The beautiful and popular beach of Cobo Bay on the north coast of the Channel Island of Guernsey marks a transition between two igneous rock types: the Cobo Granite and the Bordeaux Diorite Complex. The character of the rocks that punctuate the stretches of clean sand and clear blue water changes as you walk from one end of the bay to the other.
In the southeast, near Le Guet Quarry and Albecq the orange-pink Cobo Granite is at its most even-textured and pure with coarse grained crystals of pink potassium-rich orthoclase feldspar, light grey plagioclase feldspar, glassy quartz grains, and small black shiny crystals of biotite mica. The colours are best seen in freshly broken rock but are often obscured or muted by weathering and encrustations (lichens on dry land; algae and barnacles between the tides). The pebbles at this end of the beach are mostly brightly coloured water-worn remnants of the Cobo Granite.
The Bordeaux Diorite Complex rocks are superficially grey, composed of mostly black and white crystals with grey plagioclase feldspar, black biotite, with minor minerals such as hornblende, pyroxene, and quartz. The Cobo Granite is younger than the Bordeaux Diorite. Deep beneath the earth’s crust, the molten granite intruded into the diorite before it was fully solidified. This led to a mixing of the two types of magma, and also a breaking-off of pieces of semi-solidified diorite that became enfolded in the granite magma before cooling. Small dark grey pieces of diorite (xenoliths) can be seen in the granite to the west below the Le Guet quarry. The numbers of xenoliths increase as you walk north east. Veins of pink aplite also run through the rocky outcrops. Whole areas of rock on the beach below the Rockmount Hotel are greyish in colour where the two rock types have melded together.
Walking northeast in the direction of Port Soif, large patches of greenish-grey inclusion rock can be seen ever more frequently embedded in the granite. I think these kinds of rocks are called granite-diorite marginal facies. By the time you reach the end of the beach, the grey diorite is more evident in outcrops and boulders. An attractive stone slipway demonstrates the contrasting colours and textures of the two rock types, the dark grey of the diorite and the orange of the granite. The angular beach stones and rounded pebbles at this point also show the two rock types but with the grey diorite dominating, in contrast to the mostly orange granite pebbles at the other end of the beach.
This is just a very simple description of the geology at Cobo Bay and is intended only as a general guide to the rock features. My apologies for any inaccuracies. The expert explanation is much more complex and can be found by consulting the references given below.
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 48-51.
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 66-70.