GEOLOGY (case 3)The oldest rock dates from 2,200 million years, but much of the local rock is grano-diorite of which a small part is the rare form called orbicular diorite. Alderney is the only island in the Channel Islands group with sandstone. There is a sandstone islet to the NW called Burhou, an isolated rock called Ortac which has a gannet colony, and the famous lighthouse rocks called the Casquets which are all part of the same sandstone ridge. There are granite headlands in the NE at Bibette, and the two on which stand the batteries and forts of Chateau À L'Étoc and Corblets, but most of the island consists of grano-diorite in which there is one small patch of orbicular diorite which usually interests geologists. |