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.

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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.

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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.

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