Brittany: What Lies Beneath Part 1

The Côtes-d’Armor

A Certain Something… I Don’t Know What

The Brittany coastline has more than it’s share of the usual appeal of scenic seashores.  Painters of the impressionist’s era, including Emile Bernard and Paul Gaugin, made pilgrimages to Brittany and it’s spectacular coast where they strove to capture Brittany’s essence on canvas.  It’s a tough assignment, whether you try it with a brush, camera, or keyboard because there’s so much more to Brittany than pretty scenery.  There’s a sense of the primal, a feeling of deep pre-history that words can’t describe and images don’t express.

Je ne sais quoi is a French idiom that English speakers often attempt to coopt.  It literally means “I don’t don’t know what.” It can be used to convey a sense of mystery, as in “Susan has a mystique, a certain je ne sais quois that other girls don’t have.”   To me, at least, Brittany has this thing, this “I don’t know what” mystique that I can’t quite pin down or put into words, but it’s real and alluring.

Much of Brittany’s mystique is tied to its ancient landforms.  The peninsula was fashioned by time itself.  Not a few thousand or even a few million years, but eons, and epochs, and eras were needed to sculpt the peninsula and its rugged shores.

To get a sense of Brittany’s age and the forces that created it, you need some geologic backstory.   It’ll only take a few minutes to summarize the best parts, and if you spend those minutes now, your next trip to Brittany will have some added dimensions and a certain je ne sais quois…

Assembling Brittany – A Geology Primer

From our human perspective, the earth seems stable, immutable.  But if the earth could look into a mirror, it wouldn’t recognize its modern self from its youthful self.   The features that are so familiar to us today, like the shapes and locations of the continents, have not only changed, they’re totally different than they were just a short few hundred million years ago.

About one-fifth of the earth’s surface has managed to rise above sea level to form continents and islands.  Thanks to earth’s tectonic system, these landmasses have traveled more extensively than the average flight attendant.

In various turns, the continents have traversed the earth as one or two big pieces, while at other times the pieces have broken up and done their tectonic sightseeing in many continent-sized chunks.  Our modern distribution of continents is in flux, although it will take a few million years to see much change.

Earth’s Tectonic Plates – Credit U.S. Geological Survey

If you’re not familiar with the concept of tectonics, or plate tectonics, it’s the very well-supported theory that the earth’s surface is a patchwork of moving pieces called plates.  The plates move about on “conveyor belts” within the crust that are powered by convection.

Cross-sectional Representation of Tectonic Spreading Source: U.S. Geological Survey

The convection that drives the motion of the earth’s plates is the same process that produces ocean and air currents.  Magma from the earth’s interior is less dense than solid rock, so it rises through the cooler surrounding rock until it reaches the surface as a volcano.  About 80% of volcanos erupt below the surface of the ocean.  Many of these volcanoes are aligned in ridges, like the Mid-Atlantic Ridge, that act as “spreading centers.”   The oceanic crust on either side of the ridge is forced away from the ridgeline by rising magma.  These spreading centers are the drivers of tectonism, alternately pushing continents around the globe, then tearing them apart and shoving them back together again.

Tectonism is a slow but inexorable process.  It works because portions of the earth’s crust have different densities.  The ocean floor, which covers four-fifths of our globe, is mainly a volcanic rock called basalt.  The continents are made of many different kinds of rock, but the bulk of them are the composition of granite, which is less dense than the basalt sea floors.  The lighter continents ride atop the heavier oceanic plates, which are pushing outwards from the mid-ocean ridges.

Taken from Paleozoic Assembly of the Armorican Massif, University of Rennes, France, 2008

About 500 million years ago the portion of the earth’s crust that is now Brittany was part of a relatively small chunk of crust that geologists call a microcontinent, in this case, the Armorican Terrain Assembly or ATA.  It was likely an archipelago composed of bits of land that broke away from a much larger landmass.

Named after the ancient Gaulish moniker for Brittany, Armorica was traversing the globe in a pattern that had taken it to what is now Antartica, from whence it made its way slowly northward.

Meanwhile, what we know today as South America, Africa, Australia, Antarctica and a few others pieces of continental crust were all a part of a much larger “supercontinent” that geologists call Gondwana (the likely original source from which Armorica broke free).  Other smaller landmasses including Laurentia, Baltica, and Avalonia were floating along on their tectonic plates while dramatic changes occurred on both land and sea.   The years ticked past, and the Cambrian Period gave way to the  Ordovician Period.   As geologic fate would have it, the itinerant land masses lumbered into each other in a titanic collision that would reverberate for millions of years, reshaping global geography and geology, as well as the plants and other organisms that lived on the various colliding parcels.

By the time the collisions began, Armorica was already old, probably over 100 million years.  At this advanced age, it had the misfortune of being wedged between as many as four larger pieces of crust that had converged on it.  The collision would forever transform the ATA, but the transformation took a while.  The microcontinent’s Big Squeeze spanned the period from about 416 to around 359 million years ago.  Fifty-seven million years may not seem like a long time in geologic terms, but a lot can happen in few million years.

As Armorica endured the tortuous pounding of the collisions, parts of it began to fold under the huge pressures.   Much of Armorica’s ancient interior was raised from the depths through folds and faults.  These ancient rocks have since been exposed by weathering and erosion, giving us the benefit of having access to some very old and intriguing geologic formations from the ancient depths.

Brittany Rocks

Lovely and unusual rock formations were created by the herculean forces to which Armorica was subjected.  The mountains and uplands that rose from the folding of the little continent’s broken spine became the source of thick sequences of sedimentary and metamorphic rocks, as well as some of the rich soils of Brittany’s green and productive interior.

Earth’s deep interior is usually hidden from us by countless megatons of rocks and soil.  The wreckage of Armorica’s smash up exposed some very interesting features of the earth’s internal anatomy.  Some of the more intriguing rock types and formations include:

Ophiolites: This is not a rock type, but a sequence of rocks.  Ophiolites are pieces of oceanic plate that have been pushed upward (obducted) onto a continental plate.  In most cases, the denser oceanic plate is subducted or pushed downward beneath continental plates.  That’s why the rocks that make up ophiolites are not common at the earth’s surface.

Ophiolites include peridotite, which is the main component of the upper mantle, and the oceanic crust.

Typical Ophiolite Sequence, Courtesy Oregon State University Dept. of Geology

As drifting plates collide, upper mantle and oceanic crust can be lifted out of their deep realm by compression.  The cross-section below depicts the process.

Creating Ophiolites – Courtesy Oregon State University

If ophiolite sequences are produced as shown, it makes sense that they would be found in areas where pieces of earth’s crust collided.  Ophiolites may well mark the “suture zones” where two landmasses were joined together.

In Brittany, you can find ophiolites near Nantes, on the Bay of Audierne.  The sequence has been metamorphosed, both by the high temperatures and pressures of the tectonic collisions, and long exposure to seawater.  The Bay of Audierne is exquisitely scenic, with miles of hiking along rocky trails.  If you have time while you’re exploring Brittany, stretch your legs on the GR34 hiking trail, which has spectacular views of the bay along with some amazing examples of ophiolites.

Around the bay, you will see some great examples of the deformation and metamorphism that resulted from ancient collisions of Armorica and Gondwana.  Look for outcrops of dark colored, especially dark green rocks.  These may well be part of the ophiolite sequence that has been dated at over 400 million years in age.  These rocks may have been part of the earth’s outer mantle or deep ocean crust.  The green minerals are likely olivine, a combination of magnesium and/or iron with silicon and oxygen.  Olivine weathers relatively quickly, which is one of the reasons that geologists carry hammers.   Breaking open a rock with a weathered exterior often reveals the more or less unweathered mineral matrix.  Below is a close up of peridotite.  If you find an outcrop, a pocket-size piece will make a great souvenir of your time in Brittany.

The green color comes from olivine, which is sometimes used as a gemstone.  You may be more familiar with the gemstone name peridot, but it’s the same mineral.  Egyptian pharaohs were fond of it.

Blueschists:   Metamorphic rocks are common in Brittany, thanks to Armorica’s tortured history.  Metamorphism occurs when existing rocks, either igneous, sedimentary, or some previously metamorphosed version of them, get heated and squeezed by the sorts of processes and forces that result from tectonic collisions.

Ile de Groix

The Ile de Groix near Brittany’s south coast is made primarily of blueschists.  In geologic terms, the little island is the emerged portion of a band of high pressure, low-temperature metamorphic rocks that were produced about 400 million years ago, during the early Devonian Period.

Blueschists start out as basalt, a volcanic rock that’s common near the margins of continents and oceanic volcanos like the Hawaiian Islands.  When it’s subjected to high pressures and relatively low temperatures, the minerals in basalt re-form into new ones, including glaucophane, lawsonite, and garnet.

Blueschist

Blueschist is a gorgeous rock.  The chance to find a couple of nice specimens is more than worth a trip to Ile de Groix.

Rose Granite:  The Pink Granite Coast, or Cote Granit Rose (which sounds much nicer), is among Brittany’s most compelling attractions.  If you’re not familiar with this coastline, the name is a good summary.  It doesn’t tell you all you need to know, though.

The granitic rocks that are exposed on and near the shoreline are not only colorful, they’re sculpted and shaped into shapes and configurations that both defy and stimulate the imagination.

Weathered Rose Granite

The granites that are exposed along much of the Cote d’Armor originally cooled at some depth beneath ancient Armorica.  They’ve been exposed on the coastline by both the uplift caused by the plate collisions and subsequent erosion.   It’s likely that the granites that underlie portions of Brittany extend to 20,000 or more feet below the surface.  The exposed portions on the Cote Granit Rose have been weathered and eroded to produce a jumble of sculpted megaliths that few human artists could conceive, let alone create.

Closeup of Brittany’s Rose Granite

The rose color after which the granite was named comes from feldspar, a common mineral.  Potassium feldspar is a less common variety, and that’s what imparts the pink hue to these beautiful rocks.  Rose granite is relatively rare and is found only on Brittany’s coast, in China, Brazil, and Corsica.

Under a hand lens, the individual mineral grains can be easily discerned.  The white or gray translucent mineral is quartz, the pink grains are potassium feldspar, and the black ones are biotite or amphibole.  The individual mineral crystals are large, which is a function of the relatively slow cooling process that the granite underwent when it was originally emplaced in earth’s crust.   Not surprisingly, the folding and faulting associated with Armorica’s collisions provided the opportunity for the granitic magma to inject itself into the subsurface.  That makes the rose granite between 400 and 280 million years old.

To get some great views of the weathered rose granite, plan a half day to hike along the coast between Perros-Guirrec and Ploumanac’h.  It’s the GR34, a trail once used by customs officers to patrol for unauthorized cargo.  The tax man always cometh.  Fortunately, the nearby Traouïero Valley provided a refuge for smugglers hoping to avoid the vigilant customs officers.  Read more about the GR34 trail here,  and download a helpful English-version pdf guide to Perros-Guirrec and its environs here.

As you walk the GR34 trail, you’ll be overwhelmed with photo opportunities.  Time, wind, waves and chemical weathering have worked together to shape some spectacular artworks.  You may see Napoleon’s hat, a stack of crepes, or a stranded granite whale nearby or in the distance.  Make sure you’ve got plenty of storage and a full battery on your cell phones and/or cameras.  And be sure to collect a specimen of rose granite.  It’ll be a durable and lovely souvenir of your trip.

Mots Finaux

I’m an unabashed francophile.   I haven’t visited any part of France that I didn’t like, but Brittany is at the top just about all of my lists.   While Brittany has “je ne sais quois” it also has enough breathtaking scenery, culinary delights, and intriguing historical artifacts and monuments to satisfy every traveler.

You don’t need to know anything about Brittany’s recent or distant history to appreciate it’s beauty and allure.  But the more I learn about the factors that created it, the more attractive it becomes.

We’ve barely scratched the surface of Brittany’s geology, and the forces and factors that have shaped it.  In Part 2, we’ll explore even more fascinating facts, along with some idle speculation, about Brittany’s recent and distant past.  I hope you’ll join us.

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