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.

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