Geology & Geography
As we walk around Guernsey, very few people realize that under their feet are some of the oldest rocks in the country. The Icart Gneiss of which most of our South coast cliffs are composed is about 2.5 billion years old, one half of the age of the Earth.
In spite of what is popularly believed, only two of the nine main rock types found in Guernsey are actually Granites, the rest are Gneiss`s which are metamorphic, or rocks which have been altered by being re-heated, thus changing their chemical make up. The Granites are Igneous rocks which have originated from magma. Magma is the uppermost layer of molten rock. It is the rock that is erupted from volcanoes and has solidified either on the surface or in large chambers many kilometres deep underground.
It is the latter of these which Guernsey is composed. After millions of years of surface erosion these rocks have finally come out at the surface. They contain large amounts of Feldspar, which is a pink crystal that gives the rocks at Cobo their beautiful pink lustre. Feldspar is also a major constituent in the L`Eree Granite.
There are many sites of particular geological interest in Guernsey that are regularly visited by University groups. The geological section campaigns to preserve these.
We have also published a Geology booklet called ‘The Rocks and Scenery of Guernsey’. This is a small book describing a series of Geological excursions which are simple to follow and provide a basic knowledge of Guernsey’s rocks.
For the more academic geologist, the Guernsey Museum`s Book ‘Outline Guide to the Geology of Guernsey’ is still available from the Museum shop. This book is rather heavy going for the amateur.
There is also a Geological map published by the British Geological Survey, ‘Guernsey, Channel Islands Sheet 1’ Which also includes the islands of Alderney, Sark and Herm.
The Geology Section of La Societe Guernesiaise meets on the second Wednesday of the month at the Candie H Q during the winter and out and about at various locations during the summer. Times may vary depending on the tides.
As some of our Geological sites are now becoming protected areas where any form of sample taking is, or will soon be illegal. Any groups visiting Guernsey are requested to contact the secretary before organising visits where he can help and give advice.