Mazes of mines and catacombs beneath Paris

The Sacré-Cœur Basilica was swarming with tourists on the first day that Elaine, Heather, and I set out to explore it. The line for entry stretched all the way across the plaza and we could hear the muffled din of the crowds within the sanctuary spilling out through the doors. We made a unanimous decision to avoid the chaos by heading to the quieter sanctuary of the nearby Musée de Monmartre. It’s dedicated to the artists and cabarets that gave the neighborhood its bohemian reputation at the end of the 1800s, and was the last place I though I’d find anything geological. But lo and behold, it had a small exhibit on the gypsum mines that used to be active on the Montmartre Butte, as sketched by the Impressionists.

These mines complicated the construction of the Basilica that we had considered visiting that day. In the 1870s the Parisian government committed to building a huge Catholic monument there as an unmistakable reminder of the power of church and state. Montmartre had been the birthplace of the radical socialist Commune movement that had unsuccessfully tried to overthrow the government in 1871, and the government wanted to remind the neighborhood of who really called the shots. However before the structure could rise above the surface, the foundation required 83 pillars sunk 130 feet deep into the rock layers below the gypsum mines.

I had something of an epiphany (or more accurately, and “oh, duh!” moment) – gypsum is the main ingredient in “Plaster of Paris”! So that’s where it came from! In particular, it came from the green areas on the map below:

Mines of paris translated

Montmartre is clearly visible as the ring-shaped cluster of gypsum mines and the center top of the figure. map translated from French by C. van Stolk. By Plan: Émile Gérards (1859–1920) BnF Notice d’autorité personneDigital copy: ThePromenader – Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=28798468

 

Most of the pale building stone that makes the City of Light so distinctive actually comes from within its limits! It gave me a sense of the huge scale of mining in the city that most of what we see on the surface came from underneath it. The gypsum was mined extensively beginning in the Middle Ages to create fire-resistant covering for wooden structures. You can see the distinctive cream or yellow-ish limestone in almost all of the buildings in Paris built before the 1770s, from the Saint-Germaine church to the grand mansions between the Marais and the Place des Vosges. The limestone is riddled with little cone and spiral shaped fossils too, if you look carefully.

These quarries have been active since the Romans occupied what they called Lutece, but the city didn’t get its wake-up call until 1777 when a gaping sinkhole swallowed an entire city block near what’s now the Place Denfert-Rochereau. That’s when King Louis XVI banned mining within the city limits. He also commissioned Paris’s first mine inspector, Charles-Axel Guillaumot, to map the warren beneath their feet and shore up the weak places to prevent a repeat of the “Place d’Enfer” disaster. M. Guillaumot earned the nickname “the savior of Paris” and was one of the few royal appointees of that era to survive the guillotine – the revolutionaries considered him too useful.

Around the same time, Parisians realized that the former quarries posed a solution to another pressing problem – unmanageable overcrowding at cemeteries above ground. Between 1785 and 1814 the bones of over 6 million people who had died since the founding of the city were disinterred and moved in nocturnal religious processions into the properly sanctified sections of the tunnels designated as the municipal ossuary or catacomb just outside what was then the southern boundary of the city. The catacombs were opened to the general public for tours in 1810.

119 years afterwards, Elaine, Heather, and I set out to wait in the long line to enter the Municipal Ossuary a.k.a. the Catacombs of Paris. It’s a rare attraction that appeals the Elaine’s love of spooky things, my love of rocks, Heather’s interest in history, and all of our pressing interest in getting out of the 97 degree heat. We just managed to get into the last tour group before the gates closed for the day! Because of the time crunch we had to fly through the geology exhibition in favor of getting to the creepy bits but I took photos of the placards, some of which I have translated below. For photos of the ensuing Super Spooky Aesthetic ™ check out my travelogue post.

The rocks below Paris are younger than the rocks we clambered on in Brittany – they’re from the Eocene era between 56 and 38 million years ago, when the Paris Basin held a shallow sea that left behind characteristic limestones and shell fossils. The sea went through periods when it very nearly dried up, leaving layers of evaporate minerals such as gypsum.

translated paris paleogeography

Image photographed by author at the catacombs, legend translated

North-south tectonic pressures slightly buckled the basin in the time since the rock was formed. This created the Meudon anticline (A-shaped fold) on the southern side of Paris, which is why limestone and gypsum from different ages are mined at similar elevations on either side of the Seine.

translated cross section of paris

Image photographed by author at the Catacombs, and subsequently translated

Left: labeled layers in the limestone, Right: teensy 5mm stalactites!

Paris has a thriving community of “cataphiles” who risk law enforcement action to explore the subterranean side of the city. National Geographic did a great special on them in 2011. Over the years since the quarries were abandoned in the 1950s, they’ve made the flipside of Paris into their playground and undertaken mapping efforts. Here’s an elegant map of the “Great Southern Network”, with notes and annotations. It’s the kind of treasure map that got me into cartography in the first place.

If you’d like to learn more, Dr. Jack Share at one of my favorite blogs “Written in stone, seen through my lens” wrote two fantastic, extremely detailed posts about the geology of Paris – one focusing on the gypsum quarries and the geologic origins of the Paris Basin, and a second one on the mines and catacombs. I highly recommend those posts, and the entire blog!

Postscript:

We did eventually visit the Sacre Coeur Basilica towards the end of our stay in Paris. It turns out that the basilica is quietest directly after services in the evening, and there is a lovely organ postlude. From the inside it’s a wonderfully peculiar building. Sacre Coeur was built between 1875 and 1919 and the stained glass windows weren’t added until after WWII, and so it encompasses huge changes in the French design aesthetic. The architecture is a mix of the stately Neo-classicical style like the Pantheon and the Byzantine revival style, it got a gloss of whimsical Art Nouveau statuary at the turn of the century, and ended up with weird abstract stained glass windows from the post-war period when artists felt that the world was broken beyond repair. The building is made of travertine from the Souppes-sur-Loing quarry, in the Seine-et-Marne department about 100 km south of the basilica. Travertine is an exceedingly hard, fine-grained stone that releases chalky white calcite when it rains. So basically, it’s self-cleaning and is able to stay gleaming without pressure-washing!

English-language resources:

https://en.wikipedia.org/wiki/Mines_of_Paris

Paris: From quarry to catacombs

http://catacombes.paris.fr/en/history/geology-and-quarries

http://www.sacre-coeur-montmartre.com/english/history-and-visit/article/architecture

A really excellent post about the mines of Montmartre: http://written-in-stone-seen-through-my-lens.blogspot.com/2014/04/geological-legacies-of-paris-basin-part.html

Another very thorough post from that same author on the rest of Paris’ quarries: http://written-in-stone-seen-through-my-lens.blogspot.com/2014/06/geological-legacies-of-paris-basin-part.html

reddit post hosting an AMAZING map of all the explored catacombs under Paris: https://www.reddit.com/r/MapPorn/comments/b50j1r/detailed_map_of_the_paris_catacombs_in_english/

French-Language resources:

detailed post on mines, mining techniques, and mine inspection in Paris: http://exploration.urban.free.fr/carrieres/index.htm#exploitation

Sea caves and wild cliffs at the Crozon peninsula

After the Cote de Granit Rose, Heather and I drove about as far west as you can get in France – to the Crozon Peninsula. We settled in at a sailing club hostel in little fishing port of Camaret-sur-Mer and then hiked out to the cliffs at the Pointe de Pen Hir to enjoy the French tradition of apero (pre-dinner wine and snacks). We held on tightly to our packets of cheese crackers – 70 meters would be a long way to drop them off of these cliffs.

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The cream-colored cliffs turned golden as the sun set and I was intensely curious about them. The coast and  sea stacks were made of pale layers that were tilted about 60 degrees from horizontal towards the east and eroded in jagged shapes. Luckily, we ran into a sign for the Regional Nature Reserve with an a few answers. These cliffs are made of the Armorican Sandstone, dated at 475 millions years old. The rock is tough, but not strong enough to resist the waves entirely. The southernmost part of the cliff has been broken up into a series of six sea stacks called the “Tas de Pois”, or “Pile of Peas”.

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These tilted layers are a thing of beauty! You can see here how the purer sandstones form ridges (center right) separated by the weaker and more eroded shale layers (center). This photo was taken looking south.

The Armorican Sandstone is at the bottom of a thick stack of tilted sedimentary rocks, and we met more of them as we hiked east along the southern side of the peninsula. The rest of this stratigraphic sequence forms the Veryac’h Cliffs, which have been designated a national geologic heritage site.

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A beautiful view, and a sign that was both helpful and disappointing. It had little icons forbidding both rock hammers and sample collection on the beach!

In previous posts about the region, I’ve brought up the two mountain-building events that created metamorphic and igneous rocks in northern France. The Cadomian orogeny 750 to 540 million years ago created the metamorphic and igneous rocks near Mont Saint-Michel, and the Variscan Orogeny 360 to 300 million years ago left its mark in the granites at the Cote de Granit Rose. What I haven’t covered yet on this blog is the time that elapsed between those two continental collisions. The Armorican sandstone and the rocks of the Verac’h Cliffs were deposited in a sedimentary basin that opened up between 500 and 360 millions years ago, as the result of tectonic extension in between those two mountain-building events. Sediments eroded from the nearby Cadomian mountains and were deposited in the extensional basin. The Variscan orogeny then squeezed these horizontal layers into folds – that’s how these layers ended up tilted on their side.

French geologists are immensely proud of this 1000 meter stretch of cliffs because they represent an unbroken 50 million year record! Unbroken is the key word here. It’s rare to find an area were sediment has been laid down continuously, without the sea level changing and eroding away layers to create an unconformity. The Veryac’h cliffs hold an encyclopedia of fossils and information about the environment from the Orovician, Silurian, and Devonian eras.

I didn’t get to explore them on the ground, though. Heather’s tolerance for staring at rocks has some limits. There are some great field guides online if you read French…

If you feel like diving into a full literature review in academic French, Vidal et al. published an exhaustive geologic history of the Crozon Peninsula in 2010. It’s in French again, but has excellent figures. I’ve modified and translated one of them in the figure below (it’s large, sorry, you may need to click on it to see it full-size and read the text):

 

Crozon peninsula geology translated

It’s a great mess of a geologic map, isn’t it? The rocks were deposited in a stratigraphic sequence that was orderly enough, but in the millions of years since then they’ve been folded, broken, and shuffled around.

Imagine stacking a dozen or so carpets on top of each other. Next, recruit a few friends to shove the carpet stack from each side until it’s a rumpled mess of folds. Once you’ve done that, attack the top of the pile with a chainsaw to level it out but remove the cut-off bits as you do this. Shove the pile around a bit more for good measure and make more passes with the chainsaw, and you’ve got a representation of what happen in this corner of Brittany during the Variscan mountain building event when the ancient continent of Avalonia ran into Gondwana.

deposition folding faulting

Extremely simplified sketch of the metaphor above… figure drawn by author.

 

The rocks near Morgat on the western side of the Cap de la Chevre where we kayaked are also tilted layers of the Armorican Sandstone, but tilted to the opposite direction. The beds also strike roughly northeast-southwest but dip steeply to the northwest. I didn’t find a direct reference to this in my sources. One likely explanation is that the two outcrops of the Armorican sandstone are two limbs of a syncline ( U-shaped fold) that were broken apart in the chaotic faulting in the region during the Variscan orogeny.

Photos above: looking at the Armorican sandstone in the Cap de la Chevre from the south (left) and from the north (right).

Variations in the hardness in the sequential layers of sandstone are more or less resistant to the waves, which creates the wonderful arches and caves that we explored in our kayaks.

Image result for formation of sea stacks

Illustration of sea cave and sea stack formation, from The British Geographer http://thebritishgeographer.weebly.com/coasts-of-erosion-and-coasts-of-deposition.html

Next up on the blog: the creepy catacombs and ancient mines beneath Paris!

Sources:

https://www.presqu-ile-de-crozon.com/geologie/000-geologie-finistere-presqu-ile-de-crozon.php

https://www.saga-geol.asso.fr/Documents/Saga_312_Crozon.pdf

http://sigesbre.brgm.fr/Histoire-geologique-de-la-Bretagne-59.html

portal for geologic maps of Brittany from the BRGM: http://sigesbre.brgm.fr/Cartes-geologiques,147.html

https://sgmb.univ-rennes1.fr/vie-associative/excursions/12-excursions/51-crozon

Vidal, Muriel & Dabard, Marie Pierre & Gourvennec, R. & Hérissé, Alain & Loi, Alfredo & Paris, Florentin & Plusquellec, Y. & Racheboeuf, P.R.. (2011). The Paleozoic formations from the Crozon Peninsula (Brittany, France). Geologie de la France. 3-45.

Crazy pink rock formations at the Cote du Granit Rose

Part 2 of the geology of my summer vacation. For an idea of where this fit in our trip, check out the travelogue post. This post follows the first post on Mont Saint-Michel.

I had left all of the vacation planning in Heather’s able hands so I could focus on my thesis last spring. My only requirement (only halfway in jest) was that the vacation had to include eating pastries on rocks. And boy, did Heather deliver! Days 5 and 6 of the trip found us near Trebeurden and Ploumanac’h on the fabulous pink granite coast. The sun was shining, the pain au chocolat was as delicious as I had ever hoped for, and a giant granite playground awaited us.

croissant and rocks

my dreams came true!

There isn’t a shortage of granite on the Brittany coast – we met some in the last blog post too. Much of was grey and only visible in isolated outcrops. As we hiked east from the little port of Ploumanac’h along the coast, the grey granite gave way to crazy piles of unmistakably pink rock! I couldn’t help but start wondering what caused the change in color, not to mention the weird shapes!

It turns out that the explanations come in threes: the pink granite is made of three minerals, it belongs to one of three different igneous events in the region, and three different substances have sculpted the granite into the wild shapes at Ploumanac’h.

The pink granite gets its rosy hue from potassium feldspar, while the greyer granite has more creamy-colored plagioclase feldspar in its makeup. I illustrated their mineral composition in the figure below. The natural history museum in Ploumanac’h informed me that the pink granite is  approximately 50% potassium feldspar, 30% quartz, and 20% biotite. They didn’t give details about the less glamorous grey granite and I was too focused on getting to the pink stuff to even take a close-up of it, so I’ve only approximated its composition.

Pink Grey Granite Comparison

Both colors of granite at Ploumanac’h were put in place around 300 million years ago (mya) during the last gasps of a mountain-building event as the ancient continents of Gondwana and Laurussia crashed together to form Pangaea. I talked in depth about this massive game of continental bumper-cars in the previous post, so I’ll skip it here. Over time erosion unearthed the buried masses of granite, as shown in the figure below.

pink granite emplacement diagram.png

Photo of a diagram in the exhibit at the Maison du littoral, text translated by me.

To get even more specific, the granite in the area was put in place in three physically distinct phases around 300 mya. In the first phase, two magmas with different compositions intruded the surrounding metamorphic rock at the same time. The first was rich in silicon and formed the coarse-grained pink granite and the second was poor in silicon and formed the dark gabbro visible near Tregastel. These two igneous rock types melted in the same event from two different types of source rocks, giving them their unique compositions.

During the second phase, another silica-rich magma forced its way into joints in the now-cool first pink granite. This magma had a similar composition  to the pink granite in the first event but cooled more quickly than its predecessor, forming smaller mineral crystals.

In the third phase, a magma with a more basic (as in pH) composition intruded into an dome-shaped weakness in the cooled granite from the first two phases. This magma cooled into the blue-gray granite near Ile-Grande.

The difference between the colors of the ~520 million year old granite at Mont Saint-Michel, the ~300 million year old grey granite at Trebeurden, and the ~300 million year old granite at Ploumanac’h isn’t merely ornamental. The rocks’ mineral compositions give geologists clues to the kinds of source rocks that melted into the granite. Feldspars and quartz have high silicon:oxygen ratios in their composition, and so indicate that abundant silica was present in the source rocks.

A whole host of different kinds of minerals are built from silica and oxygen, ranging from the densest minerals with 4 oxygen atoms  for every 1 silicon atom to the less dense minerals with only 2 oxygen atoms for every 1 silicon atom. In general, the less dense silicon-rich minerals are more represented in the continental crust, while the denser silicon-poor minerals are more common in the oceanic crust.

You can see these relationships between minerals’ properties and igneous rock types below in the igneous rock classification chart every mineralogy student learns by heart by the end of the term. It’s only a guideline – if a mineral was missing from the source rock, it will not show up in the igneous rock created from its melting. For example, amphibole and muscovite are missing from the pink granite.

This indicates that the pink granite was formed predominantly by the melting of low-density, high-silica rocks at low melting temperatures. The grey granite at Trebeurden is a little bit to the right of the pink granite on the classification chart – still a granite, but including more minerals with higher melting points and less potassium feldspar (a.k.a.  orthoclase feldspar). The gabbro at St. Anne is even further to the right, and likely formed from the melting of a chunk of oceanic crust. Sometimes rocks are completely off this chart. For example the magma that formed the pale granite that we saw at Mont Saint-Michel either melted at low temperatures (geologically speaking) of ~600 C or melted from source rock whose chemistry didn’t allow for the formation of dark mica or amphibole crystals.

So I figured out why the granite was pink instead of grey. But what created its otherworldly shapes? And where did all these boulders come from?

Usually boulders are created in steep landscapes where chunks of rock falling off the canyon walls are tumbled aggressively in mountain streams and carried long distances. In contrast, these boulders have barely moved relative to each other since the granite cooled! They were formed in place by erosion, shown in the diagram below. The technical French term for this formation is “un chaos”, which seems very appropriate.

granite chaos creation

The important factor here is a change in the rate of weathering and erosion. In this case, the erosion regime changed from slow dissolution of the rock by groundwater (shaping the granite into boulders underground) to more rapid erosion as the waves crash on the shore (exposing the boulders).

Once the boulders are exposed to the elements, two slower types of chemical erosion nibble them into even more convoluted shapes. Chemical reactions between salt spray and the the mica and feldspar crystals in the rock transform them into weaker clay minerals that wash away, creating divots and creases in the rock wherever salt collects.

As saltwater works on the rocks from the top, organic acids in soil eat away at the rocks at ground level over tens of thousands of years to create subtle mushroom shapes.

acidic soil erosion

The end result is an utter delight to explore!

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Heather points out a quartz vein in the pink granite. The boulder on the center left shows a distinct salt weathering divot on its top.

pink granite castle

Climbing to the top of a formation, I found a 2-foot deep crenelated “crow’s nest” formed by salt weathering!

Sources (all are in French):

Great summary from the local natural history museum, the Maison du littoral: http://ville.perros-guirec.com/fileadmin/user_upload/mediatheque/Ville/Maison_du_littoral/refonte_page_environnement/expo_origine_du_granit_roseBD.pdf

Less technical summary from the local tourist board: http://www.cotedegranitrose.net/la-cote-de-granit-rose/geologie-le-granit-rose/

Long and extremely thorough field trip guide published by the Geological and Mineralogical Society of Brittany: https://sgmb.univ-rennes1.fr/geotopes/decouvertes/23-decouvertes/67-ploumanac-h

Short summary/technical field trip guide: http://www.saga-geol.asso.fr/Geologie_page_conf_Ploumanach.html

 

Why can Mont Saint-Michel withstand the tides?

mont st michel

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…

Mont St Michel surface geology IMS 2017

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:

BRGM Mont Saint-Michel Cross Section

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.

Cadomian Block Map Chantraine et al 2001

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??”

formation of Laurussia caledonian orogeny

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.

variscan orogeny MSM note

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 Camodian 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.

BRGM baie de MSM 10000 ya traduitAround 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.

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!

 

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

Twin Trek 2019: France!

This is a “travelogue” post – more geology specific posts to follow!

My sister and I had a fantastic opportunity for out annual “Twin Trek” this year! My family was having a reunion in Ireland, so our transatlantic plane tickets were covered… it opened up a whole new continent of possibilities. I handed all the responsibility for choosing a destination over to Heather, pleading that I didn’t need such a tempting distraction while finishing my thesis. I told her that as long as I could eat pastries while sitting on rocks at some point I would be happy. She’s a gem and put together a fantastic itinerary in France! Both of us had studied abroad in the south of France in college, and she had spent a year teaching English in Normandy. This time, she decided that we would explore a beautiful region that she had briefly visited and wanted to return to – Brittany, in the northwest. We hostel-hopped from Rennes to Mont-Saint-Michel to St. Malo to the Pink Granite Coast to Finisterre, then back to Rennes and on to Paris. Being over 25 and being able to get a rental car felt so luxurious… the last time we were in France as college students we got an education in foreign public transit out of necessity.  I created an ArcGIS Online map of our route and have included a link to it below (unfortunately, free WordPress accounts can’t embed maps). I love the new watercolor base map that is available! The link is followed by screenshots.

(You can reach the map of our trip location directly at this link)

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I’m looking forward to writing several posts about this trip. I’m sure the research will stretch my command of the French language in new directions, but it will be a fun scavenger hunt to see what information I can find!

  1. What geologic features allow Mont-Saint-Michel to rise above the tidal flats?
  2. Why is the granite in Ploumanac’h and the rest of the “Cote de Granit Rose” so pink?
  3. When created the spectacular white cliffs near Camaret-sur-Mer on the Presque-Isle de Crozon?
  4. Why are there so many sea caves near Morgat, also on the Presque-Isel de Crozon?

But in this post, I’ll just share the travel diary part of the story.

I was cranky, jet lagged, and hadn’t slept in 20 hours when Heather picked me up from the train station in Le Mans. I’m not sure which one of us was more frazzled – she had spent the previous few hours reintroducing herself to driving stick shift in a tiny car on tiny roads after six years driving exclusively an automatic. So as glad as we were to see each other it was a very quiet car ride to Rennes, where we checked into the hostel and went in search of Brittany’s specialty: buckwheat crepes filled with delicious things. We felt significantly better about the state of the world when our food arrived, accompanied by traditional Breton teacups of hard cider.

Rennes was a wonderful place to recover from jet lag and feel like I was truly in France. Brightly painted timber-and-plaster houses lean crookedly against each other like they’ve had too many teacups of cider and surround gothic-style churches and squares full of cafe tables. After getting lunch (crepe-wrapped sausages) at the huge Saturday market at the Place des Lices, Heather and I wandered through the shopping district to the Jardin de Thabor. Once a monastery garden, the public gardens got a scenic 19th century renovation to include paths, grottoes, a botanic garden, and a delightfully random aviary. The lawns were packed with people escaping un-airconditioned apartments to catch breezes in the shade. We parked ourselves on a shady bench by the rose garden to finish the rest of the basket of strawberries. By that time my internal clock was in revolt. I went back to the hostel to crash until Heather lured me out of the room with promises of  new kinds of crepes and a glass of rose.

The next day we set out on the Twin Trek in earnest. Heather was excited to finally see Mont-Saint-Michel in sunny weather, and I was curious as to whether it would equal the hype. It turned out that getting there early on a Sunday was a great decision – the tour buses from Paris must have been running late because there were pleasantly few other tourists there. We could really imagine that we had stepped back in time. The stories on the audioguide of the Abbey made the small fee well worth the money. There aren’t many interpretive signs to bring the impressive but stark walls of the abbey to life; the audioguide explains not only the construction of the abbey but the history that it witnessed and the lives of the religious orders that lived there. We had lunch on the ramparts beside a family of seagulls who watched us with great interest and eventual disappointment when we refused to share.

Heather and I headed back to the car once tour groups started to flood the island in earnest – the small streets were so crowded that we had trouble elbowing our way back down to the gate. We drove to the storied port town (and pirate hideout) of St. Malo, settled into the hostel, and walked down the beach’s boardwalk to find a crepe place in the historic walled city. It’s amazing – the city was 75% destroyed during WWII, but was painstakingly rebuilt stone by stone so it looks unchanged since the 1600s! We missed the last bus back to the hostel and stayed to watch the Bastille Day fireworks. The fact that the sun set at 11 pm was really throwing me off!  Especially because we had big plans for the next day – a hike from St. Malo to Port Mer along the coast.

We took the number 8 bus to the Ilots stop, and then hiked the GR (Grand Randonee) 34 to Port Mer where we caught the bus back to the hostel. We weren’t using a map, but it turns out that it was over 11 miles. It was a hot, sunny day and the coast was beautiful – all sheer cliffs, ruined castles, and sailboats tacking between tiny islands. Also, as it turned out, nude beaches. So despite the ocean views, there were some parts of the route where we chose to admire the landward side of the trail. I’ve included an interactive web map below. Although it may look like we walked on water, those parts of the route actually indicate tidal flats. There’s such a huge tidal range here! When we left at the morning the sea was a between 1/2 mile and 1/4 of a mile away from the boats stranded on the tide flats, and in the evening the boats were floating. Heather and I agree that we would recommend taking the bus one stop further to La Guimorais to get straight to the prettier parts of the hike.

(You can link directly to the interactive map here)
st malo port mer map
After a well-earned dinner and beers at Port-Mer, we took the bus back to St. Malo and slept very well that night. If you weren’t doing this hike in the summer, you’d have to go all the way to Cancale to catch a bus back to St. Malo – the bust line that serves the beaches is seasonal.

After a morning exploring the ramparts of St-Malo and hunting down ermine-themed souvenirs, we started the drive west to our next hostel in Trebeurden on the Cote de Granit Rose. Once we reached the hostel, I switched into the driver’s seat and Heather navigated us to the surreal-looking geologic destination that she had been promising me – the pink granite near Ploumanac’h. We had a leisurely happy hour, hike, and dinner while watching the sun slowly set over the Channel.

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We met more opportunistic seagulls while eating our picnic dinner on the pink granite

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Heather hanging out with a “chaos” of pink granite boulders in the background. The boulder had been sculpted into crazy shapes by water and wind!

The next morning, I dragged Heather out of bed bright and early so that we could go back to the geologic museum I had seen at the Maison Littorale along our hike the previous evening. It gave me plenty of material for a future blog post on the granite we were scrambling over, and the Heather bought me a lovely small piece of polished local granite from the gift shop as a birthday present. She knows me well, and yes I am literally that person who fills their suitcase with rocks. In my defense, it wasn’t much bigger that a bar of soap. The museum also had an exhibit on how the park was trying to restore vegetation, so Heather and I tried to be good stewards when we were using the boulders as adult-sized jungle gyms. There were still plenty of rocks and tide pools that we could get to appropriately! The tide pools here look different than the ones in Oregon – the coralline algae is grey instead of pink, and the predominant anemones are smooth, dark, and glossy instead of rough and green.

It was hard to drag ourselves away from that amazing coastline, but we also know we needed to make it to our next stop that night. We had lunch with the chickens at the hostel, loaded up the car, and drove a two hours to the small fishing port Cameret-sur-Mer on the Presque-Isle de Crozon. We went on a hike before dinner with a plan to explore a surrealist poet’s ruined mansion, and menhir alignment, and the Point de Pen Hir. Along the way, we stumbled across a huge complex of WWII bunkers and sobering memorials to the 638 French merchant marine ships and many Bretons lost in the war.

The next day dawned grey and cloudy, and Heather had planned for us to hike near Kerloc’h and then rent kayaks. That original plan was foiled when the boat rental shop told us that the westerly wind was too strong to rent kayaks from Kerloc’h, and so we went to Morgat on the opposite side of the Cape de Chevre where the wind was more favorable.The day was still cloudy and cool when we hauled our kayaks to the edge of the tide flats. That rental staff looked at us like we were crazy and suggested renting wetsuits, but the sun came out a few minutes after we launched! It turned into an absolutely perfect day to be on the water. The wind was still unpredictable though – one sneaker wave tossed Heather and her kayak into a complete somersault as she was pushing off from a beach, scraping up her arm and scattering her belongings across the waterline. She maintains that it was OK because getting a scar at sea ought to make her an honorary Breton pirate.

Relocating our kayak adventure to Morgat had a major unexpected silver lining: sea caves!! The coastline was steep, convoluted, and carved into fantastic arches and caverns. When the tide is high you can paddle into some of them…although the incoming tide created significant whitewater in some of them. Heather and I had a blast surfing the waves in the more exciting caves but it may not have been the smartest thing to do. When a family with small kids on the bows of their kayaks asked us where the “Devil’s Chimneys” were, we crossed our fingers behind our backs and feigned ignorance.

Th next day, the clouds of the previous day turned into genuine Breton downpours. We gave up the idea of outdoor adventures in favor of taking a bouncy ferry ride across the inlet to the huge port of Brest. Unlike Rennes, it doesn’t have that old-world scenic French flavor. It was bombed completely flat during WWII and hastily rebuilt in cubic concrete except miraculously for one thing – the ancient fort. It now houses the French naval offices and also a great maritime museum. I wish I could have teleported my dad there to enjoy the exhibit on around-the-world racing in catamarans for the Jules Verne Trophy.

The next day was pretty tame… we poked around the many art galleries in Camaret-sur-Mer, and then drove back to Rennes. The following morning we took the train to Paris to meet up with Heather’s girlfriend Elaine.

While it was relatively warm in Brittany, the “canicule” (poetic French term for heat wave) was merciless in Paris during the five days of our stay. A change from the usual atmospheric patterns caused more hot air than usual to push its way north from the Sahara into countries much worse prepared to deal with it. The daily high temperature ranged from 97 to 108 degrees F , while the average high for July is 78. This forced us to change our usual travel patterns and take a more relaxed approach to Paris than we had planned. We made it through the week with strategic applications of siestas, Orangina, and ice cream.

Over the course of the visit the three of us visited the Pantheon (mercifully cool, and with a fascinating exhibit on deaf history), the Musee d’Orsay (packed, but worthwhile for the amazing exhibit on Berthe Morisot), Sacre Coeur (overrun by tourists diverted from the closed Cathedral de Notre Dame, and quieter directly after services), Musee de Montmatre (an quiet oasis well worth the admission cost with delightful exhibits about impressionists and the neighborhood), and the Catacombs (Elaine’s favorite for the Spooky Aesthetic ™, and a standout for me for the ancient history of mining). I’ll definitely write another post about the elaborate system of mines and tombs under Paris!

On the last day of our stay, we successfully navigated a packed metro with our luggage, Heather led the way to the most well-hidden municipal bus depot I’ve ever encountered, and we headed north to meet my parents and visit my grandmother in Belgium. Thank heavens the bus was air-conditioned.

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Can’t I just teleport back to Ploumanac’h?

Next up: geology posts.

After that: Ireland!

Smoked in at Crater Lake

My sister is so patient with me and my geologic pilgrimages. She spent two days in 2014 tolerating me taking selfies with every geologic contact I saw in the Grand Canyon. Last summer, the two of us hopped in the car and drove south through the wildfire smoke to another one of my bucket-list stops: Crater Lake.

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I knew it was the deepest lake in the USA, but nothing really prepared me for the size of the view when we arrived at the rim of the caldera! The ferry to Wizard Island looked absurdly tiny as it cruised past, 800 ft below our feet. The lake itself covers 20 square miles – it’s large enough to swallow the entire town I’m living in.

crater lake scale2

Heather sat on a bench and befriended the ground squirrels while I spent some quality time in the geology museum carved into the side of the caldera to get my bearings.

Crater Lake was a very different landmark 7,700 years ago: it was a 12,000 foot volcano that we retroactively refer to as Mt. Mazama. Around 7,700 years ago it catastrophically blew its top, spewing 12 cubic miles of magma into the atmosphere. That’s enough ash to cover the entire state of Oregon in a layer 8 inches deep if it had settled perfectly.

crate rlake eruption volume

Once the explosive eruption had come to a close, the rim of Crater Lake stood only 8,200 feet at its highest.

The upside of that massive evisceration of the volcano, besides scenic views, is that it gives us an opportunity to see the old plumbing of Mt. Mazama up close and personal!

devil's backbone plumbing

crater lake llao rock

When we hiked up to Garfield peak, we traversed jumbled cross-sections of Mt. Mazama’s ancient eruptions.

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In some places there are very obvious sloping structures where lava flowed down Mt. Mazama’s flanks (below), while other parts of the slopes are messes of lighter-colored welded ash and pumice (above).

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We had fun taking perspective shots with the Phantom Ship landmark, which is the scant remnant of a fissure that was filled with lava and cooled into a more resistant fin.

Crater Lake is one of the best-appreciated geologic sites in the nation; it’s a centerpiece of scores of books and papers. Nothing I could write in this blog would come close to doing justice to all that, so I’ll keep it short! Here are some of the more noteworthy online resources I found to learn more about the park’s geology:

Ian Madin wrote a great blog post on cycling around Crater Lake at Cycle Oregon, and it includes GIS reconstructions of Mt. Mazama as well as detailed descriptions of the eruption.

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The USGS created a beautiful and informative poster using the LIDAR data (super-detailed laser-measured elevation) for the park, with extensive landscape notes. Click on the link in the previous sentence to get the full-sized poster.

usgs crater lake lidar

As soon as we finished the hike up Garfield Peak the wind’s direction changed. During the day the wind blew fresh air from the west, but in the evening the wind shifted to usher in the wildfire smoke from the huge fires in northern California and southern Oregon. Wide vistas were replaced with a blanket of smoke so think that I couldn’t see my car from across the parking lot. Everything besides the road might as well not have existed as we drove cautiously northwest out of the park to head back to Corvallis.

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Heather, Jo the Adventure Civic, and what would be Diamond Peak if it wasn’t shrouded in smoke.

The smoke mostly cleared by the time we reached Diamond Lake. Heather and I really wished we could blow off her flight back to the east coast to spend a few days camping here! It’s always hard dropping her off at the airport. Separating was made a bit easier when a few weeks later we got news that the whole family would be going to a reunion in Europe the next summer.  The two of us started plotting new adventures in France and Ireland!

 

 

Goodbye, OSU! It’s been great!

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My thesis advisor, Dr. Michael E. Campana, and I both braved the heat and walked at graduation. He was there to hood his PhD student Dr. Maria Gibson, who did fascinating work on aquifer storage and recovery in the Yakima Basin.

I made it! On June 5th I defended my thesis, Evaluation of Compartmentalized Aquifers in the Walla Walla Subbasin of Oregon Using Isotopic and Geochemical Tracers, and on June 15 I walked across the stage in Oregon State’s 150th graduation ceremony to get my M.S. in Geography.  The past year has been a whirlwind of fieldwork, coursework, teaching, writing my thesis, and jumping through all the administrative hoops to get a diploma. While much of my free time this summer will be spoken for as I try to find a groundwater resource management job here in the PNW, I’ll also be finally writing for the blog again! Keep an eye out for:

  1. Crater Lake adventures…. from last summer. Better late than never, right?
  2. More skiing thoughts – why are Mt. Hoodoo, Haystack Butte, and Mt. Washington so close together but such different shapes?
  3. Looking to the future – This year’s Twin Trek will be in France! Heather and I are going to travel around her old haunts in Britanny and Normandy, and she has promised me adventures involving pink granite cliffs and chocolate croissants.
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I added glitter to my mortarboard so my folks could find me among the 4,200 graduates. It worked pretty well!