Rocks at Marble Bay 1

There is no marble at Marble Bay in the Channel Island of Guernsey! It looks as if there is but really there is none. The name is thought to be due to the massive vein of white quartz that crosses the beach. Equally, the name may have arisen from the phenomenon of encrusting bio-films of various types (algae, bacteria and lichens) that coat the rocks with vivid coloured patches of red, orange, yellow, and black.

The main bedrock in the bay is in fact Icart Gneiss with its large squashed pink-orange feldspar crystals (as found in the nearby Moulin Huet Bay on the other side of the Jerbourg Peninsula). This metamorphosed type of granite is riven by a single massive 2-3m thick vein of quartz in a fault zone that extends right across the peninsula so that the same vein reappears at Petit Port adjacent to Moulin Huet. Smaller branching veins of quartz also appear in the Icart Gneiss. What seems to be a large dolerite dyke with grey fine-grained texture and smooth surface additionally crosses the beach. The true appearance of each of the rock types is mainly masked by the bio-films and larger seaweeds attached to the rocks. Inter-tidally, however, some outcrops remain clear of growth, and the location of the wave-cut notch at the base of the cliffs is especially good for viewing the Icart Gneiss natural pattern and texture.

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.