St Michael''s Mount is a tidal island located 366 m (400 yd) off the Mount''s Bay coast of Cornwall. It is united with Marazion by a man-made causeway, passable only at mid to low tide, made of granite setts. The island exhibits a combination of slate and granite.
Historically, St Michael''s Mount was a Cornish counterpart of Mont Saint Michel in Normandy, France.
The Mount may be the Mictis of Timaeus, mentioned by Pliny the Elder in his Naturalis Historia. The Greek Pytheas visited the island in the fourth century BC.
The rock exposures around St Michael''s Mount is made of the uppermost part of a granite intrusion into metamorphosed Devonian mudstones or pelites. The granite is itself mineralised with a well-developed sheeted greisen vein system.