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].
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.