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Mont St-Michel

Turning back the sands of time

The Abbey of Mont St-Michel, off the coast of Brittany, is perhaps France’s most iconic image after the Eiffel Tower, perched high on a granite island left by the sea’s erosion of the surrounding terrain. It was on this island, so the story goes, that the Archangel Michael instructed St-Aubert of Avranches in 708 to build a church. The monastery at Mont St-Michel is depicted in the Bayeux Tapestry, and it has had a colourful history over the centuries, including being converted into a prison during the French Revolution which was finally closed in 1863. It is the most popular tourist site outside of Paris, attracting 3 million visitors from all countries each year, and enjoys the double distinction of having been listed in 1979 as one of UNESCO’s World Heritage Sites, under criteria for both cultural and artistic heritage as well as natural beauty. The dramatic beauty of its solitary position, jutting up from the waters of the bay one kilometre outside the mouth of the river Couesnon near Avranches, however, has been under increasing threat from forces both natural and man-made leading to the relentless ensablement, or “sanding-in”, of the bay around its base. A large-scale project, the Projet Mont St-Michel, is now under way to reclaim its magnificent isolation from the mainland, and “restore the aesthetic and spiritual dimensions” of the site.

The Bay of Mont St-Michel has the fourth largest tidal flow in the world, with the spring high tides reaching a level of 46 feet, and in the words of a famous visitor, Victor Hugo, racing in “with the speed of a galloping horse”. The incoming tides bring in sand and silt, which mixes with that brought down by the three rivers that empty into the bay, the Couesnon, the Sée, and the Sélune; it is estimated that 700,000 cubic metres of this aggregate are deposited each year. In the distant past the flow of the rivers into the bay was sufficient to disperse most of this accumulation, but centuries of polderisation of the coastal area, to reclaim land for agriculture, has weakened this natural process. In the 1850s a dike was built to divert the Sée and the Sélune rivers to the north in order to gain further terrain, and in 1879 the natural land bridge to the Mont was fortified into a permanent causeway. In the 20th century the Couesnon was channelled to strengthen its banks, and in 1969 a dam was built across it, which all but eliminated the river’s power to disperse accumulated sand from its mouth. The threat of irreversible sedimentation of the entire bay around the Mont, and the loss of its exquisite maritime setting, had to be addressed.

The Mont St-Michel Project, inaugurated in July 2003, with work commencing 16 June 2006, is an extraordinary and ambitious experiment in large-scale restoration of a natural landscape, taking in extensive studies of the hydrology and sedimentology involved, as well as evaluating environmental considerations and, of course, tourism. A four-year study of the sedimentology required the construction of a huge physical model to reproduce and predict the dynamics of sand and tides; it covered 900 square metres and was used to model 45 annual cycles in the bay. The conclusion was that by the year 2042, if nothing were undertaken, the bay would be completely and irreversibly silted up; four square miles of vegetated land had accumulated in the past several decades.

The Project, scheduled to be completed by 2015 and at a cost of €164M, involves a number of initiatives. The principal focus is to restore the natural flushing power of the Couesnon. A new dam has been built which stores water from the incoming high tides as well as from the river itself, building up a head of water pressure to be released at intervals calculated to maximize the redistribution of accumulated sediment into the further reaches of the bay. The Couesnon is to be dredged both upstream and downstream of the dam to increase its holding capacity and flow; the salty “tangue” dredged up is to be used in the polders as agricultural fertiliser. A dike is being built below the dam to divert the seaward flow into two narrower (and more powerful) channels. Excess water behind the dam will be diverted further upstream into Moidry Cove, to feed back into the Couesnon as required.

The next phase will be to remove the causeway which links the Mont to the mainland, and unfortunately interferes with the natural tides and currents, and to replace it with an access route set above the normal water level at high tide. This route will permit access by normal service and emergency traffic, as well as a dedicated shuttle service to the island from the parking area.

Most visitors will however prefer to walk the 2.5 kilometre distance from the carpark to the Mont, enjoying the lovely vista as they near the Abbey viewed from across the waters. The last stretch of this pleasant stroll will be a 700-metre "jetty", as its designers call it, a weathered oaken "boardwalk" standing on a row of thin piers, barely visible from a distance. At the highest of the annual spring high tides, already a popular visitors’ attraction, it will be submerged, and for a few hours each year Mont St-Michel will reacquire its past glorious isolation from the mainland and stand utterly surrounded by the waters of the bay. In the words of the Project manifesto, the Mont then “recovers pride of place as the central feature of the Bay, amid a seascape of sands constantly being reshaped by tidal and river water.”

The results will be gradual, but in 30 years or so the sea will have recaptured 50 hectares of land, effectively taking decades on the site’s age. Traditional local activities will be protected: oyster farming and shrimping will continue, and the polders will still be grazed by flocks of sheep destined for the local table in the form of the highly tasty “salt meadow lamb”. The local wildlife refuges will be protected; they are shelter to many unusual migratory birds.

For more information on Mont St-Michel itself, visit:

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Aerial view
Daniel Fondimare

Current Couesnon dam
Thomas Jouanneau

Aerial view
Daniel Fondimare

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