Foreshore area covered and uncovered day after day by the tide.

Out of the sea currents, in the pocket formed by Brittany and the Cotentin peninsula, Mont St Michel Bay (400 km2) opens out into the English Channel. The exceptional tidal range can reach 15 m during spring tides. The intertidal area covers 250 km².

This Outer bay includes a 50 km2 Inner bay extending from Chapelle Sainte-Anne (Cherrueix) to Bec d'Andaine (Genêts). Covered in sands and mudflats, and salt marshes and meadows, it is also an estuary for three rivers, the Sée, the Sélune and the Couesnon.

Sometimes called an eagre. This phenomenon occurs on the rising tide about one hour before high water; it is a wave formed when the incoming tide rushes up against ebb currents. It forms in the mouth of the Couesnon and in the Sée and Sélune estuary (Roche Torin and Grouin du Sud headlands).

The halophilous (salt-loving) plants that grow in the salt marshes come from the land and have adjusted to colonize the upper intertidal area (the strip of coast between the low and high water mark), turning it into salt meadows.
The salt meadows are used for grazing sheep and hence known in French as "prés salés".
Different plants grow at various heights above sea-level up the slope, depending on how long they can survive underwater and how sensitive they are to salt. Some of the hardier plants like Spartina grasses and salt-wort (Salicornia) can grow at around datum level +5m in calm waters where sedimentation is fairly sluggish.
The Mont St Michel salt marshes cover an area of around 4,000 hectares, each year laying a further 20 hectares of mudflats (i.e. stretches of bare sand) into the Outer Bay.

Hydrosedimentary brochure

Inlets in the salt marshes that fill up with the rising tide.

Over 600 samples sifted directly on the spot were found to contain 12,000 specimens belonging to 12 different species.
The bottom fauna in the bay is made up of bivalves: Macoma and cockles - molluscs that filter particles out of the water through their branchiae (gills), keeping the organic matter then putting the minerals back; also small crustaceans that feed on debris and resemble "sand fleas" (tiny crustaceans a few millimetres in length which feed on organic matter on the surface of sand deposits); finally a worm, the nereid.
Benthic invertebrates feed on micro-organisms - diatoms, tiny algae, crustacean and mollusc larvae - living in the sediment or suspended in the sea water. They are in turn eaten by sea birds and fish.


Environmental brochure

Diatoms are unicellular algae that form in their billions on the bed of a bay a brown film invisible to the naked eye. Their size ranges on average somewhere between 20 and 150 thousandths of a millimetre.
These microscopic algae are crucial to the bay's ecosystem. Oysters, mussels, benthic invertebrates and even certain fish like mullet feed on diatoms, on which a whole branch of the food chain thus depends.
Unlike the plants of the salt marshes which are luxuriant and slow to reproduce, and only a fraction of which can be grazed by sheep, diatoms have an extraordinary ability to multiply and are very unobtrusive.