the same geological period in different parts of the world, and see their differences and similarities, whether in situ or in buildings, walls and other structures. I begin to feel an enormous sense of wonder and awe, as well as an enormous feeling of humility, at this hugely significant part of the natural environment, a part on which everything else in nature depends or by which it is affected.

Rocks at Fermoyle on the Dingle Peninsula

Red Devonian sandstone rocks at the beach with fucoid seaweeds

The place where I took these photographs is marked on the map as an island but it is actually just a tiny promontory near to the village of Fermoyle, along the Dingle Way, on the north coast of the Dingle Peninsula in Ireland. I am sure that most people visit the location for its wonderful long unsullied sandy beach. However, I was drawn to this particular part, at the extreme western end of the beach, because of its fascinating geomorphology. The rocks are sandstones and conglomerates (mostly but not exclusively red) of the Glengarriff Harbour Group from the Devonian Period. The bright olive, lime, yellow and orange colours of the seaweeds, and the black, yellow and white of encrusting lichens, clash garishly with the red rocks. The rock strata are clearly defined: sometimes on-end, sometimes as flat bedding planes, and in one place a dome of strata lies cut-away and exposed. Beach stones rather than pebbles cover a portion of this area; and there are also occasional huge boulders composed of conglomerate scattered along the shore nearest the inlet from Brandon Bay.