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!

Road Trip Part 4: Columbia River Gorge

This is the final installment of my series following my father and my cross-country road trip from Tennessee to Oregon so I could start my master’s program at Oregon State University.

Road Trip Part 1: Why are the high plains so flat?!

Road Trip Part 2: Wyoming’s Great Divide Basin

Road Trip Part 3: The Wasatch Range

Day 6: Salt Lake City, through Idaho, to Pendleton, OR. Sorry Idaho, I’m skipping your geology, maybe another blog post…

Day 7: Pendleton, OR to Corvallis, OR!

The last big geologic conundrum of my trip was the giant layer cake of volcanic deposits that came into view along Highway 84 just past Boardman. The Columbia River sliced through it like a knife, revealing stair-stepping steep cliffs.

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Highway 84 clings to the side of these cliffs for dear life, and every now and then a spur road would snake up the cliff to a town perched high above.

Welcome to the Columbia River Gorge! The river has cut 4,000 feet down into almost  basalt deposits up to 2 miles deep over the past 15 millions years, and the results are amazing.

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Tennessee’s only volcanic rocks are thin ash deposits, so this landscape was utterly foreign. My research on the topic was delayed by the first three blog posts and a 20-page paper on the philosophy of geography, but during the week of final exams I found Central Washington University professor Nick Zenter’s engaging video series on YouTube.  He gives a wonderful introduction to the geologic world of the Pacific Northwest in a format that’s friendly to both non-geologists and geologists whose brains are too fried by studying to read off-topic academic journals. Manatash Mapping out of Ellensburg, WA made some of the best maps I found of the basalt flows to accompany his lectures: the one below shows the total extent of the Columbia River basalts! The Columbia Gorge is not indicated on these maps, but it defines the OR/WA border from just south of Pasco, WA to the Pacific Ocean.

Zentner_CBG_ExtentMapC

Brown shading = the sum of the area covered by over 300 basalt flows. 63,320 square miles in all!

This giant pile of 41,985 cubic miles of basalt was belched out by a swarm of “dikes”, or vertical ruptures in the Earth’s crust where lava escaped, between 17 million and 6 millions years ago. 80% of this lava came to the surface between 16.5 and 15.5 million years ago as part of the Grande Ronde Member, which we saw as we drove through the Columbia River Gorge. The Grande Ronde basalts flowed out of the dikes in the area where Washington, Oregon, and Idaho’s borders meet on the map below.

MapEFinal_WithAllDikes_20161012

Orange = area covered by Columbia River Basalt Groups, Black lines = approximate locations of dikes

The next map shows the approximate depths of these lava flows, focusing on the Washington-Oregon border. While depths in the Gorge are between 0.5 and 2 miles, the flows are 3 miles thick in south-central Washington!

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These flows continued east, following the path of the Columbia all the way to the Pacific. However west of the Cascades, as we approached Portland, the wetter climate hides the sheer cliffs with a carpet of trees.

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Looking upriver from an overlook near Hood River, Oregon.

But what caused the Earth’s surface to split open and spew out vast sheets of lava? 16 million years ago in the middle of the Miocene period of geologic time, northern Oregon and southeastern Washington would have looked a lot like the fiery slopes of Mt. Kilauea in Hawaii, or Mt. Bardarbunga in Iceland.

Bardabunga-volcano

Pendleton, OR in the Mid-Miocene?

Geologists don’t have a definitive answer yet, although many interacting geologic events have been proposed to have contributed to the eruptions.

  1. The eruptions may be related to the historical path of the Yellowstone Mantle Plume, or “hot spot”. The oldest dikes in southern Oregon opened up just as the Yellowstone hot spot was erupting in what is now northern Nevada, directly south of the dikes.
  2. As the North American plate moved to the southwest over the hot spot towards its current position, cracks in the crust radiated northward, likely along lines of weakness between accreted terranes (bands of islands and sea floor scraped onto the continent by subducting plates) and the core of the continental shield.
  3. As the Farallon oceanic plate collided with and sank beneath the North American plate, crumpling the Coast Ranges and creating the stratovolcanoes of the Cascade range, these stresses could have helped open up these dikes. The majority of the dikes are perpendicular to that west-to-east direction of stress, which would be typical, and the eruptions happened directly after the collision.
  4. It’s possible that after the Farallon Plate slid under North America, parts of it tore open along long north-to-south trending lines. A tear in this subducted plate could allow hot rock to rise up from the upper mantle and punch through weaknesses in the crust.

Luckily for us these dikes have been quiet for the past 6 million years, and don’t show signs of starting back up. Nowadays, only water flows through the Columbia River Gorge. I’m looking forward to going back and exploring the many waterfalls that feed into it this spring as during our road trip in late August 2017 the area was ablaze for a different reason –  forest fires!

References:

Columbia River Flood Basalts | Volcano World | Oregon State University. (n.d.). Retrieved December 7, 2017, from http://volcano.oregonstate.edu/columbia-river-flood-basalts
Liu, L., & Stegman, D. R. (2012). Origin of Columbia River flood basalt controlled by propagating rupture of the Farallon slab. Nature, 482(7385), 386–389. https://doi.org/10.1038/nature10749
Zentner, Nick, Narrator. Flood Basalts of the Pacific Northwest. , Central Washington University, 2017, https://www.youtube.com/watch?v=VQhjkemEyUo&t=2967s. Accessed 15 Jan. 2018.

Road Trip Part 3: The Wasatch Range

Road Trip Part 1

Road Trip Part 2

Days 4 and 5: Salt Lake City, Utah

Of all the places for my car to start hemorrhaging power steering fluid, Salt Lake City turned out to be one of the better ones.  I dropped it off at the mechanic and then my cousin Scott distracted me with a trip up Little Cottonwood Canyon to one of his all-time favorite places – the Snowbird ski resort.

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View from the top of Hidden Valley Peak!

Scott said he always enjoyed introducing out-of-staters to his hometown, but I hope that my constant stream of “oh my GOSH WOW” coming from the backseat on the way up the canyon didn’t get too annoying.  I mean, what’s a geologist to do? We passed glacial moraines AND fault scarps AND giant granite intrusions AND hanging glacial valleys AND massive thrust-faulted hodgepodges of sedimentary rock AND not to mention that view of Salt Lake to the west…

All this is possible because Salt Lake City and the adjacent Wasatch Range are perched on a unique boundary – the very eastern edge of the Basin and Range Province of the USA. Yep, you guessed it, it involves the Laramide Orogeny like everything covered in my last two posts, but also the Laramide’s fraternal twin mountain-building event. Meet the Sevier Orogeny.

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(courtesy of the Wyoming State Geological Survey)

Both the Sevier and the Laramide  happened at roughly the same time (70-50 million years ago) on account of the same pressure (the subducting Farallon Plate). However, two areas of the USA responded differently to the pressure. In areas further east, such as Colorado, eastern Wyoming, and Montana, that pressure hit areas of the continental basement which had been weakened when the supercontinent Rodinia was ripped apart 750 million years ago (mya) and the Ancestral Rockies rose around 300 mya. The weakened continental basement rock buckled under the stress. Geologists refer to this as “Laramide-Style” orogeny, and I saw its results in the Colorado Rockies and the “basement-cored” ranges in the South Wyoming such as the Rawlins and Rock Springs uplifts.

The pressure from the colliding and subducting plate manifested differently further west (Utah, Western Wyoming and Montana) where the continental basement rocks had not been cracked by previous mountain-building or continental rifting. Here, the many layers of sedimentary rock deposited in the Cretaceous Seaway took the strain as the basement rocks got scrunched together. These thin layers cracked and thrust over each other like shuffled decks of cards, creating the thin-skinned “Sevier-Style” orogeny. This style is evident in jumbled, repeated bands of rock in the Wasatch range. The corresponding geologic map looks like one of those scribble-and-fill masterpieces that happened when I first discover MS Paint in 6th grade.

Snowbird geo

Geologic units on the Snowbird property (blue boundary) – note the repeated purple, lilac, and mauve bands of rock. These represent sedimentary units between 1 billion and 350 million years old! The yellow blobs on top are bulldozed bits of sediment from glacial activity ~15,000 years ago

The Western USA breathed a sigh of relief once the Farallon plate completely disappeared under the North American Plate around 50 million years ago.  The continental basement, full of north-south trending cracks and pent-up tension from the insistent force of the collison, relaxed westward and flexed downward along those lines of weakness.  A simplified version of that is shown in the diagram below…

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Image from the University of Georgia, http://www.gly.uga.edu/railsback/1121Lxr37.html

This had some peculiar consequences for Utah, Nevada, and bits of the surrounding states. You can see this from space!

Basin and Range

ESRI basemap + USGS physiographic province data.

The decompression of the earth’s crust caused a maze of roughly north-south trending valleys and mountain ranges. Additionally, it dropped this whole area to a level where water could not get over the Sierra Nevadas to the Pacific or the Continental Divide to the Atlantic.  The Basin and Range Province became a giant version of the Great Divide Basin where water can only flow into its local valley and evaporate, and the Great Salt Lake is the poster child.

The formation of the Basin and Range landscape isn’t anywhere near done, to the dismay of city planners in Salt Lake City. Utah’s capitol sits right on top of the fault zone where the Great Salt Lake’s basin is sporadically sliding down the edge of the Wasatch Range. This is evident along the edge of the mountains where you can see (geologically) recent fault scarps from the highway.

faults and landmarks

Everything right of the brown lines is rising, and everything to the left is sliding down…

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My photo didn’t come out, so here’s a better one from TaylorScienceGeeks with yellow arrows pointing to the faults I saw from Hwy 215

In the Little Cottonwood Canyon part of the Wasatch Range, to add insult to injury, a giant blob of magma rose from the tail of the subducting plate 30 million years ago and punched through the already disheveled layers of sedimentary rock. Most locals refer to the rock as the “white” or “Temple” granite, but the smooth, bare cliffs are actually made of a relative of granite called quartz monzonite that has less quartz and a more even balance of two kinds of feldspar minerals. This massive batholith (geology-ese for “giant blob of magma), now unearthed by millions of years of erosion, is currently home to some world-class rock climbing routes and a Church of the Latter Day Saints top-secret genealogy bunker.

 

I couldn’t manage to get a good photo of the Little Cottonwood formation without the car door in it, here’s a beautiful one from seekraz.wordpress.com (c) Scott

On the way back down the valley it was easy to see traces of the latest force of nature in the canyon. During the last glacial maximum 15,000 years ago, the road we drove on would have been under hundreds of feet of ice! Both Big and Little Cottonwood canyons were occupied by huge glaciers fed by precipitation fueled by the ancient Lake Bonneville, driven up the mountains by western winds, and dumped in the Wasatch Range as snow. As these well-fed rivers of ice scraped downhill they carved out the dramatic steep-walled valley that we see today. The piles of pulverized rock shoved ahead and to the sides of the glaciers remain at the mouths of the canyons and are now mined as construction fill. The same climate pattern bears out today, with a warmer average temperature and a smaller lake, as the powdery snow that Scott loves to shred down at Snowbird.

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If you were wondering, the cable car ride to Hidden Peak is totally worth it, and not just for the thoughtful signs!

On Thursday, with my car still up in the air, Scott took my dad and me downtown to see the famous monument built out of Little Cottonwood Canyon’s quartz monzonite – The Church of the Latter Day Saint’s Temple Square.

When the congregation outgrew the original temple they moved to a giant structure that could hold 20,000 Saints at a time and has a forest on the roof! In order to avoid the weight of organic soils up there, the engineers used ground-up shale from the Wasatch Range to anchor the plants and  then pile on the fertilizer.

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Scott and I with the guide

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Pulverized shale “dirt”, at 1/3 of the weight of the real thing

 

Just as we were leaving the conference center I got the call saying that my car was ready to roll again. That afternoon we said goodbye to Scott and Salt Lake City, and headed north to a very different landscape indeed. Goodbye mountains, hello giant lakes of (cooled) lava!

Stay tuned for Road Trip Part 4: Snake River Plain and Columbia Gorge.

References:

“Glad You Asked: How Was Utah’s Topography Formed? – Utah Geological Survey.” Accessed October 25, 2017. https://geology.utah.gov/map-pub/survey-notes/glad-you-asked/how-was-utahs-topography-formed/.
“Little Cottonwood Canyon – Utah Geological Survey.” Accessed October 25, 2017. https://geology.utah.gov/popular/places-to-go/geologic-guides/virtual-tour-central-wasatch-front-canyons/little-cottonwood-canyon/.
“Wasatch! Part 1 – Geological Evidence of a Fearsome Fault.” The Trembling Earth (blog), May 8, 2013. http://blogs.agu.org/tremblingearth/2013/05/08/wasatch-part-1-geological-evidence-of-a-fearsome-fault/.
Eldredge, Sandra N. The Wasatch Fault. Vol. 40. Utah Geological Survey, 1996.
“Knowledge of Utah Thrust System Pushes Forward – Utah Geological Survey.” Accessed October 30, 2017. https://geology.utah.gov/map-pub/survey-notes/knowledge-of-utah-thrust-system-pushes-forward/.

 

https://seekraz.wordpress.com/tag/white-granite-mountains/

Road Trip Part 2: Wyoming’s Great Divide Basin

Day Three: Boulder, Colorado to Salt Lake City, Utah

We headed out of Boulder early in the morning, and as my father drove first I clutched my thermos of tea and looked over the map for the day. I hadn’t ever looked at southern Wyoming with any interest before, but we were going to be driving through most of it. The mountain ranges and high plateaus in Wyoming were created by the same processes that created the Colorado Rockies: the Laramide Orogeny that elevated the American West between 70 and 60 million years ago. The atlas had the Continental Divide marked in bright yellow, and to my surprise it seemed to acquire a split personality just north of the Sierra Madre Mountains, skirt a vast empty area on the map, and then reunite south of the Wind River Range.

A few hours later I took the wheel in Rawlins, and signs announced that we were crossing the Great Divide for the first time today and entering the Great Divide Basin.

Day 3 itinerary

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Welcome to the Great Divide Basin! A whole lot of flat sage brush 7,000 feet in the air…

If I had poured out my thermos onto the ground in Rawlins, it would eventually flow towards the Atlantic.

If I dumped that same tea out in Green River, on the western side of the Great Divide basin, it would flow towards the Pacific.

But if I poured it out by one of the many oil derricks dotting the Great Divide basin… it would go pretty much nowhere.

So why does the defining drainage divide of the continent have a hole punched in it in the middle of Wyoming?

Google was less useful than usual on this question, so I had to wait until I got my journal access through Oregon State (SCORE!) to do some serious database sleuthing. And even there I couldn’t find much – I guess there aren’t many scientists considering the middle-of-nowhere Wyoming. However I did find a 2010 article by Paul Heller, Margaret McMillan, and Neil Humphrey at the University of Wyoming and University of Arkansas that presented a potential cause.

These authors propose that the Great Divide Basin originally drained through Sand Gap, on the northeast side of the basin, to the Platte River around 50 million years ago in the early Paleogene period. (shown in figure 1 below) They based this on a comparison of bedrock elevations at the 4 most likely historic outlets of the basin.

Heller et al figure 1 captionHeller et al figure 1

The next crucial step is climate: The high elevation but relatively low relief of the Wyoming basins meant that they have gotten little precipitation throughout the past 50 million years compared with the neighboring high peaks to the east. This leads to a difference in erosion between the basin areas and the majority of the area of the North Platte River headwaters and watershed. More sediment was removed north and east of the Great Basin, causing the Earth’s crust to bounce back in those areas by a few hundred meters over millions of years. The science-y ways to name these processes are differential erosion and isostasy.

By around 10 to 8 million years ago, this uplift east and north of the Great Divide basin tilted the basin to the south just enough that water no longer had any reason to flow out of Sand Gap. Instead, it flowed into lakes with the basin itself and evaporated, causing the saline soil that confounded settlers’ effort to cultivate the area. Figure 6 from Heller et. Al, below, shows the direction of that tilt…

Heller et al figure 6Heller et al figure 6 caption

Going back to the tea theme earlier in the post, I found it easier to think about this in terms of a teacup (the Great Divide Basin) with a chip in the edge (Sand Gap) on a balance (the earth’s crust). This is a farfetched analogy, but hang with me here. In the beginning the balance is evenly weighted – tea is poured into the teacup and flows out the chip in the side, and there is an equivalent weight on the opposite side of the balance that keep the bar level.

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However as weight is removed by the North Platte River from the northeastern side of the balance, the opposite side tilts down to the southwest. In this tilted position the bottom of the chip is at a relatively higher elevation than before, and with the cup being refilled less often than previously tea can no longer flow out of the chip. Instead it evaporates there and leaves behind residue, much like what I find on Monday morning when I don’t wash out my mug before leaving my grad student office on the previous Friday…

After almost two hours of driving through the basin we drove past the sandstone formations of the Rock Springs uplift and passed the *other* continental divide into the Green River Basin.

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Around the Wyoming/Utah border we started descending from the Rocky Mountain plateau down into the Basin and Range geologic province. The western side of this plateau gets relatively much more rain, so we saw our first tree-covered mountains since Laramie earlier in the day!

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Unfortunately, my valiant little Honda Civic had some seriously weird noises going on after we swerved and braked hard to avoid an accident that day.

The downside: We had to spend an extra day in Salt Lake City while a mechanic checked it out.

The upside: We have family there, and they had the time to take us up into Little Cottonwood Canyon in the Wasatch Range to play tourist.

More details about the fantastic landscape around the Great Salt Lake to come in Road Trip Part 3!

Source Cited:

Heller, Paul L., Margaret E. McMillan, and Neil Humphrey. “Climate-Induced Formation of a Closed Basin: Great Divide Basin, Wyoming.” Geological Society of America Bulletin 123, no. 1–2 (2011): 150–157.

Road Trip Part 1: Why are the high plains so flat?!

Day 1: Memphis to McPherson, Kansas

Day 2: McPherson to Boulder, Colorado

My father and I pulled out of Memphis early one Monday morning and I, having procrastinated packing into the wee hours of the morning, slept through Arkansas as he drove. I only woke up when the tail end of Hurricane Harvey dropped a solid curtain of rain on the car somewhere around Forrest City. I’ve already explored Arkansas’ landscape a bit in my Petit Jean State Park blog post, so I don’t feel too guilty about skipping it in this account.

I’m familiar with the Ozarks in western Arkansas and eastern Oklahoma, but the endlessly rolling hills of Kansas were a new phenomenon to me. The early pioneers weren’t exaggerating when they described a “sea of grass”!

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With most of my geological education focused in Tennessee, Montana, and the highways between the two, I have to admit my knowledge of the Midwest was mostly limited to knowing it is FLAT. In Iowa this endless pancake of a landscape was bulldozed by glaciers, but what about the non-glaciated, pancake-flat parts of the Great Plains? What’s with them?

USF glacial drift mod

Glacial drift map from University of South Florida https://etc.usf.edu/maps/pages/4500/4546/4546.htm

It turns out that unlike areas where glaciers shoved sediment in from the north, the sediments under the High Plains of Kansas, Oklahoma, and Colorado came from the west.  In order to understand these plains, we have to turn to their opposite – the Rocky Mountains. Luckily my dad and I were driving straight to them! We met my friend Alyssa for dinner on Day 2 in ground zero of the eroded source of the High Plains – Boulder, Colorado.

Boulder is perched right on the boundary between tilted layers of 315-70 million year old rocks rocks, and the masses of Precambrian granite continental basement that were uplifted during the Laramide Orogeny that reshaped the American West between 70 and 60 million year ago. In that period, the Farallon oceanic plate dove under the North American plate at an unusually shallow angle, resulting in volcanism unusually far inland.  Additionally, the friction between this subducting plate and the overlying continent formed the Colorado Plateau as it rumpled the North American plate like a rug on a hardwood floor. The figures below show the shallow angle of the Laramide Orogeny, a cross-section view through Boulder, and a map view of those tilted layers exposed in Boulder along with their names and ages.

Diagram from https://en.wikipedia.org/wiki/Laramide_orogeny

cross-section view of Boulder….

 

 

 

 

 

diagram from http://bcn.boulder.co.us/basin/watershed/geology/

Map-view of exposed rocks in Boulder….

diagram from http://bcn.boulder.co.us/basin/watershed/geology/geolmap.html

Unfortunately I didn’t have time to go hiking with Alyssa in the foothills, but two weeks later my sister went adventuring with her in the Flatirons! Heather took tons of amazing photos including several in the Flatirons area where those tilted rocks are dramatically exposed

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View of the Flatirons, (c) Heather van Stolk

sketchbook C in Flatirons

My wonderful sister brought me along in sketch form! I can’t wait to go there in person! image (c) Heather van Stolk

Nowadays we see those layers of sedimentary rock cut through and exposed at the surface, but originally they would have extended westwards and upwards to cover those older granite rocks.  However, over the 60 million years since mountains were uplifted streams have been hard at work eroding those rocks from the higher areas and washing them downstream to lower elevations. This effect is called a “sediment apron” of a mountain range.  The Rocky Mountains are enormous, so that sediment apron extends all the the way through central Nebraska! The diagram below shows the general process of the sediment removal from the mountains, deposition on the High Plains, and gradual erosion from the High Plains.

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Schematic cross-section of the Colorado Front Range and adjacent High Plains (from Anderson et al., 2012, Figure 4). ‘LGM’ stands for “last glacial maximum”, when glaciers had their maximum impact on the North American landscape.

Cross section source here

This thick apron of sediment is the cause of the gradual, sloping rise up to the base of the Colorado Front Range of the Rockies. This area is in the “rain shadow” of the Rockies where precipitation is pretty scarce, and so the High Plains have not been as highly dissected by streams as the Cumberland Plateau which I am more familiar with and have written about on this blog. The relatively soft, homogeneous composition the sediment causes the High Plains area to be eroded gently and gradually by what streams there are.

I found a helpful introduction to the High Plains thanks to the writer at “In the Company of Plants and Rocks”, who did a great write-up of their trip through the high plains of Colorado.

In retrospect, the Plains landscape would be much easier to understand if we had been driving west to east instead. However after Boulder we had our sights on Salt Lake City and took the northern route across Wyoming to get there.

Up next:

Why is there a basin on top of the Continental Divide?

What’s with the colorful jumble of rock in the Wasatch Range?