Rock Texture & Pattern at Dog’s Bay

The rocks at Dog’s Bay in Connemara, Ireland, are part of the Galway Batholith. In particular they are composed of the Errisbeg Townland Granite riven by faults and many dikes containing other younger intrusive igneous rocks. The juxtaposition of the different rock types is a phenomenon marked by contrasting, colours, textures, and patterns. The whole rocky terrain has been levelled off and smoothed by ice sheets and reflects many glaciation features.

Feely, M. Leake, B.E., Baxter, S. Hunt, J. and Mohr, P. A Geological Guide to the Granites of the Galway Batholith, Connemara, western Ireland.Geological Survey of Ireland, 2006.

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 Guernesiaise, 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.

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 Guernesiaise, 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.

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 Guernesiaise, 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 Moulin Huet Part 2

Contrasting rock textures at Moulin Huet Bay

The metamorphosed igneous rocks of Moulin Huet Bay in the Channel Island of Guernsey, the Icart Gneiss, are traversed by later intrusions of molten volcanic rock that filled spaces where fractures opened up in the gneiss. These intrusive rocks are sheet-like formations of varying extent and thickness, appearing on the weathering rock surface as narrow bands of contrasting colour and texture, and they are known as dykes. Dykes are igneous rocks that can be composed of different combinations of minerals, and they can also be metamorphosed later into yet more compositions. On Guernsey there are apparently six different types of basic (as opposed to acid) dyke and it is difficult to distinguish between these types when just observing in the field. According to the simple guide written by Pomerai and Robinson (1994) most of the dykes at Moulin Huet are made of dolerite, as shown in the examples illustrating this post. [There are also a couple of lamprophyre dykes which I will show in a separate post]. I am aware that this identification as dolerite may be an over simplification but will investigate further.

The photographs here show the contrasting textures and colours of the rocks, with the relatively fine-grained, smooth, and homogenous grey-green dolerite dykes within the coarse-grained Icart Gneiss and its large, squashed pink-orange feldspar crystals. In some instances, there are pieces of the Icart Gneiss within the dolerite, these having broken off the sides of the bedrock and become incorporated into the molten lava as the dyke was formed – these inclusions are called xenoliths.

REFERENCE

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