The island lies approximately one kilometre one-half nautical mile off France's north-western coast, at the mouth of the Couesnon River near Avranches and is 7 hectares 17 acres in area. The mainland part of the commune is hectares acres in area so that the total surface of the commune is hectares acres.
The commune's position—on an island just a few hundred metres from land—made it accessible at low tide to the many pilgrims to its abbey , and defensible as the incoming tide stranded, drove off, or drowned would-be assailants. The island remained unconquered during the Hundred Years' War.
A small garrison fended off a full attack by the English in Now a rocky tidal island , the mount 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. Mont-Saint-Michel consists of leucogranite which solidified from an underground intrusion of molten magma about million years ago , during the Cambrian period , as one of the younger parts of the Mancellian granitic batholith.
The mount has a circumference of about m 3, ft and its highest point is 92 m ft above sea level. The tides vary greatly, at roughly 14 metres 46 ft between highest and lowest water marks. Popularly nicknamed "St.
Michael in peril of the sea" by medieval pilgrims making their way across the flats, the mount can still pose dangers for visitors who avoid the causeway and attempt the hazardous walk across the sands from the neighbouring coast.
