Now a rocky tidal island, the Mont occupied dry land in prehistoric times. As sea levels rose, erosion reshaped the coastal landscape, and several outcrops of granite emerged in the bay, having resisted the wear and tear of the ocean better than the surrounding rocks. These included Lillemer, the Mont-Dol, Tombelaine (the island just to the north), and Mont Tombe, later called Mont Saint-Michel.
Mont Saint-Michel is made of leucogranite, which solidified from an underground intrusion of molten magma about 525 million years ago, during the Cambrian period, as one of the younger parts of the Mancellian granitic batholith. (Early studies of Mont Saint-Michel by French geologists sometimes describe the leucogranite of the Mont as "granulite", but this granitic meaning of granulite is now obsolete).
The Mont has a circumference of about 960 metres (3,150 ft) and its highest point is 92 metres (302 ft) above sea level.