The rocks at Spur Bay in Guernsey, Channel Islands, are composed of igneous St Peter Port gabbro which was formed from 500 – 550 million years ago in the Cadomian phase of activity. This dark grey rock has characteristic large hornblende crystals that often occur in layers alternating with feldspar at this particular location. The distinct dark crystals also give this rock the common name of “bird’s eye” gabbro. In fact the gabbro has quite a variable composition, texture and patterning within a relatively small area. Some of the natural patterns have been caused by the infill of later-forming cracks and fractures by different molten rocks such as aplite, and the inclusion of rock fragments within the aplite.
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 40-42.
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 1 – The St Peter Port Gabbro, 76. Guernsey Museum Monograph No. 3, Gloucestershire: Alan Sutton Publishing. ISBN 1 871560 02 0, pp 45-52.