Mont-Saint-Michel in the context of "St Michael's Mount"

A tidal island is a raised area of land within a waterbody, which is connected to the larger mainland by a natural isthmus or man-made causeway that is exposed at low tide and submerged at high tide, causing the land to switch between being a promontory/peninsula and an island depending on tidal conditions.

Because of the mystique surrounding tidal islands, many of them have been sites of religious worship, such as Mont-Saint-Michel with its Benedictine abbey. Tidal islands are also commonly the sites of fortresses because of the natural barrier created by the tidal channel.

The Couesnon (French pronunciation: [kwenɔ̃] ; Breton: Kouenon) is a river running from the département of Mayenne in north-western France, forming an estuary at Mont-Saint-Michel. It is 97.8 km (60.8 mi) long, and its drainage basin is 1,124 km (434 sq mi). Its final stretch forms the border between the historical duchies of Normandy and Brittany. Its historically irregular course, shifting between two beds to the north and south of Mont-Saint-Michel until eventually settling on the southern one, inspired the saying Le Couesnon en sa folie mit le Mont en Normandie (“The Couesnon in its madness placed the Mont in Normandy") as the Mont is just to the Norman side of the river’s current mouth. However, the modern administrative boundary separating the two regions does not follow the river course; it is some six kilometres west of the Mont.