Thunder Mountain Lakes Blew My Mind

Note to the reader – this blog post describes a trip to fragile ecosystem via a risky trail. If you decide to follow me here, please use caution and turn up your leave-no-trace skills to 11. 🙂

The third time really is the charm. In June I tried to make it here but the road was still snowed in. In early July I started from a different lower-elevation trailhead, but the pass was…. still snowed in. The snow finally melted in early August – success!

I finally completed this truly magical backpacking trip.

I started from the Hope Lake Trailhead up Tunnel Creek Road off of Highway 2, just a few miles west of Steven’s pass. The Hope Lake trail heads uphill to connect with the Pacific Crest Trail, where I turned south. There’s a rockfall just south of Hope Lake that has a booming and adorable small mammal population – I saw 2 families of marmots and a family of pikas! After the steep first 2.5 miles of the hike the trail flattens out, the trees open up, and all of a sudden you expect Julie Andrews to pop out and start singing the Sound of Music.

The hike from here to Trap Lake is a delightful ramble through wildflower meadows and groves of pine trees.

I got to Trap Lake around noon, claimed a campsite, and made ramen for lunch. And then I started to climb. To get to Thunder Mountain Lakes you head up the PCT to Trap Pass, and then turn south onto a seriously sketchy trail that follows the county line south and up to Thunder Mountain Ridge. Sketchy as in if you slip, you fall 500 feet. Not for the faint of heart, and it could be risky for dogs.

The trail takes you up above the snow line, and starts to be marked with cairns to guide hikers through the snow field and bare granite. There are a few islands of vegetation, including stunted pine trees, pink heather, and blue dwarf lupine. The bees were going wild on the heather – I guess they only have a short window for foraging up here.

At this point I was 6.5 miles from the trailhead and 3,300 feet higher up in elevation. Just as I was having to start counting my paces to keep myself going, I turned a corner and my jaw dropped.

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I had made it to Thunder Mountain Lakes! I could see all the way to Mt. Daniel and Hinman Glacier in the distance.

I hiked down to the lake and sprawled on the nice warm granite to eat a chocolate bar. As I got up to look down at the lower lake, I heard a rustle….

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… and there was a mountain goat with her fuzzy baby! They’re such lucky creatures, getting to live here full-time. I hung out with them until they made their way along the ridge. The baby kept trying to sprint up onto rocks only to fall – luckily straight into its mom. It takes true love to be your offspring’s bouldering mat.

I lingered up by the lakes until the sun started to sink close to the ridgetop. The nice thing about hiking downhill is that it leaves me with more energy to spare to admire the rocks. The downside of admiring the rocks is that I get distracted and loose the trail. But these rocks were great!

They’re all granite, but conceal a subtle story. Textbook illustrations often depict magma bodies as single lumps that rise and cool neatly independently. Reality, as always, is more messy. Magma bodies (or batholiths, as they’re referred to once they cool down) in reality can merge, or pick up bits of other batholiths, or be reworked once they’ve cooled.

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For example, this photo (above) shows a light gray batholith that cracked under pressure once it had cooled. A different kind of magma forced its way through the cracks as it rose, forming the dark grey stripe. Later the line of weakness was reopened and filled with hydrothermal quartz, creating the white streak. It’s kind of like a turducken of igneous rock.

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And in this photo, the dark grey magma body ripped up chunks of a paler batholith as it rose and incorporated them into its mix, Pac Man style.

As I headed north back to Trap Pass I got phenomenal views of Glacier Peak as well as glimpses of Mt. Baker to the northwest.

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Heading downhill back to my camp I met several fat marmots. Two of them were engaged in a clumsy but effective high-speed chase and high-volume screaming match. Their counterpart above Trap Lake obviously don’t think much of humans and screamed at me to let me know it. (lower left photo)

Trap lake was an idyllic place to spend the night, especially on a Sunday night when most of the weekend backpackers were long gone. I really got a prime campsite (above right) and enjoyed the luxurious packable chair that my sister (the one featured in all the Twin Trek posts on this blog) gave me for our birthday. Glamping all the way!

I spent a leisurely Monday in camp and then headed back to the trailhead at an equally leisurely pace. I really didn’t want to leave. Between snack, scenery, lunch, and pika -appreciation breaks I managed to stretch the 5 miles into 6.5 hours.

I may have had to drive back to civilization that day, but I’m definitely coming back here!

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Rampart Ridge Rocks!

At Vanderbilt University’s Wilderness Skills club we classified adventure into two types of fun. Type 1 fun was fun to experience and fun to remember. Type 2, the slightly more common type, was miserable while it happened but either has a great reward or created a story that got you attention at parties.

The hike up to Rachel Lake after work with a 30-pound pack on my back was decidedly Type 2.

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Where in the world was I? Trailhead marked in blue, trail in dark green. map created in ArcMap by C. van Stolk using trail shapefiles created by the US Forest Service and ESRI’s Streets basemap.

Rampart Ridge trail map

Trail in bright green – map created in ArcMap by C. van Stolk using trail shapefiles created by the US Forest Service and ESRI’s terrain basemap.

I got off work early on that Friday, headed east on I-90, turned off at the exit for Kachess Lake, bounced up potholed gravel roads to the trailhead, and set out for adventure. I knew the hike went from 2,800 to 4,800 feet in four and a half miles. What I had foolishly overlooked is that it gains 1,400 feet in the last 1.2 miles. A significant distance of that 1.2 miles is literally in a creek bed. It had me questioning my life choices. I had planned to go all the way up to Rampart Lakes at 5,100 feet but I was absolutely done by the time I got to Rachel Lake. I was too cranky to eat my ramen noodles. I set up my tent at the outskirts of an inordinately crowded back country campground just as it got dark, made a cup of tea, and turned in for the night.

The next day was 100% heavenly Type 1 Fun.

I left the burden of my camping stuff where it lay and headed uphill into a clear blue day. My goal was Alta Peak at the northern end of Rampart Ridge, elevation 6,152 feet. I climbed up the ridge past fields of glacier lilies and heather and creeping phlox. It took me about two hours to make it to the top, with views of Mt. Rainier, Mt. Adams, and Glacier Peak. I settled in for an hour with my chair and my map to plan further adventures while I could see so much of the landscape!

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Alta Peak, looking south at Mt. Rainier (center) and the Summit at Snoqualmie ski routes (center right).

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Alta Peak, looking north (the white tip of Glacier Peak is peeking out from behind the Four Brothers on the center right)

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Looking down past the snowdrifts to the basin below Alta Peak

I then rambled downhill to have lunch at Lila Lake. It was hopping with backpackers but I found a nice spot to eat my snack assortment and get yelled at by a marmot.

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Once I had my fill of exploring the bouldery outcrops at Lila Lake, I headed south to Rampart Lakes. I spent the rest of the afternoon basking on a rock with a view at the Rampart Lake furthest along the trail. I braved the water for a swim and dried off in the sun while reading a mystery novel and enjoying my sippy flask of rosé. I did not join the hikers who were penguin-sliding down the snowdrift straight into the lake, though.

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Rampart Lake, looking up at Rampart Ridge.

Around 6pm I headed back down to Rachel Lake to cook dinner and explore the campsite possibilities in the main campground for next time. I went to sleep in an exponentially better mood than I had the previous night.

Sunday morning I hoped to make it down the horribly steep creek bed part of the trail before the day hikers would be heading up it. Mission successful! I took loads of photos of wildflowers. I set up to finish my novel and eat lunch at a rocky waterfall overlook about a mile from the trailhead.

By this point Friday’s uphill slog was completely a thing of the past. All things considered, the trip had been a delight.

But wait! I couldn’t ignore the cool rocks. You know me.

The rocks exposed up at Rampart Ridge were gray with white clusters of larger elongated crystals. I thought they were really distinctive, but didn’t know their name.

It turns out these rocks have the epic moniker of “glomeroporphyritic basalt”. Glomeroporphyritic translates out of science Latin into “collected-together larger crystals”. In geology-ese, “porphyritic” refers to an igneous rock texture where larger crystals are set in a matrix of rock crystals with a much finer texture, like blueberries in a muffin.

Porphyritic igneous rocks form in two stages – the first one at deep in the earth’s crust, and the second in a shallower, cooler zone at or near the earth’s surface. The large white crystals in Rampart Ridge’s basalt formed when the magma was deep underground. They had plenty of time to slowly cool into large crystals in the hot environment at depth. However, some igneous or tectonic process suddenly shoved the magma body up towards the surface. This made the rest of the magma cool suddenly. Because these newer crystals did not have time to grow, they stayed very small.

But why did this one white mineral form crystals at depth, and not the others?  I turn to a familiar chart from my geology textbooks for the answer. It’s called Bowen’s Reaction Series, and describes the order in which minerals crystallize out of molten rock. This series springs from painstaking experiments involving pulverized minerals, a very very hot oven, and more patience than I possess. They revealed that minerals form into crystals at the different temperatures along a gradient.

The elemental mix of magma that becomes basalt creates the white mineral calcium plagioclase and the dark gray/black mineral pyroxene, with only trace amounts of other minerals. Calcium feldspar has a higher melting temperature, and so solidifies at a higher temperature while pyroxene has not yet formed into crystals. An important caveat is that not all magma contains all the elements necessary to make every rock in the series, so several minerals may be “skipped” in a certain magma body.

For example, quartz has the lowest melting temperature of all the common minerals, which is why it often forms decorative crystals or veins in the voids left when other minerals have already crystallized.

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Quartz veins in volcanic rock higher up on Alta Peak.

The two kinds of rock I saw on this hike date mainly from the Eocene and Oligocene time periods between 55.8 and 23 million years ago. Washington was roughly at it’s current location on the globe back then and the volcanoes of the Cascades were starting to rev up. Since then, these rocks have been folded by tectonic forces, broken by faults, and eroded until they cropped out in the patchwork patterns that geologists map today.

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Summarized in MS Paint from the original Snoqualmie Quadrangle Geologic Map by Tabor, Frizzell, Booth, and Waitt of the USGS: https://pubs.usgs.gov/imap/i2538/

The glomeroporphyritic basalt dates from the late Eocene period. It’s colored medium green and marked as Tnbg on the map above.

Tv and the light pink color stands for Oligocene volcanic rocks – an igneous jumble that’s a few million years younger than the glomeroporphyritic basalt. The rocks on Alta Peak are describe in the USGS pamphlet for the Snoqualmie quadrangle as “coarse volcanic breccia and tuff with minor ash flow tuff). They look almost like concrete made with blocky, angular aggregate. Breccia describes rocks created when magma shattered and engulfed surrounding rock as it erupted. Tuff forms when ash becomes cemented by its own heat, like how I described in the Smith Rocks post from 2018. Breccia makes up the ridgeline of the photo below – you can really see how this one rock classification encompasses a bunch of different kinds of rocks that erode differently to create a mix of straight ridge lines and messy talus slopes.

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It’s hard to get a sense of the scale of the waterfall in the center from the photo. I could hear it roaring down the rocks from a quarter mile away!

I’m still doing research about how these rock types ended up juxtaposed. Western Washington’s rocks tell a complex story of bits of foreign continents (called accreted terranes) that were stuck onto the rest of North America by subducting plates, then covered with volcanic rocks and shuffled around by faults. It’s the northern relative to the process in Southern Oregon that I wrote about in my accreted terranes post. Up here, the terranes were even more altered by volcanism and faulting.

It definitely created a fantastic landscape!

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Volcanic breccia on Alta Peak

Resources:

Definition of glomeroporphyritic basalt: https://blogs.agu.org/georneys/2011/07/14/geology-word-of-the-week-g-is-for-glomeroporphyritic/

USGS map and pamphlet for the Snoqualmie Quadrangle: https://pubs.usgs.gov/imap/i2538/

Info on volcanic breccia: https://en.wikipedia.org/wiki/Breccia#Volcanic

Information on Bowen’s reaction series: https://courses.lumenlearning.com/physicalgeology/chapter/3-3-crystallization-of-magma/

 

Ireland: Rocks of Newgrange

This post is the last in a series that covers last summer’s van Stolk family trip to Ireland. Here’s the highlight map to get you oriented:

irelandtripmap

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We don’t know much about the people who built the massive passage tomb at Newgrange over 5,000 years ago. They left no written record, and we can only guess at how they used the ritual passages aligned precisely to the winter solstice. However one thing is obvious – they had a particular eye for beauty and detail.

How do we know this? The story is all in the rocks. Black and white, rough and smooth, plain and intricately carved. The massive “kerb stones” along the bottom of the monument were not shaped by human tools except for detailed swirling patterns created with small chisels. The builders must have scavenged all over the countryside for boulders of the appropriate size to decorate. And many of the rocks come from dozens of miles away!

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Swirling patterns on a kerb stone

Bright white quartz cobbles from the Wicklow Mountains (near Glendalough, which I wrote about in the last post), dark speckled granodiorite cobbles from the Mourne Mountains, and dark gabbro cobbles from the Cooley Mountains form designs on the outer walls of the monument. Smooth greywacke from Clogherhead in County Louth forms the inner passageway and outer kerb stones. Finally, the interior of the huge mound was built up from local gravel from the banks of the Boyne River.

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Keep in mind, Newgrange was built before the invention of the wheel made it to Ireland. Archaeologists think that the rocks were carried as far as possible by boat before being loaded onto log rollers or sledges to get to their final destination.The 97 kerb stones each weigh at least a ton, and it’s estimated that Newgrange contains 200,000 tons of stone. These ancient people committed themselves to a serious labor of love!

Archaeologists debate the period of time that people used Newgrange as a spiritual and cultural site. However they agree that it was “lost” around the 5th century and then rediscovered at the turn of the 18th century when a local landowner went looking for stones to expand his buildings. Amateur “antiquarians” from around the British Isles visited the site from its rediscovery in 1699 to the 1920s. A formal archaeological survey started in 1962 and was complete in 1973. During this process, the tomb was “restored” to the way that we see it today by educated (and controversial) guesswork. The researchers found the cobbles piled down by the base of the mound and posited that they had originally been pressed into the side of the mound as a kind of retaining wall.

newgrange rock types

So who make sup this cast of characters? Distances measured in ArcMap, from a generalized geologic unit to Newgrange assuming that the builders used boats as much as possible to transport the rocks.

Quartz: Distance traveled: about 65 miles. These are the same quartz veins that I wrote about in my Glendalough post!

Gabbro: Distance traveled: about 35 miles. This is a dark coarse-grained intrusive igneous rock.  It’s the most plentiful rock in the ocean’s crust, and when found on land is associated with rift zones where continents have torn apart from each other. It has the same mineral composition as basalt (like at Giant’s Causeway) but cooled slowly underground instead of solidifying quickly above the surface. It’s mostly made of the black mineral pyroxene, with smaller amounts of white plagioclase feldspar and green olivine. This gabbro even shares the same age and source as the Giant’s Causeway – about 60 million years old, and formed when Pangaea ripped apart to form the modern Atlantic Ocean.

Granodiorite: Distance traveled: about 40 miles. This rock is very much like granite but has a different balance of the two feldspar minerals. It has more plagioclase feldspar that granite does, and so appears whiter. This contrasts strikingly with the black amphibole and pyroxene minerals in the rock, so granodiorite puts me in mind of a dalmatian’s color scheme. The other kind of feldspar, orthoclase, can add a pinker tinge to granite. The Newry Granodiorite complex crystallized in the Caledonian period – about 400 million years ago – as a byproduct of the closure  of the ancient Iapetus Sea.

Graywacke: Distance traveled: about 15 miles. This particular kind of sandstone forms in deep marine environments when undersea avalanches, called turbidity currents,  mix up all sizes of particles – silt, clay, sand, and gravel. This rock was formed during the Silurian period between 433 and 419 million years ago, and got shuffled to the surface since then.

I can only wonder what these rocks meant to Newgrange’s builders. Some thoughtful ancient travelers noted their favorite rocks around the island, and decided that they ought to be a part of their community’s treasure. I think we might have gotten along.

Resources:

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

https://www.ancient.eu/Newgrange/

Granodiorite at the Mourne Mountains: http://www.mournelive.com/how-the-mournes-were-formed.htm

Gabbro on the Cooley Peninsula: http://www.3roc.net/assets/CooleyPeninsulageology.pdf

 

Ireland: The Giant’s Causeway and Carrick-A-Rede

There are some geologic features that are just way too cool for humans NOT to write a myth about them. The long-standing explanation of the columns of the Giants Causeway was that the giant Finn MacCool wanted to build a bridge to attack his rival in Scotland and that they destroyed the bridge in their fighting, leaving only the Causeway in Ireland and Fingal’s Cave in Scotland remaining. Then that long-standing story gets to rub shoulders with whatever geologic explanation comes along a few centuries later.

On this vacation, we got to road-trip to one of the most stunning geologic sites in Ireland – the Giant’s Causeway.  While we were up there anyways we decided to check out the Carrick-A-Rede rope bridge too, where I got an unexpected extra dose of cool geology.

Ireland trip

Giant’s Causeway is a phenomenal exposure of basalt. Sure, the rock type itself is nothing new. I wrote my thesis about basalt in northeastern Oregon. This particular basalt became famous for the huge field of well-exposed basalt columns, a feature that forms when lava gets the chance to cool slowly. And not only do you get to see the sides of the columns like you can in Oregon, but you can clamber over the tops of them too. It’s well worth the money for the audio tour. There are two versions for two different audiences- normal person and geology nerd – and both are entertaining and informative.

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This basalt flowed out of cracks in the local limestone around 55 to 60 million years ago, when the Atlantic Ocean was opening up. It took two stages to create the landscape here. First, about 6 basalt flows spread evenly across the landscape. Then volcanism stopped for a while; there was enough time for a the top part of the basalt to be weathered a rusty brown color as chemical reaction changed the basalt to laterite, lithomarge and bauxite. This zone is called the Interbasaltic Formation here. There was also time for water to carve a valley into the landscape. Later, volcanism restarted and a huge volume of lava poured into the river valley! At the time of formation, this deposit would have been a lava lake 90 meters (295 feet) deep! We know it had to have happened all in one event, because there are no weathered layers within it.You can see a basalt cliff at the level of my head in the background of the photo to my right – that basalt was deposited on top of the plateau beside the river valley. The thickness of the lava in the river valley meant that it cooled quite slowly, allowing the formation of the regular 4 to 7 sided columns.

You can see what I just described in the photo below. Note that you can see all three layers behind my mom, but that she’s standing on the Giant’s Causeway basalt that flowed into the valley that was cut into the Lower Basalt Group.

Giant's Causeway basalt annotated photo

My mom Lise, always the best tour guide, is here to introduce you to basalt geology! We didn’t hike up to the Organ Pipes, but they’re a beautiful exposure of both the colonnade and entablature layers of the Giant’s Causeway Basalt formation. Finn MacCool’s Chimneys are precarious free-standing basalt columns whose companions were eroded away on the headland.

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Checking out the lower basalt groups – they were physically weathered into rounded shapes as groundwater percolated down into the formation. The fancy name for this is “spheroidal weathering”. You can see a little bit of the Interbasaltic group in the top left corner of the photo, where the dark gray basalt has been chemically weathered into orange/brown minerals.

Whether they’re the ones I studied in Oregon or the Irish ones in this post, basalt flows have a certain internal structure to them. It’s determined by the fact that the molten rock cools mostly from the top down, with a little bit of cooling driven by the cooler ground underneath the flow. The quicker a region of the lava cools, the smaller the size of the cooling features. Air or contact with underlying rock rapidly cooled the lava at the very top or bottom of the flow, giving it a crumbly texture. In the diagram below, these areas are the vesicular top and bottom. Just below the vesicular top, the rock was cooling more slowly. Small cracks propagated from the top of the flow downwards as the rock cooled and shrank. The rock was cooling just quickly enough that the pattern of cracks was somewhat chaotic, creating blocky shapes or the curved and twisting “fanning columns” in this “entablature” zone.

basalt flow interior

Illustration of a basalt flow interior with vesicular zones, entablature, and colonnade from: https://jgs.lyellcollection.org/content/157/4/715

At Giant’s Causeway you can see huge boulders from the entablature zone that have fallen down among the columns. They have kind of a “giant meatball” rubbly texture that contrasts with the smooth elegance of the columns but makes them easy to climb.

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Below that is the sweet spot of cooling, whose features dominate the Giant’s Causeway. In the “colonnade” zone of the basalt flow the cracking pattern propagates neatly downwards. A hexagonal pattern develops when cooling contraction occurs at centers of mass that are evenly spaced in a homogeneous body of lava. If there are variations in the thickness or composition of the lava then other geometries of fracture may occur, with anywhere from 4 to 8 sides.

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Giant’s Causeway’s famous colonnade layer, with geologist for scale.

You can see in the picture above that the columns are broken by joints perpendicular to the ones that define the columns. These are subtle ball-and-socket joints that formed when the area was subjected to horizontal stresses after the rock had cooled and the columns had formed. These ball-and-sockets allow the formation to bend a bit.

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concave “socket” column top on the left, convex “ball” column top on the upper right.

Later that day the sun came out and we decided to check out the Carrick-A-Rede rope bridge. It’s absolutely touristy but worth it on a beautiful day like that. The bridge and island are only accessible by paying for a group tour that departs at regular intervals. You end up parking in a nearby abandoned limestone quarry called Larrybane Quarry.  It has beautiful seaside views, and a fight scene from Game of Thrones was filmed here.

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Unfenced drops? Cliff edges? Count me in. Spoiler alert, our mom did not keep us close…

How can you ask a geologist to stay on the path when there are cool chert nodules to investigate in sea caves? In the right-hand photo, the black rock is chert. In the photo with my sister and me in it on the left, you can see how these chert nodules are distributed in horizontal bands in the limestone. These nodules are formed after the remains of microscopic organisms like coccolithophores, radiolarians, and diatoms are laid down and start to solidify into rock. Water in the formation dissolves some of the silica-rich remains and redeposits them in layers around impurities in the sediment, forming blobs of chert that are often called “flint” when they’re found in chalk beds like these.

In the top right corner of the photo you can see stalactites – this is the only known location in the Ulster Limestone (a.k.a. Upper Cretaceous Limestone) along the Causeway Coast with these distinctive formations.

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When the line of tourists waiting to get across the bridge is longer than the bridge, it might be a tourist trap.

This was just a detour to the main attraction – the Carrick-A-Rede rope bridge. It was constructed in 1755 to allow fishermen access to fruitful salmon fishing groups, and has been developed as a somewhat oversold tourist attraction. It has a neat geologic history though. The little island stands tall because it’s the eroded neck of a volcano! It formed 62 million years ago, just a little bit before the basalt at the Giant’s Causeway. The island is formed of dolerite, a kind of intrusive volcanic rock. When you’re on the island and looking back to the mainland, you can see large blocks of basalt and limestone suspended in a matrix of volcanic ash.

 

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The short hike to the bridge takes you down rocky staircase and along gravel paths with lovely views out to sea. The tour guides don’t encourage the group to spend too much time on the island itself, but we got to take twenty minutes or so to spy on the seagull nests, eat some snacks, and admire the view out to Rathlin Island.

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Here’s a map of an overview of the geology around Giant’s Causeway and Carrick-a-Rede that I put together in ArcMap. This is from the 1:500,000km shapefile available from the Geological Survey of Ireland. The more detailed 1:100,00K map excludes Northern Ireland… I guess political animosity somehow spilled over into data collection and sharing here. That’s a shame. So Carrick-A-Rede itself isn’t mapped here.

Northern Ireland Geology

Stay tuned for a geology/archaeology crossover event! My family headed to the Newgrange Stone Age Passage Tomb later that week.

Resources:

https://www.geolsoc.org.uk/GeositesGiantsCauseway

booklet on the geology of Northern Ireland’s Coast: http://ccght.org/wp-content/uploads/2012/05/geology_booklet.pdf

A field trip at the Sand Atlas blog whose author took some better photos of the formations: https://www.sandatlas.org/giants-causeway/

Debbie Hanneman over at GeoPostings did this trip back in 2017 and shared some great photos: https://www.geopostings.com/category/giants-causeway/

AGU resource on columnar jointing: https://blogs.agu.org/georneys/2012/11/18/geology-word-of-the-week-c-is-for-columnar-jointing/

Information about the geology at Carrick-A-Rede: http://www.habitas.org.uk/escr/site.asp?item=1145

Article about the formation of chert nodules in carbonate beds:

Maliva, Robert G., and Raymond Siever. “Nodular Chert Formation in Carbonate Rocks.” The Journal of Geology, vol. 97, no. 4, 1989, pp. 421–433. JSTOR, http://www.jstor.org/stable/30078348. Accessed 14 May 2020.

Cool podcast about the tiny creatures that become chalk, courtesy of Radiolab: https://www.wnycstudios.org/podcasts/radiolab/articles/190284-war-we-need

Ireland: Mining’s legacy in Glendalough

After Heather and I explored Brittany we headed north to join 35 other van Stolks and their partners in Ireland for a family reunion. No, we aren’t Irish, but the Dutch family wanted to vacation outside of the Netherlands and the American part of the family wanted to spend time in a scenic part of northern Europe. Ireland was a delightful compromise. We converged on a holiday cottage complex just north of Dublin where we spent a convivial time moving from porch to porch catching up on years of news. The whole bunch of us set out in a rather unruly convoy to highlights like Newgrange, Slane Castle, and a sheep herding demonstration further afield in Glendalough.

Ireland trip

Glendalough is a jewel of a lake in the mountains south of Dublin in County Wicklow, a rugged contrast to the gently rolling green hills usually associated with Ireland. We all oohed and aahed at the sheepdogs and their  puppies, and then a smaller group of cousins set out on a hike to work off the cabin fever.We did the “white trail” around the upper and lower lakes at Glendalough. It’s a stunning 7.8 mile loop! We did it counter-clockwise, which results in a gentler upward climb. If you hike this clockwise you have to climb up the hundreds of wooden stairs on the eastern side of the lake… not my idea of a great time. Going counter-clockwise also results in beautiful views of the monastery site from the top of the cliff!

Glendalough white trail

Trail map from Alltrails.com

It turned out to be a lesson in the importance of looking at the scale of the contour lines on the topographic map at the visitor’s center. We thought “oh, we only cross one topo line on the map, the trail must stay close to the lake.” Well it turned out that the distance between topo lines on that map was 0.4 kilometers – about 1,500 feet. My cousins with a strong aversion to heights were absolute troopers. The views from the top were amazing!

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Hanging out in a textbook-perfect glacial valley, with the old mine buildings in the background. Halfway to the top!

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At the top! Two cousins aren’t in this picture because they didn’t fancy spending more time at the top of this cliff than absolutely necessary and they also are much fitter than the rest of us.

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The southern cliff is topped with blanket bogs. They’re an extremely soggy and sensitive landscape, so the park put in a couple miles of boardwalk to minimize human impact. I felt like I was somewhere in Tolkien’s Middle Earth!

Glendalough seems like a valley outside time, once you step away from the tourist shops. The paths take you by an old monastery, streams in strange mossy landscapes, and the lake itself surround by hills and forests. It came as a surprise to me when we found the remnants of an abandoned mine at the eastern end of the lake.

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It turns out that between the 1790s and the 1920s this area a hive of mining activity. You can easily tell what the miners were looking for in the landscape – the scars of white quartz rubble are the giveaway (see the slope on the right side of the photo above). The miners were looking for lead, silver, and zinc. Specifically, they found it in the minerals galena and sphalerite.

Image of galena (dark gray) and sphalerite (orange-ish brown) in quartz (white) from the Glendalough mine from the National Museum of Ireland.

Galena is a mineral composed of equal amounts of lead and sulphur (its formula is PbS). In this area silver substitutes for lead in the crystal structure around 5% of the time, making it a valuable ore for silver as well as lead. Sphalerite is made of equal amounts of zinc and sulphur (formula is ZnS). But how did they get here, and why are they only found in the quartz?

Let’s take a step back an look at the history of the local landscape over the past few million years, courtesy of an interpretive sign at Glendalough’s ranger center. The information is great so I didn’t bother re-typing it, but you may have to click on the image and zoom to read it if you’re reading this on your phone.

 

Photo of an interpretive sign of Glendalough's geology at the ranger center.

Photo of an interpretive sign of Glendalough’s geology at the ranger center.

Two types of rocks form the foundation of this landscape: a metamorphosed version of mudstone or shale called schist, and the granite which muscled its way up into those rocks during the Caledonian orogeny. Remember that from a few blog posts back? This Irish granite is a cousin of sorts to the granite that became the Mont Saint Michel. It too was formed as the heat created by the collision of Laurentia, Baltica, and Avalonia created magma that rose up into overlying rocks and cooled into huge lumps of granite (called batholiths in geologist jargon). In the map below, the granite is shown in red. It also shows just how many mines were once active in this area!

Glendalough mining

A map of mining activities near Glendalough – we hiked past #7 and #8, Glendalough Valley mine and Van Diemen’s land mine. This map also shows the geologic contrast in the region between the schist (light pink) to the east and the granite (red) to the west of the lake. This map is from https://secure.dccae.gov.ie/GSI_DOWNLOAD/Geoheritage/Reports/WW065_Glendalough_Glendasan_Glenmalure_District_Overview.pdf

This particular batholith is called the Leinster Granite batholith and underlies much of County Wicklow. It’s harder than the surrounding schist and creates more rugged cliffs when assaulted with millions of years of wind, rain, and glaciers. In the Wicklow mountains the granite slopes tend to be covered in boulder fields, and the schist slopes are covered in heather and other creeping low bushes. Neither type of rock weathers into particularly inviting soil for plants, at least not in the geologically short period since the last Ice Age.

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Rock specimens that I couldn’t resist at Glendalough: Schist with neat protruding flexible sheet of mica (left), granite (center), bits of waste quartz from the mining operation (right)

Here’s a map of the mine site that is #7 on the map above, and the first area we came to on our hike. This map was put together by the educational group “Glens of Lead”. This group put up some great historical signs in along the park about how the old mining operations worked.

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And here’s a map of the second area on the hike, #8 on the map.

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Very little of the original infrastructure remains today, except for the stone buildings at the Glendalough mine site and the bright white quartz of the tailings rubble from the mines. The shafts and tunnels have been blocked off and the old tramways completely dismantled. The site seems very wild again.

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Standing on the schist side of the valley, looking over to the steep granite cliffs and the piles of quartz tailings below the exits of the old mine shafts 1,000 feet below.

Above right: granite with vein of hydrothermal mineralization (foot for scale) in the mining area, compared with schist exposed at the top of the cliff on the south side of the lake.

But how does lead ore get into quartz veins? I’ve written about continental collisions and granite before in this blog, but not really about smaller processes of metamorphism. It’s time to fire up MS Paint again.

subduction

Magma bodies (red) rise off of the subducting oceanic crust and cool into intrusive igneous rocks (pink). Water (blue speckles) in the oceanic crust allows the crust to melt at lower temperatures than the surrounding rock, and travels upwards as a part of the magma. Diagram by C. van Stolk.

Back around 300 million years ago, the ocean Iapetus was closing as the old continents Laurentia, Gondwana, and Avalonia moved towards each other. The oceanic crust under Iapetus had to go somewhere; it subducted under the continents. After a few million years of being underwater that oceanic crust was pretty soggy as rocks go. The conveyor belt of plate tectonics drove the heavy oceanic crust down under the lighter continental crust.  It began to melt as it sank beneath the continent and into the upper layer of Earth’s mantle called the asthenosphere.

It turns out that this water trapped in the crust is kind of the “secret sauce” of metamorphism. The presence of water allows rocks to melt at lower temperatures than they would otherwise. Metamorphism boils down to two variables – heat and pressure. Both increase vertically with depth in the earth’s crust. Pressure also increases horizontally in collision zones. In the presence of equal amounts of heat and pressure, wet rock will melt to a greater degree than dry rock.

Anyone who has taken a ride in a hot air balloon learns that heat rises – the hot air in the balloon keeps the passengers aloft in the cooler surrounding air. The blobs of magma rising from the subducting wet oceanic crust are much like extremely dense, slow-motion hot air balloons – they rise through any weakness they can find in the surrounding cooler and drier rock. The blobs of magma become batholiths of intrusive rock when they cool, like the granite here. As the granite cooled, the heat had to go somewhere, just as the oceanic plate had to go somewhere as the ocean closed. The magma “cooked” the shales that surrounded it into the metamorphosed version – schist (see purple “contact metamorphism” on the diagram below). However the story of the water that magma contained isn’t over.

subduction with contact metamorphism

If water can’t fit into the crystal structure of the magma as it cools into intrusive igneous rocks, it is released from the melt. It takes along ions that can be dissolved in it and travels into cracks in the surrounding rock. Often these “cracks” are caused by faults or by joints caused by horizontal pressure. One of the most common elements carried along this way is silica, which in combination with oxygen forms quartz veins as it cools. This mineral-rich hot water is called a “hydrothermal solution”.

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Diagram of hydrothermal alteration from https://opentextbc.ca/geology. (a) Shows a magma body that has risen into cooler rock and is “cooking” it shown by the purple aureole. (b) Shows magmatic water being released from the magma body through veins. (c) Shows how groundwater moving past the magma body can also carry dissolved minerals away from it to other locations.

As the hydrothermal solution rises and cools, minerals form out of the solution like rock sugar forming out of hot sugar syrup as it cools down. Not every part of the solution is really well mixed – some parts of the solution are like oil and water and stay somewhat distinct as they travel together. Examples of this are silicate minerals (i.e. quartz SiO2, feldspar KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) and sulphide minerals (galena PbS, sphalerite ZnS). As the solution cools, these two types of minerals form adjacent but separate structures.

So here at Glendalough, you have granite and schist cut through with veins of hydrothermal rocks that contain chunks of sulphide ore minerals in the more abundant quartz. The miners followed these hydrothermal veins to find the valuable ore, and discarded the attractive but comparably worthless quartz in tailings piles at the site after the rock was put through a huge crusher to break out the softer sulphide ore minerals. I was sorry to notice that they were so thorough that a casual geologist really can’t find any of that ore nowadays.

I was happy enough to take away great memories, beautiful views, and a few new rocks in my pocket.

Resources:

A good brief history of the mines here: https://www.mindat.org/article.php/368/A+History+of+Glendalough

Brief intro to geology of Glendalough here: https://www.wicklowmountainsnationalpark.ie/nature/geology/

https://www.gsi.ie/en-ie/geoscience-topics/geology/Pages/Geology-of-Ireland.aspx

detailed geological survey of mining in the region: https://secure.dccae.gov.ie/GSI_DOWNLOAD/Geoheritage/Reports/WW065_Glendalough_Glendasan_Glenmalure_District_Overview.pdf

detailed geological survey of mining at Glendalough specifically: https://secure.dccae.gov.ie/GSI_DOWNLOAD/Geoheritage/Reports/WW025_Glendalough.pdf

geological map of Ireland: https://secure.dccae.gov.ie/GSI_DOWNLOAD/Bedrock/Other/GSI_BedrockGeologyOfIreland_A4.pdf

Maps of lead mining in the area by the local education group “Glens of Lead”: https://www.avenzamaps.com/vendor/209/glens-of-lead

More about the Iapetus Suture, which connects the half of Ireland/Scotland that was once Laurentia (proto-North America) with the half that was once Avalonia (proto-Europe). https://en.wikipedia.org/wiki/Iapetus_Suture

Interesting map of the terranes that make up Ireland: http://www.askaboutireland.ie/reading-room/environment-geography/physical-landscape/Irelands-physical-landsca/the-formation-of-the-phys/

Ireland through geologic time: https://www.gsi.ie/en-ie/education/the-geology-of-ireland/Pages/Ireland-through-geological-time.aspx

Specimen of galena in quartz from the mine: https://www.museum.ie/The-Collections/Documentation-Discoveries/July-2016-(1)/Lead-Bearing-Minerals-from-Glendalough

More about contact metamorphism/hydrothermal alteration/sulfide ore bodies

Contact metamorphism with good diagrams: https://opentextbc.ca/geology/chapter/7-5-contact-metamorphism-and-hydrothermal-processes/

http://www.geologyin.com/2014/11/veins-and-hydrothermal-deposits.html

Overview of sulfide ores: https://uwaterloo.ca/earth-sciences-museum/resources/hydrothermal-minerals

Cliffsnotes version of hydrothermal metamorphism: https://www.cliffsnotes.com/study-guides/geology/metamorphic-rocks/hydrothermal-rocks

Detailed review of ore genesis, including immiscible solutions. https://courses.lumenlearning.com/wmopen-geology/chapter/outcome-ore-and-mineral-resources/

Mazes of mines and catacombs beneath Paris

The SacrĂ©-CĹ“ur 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 MusĂ©e 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 GĂ©rards (1859–1920) BnF Notice d’autoritĂ© 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

Click to access 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 (hint = look for the red dots)…

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 tachetĂ©s” = 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 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.

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, GĂ©rard & 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)

twin trek map zoomtwin trek map zoomed out

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!