Marble Bay, also known as Pied du Mur, lies on the east side of the Jerbourg peninsula on the south coast of the Channel Island of Guernsey. There is a major fault zone extending from Marble Bay across the peninsula to Petit Port; and this fault has given rise to an area of fractured Icart Gneiss with many small sheared quartz veins and one major quartz vein about 2-3 metres thick. The sea has worn away this zone of weak fractured rock to form the bay.
Evidence of the process of erosion can be seen in the smooth curved notches present at the base of the cliffs and also of some large outcrops of bedrock on the shore. These notches indicate how the rock is still being eroded to this day by the waves with their stone load. Visible beneath the pebbles on the beach are exposed areas of wave-cut rock platform that show the long-term effect of of this eating-away of the cliffs in relatively recent geological time.
The promontory on the northern side of the bay features a wide ledge at about 8 metres above sea level which is a remnant of an earlier wave-cut rock platform, created when the sea was at a much higher level than it is at present. The erosion of the bay can additionally be detected in the caves that have been sculpted out along minor faults in the cliffs, one of which has a collapsed roof (blow-hole) leading to the formation of a so-called geo on the outer surface of the cliff.
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.
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.
6 Replies to “Wave-cut Platforms at Marble Bay”
Thanks for another interesting lesson.
Thank you, Linda.
Causes one to wonder when it was that the water was at the higher level.
I was curious about that, too. The date of the raised wave-cut platform at Marble Bay wasn’t immediately apparent in the short guide book I have been using. Sea levels have risen and fallen many times over the last 65 millennia so that the Channel Islands have at times been merely hills in an expansive plain in a single European landmass while at others variously exposed islands when sea levels rose to fill the English Channel. More recently, in the Pleistocene, during the Ipswichian interglacial period about 130,000 to 125,000 years ago when glacial ice waters melted, the sea rose and left beach deposits of pebbles and sea shells on mainland UK at heights between about 6 and 10 metres – I have featured posts on such raised beaches on the Isle of Portland and the Gower Peninsula. I am thinking that the higher wave-cut platform at Marble Bay may belong to the same Ipswichian period….but I need to check that out.
Interesting insight Jessica, and looks like a beautiful spot.
Thank you, Aidy. These features occur on all sorts of shores in the British Isles and elsewhere but they were easy to distinguish at this site.