Mont Saint-Michel is surrounded by water due to the tide, a natural phenomenon where large amounts of seawater rise and fall daily, thus forming a temporary island character to the mount.
Mont Saint-Michel is actually made up of a huge block of granite, several hundred million years old, formed during the Paleozoic era. In that distant time, hot magma from the depths of the Earth slowly rose, cooling and gradually creating this solid rock. Then, over time, rain, wind, and waves came to patiently wear away the surrounding land, eroding the softer soils to leave only this ultra-resistant rock emerging. As a result, a kind of natural rocky island was formed, rising in the middle of a vast sandy bay that is regularly covered by the tides. Even today, it is this contrast between the tenacious rock and the easily displaceable surrounding sediments that explains why the Mont stands out so sharply in the waters and its distinctive silhouette in the landscape.
Mont Saint-Michel is famous for its exceptional tides, among the largest in Europe. Here, the difference between high tide and low tide can exceed 14 meters, quite an amplitude! This occurs due to the bay's particular funnel shape, which concentrates the waters during rising tides. During spring tides, the water rises very quickly, sometimes at the speed of a galloping horse, resulting in the impressive phenomenon known as mascaret. It is this amplitude and these very strong tides that periodically isolate the Mont, regularly transforming it into an island for a few hours. At low tide, one can even walk on the sand around the Mont, but beware of being caught off guard when the sea returns very quickly!
Mont Saint-Michel is clearly influenced by several waterways, particularly the Couesnon, which plays a major role in the bay. This river constantly carries sediments (such as mud and fine sand), which it deposits in the bay. Over time, these deposits gradually change the landscape by creating sandbanks or even threatening the island character of Mont Saint-Michel by promoting its connection to the mainland. The phenomenon is natural but amplified by surrounding agricultural practices. This is referred to as silting, and frankly, it is not ideal for preserving the uniqueness of the site. Moreover, it partly explains why recent work has been done to regulate the flow of the Couesnon, in order to preserve the maritime identity of the famous mount.
Over the centuries, man has significantly altered the landscape around the Mont. The most striking example is the construction of a causeway at the end of the 19th century. It connected the Mont to the mainland to facilitate permanent access, but it disrupted the natural flow of water. As a result, water stagnated, silt accumulated, and in short, it gradually filled in the bay. To correct this and restore the Mont to its original appearance, a major project was launched in the early 2000s, which involved the partial demolition of the old road and the installation of a footbridge on stilts. Now, tides can once again flow freely under the bridge, facilitating natural silt removal and enhancing the island-like appearance of the Mont.
Around Mont Saint-Michel, there is a super-rich marine ecosystem with an impressive variety of species. The tides play a key role, providing perfect spots for certain species like migratory birds to rest or feed. On the fish side, the wetlands are teeming with eels, bass, and even mullet. On the mudflat, you can easily find crustaceans like grey shrimp, crabs, and mollusks (clams, scallops). A real pantry for birds! Moreover, nearly 130 species of marine and terrestrial birds regularly inhabit this area. And on the flora side, the salt marshes host salt-resistant plants — the famous salicornes, which are also affectionately called "sea beans." The result: quite a unique biodiversity thanks to the meeting of land, sea, and marsh.
During certain periods, known as equinox tides, Mont Saint-Michel becomes an island for a few hours, a spectacular phenomenon called 'mascaret' or 'tidal wave.'
The bay of Mont Saint-Michel is home to around 130 species of migratory birds that take advantage of the rich marine biodiversity and the surrounding wetlands to feed and rest.
The sand removal work undertaken since the 2000s has allowed Mont Saint-Michel to regain its maritime character: once threatened by siltation, the bay has regained its aquatic vocation thanks to these carefully planned human interventions.
Mont Saint-Michel has been listed as a UNESCO World Heritage site since 1979 due to its exceptional beauty, unique history, and the richness of its natural and cultural environment.
It is possible to walk to Mont Saint-Michel at low tide, but it is essential to be accompanied by an experienced guide, as the tides rise quickly, and the muddy areas can be dangerous.
The bay benefits from a unique geographical location and a funnel shape, which concentrate and amplify the phenomenon of tides, resulting in some of the largest tides in Europe.
Human interventions such as the construction of dikes, dams, and roads have affected currents and sedimentation, temporarily reducing the insularity of the Mont. However, recent efforts to restore natural insularity have helped preserve its maritime character.
Like all coastal regions, the Bay of Mont Saint-Michel is affected by rising sea levels related to climate change. Authorities regularly monitor the situation's evolution to anticipate any necessary measures for the preservation of the site.
No, Mont Saint-Michel is surrounded by water only at high tide when the sea floods the bay. At low tide, the island is surrounded by stretches of wet sand, accessible on foot.
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