Accreted terranes: a slow-motion pileup on the Pacific Coast

Oregon became a US State in 1859, so you would think the underlying rock would at least be North American. It turns out that like the modern population of the state, though, southwest Oregon’s bedrock is an international melting pot.

Let’s take a step back and deconstruct that piece of jargon I threw out there in the post title.

Accreted = added on, and terrane = small bit of independent continent.

If you’re familiar with the theory of plate tectonics, it’s often simplified into a huge shifted puzzle of large plates that either collide violently or slide under each other neatly. However there are actually some smaller pieces that get swept up in the cycle of creation and subduction. These could be pieces of oceanic crust that got scraped off of a subducting plate or a volcanic arc like Japan, for instance.

When the supercontinent Pangaea was torn apart by rifts starting around 200 million years ago it started a planet-wide game of bumper cars. The Mid-Atlantic rift separated North America from Europe and it pushed North America westward; this sped up activity along the subduction zones on the continent’s western coast. The Oregon coast shows evidence of the odds and ends of lithosphere that the newly liberated North America plowed into on its journey west.

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Figure showing how accretion works, from Miller 2014

The southwest coast of Oregon where my family vacationed is a giant 11-car pileup of accreted terranes ranging in age from 180 million to 100 million years old. When we hiked on Cape Sebastian we were standing on the Gold Beach Terrane, which took an unconventional path to Oregon. Unlike the neighboring terranes that were scraped onto the continent by converged plates from similar latitudes, studies of the rocks found in the Gold Beach Terrane show that they originated near southern California and were transported north on a transform fault similar to the San Andreas!

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Closeup of southern Oregon and northern California from Miller 2014 showing the sequence of terranes plastered onto the coast

The most prevalent family of rocks in the Gold Beach terrane is the Otter Point Formation. This melange formation (melange being geology-ese for “ungodly mess of rock types”) contains mostly sandstone with dashes of conglomerate, mudstone, bits of interleafed sandstone and shale, and blocks of misplaced metamorphosed oceanic rock. The sandstone from this formation creates many of the dramatic sea stacks that we saw at Secret Beach, Arch Rock, and near Port Orford.

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Arches, mussels, and starfish on the Otter Point Sandstone at Secret Beach

On Myers beach we saw another Otter Point rock that, although softer than the sea stack sandstone, still had a great story to tell.

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Turbidite on Myers Beach

These rocks are called turbidite, and were originally deposited in a deep, quiet environment underwater on the continental shelf. The water was so calm at that depth that any storm deposits that rushed out of a delta upslope sorted themselves gently into larger, heavier particles on the bottom and lighter particles on top. Eventually they formed parallel layers of sorted sediment, one per storm event, and were cemented together by pressure and mineralization.

And then their quiet neatness was ruined when the rocks were scraped onto another continent a few million years later.

Now that turbidite has been bent at 90 degree angles, faulted, hoisted above sea level, and is eroding into nice fine beach sand. Not the retirement it was hoping for, I think.

Source:

Miller, Marli B. Roadside Geology of Oregon. 2nd ed., Mountain Press Publishing Company, 2014.

 

 

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Forget rising tides, what about a rising coast? Uplift at Cape Arago

So in my last post I showed off some pretty pictures from Shore Acres State Park… that raise questions.

What’s with the rock blobs?

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Why are the cliffs tilted?

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And further south, why are all the sea stacks about the same height, and the same height as the mainland?

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Unfortunately those blobs are not about to hatch some rock-type Pokemon. They have something much less exotic at their center – small irregularities like pebbles or shell fragments. As groundwater slowly flowed though the sandstone it preferentially deposited minerals on larger particles, creating a snowball effect around imperfections in the otherwise relatively homogeneous rock. This extra “cement” makes those areas harder and more resistant to erosion than the surrounding rock.

The tilting rocks are a reminder of the pressure that the coastline has been under over the millennia – it’s the western edge of north-south trending downward fold, or syncline, that includes all of Cape Arago.

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Pressure from the colliding Juan de Fuca and North American plates farther offshore has made the coast buckle and rise over the millennia. Over time the waves wear a flat platform in the rocks, only to have that platform eventually raised out of their reach. There are five different such platforms visible in the Cape Arago area which have been uplifted at a rate of about 3 feet per thousand years.

The lowest visible terrace in the area, called the Whiskey Run Terrace (Q1 in the diagram above), rose from the sea about 80,00 years ago. Although its top might have been elevated above the waves they continued to erode its sides, eventually breaking much of it down into individual sea stacks. Similar terraces and the same wave action occur all along the Oregon coast, creating families of sea stacks with matching elevations.

The uplift isn’t a completely steady process – when the North American plate jolts forward and releases the tension with the subjecting Juan de Fuca plate the coast can plummet a few feet in elevation. However based on the syncline and pattern of older terraces at higher elevations, it seems like upward motion has won the long game.

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Image from Leonard et al. 2004, showing subsidence after quake

I missed an amazing photo op at Sunset Cove near Shore Acres State Park. Apparently at low tide you can see the stumps of trees that were submerged during the Cascadia mega-quake in 1700. I’ll just have to visit again to meet them in person…. not a hardship at all!

Source:

Miller, Marli B. Roadside Geology of Oregon. 2nd ed., Mountain Press Publishing Company, 2014.

Lucinda J. Leonard, Roy D. Hyndman, Stephane Mazzotti; Coseismic subsidence in the 1700 great Cascadia earthquake: Coastal estimates versus elastic dislocation models. GSA Bulletin ; 116 (5-6): 655–670. doi: https://doi.org/10.1130/B25369.1

 

Family fun on Oregon’s otherworldly southern coast

Like with my Petit Jean State Park trip, I’m going to break this trip up into this travel blog post and another geology post (or posts!).

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The last time I got to play in tide pools with my mom I was blond and about 3 feet tall. Twenty years later, it was a wonderful treat to be able to catch hermit crabs with her again! In honor of my sister and my shared birthday as well as our parents’ milestone anniversary, my family flew out to the west coast and stayed in a lodge near Gold Beach, Oregon. From there we explored the Dr. Seuss-ish landscape of the Samuel Boardman Scenic Corridor – a coast full of hidden beaches, huge arches over the crashing waves, and trail-side berry feasts.

We saw so many phenomenal rocky landscapes! I’ve mostly noodled around the headlands of the northern coast – Yaquina Head, Cascade Head, Cape Perpetua, and Cape Lookout – where isolated chunks of basalt form prominent highlands that more successfully resist erosion. There’s no basalt here but the rock formations are phenomenal! I’m looking forward to researching them for future posts.

Shore Acres Park near Coos Bay was a beautiful start to the trip – botanical gardens to keep my mom happy and views that wowed all of us. The hike down to the beach was worth it for the cool spherical features within the sandstone – I’ll definitely have to look up what created them! The park is one of three along a short rugged section of coastline – Cape Arago, Shore Acres, and Sunset Cove would make a wonderful weekend trip. We were just passing through on the way from Florence to Gold beach, so we picked one.

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Fantastic sandstone formations

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Checking out the mysterious “eggs” in the sandstone at the beach…

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My mom loved the dozens of types of dahlias at the Shore Acres gardens

The next day we headed out to Myers Beach south of Gold Beach. My dad felt very much at home – the 55 degree beach weather is pretty similar to what he grew up with in Holland. The views however, were far from it! It’s a nice place for a walk and we found lots of hermit crabs in the tide pools about 1/2 mile north of the parking pull-out. The geology was pretty cool too, and I’ll definitely touch on that in following posts!

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The tide pools here were shallow and wave-tossed – good for crabs, but we didn’t see many anemones or snails. The lack of snails seemed to be a bit of a problem for the hermit crabs. Many were stuck in shells a few sizes too small! We were able to catch a few who were too involved in duking it out with a shell competitor to notice the humans crouched above them.

Cape Sebastian features a 3.2 mile round-trip hike from the top of the promontory down to dramatically tilted slabs of sandstone at its base.  However the tide pool creatures were worth it! We saw two chitons – molluscs that look like armored sea slugs and have been around for the past 400 million years. Some of the slabs of rock had tumbled down, creating hidden pools filled with anemones, sea urchins, starfish, and shy purple shore crabs. The downside was that there was an awful lot of “up” on the way back to the parking lot, but it wasn’t too bad since we were fueled by the bounty of ripe huckleberries, salal berries, and thimbleberries along the trail.

 

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Heather’s first thimbleberry

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finding chitons!

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One of the most diverse tide pools we saw on the trip – 2 types of anemones, sea urchins, starfish, purple shore crabs, and several varieties of sponges.

 

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Clambering up the tilted sandstone slabs at the base of Cape Sebastian

In order to experience the magic of Secret Beach, you have to get your timing just right. It’s only accessible when the tide is below the “zero” of the chart, but it’s definitely worth the hassle. A one mile trail leads you down to a rocky spit overlooking an otherworldly bay full of towering sea stacks topped by battered pine trees and flocks of cormorants. During the lowest tide you can access a string of small beaches and phenomenal arches. We saw whole constellations of star fish here too. Caution is needed though- if you get too engrossed in the sights and the tide comes in you’re stuck until the next day.

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We also pulled over at two viewpoints near Secret Beach – unfortunately my photos didn’t turn out though. Arches Rock is the opposite of Secret Beach – it’s an island that you can see it from a short paved trail any time you please. It would make a great photo op when huge waves are breaking on it. I’d recommend bringing binoculars to scope out the birds nesting on top. The Natural Bridges are a beautiful, convoluted set of three arches. Brave souls can take the sketchy unofficial trail from the official lookout that takes you over the narrow top of the bridges. You would have to risk wind, poison ivy, and eroded paths… not something I felt like making my mom watch me do.

After a few days we headed back north towards Corvallis. Along the way we stopped to see the Hecata Head lighthouse:

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had an early dinner at Local Ocean in Newport, which was amazing:

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Poor Heather, we were all snitching from her monumental seafood stew

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My parents in Newport’s harbor. So many beautiful boats!

And after dinner we headed to Yaquina Head to hang out with the seals and a couple thousands murres.

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The lighthouse, with the cliffs full of sea birds

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Spying on the seals and their adorable babies

 

Once in Corvallis we were pretty ‘boring’ as far as travel photos go. Mostly just eating good food and enjoying each others’ company, but our last touristy excursion was to Willamette Valley Vineyards. We had a wonderful guide who let me pepper her with questions about the soil and then gave us samples from different terroirs owned by the company.

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After a marvelous week, it was so hard saying goodbye to my parents. At least I got to hang onto Heather for a few more days. The mountains were calling, and it was time for our annual twin camping trip. More on that later!

 

 

 

 

Dome Rock and the continuing trials of Jo, the Adventure Civic

Dome Rock Hike: 10/10 would hike again, magnificent view

Drive to Dome Rock north trail head: 10/10 would NOT attempt again in a 2001 Honda Civic

I had originally planned to hike the 10-mile round trip trail from the Detroit Lake information center up the ridge to Dome Rock, but the ranger at Detroit Lake State Park was quick to discourage me. He suggested that it would be much easier to take the Forest Service road to the northern trailhead and just do the prettiest 3 mile section along the ridge top. Sure! Why not?

18% average slope on gravel roads is why. Having to stop and restart my crotchety old car multiple times on said 18% slopes to move away the fallen rocks so I could get clearance is another good reason.

Luckily the beauty of the hike brought my blood pressure back down again within a mile or so. The trail wound though firs, maples, and thimbleberries (snack time!) along the ridgetop above Tumble Lake.

Map of Dome Rock trail

Map from Willamette National Forest USFS website for trail 3381

Directions to the Tumble Creek North Trailhead can be found here on the USFS site. They aren’t kidding when they say “up steep mountain roads”.

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At the top of Dome Rock, selfie with Tumble Lake!

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View of Mt. Jefferson and the Three Sisters from the top of Dome Rock.

I was hiking in the Western Cascades, which form the more eroded volcanic predecessor to the striking peaks of the younger High Cascade mountains. Magma rising from the subducting Farallon plate created both zones of the Cascades, but the two stages of that subduction made them distinct. Between 35 and 8 million years ago the plate sank under North America at a slightly steeper angle, resulting in the location of the Western Cascades. Around 7 million years ago that angle became shallower, which moved the depth at which the magma rose off of the melting plate to location further east. (Devis 2013)

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Figure from page 110 of Marli B. Miller’s classic “Roadside Geology of Oregon” – the area of Dome Rock  is circled in yellow

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Figure also from “Roadside Geology of Oregon”, page 113, showing how the change in subduction angle influenced the location of volcanoes further inland.

All the classic cone-shaped volcanoes of the Cascades such as Mt. Jefferson, the Three Sisters, and Mt. Hood are part of the High Cascades. In contrast, a few more million years of exposure to rivers and glaciers created the more subdued landscape of the Western Cascades. Any volcanic cones from that era have long been ground down to their roots.

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Standing in the Western Cascades, looking at Mt. Jefferson in the High Cascade mountains. Photo taken during one of my stops to move rocks off the road…

Dome Rock itself is one of those “roots” – an isolated piece of 10 million to 17 million year old andesite where newer magma punched through a 30 million to 17 million year old area of tuff (cemented volcanic ash) and basalt. (Walker, G.W., and Duncan, R.A., 1989) It’s relative toughness meant that it withstood the 10 million years of weathering since its formation better than the surrounding formation’s softer tuff with basalt, creating the bare knob with spectacular 360 degree views.

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Andesite near the top of Dome Rock… next time I’m hiking with my rock hammer.

Jo’s engine may have nearly overheated on the way up, but at least I didn’t have to use the engine at all for seven miles on the way down. After creeping back down the forest service road using a combination of second gear and brakes, I stopped at a peaceful little day use area along Frenchman Creek to eat my lunch. Judging by the size of the boulders in the creek bed, the stream hasn’t always been so tranquil!

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Frenchman Creek day use area, about 1.5 miles north of the intersection with Hwy 22

Zach Urness of the Statesman wrote a helpful article on the Dome Rock/Tumble Lake hikes with more information about the lake and its campsites. I didn’t go down to the lake this time, but maybe next trip.

With all the time that skipping the extra 7 miles of the hike saved me, I stopped by Marion County’s Niagara Park on the North Santiam on the way home. My phone was dead, so no pictures this time, but if I’m by there again I’ll definitely stop to take some. The site was ambitiously called “Niagara” by hopefuls in the late 1890s aiming to build a dam where the Santiam is funneled through a 4-foot-wide crack in the underlying rocks. The dam failed repeatedly and they gave up in 1912, leaving a park with picturesque ruins. About a half-mile up the stream from the failed dam lies a  misshapen mound of rocks eroded into a perfect picnic spot and place to cool your feet off in the river.

I was sorry to have to leave the parks and head back home… and on the way back I got Jo a well-deserved car wash.

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My sister posing with Jo the Adventure Civic on another trip that I’ll be blogging about soon!

Field work: Week 2

This past week Jen and I headed back out to the Walla Walla Basin, but primarily for another project: doing the quarterly water level check and data collection download at observation wells in the Umatilla and Walla Walla watersheds. In between monitoring wells we collected the five remaining samples budgeted for the geochemistry project.

After visiting 60 or so wells, each with their standard blue wood-and-aluminum housing, they started to blur together. A few things stood out…

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Granite boulders 200 miles from where they ought to be

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and the earth-shaking exploding munitions that I was discouraged from photographing.

Those munitions were on the Umatilla Army Chemical Depot, where OWRD has a handful of  monitoring wells. Atlas Obscura has an intro and some interesting photos… The site was created during WWII to store weapons and supplies, and since then has been the location for disarmament from weapons stockpiled for use in the Pacific theater of the second World War as well as the Cold War. Our guide said that they had indeed gotten rid of all the chemical weapons stored onsite, and the current mission on the base is making sure that the old explosive weapon destruction pits are done exploding. That explosion while we were sampling was proof that the second look was necessary. The end goal for the site is to render it harmless enough for limited non-military use such as stock grazing.

Not much explanation is needed for the well housing tenants – the structures form hospitable shelters in the middle of wide-open grain fields. A perfect bed-and-breakfast for four-legged or eight-legged creatures. Luckily we didn’t see any of the region’s black widow spiders – just harmless, fuzzy Phidippus audax. For the sake of my arachnophobe friends I won’t post a portrait, but google them if you’re curious. They’re actually kind of cute.

That granite boulder, on the other hand, is a long way from home. Like, 100 to 200 miles. And it’s not a small boulder – it’s about the size of an oven.  Below is a map showing the “closest” granite outcrops in purple, and the location of this lonesome rock with a pink star. What on earth is it doing by a well in Morrow County, Oregon?

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Like so many geological oddities in the Columbia River basin, it hitched a ride on the epic Glacial Lake Missoula floods (shown in blue below)! It likely came from somewhere around Spokane.

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Glacial flood extent created by ESRI user jcleveland0, accessed via ESRI Online. Granite outcrops selected from the USGS Preliminary Integrated Geologic Map Databases of the United States shapefiles for OR, WA, MT, ID.

The Missoula Floods were an amazing manifestation of the latest Ice Age between 13,000 and 15,000 radiocarbon years ago. An ice sheet repeatedly dammed a predecessor of the Salmon river at its headwaters in Montana, creating a lake over 200 miles long. Then as water likes to do it eventually blasted through. Again. And again. In each flood event water racing at over 10 million cubic meters per second scoured the landscape in northern Idaho, Eastern Washington, and northern Oregon. These floods meant business, creating ripple marks bigger than houses, amphitheater-sized waterfalls, and topsoil stripped from Spokane to be deposited in Salem. That flow picked up boulders the size of buses only to set them down them hundreds of miles away, so the moderately sized one we saw on our rounds would have been a piece of cake.

The Washington Geologic Survey created a beautiful, user-friendly introductory website for the floods here. I really recommend it! It not only shows the scientific knowledge surrounding the floods, but the process of science that connected all the disparate observations into one phenomenal story. At least, phenomenal if you’re as nerdy as I am.

Our own research for the week was unfortunately nowhere as riveting as this rock’s journey. In the coming weeks I’ll wait with bated breath for the laboratory results, learn how to process four months of water level transducer data for a few dozen wells, and start my literature review. However any blockbuster geologic story like the Missoula floods was assembled out of thousands of seemingly trivial observations, so I’m happy to work away in my own little corner of science.

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It’s not a bad-looking corner at all, just a bit hot…

Grad School Gets Real: Thesis Field Work Part 1

You never know where a conversation will take you. This one started with a conversation about the best puffy jackets at a conference, and ended with me perched on a tailgate wrestling with pipe fittings in an apple orchard.

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So even I, an avowed introvert, have to give small talk some credit. This summer I’m working with the Oregon Water Resources Department (OWRD) evaluating the chemistry of groundwater in the Oregon side of the Walla Walla Basin! It’s a win-win scenario: they get a low-cost field technician, and I get to use the data we’re collecting to support my thesis looking at the spatial distribution of the water chemistry. I’m really excited to learn more about how the state manages these resources, and am grateful for access through them to the ability to sample from both public and private wells during their routine visits.

To learn more about what this research entails, check out the link below, and then come back to this page to read more about the field work that will make it possible.

Click here to go to the “story map” website I created to introduce my research

In short: agriculture in the Walla Walla Sub-Basin of Oregon (WWSB) depends heavily on groundwater from the deep layers of basalt. Those groundwater levels are declining at alarming rates, and the OWRD is trying to figure out how the aquifers work and what would be the fairest way limit use to more sustainable levels. My research focuses on a spatial approach to this problem: how does groundwater interact with the faults in the basalt, based on chemical, geologic, and hydrometric data? Should the faults be taken into account when allocating water rights?

This project is spearheaded by Jen Woody, a hydrogeologist at the OWRD, and she and I are also collaborating with the US Geological Survey to build on their studies in the region and laboratory capabilities. In return for the ability to put our results in their databases, the USGS is giving the OWRD team technical support, the use of their sample bottles, prep lab access, and assorted sampling gear. With training complete and the gear piled into the back of a pickup truck, Jen and I spent the past week based out of Milton-Freewater. It’s also known as “Muddy Frog-Water” (don’t ask me why…) and grew along with its wheat fields and apple orchards on the banks of the Walla Walla River.

 

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I love this statue welcoming travelers into central Milton-Freewater. It’s the winner of the yearly “chainsaw frog sculpting” contest!

A day of sampling started a 7:30, when Jen went into the Safeway to get a couple bags of ice for the coolers and I set up shop on the tailgate calibrating the pH, conductivity, and temperature meter in brightly colored solutions. We then hit the road and headed out to the first well of the day.

We had picked out wells to sample ahead of time based on geology, GPS well locations, and well log characteristics, but those plans had to be flexible. Things that look so simple in ArcGIS are always more complicated in reality. Access to wells depended on who Jen had been able to get in touch with, whether the farmers were pumping that day, and whether there were sample ports on the well. Wells in the basalt aquifer here are deep – most are between 600 and 1000 feet – and the huge turbine pumps that access that water can cost up to $10,000 per month to run. They’re not something that we could turn on casually to get samples!

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Jen taking notes at one of two wells that water almost 2,000 acres of grape vines.

After we beat back the tumbleweeds, found an outlet on the well, and ‘MacGyver’ed some combination of hardware to linked that outlet with our sampling pressure fitting, it was time for science. At each well we took measurements for pH, conductivity, and temperature, and filled bottles with water to analyze for alkalinity, ions, oxygen and hydrogen isotopes, tritium, and carbon 14. Those bottles then got safely stowed in coolers until we drove them to the USGS lab on Friday. If you skipped the story map link, the images below outline what we’re sampling and why.

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Our record was five wells and a surface water sample in one day. We had a break before a 5pm appointment at a well that day, so we just had to drive out of the blazing hot valley and up the canyon to a cool and shady park to take another isotope sample from the river… life is tough.

South Fork Walla Walla river

Finding the secondary well outlet sounds easy in theory… and then we ran into these mazes of pipes.

In addition to collecting samples it was also fun collecting stories from the farmers we met, whether it was gossip about the new vineyard in town or a tour of a retired farmer’s studio for restoring antique saddles.

It rained on Thursday, which brought the temperature down nicely but meant that pumps weren’t running. No irrigation meant no sampling, so Jen and I took a geology break up in the hills above Walla Walla to admire outcrops of the basalt that also form the aquifers deep below the valley.

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The goal of our sampling is to evaluate how the faults in the region affect groundwater flow, so it was great seeing examples up here in the hills. Faults were visible as breaks in the horizontal structure of the cooled lava flows, but their size varied widely. Some faults looked simply like the rock had been cut and one side moved, while others took the form of 50 meter wide zone of pulverized rock! This was definitely food for thought when considering the faults on the map. I’ll have to do more research to see whether the ones I’m studying are of the 5 inch or 50 yard variety.

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This is a fault outcrop to the left of the road, believe it or not…

 

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An incredibly scenic lunch break beside the Hight Fault Zone in the Blue Mountains

At the end of the day we retreated to the air conditioning and giant plates of food at La Ramada, which along with the Dairy Queen and El Sombrero represented the sum total of dining options of Milton-Freewater. The Hank FM country station was kind enough to info us that the local cider brewery, Blue Mountain Cider, had a tap room open on Thursdays. It was a treat to finally taste the apples we had been working among all week, especially in liquid form after a hot day.

We got a total of 12 wells sampled last week. Jen and I are heading back out later this month to try to knock out the next 9 samples and to see if there are any additional wells where we can just take samples for oxygen and hydrogen isotopes. Additionally, we’re doing the OWRD’s quarterly check of water level recording equipment in wells around the area. Stay tuned for Part 2!

 

 

Strike, Dip, and Skis

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Mt. Washington selfie on top of Hoodoo Butte!

It’s been something of a theme in my recent blog posts that the west-coast mountains are a source of nerdy joy for this recently transplanted geoscientist. This winter I went on my department’s grad student ski weekend to try out this whole zooming-down-mountains-on-sticks thing and get a chance to hang out at Mt. Bachelor with a convivial bunch of folks. I got thoroughly hooked despite spending much of my time crashing on the bunny hill and watching the 4-year-olds effortlessly ski past me. It’s a whole new way to appreciate the volcanic history of the northwest! (hmm, that sounds like a blog post series once this term is finished…)

Two weekends ago I progressed to wiping out on the blue-level slopes instead of the greens (baby steps!), and a thought hit me out of the blue that dramatically improved my control over my skiing.

‘What would a Brunton compass tell me about this line?”

I promise I hadn’t hit my head and lost my marbles on the previous “yard sale” fall where my scarf, poles, and skis ended up strewn around me. For my non-geologist readers, a Brunton compass is the strange but useful combination of a compass, mirror, and level that geologists use to measure the dip (steepest angle) of a tilted piece of rock as well as the strike (cardinal direction perpendicular to dip, and the direction where the Brunton is completely level).

I was skiing down the Glade route at Timberline and my trusty “aim to the edge of the run to slow down” method was failing me miserably half of the time. I had a flashback to the last time I was in mountains, swearing at my Brunton at Indiana University’s field camp in the mountains of Montana. In that moment of clarity I realized that if the mountainside was a tilted rock layer I was expecting the route to be perpendicular to the strike (true dip, i.e. straight down) while it was actually at an angle (following a shallower apparent dip). By aiming to the edge I was going to the steeper true dip and accelerating – exactly the opposite of what I wanted to to do!

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It’s a universal truth in physics, particularly noticeable for people who ski or mountain bike, that the steeper your trajectory the faster you’ll accelerate. Friction determines the maximum speed. In my case gravity conspires with my properly waxed skis to set my maximum speed on the steeper blue routes much faster than I have the skill to control, hence the wipe-outs.

If the route is straight downhill following the “true dip” of the geologic example, I can ski either to the right or left to decelerate to a more comfortable speed. This holds true for the “Over Easy” route I first skied/slid down at Mt. Bachelor, most routes at Mt. Hoodoo, or for the Magic Mile routes at Timberline. These routes are aimed pretty much straight down the side of the mountain.

true dip skiier

However if the route is at an angle to the steepest possible line, then it is following the geologic “apparent dip” of the landscape. In that case, if I ski to my right, away from the angle of the route, I would find myself skiing at a steeper angle towards the true dip and accelerating into a snow bank.

apparent dip skiier

Geology students find ourselves tripped up in less spectacular ways when faced with eroded inclined structures, where the true dip of the rock bed isn’t perpendicular to the eroded top of the formation.

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Week 5 of field camp: why the heck is the hillside shaped like an “M”?

On a contorted landscape like these hills near Doherty Mountain in Montana, it was all too easy to ignore the wobbling level bubble in my Brunton compass in the rush to finish surveying an area. After the first day we all put our notes together and realized that none of us agreed on the dip of the hillside. We had been fooled into interpreting the easiest path to walk across the hill as “horizontal” and measured various apparent dips.

apparent dip geologist

As enjoyable as my class made it to trek up those hills through the bushes and snakes, wouldn’t it be even more fun to ski  down them over a nice smooth blanket of snow…

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