The Chausey Islands form an archipelago northwest of Granville in the Baie du Mont-Saint-Michel and are part of the northern domain of the Massif Armoricain. The islands are made of two types of granitic rocks: a granodiorite (bluish-gray rock widely exploited to make sidewalks, quays, etc.) and a porphyritic granite (rich in feldspar megacrystals never exploited). Dated some 596 million years, the rocks have little evolved and constitute a goldmine of information on the Precambrian.