Why can Mont Saint-Michel withstand the tides?

I’m on the left, Heather’s on the right, with Mont Saint-Michel!

It’s hard to miss the stunning abbey/fortress of Mont Saint-Michel as you drive along the coast towards it. It stands proudly above the surrounding flat estuary with flocks of particular salt-tolerant sheep grazing on the marshes.  The abbey and town grew to cover almost all of the original rock exposed on the Mont – they’re built out of rock from the Mont itself and from nearby islands in the English Channel. It seems incongruous and bold beyond belief that someone would built it so far out onto the marshes and the tidal plain, far from dry solid land. So why were the abbey and fortress built here? What allows them to stand the test of time and tides? It turns out, it’s the geology. Look for Mont Saint-Michel on the map below (hint = look for the red dots)…

Surface geological map of the area around Mont Saint-Michel, taken from the proceedings of a 2017 field trip of the International Meeting of Sedimentology prepared by Bernadette Tessier and Pierre Weill

It was built on an outcrop of hard granite that stands tall as the tides shift the soft sand and silt around it.

Beneath the veneer of Quaternary sediment from the estuary, the region is made up of mudstones and sandstones that were transformed into metamorphic rocks between 600 and 570 million years ago at the root of an ancient mountain chain formed by an oceanic crust – continental crust subduction zone. At that time, this chunk of northwestern France was connected to the ancient continent Gondwana, and located near the south pole. Around 525 million years ago, magma rose off of the subducting oceanic plate and pushed up through the cooler, denser metamorphic rocks. This magma cooled to form the igneous intrusions that would become Mont Saint-Michel and the nearby Mont Dol and Tombelaine. These instrusions were made of a unique rock named leucogranite, notable for the lack of dark felsic minerals such as amphibole or pyroxene. Pink feldspar, grey quartz, and clear quartz give Mont Saint-Michel’s rocks a beautiful pale color.

Intrusive igneous rocks such as the leucogranite at Mont Saint-Michel are much more resistant to erosion than the shales, schists, and sandstones that they intruded into. Over time, this difference formed hills, cliffs, and outcrops along the coast of Brittany. This is evident in a cross section of the Bay of Mont Saint-Michel compiled by France’s geological survey below:

Translation – “Geologic Cross Section across the bay, passing by Mont-Saint-Michel and Tombelaine”. “schistes tachetés” = speckled schist, “digue des polders” = polder seawalls

It turns out that these rocks have been on a long, strange journey.  This part of Brittany and Normandy belongs to a tectonic fragment defined by its experience as part of the Avalonian-Cadomian belt  around 600-500 million years ago close to the South Pole. These rocks – schists, sandstones, and intrusive volcanics – were formed at the roots of a mountain chain at the northern edge of Gondwana , as oceanic crust subducted beneath regions of Gondwana that now form northern Africa.  You can see a reconstruction of its historical place on Gondwana in the inset map of the figure below, and the main figure shows the modern position of that block in northwest France and underneath the English Channel.

This figure shows the Cadomian terrane shortly after it began to split, around 490 years ago. Image from The Formation of Pangaea by G.M. Stampfli et al, 2013, via https://quatrevingtans.net/2014/04/

Baltica, Laurentia, and the Avalonion Terrane shown on the map above later collided to form the continent Laurussia during the Caledonian Orogeny around 410 million years ago… with our featured Camodian block steadily heading northward but not quite there yet. On the figure below, it’s part of the lump labeled “Armorica??”

By Woudloper – Own work, CC BY-SA 1.0, https://commons.wikimedia.org/w/index.php?curid=5038110

This piece of the Cadomian terrane didn’t get sutured onto the rest of France until about 320 million years ago – it had rifted off of Gondwana and ran into Laurussia as part of the Variscan Orogeny that finished the formation of Pangaea. The aftermath of the Variscan orogeny is shown in the figure below, with our featured location indicated by the teal dot.

Close up of the collisions between Gondwana and Larussia, with Baie de Motn St Michel as a teal dot. Current continental outlines are approximated with grey lines. Picture By Woudloper – Own work, CC BY-SA 1.0, https://commons.wikimedia.org/w/index.php?curid=5330107, edited by the author

Since then this fragment of the Cadomian terrane has hung on tight to the rest of France as Pangaea ripped apart and the continents shuffled around to their modern configurations. Through these 600 million years Mont Saint-Michel’s geologic setting moved from the south pole to around 45 degrees north, switched continents while remaining intact, survived the breakup of Pangaea and the opening of the Atlantic ocean, and eroded to its modern form.

This area doesn’t preserve any of the geologic record from the Paleozoic or Mesozoic eras, and the only record of the Cenozoic era are certain Oligocene marine sediments in the bay. However, its Quaternary sediments since the last glacial maximum give scientists plenty to study, and account for much of its dynamic recent history. At the height of the last ice age around 15,000 years ago, wind-blown loess and sand covered much of the ancient geologic platform.This is shown in the map below – you may have to click on it for the full version in order to read the text. I added English translations in blue text.

Around 8,000 years ago the sea level rose to intrude into the bay, creating the topography that we see today. The defining sediment around the Mont Saint-Michel nowadays is “tangue” – a salty fine-grained mix of clay, silt, and shells. It’s created by the competing forces of the three rivers discharging sediment into the bay and the force of the tides which rework that sediment and add the pulverized shells. Elsewhere in the bay, the dominant sediment is bioclastic sand, which is a fancy way of saying sand made up of bits of shells.

The Baie de Mont Saint-Michel has the 5th largest tidal range on earth thanks to its position at the mouth of the English Channel – 14 meters! This huge tide, in combination with the sediment flowing out of the rivers See, Couesnon, and Selune, adds 400,000 to 700,000 cubic meters of marine and terrestrial sediment to the bay each year. This natural influx has slowly filled in the tidal area that isolated the Mont, but human actions have accelerated this process. In the 1850s, polders and dikes were built to extend the arable and pastoral land around the three rivers in the estuary. This ate up area on the tidal flats. Additionally, a dam was built on the Couesnon River in 1969 that eliminated its ability to flush sediment out of its mouth in the bay. To add insult to injury, a permanent parking lot was built up above the tide adjacent to the Mont to allow visitors easy access. It seemed imminent that Mont Saint-Michel would become a part of the mainland, a peninsula when it was once an island.

In 2006, work began on projects to preserve the maritime character of Mont Saint-Michel. This included relocation of the parking lot from adjacent to the Mont to further inland, constructing an elevated causeway that allowed water and sediment to flow underneath it, dredging the channels of the Couesnon and adding riprap structures to split the flow of the Couesnon in two near the Mont, modifying the dam on the Couesnon so it could allow the river to flush sediment more powerfully at the receding tide, and restoring marshes on the Couesnon to trap sediment upstream. The goal of all this was to deepen the water directly around Mont Saint-Michel by increasing the erosive power of the Couesnon River and removing obstacles that collect sediment.

Image from https://throughthesandglass.typepad.com/through_the_sandglass/2009/09/montsaintmichel-a-massive-sedimentology-experiment.html

The following map shows the difference in elevation around the Mont, measured by LIDAR, between February 2009 and September 2019. The project has been quite successful so far!

 

Map from http://www.projetmontsaintmichel.fr/les_travaux/hydrosedimentaire.html

translation of the text box:

• The erosive fringe to the right of the eastern grassy area is still present but stable in the absence of an active channel in the zone.
• The zones of erosion directly to the north of the Mont have increased (140m in width and 1.80m in thickness in places)
• The zone of erosion to the right of the western grassy area has grown (150m in width and 2.5 m in height), with a significant reactivation of the western stream..
• Erosion through the large western bank was increased and the area was enlarged.
• The western and eastern channels rejoin to the south of the Mont, creating strong erosive forces in the zone, -4m in places.
• Zone of enlargement of the large western bank to the north of the Mont still present and growing (until +2.5m).

All of this does not reverse the sediment deposition in the bay – there’s no way for us to permanently fight the influx from the incoming tide and the three rivers in the bay. However, it does reverse the human-caused processes that were accelerating the accumulation of sediment around Mont Saint-Michel.

And just from a touristy viewpoint, I enjoyed the pedestrian bridge and the removal of the parking lot and visitors center from directly in front of the historical site. It makes me feel more like I’m approaching a medieval fortress and less like I’m approaching a historical theme park. The new parking lots and visitors center are surrounded by marshes and trees, and the short walk to the Mont is beautiful.

This UNESCO world heritage site was more than worth the drive just for the history and the fun of exploration, and seeing its unique place in the landscape was also fascinating! I was thrilled to check this place off my bucket list!

Resources:

Extremely thorough French-language geologic and sedimentologic paper and maps of Baie de Mont Saint-Michel by France’s geological survey: http://ficheinfoterre.brgm.fr/Notices/0208N.pdf

Great, detailed English-language resource of the geology and sedimentology of the bay: https://www.unicaen.fr/m2c/IMG/pdf/field_trip_mtstmichel_bay_ims2017_toulouse.pdf?916/99338f5f109256e86ac5bb88aa170b32c7a5714e

Chantraine, Jean, et al. “The Cadomian active margin (North Amorican Massif, France): a segment of the North Atlantic Panafrican Belt.” Tectonophysics, vol. 331, 8 Oct. 1999, pp. 1-18.

Stampfli, Gérard & Borel, G.D.. (2002). A plate tectonic model for the Paleozoic and Mesozoic constrained by dynamic plate boundaries and restored synthetic oceanic isochrons. Earth and Planetary Science Letters. 196. 17-33. 10.1016/S0012-821X(01)00588-X.

Excellent summary of the history, sedimentology, and restoration of the bay: https://throughthesandglass.typepad.com/through_the_sandglass/2009/09/montsaintmichel-a-massive-sedimentology-experiment.html

French-language field trip guide to the bay: https://sgmb.univ-rennes1.fr/vie-associative/excursions/12-excursions/47-baie-du-mont

French-language resource on the project to restore the bay: http://www.projetmontsaintmichel.fr/index.html

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