in prehistoric times, the "mont" was on dry land. it is now a rocky tidal island. sea levels became higher and erosion changed the landscape of the coast. several layers of granite or granulite came to the surface in the bay. these layers of rock resisted the damage by the ocean better than the other rocks around them. these included lillemer, the mont-dol, tombelaine (a small island just to the north), and mont tombe, which later became mont-saint-michel. the mount has a circumference of about 960 metres (3,150 ft). it is 92 metres (301 ft) above sea level at its highest point.