A Vogesite Dyke at L’Eree

Detail of a vogesite dyke at L'Eree in the Channel Island of Guernsey

Dykes formed when molten lava flowed into deep cracks and fissures in pre-existing rocks millennia ago. The lava set in a sheet form within the other rock but frequently dykes are seen on the surface today, after many years of erosion and earth movements, as lines of contrasting rock type. Dykes are composed of many different mineral combinations. When I visited Guernsey in the Channel Islands last year I discovered three types of dyke cutting through the predominantly igneous and metamorphosed rocks. I have already shown some pictures of dolerite, albite dolerite, and lamprophyre dykes, all of which seem to be quite common on the island. However, on an expedition to L’Eree on the north-west coast I spotted a dyke with a very different texture cutting east-west across the other  north-south dykes. This proved to be a vogesite dyke.

Vogesite has a very characteristic texture made up of rounded mineral inclusions – ‘large euhedral amphibole phenocrysts’  – set in a fine grained groundmass of plagioclase feldspar, alkali feldspar and quartz (Roach et al. 1991).

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, 30-34.

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

Carboniferous Limestone Textures at Threecliff

Rock pattern and texture in Carboniferous Limestone

A further assortment of interesting rock patterns and textures in Carboniferous Limestone strata of the cliffs on the east and west sides of Threecliff Bay on the Gower Peninsula in South Wales. Of particular interest is the honey-comb texture of erosion along the bedding planes of some of the dipping layers, and the red colouring due to iron bearing minerals of other areas,  on the east side of the bay. On the west side of the bay the rocks seem less weathered and are often encrusted with patches of black lichen.

Bee Burrows in Redend Point Rocks

Soft red and yellow sandstone with holes made by bees

These curious rocks occur at Redend Point between Middle and South Beach at Studland Bay in Dorset, England. I have featured them several times in the blog before. The bright red and yellow patterned sandstones are Eocene Creekmoor Sand (also known as Redend Sandstone) and are part of the Poole Formation in the Bracklesham Group. The sandstone is very soft and easily eroded. It is also easily carved and provides a surface for much graffiti. However, the busiest carvers are small bees which excavate burrows in which to lay eggs. Now the sandstone has weathered away you can see the empty pupal cases from last year.

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.