Geology of Guernsey

Near Vale Castle the rock is of a type called bojite with interlocking hornblende and plagioclase crystals.

The island was only separated from the continent of Europe by rising sea levels at about 5000 BC during the new stone age.

At the western coast of the island, a shear zone is exposed at the contact between the Perelle Foliated Quartz Diorite and the Icart Gneiss.

The younger quartz diorite is mylonitised, where field evidence suggests that it was most likely deformed synchronously with its intrusion.

Guernsey has had an active quarrying industry over the years, largely removing rock for use as building materials.

The only remaining active quarry is Les Vardes on the west coast of the Island, operated by Ronez.

Mont Cuet is another former quarry, now used as a landfill site to dispose of the majority of the Island's non-recyclable waste.