This little black pebble, now sitting on my desk, traveled a lot today. After picking it up beneath a cliff face in southwestern Montana, I carried it in my pocket for a few hours and then drove some 20 miles to this college dorm where I’m staying. It’s the most this pebble has moved for millions of years.
Some time ago–this morning, a week, a year, 10 years, 100 years—the pebble weathered out of a much larger rock and fell to the ground. Its worn, rounded edges tell me that before it became part of that larger rock, it traveled down a stream bed –and its size tells me that its source probably wasn’t too far away. As is typical of stream gravel, its movement was irregular, marked by short bursts of movements during floods separated by longer periods of rest on a gravel bar or in the channel itself. Somewhere along the line, the pebble became buried by more sediment, probably because the land subsided or the river channel switched to another position. Eventually, the pebble and the rest of its surrounding sediment turned into rock.
All this took place some 125 million years ago early in the Cretaceous Period during uplift of an early version of the Rocky Mountains. As the mountains rose, they eroded, shedding material off their flanks and ultimately into streams, where they became worn and rounded, my pebble among them. Today, we can see the folded and faulted remnants of those mountains. They include a sequence of sedimentary rock that records the area’s geologic history dating back more than 500 million years. One of the layers in this sequence, called the Phosphoria Formation, provided the raw material for the pebble. Another layer, called the Kootenai Formation, contains the rock that held the pebble.
Using a handlens, I look closely. The pebble is incredibly fine-grained, with a texture more like a piece of caramel than most typical rocks. It’s chert, made of sub-microscopic silica from single-celled organisms called radiolaria. These creatures float in ocean waters and settle to the bottom when they die. The Phosphoria Formation formed during the Permian Period about 270 million years ago when more than a hundred feet of chert accumulated on the sea floor.
I think that’s what fascinates me the most about this pebble: its very existence speaks to a geologic history from ocean bottom to mountain range. Not only that, but it exists in two time periods: today and the Cretaceous. Throw it on a gravel bar and you couldn’t tell it apart from the modern pebbles.
Perhaps even more extraordinary is how non-extraordinary this pebble really is. Take any handful of loose sand that’s weathered from a nearby sandstone outcrop and look closely: those tiny sand grains are just like the pebble, only smaller. They’re sand grains now, just as they were when they became a part of the nearby rock. And like the pebble, they came from a still older rock –which itself has a history. And it’s all in transit.
To view and/or download more images of sedimentary rocks –or any kind of geology for that matter–please take a look at my website: geologypics.com
I’m very happy to announce that my new book, Oregon Rocks! A guide to 60 Amazing Geologic Sites, is out there and available. Although the title always makes me cringe a little, I’m excited and proud of this project, which took me three years to complete and took me all over my beautiful state. At the risk of being overly exuberant, here are some excerpts!
If you click on the image below, you can actually read the table of contents (left) and study the site map. Both these illustrations group the sites in one of Oregon’s six physiographic provinces: The Coast Range, Klamath Mountains, Cascade Range, Lava Plateaus, Blue Mountains, and Basin and Range.
The site numbers are especially important –not only because they key to the map location and page numbers in the book –they also refer to the timeline and schematic cross-section across the state. I’m especially proud of this part because it lets you place each site into the context of how the entire region evolved. Click on the timeline and maps below to see what I mean!
Something different here: an essay I wrote about teaching field geology on public lands– published under the same title in the March, 2021 issue of Desert Report with a few extra photos added (please click on the photos to see them larger). I guess I’m hopeful –even optimistic– that I’ll be able to teach field camp again this summer! ******************
Six pairs of eyes stared blankly at me. Cows. Amazing how big those creatures are, especially when you’re sitting on the ground so I was grateful they hadn’t already tried to share my tiny bit of shade. I stood up, shouldered my pack, and walked into the bright sun.
I’m in southwestern Montana, teaching a field geology class. It’s public land, a place I bring my students year after year without fear of locked gates or fences. Besides the great geology, we see awesome views in all directions, innumerable cacti, wildflowers, and sagebrush, occasional wildlife, and of course, cattle. But we come for the rocks: layer after layer, deposited as seafloor sediment, coastal dune, or river gravel over a period spanning some 300 million years. We’re in what’s called the Rocky Mountain Fold-Thrust Belt, where the layers show twists and turns you can’t imagine, and fault zones that caused mountains to rise long before the present landscape even began to form.
And because the area’s managed by the Bureau of Land Management, we can go pretty much anywhere and everywhere over this 3 or 4 square mile area, just like the cows. Each student creates a detailed geologic map. They identify the rocks, draw lines on their maps to show the boundaries between different rock units, and use their maps to interpret the geologic history. It’s difficult work and highly rewarding ‒ and most students complete the course with a new-found sense of confidence and competence.
I spot some students on a nearby ridge and veer off the gravel road to meet them. The sagebrush near the road is high, but as the slope steepens, it gives way to grass and ledges. On cresting the ridge, I call out:
“How’s it going?”
“Okay” one of the students replies. We look at each other stupidly. I have no idea who it is.
“Hope you’re having a good day!” I say cheerfully and walk on.
These students aren’t mine. They’re from one of the many other universities that come here to teach field geology skills. With so many students tramping over this landscape every summer, following existing paths to key outcrops or creating their own, breaking pieces off the bedrock so they can inspect fresh surfaces, I sometimes worry that we might love this place to death.
I feel a slight cooling breeze and sit down. In front of me, the rocks form a rounded arch shape ‒ a product of crustal compression that built mountains here some 80 million years ago. The frustration of chasing down the wrong students morphs into gratitude. I think of the many students who’ve told me, years later, how much they learned here and how much it shaped their careers. Many describe how blending physical exertion with their academic backgrounds led to a sense of discovery they’d never before experienced.
I watch a pickup truck approach the cows down on the road. It slows briefly to pass by and then continues up the gentle grade. Multiple use ‒ that’s the BLM mantra. Besides grazing and wandering geology students, this place is open to energy development, timber harvesting, and all sorts of recreation. By comparison, our impact is small ‒ and the geology here will outlive all of us, no matter how many cattle trails we follow, or create, or how many outcrops we hammer. I’m grateful for our freedom to wander, pursuing education anywhere and everywhere here, and to arrive anytime without having to fill out any kind of paperwork.
Ten days later, I’m sitting on a block of limestone, listening to water cascade over some ledges before coalescing into a narrow stream. Above me, soaring cliffs of Helena Dolomite form the headwall to the once mighty Grinnell Glacier. Some of the glacier persists high up in this cirque, but the retreat has been so rapid and recent that the glacial scratches across the bedrock are still fresh. Two of my students sit silently, gazing upwards to the headwall. Another studies a rock with a hand lens. Two others slowly make their way towards my perch.
Glacier National Park hosts a vast wilderness of landforms designated by glacial terminology: peaks called horns, ridges called arêtes, cliffs called headwalls, bowl-shaped valley heads called cirques, rock-bound lakes called tarns, deep U-shaped valleys called U-shaped valleys ‒ all carved by glaciers into this amazing bedrock. I ponder what this place must have looked like 15,000 years ago when glaciers were at their maximum. The peaks and some ridges would be recognizable but the valleys between them would be filled with ice that extended in long strands to the edge of the Great Plains.
One of my students practically shrieks with excitement. I’ve been waiting for this: they found the stromatolites, fossilized algae ‒ Earth’s oldest easily visible life form. Resembling an onion in cross-section, these thin concentric layers of rock formed when dome-shaped bodies of algae in shallow clear water trapped sediment as they grew. The ones here, which reach a meter or more in diameter, are the largest and most dramatic I’ve seen.
I watch the other students hurry over as the contemplative mood lights into a spontaneous revival. One student starts laughing. Another exclaims, “Look at this one!” another says, “Oh God!” another: “And the glacial striations go right across them!” Another turns to me quizzically: “How old are these rocks?”
I smile. “I just read that they were all deposited between 1.4 and 1.47 billion years old ‒ so somewhere in there.”
Another question: “And these glacial striations, maybe 10 or 15,000?”
I keep smiling. This spot is one of my all-time favorite places. We hiked the five miles up here for sunrise and now, mid-morning, still have the place to ourselves. The landscape is so fresh and raw ‒ so untouched ‒ that we feel a primeval kinship with this rock and ice.
I finally answer. “Or younger. This place was under ice just thirty years ago. There’s still some glacier left, just across the lake. So much change, huh?”
Many of my students have visited national parks before, but this is the first time they’ve come as geologists. They now see things through the lens of geologic time and process ‒ and for the first time, they are applying their knowledge to a landscape that not only surrounds them, but is beautiful and pristine. They learned important skills earlier in the course on BLM land and here they combine those skills with their human spirits.
We cross the stream and start picking our way up the ledges on the other side, finding more stromatolites as we go. I hear my students talking about landscape and time. Like me, they can relate to today’s landscape, and can imagine the scene during the glacial maximum some 15,000 years ago, but the shallow inland sea in which these rocks formed seems beyond comprehension. Its age, about 1.4 billion years, makes it all the more inconceivable. And what does the word “about” mean in this context anyway? If the rocks formed 1.41 instead of 1.40 billion years ago, they would be a full ten million years older. As geologists, we bandy these ages around with comfort, but when we really think about it, we can’t comprehend 10 million years, let alone a billion.
I stop and consider a series of stromatolites in front of me. They’re exposed on both the front and upper sides of the ledge, giving three-dimensional views of their mounded shapes. Weathering and erosion rounded off many of the broken edges, accentuating the stromatolite’s appearance. For a second, I think I can see them breathe.
Our little group falls silent as we watch a rain squall farther down the valley. What a vast open-air cathedral we’re in! Public lands, be they national parks or forests or open BLM land, house these wonderful places and welcome everyone who will make the journey.
To see more geological photos from from SW Montana or Glacier NP, please see my geology photo website, where you can use keywords to search among >4000 images and download them for personal or instructional uses for free.
Oregon sits at the very western edge of the North American Plate, an “active” plate margin in the truest sense of the term. There’s active uplift on the coast, active volcanic activity in the Cascade Range, and active crustal extension to the east—not to mention the active subduction just offshore that’s driving most of it. And the products of all that activity are today’s amazing beaches, forests, sand dunes, playa lakes, plateaus, mountain peaks, rivers –the list goes on.
Collectively, those landscapes paint a picture of Oregon and its geology today. But Oregon’s geologic history stretches back some 300 million years to its oldest rocks of Devonian age and will continue into the future until who knows when. We live in a snapshot of an unfolding geologic history –and while we can’t see the landscapes of the future, we have the rock record to show us some of the landscapes of the past.
The schematic cross-section above outlines Oregon’s geology, with each different color signifying a different grouping of rock, and therefore, a different part of its geologic history. The heavy red dashed line marks the boundary between Oregon’s “basement rock”—a term that refers to the deepest level crustal rock in a given area—and its cover. Oregon’s basement rock consists of disparate crustal fragments called “terranes” that were accreted to the edge of North America since about 200 million years ago or igneous bodies called “stitching plutons” (in pink) that intruded the terranes. The cover consists of sedimentary and igneous rocks that formed after accretion and over the top of the terranes.
Fun fact: Oregon has the shortest geologic history of any state in the conterminous US! That’s because its geologic history only goes back as far as the oldest rock of its oldest accreted terrane, which is some Devonian (419-359 million years) limestone in the Blue Mountains. All the other states have basement that includes rock of Precambrian North America. In many states, this older rock isn’t exposed, but we’ve seen it in well cuttings or on seismic lines. In Oregon, it’s simply not there! –the basement has all been added onto the edge of the ancient continent.
All told, we humans have discovered more than 5000 mineral types. I challenged myself to list the five most important minerals when it comes to the formation of rocks and their subsequent weathering and erosion into landscape.
It’s an impossible task –and subjective, of course –and I fudged a bit by grouping some minerals together. But here they are: the silicate minerals quartz, mica, feldspar, and mafic minerals, and the carbonate mineral, calcite. I invite you to take me to task in the comments section. Even so, these five minerals are easy to identify and are critical to any discussion about rocks –which makes them important to understanding weathering and erosion. And, I’m adding them to my already packed curriculum for my introductory class on surface processes next term, so five seems to be a good upper limit on the numbers.
Oh! Click on any image to see it enlarged on a separate page.
Weathering, by the way, is the in-place physical and/or chemical breakdown of rock; erosion is its removal –so they go hand-in-hand. A lot goes into how susceptible a rock is to these processes—not just the mineral content—and when it comes to chemical breakdown, the main factors always involve water. If a rock is accessible to water, it will break down more quickly; if it’s not accessible to water, it will be more resistant.
Because some rocks will weather and erode more quickly than others, landscapes form with cliffs and valleys and steep slopes, gentle slopes –all sorts of variation you can imagine—depending in part on the bedrock. We call the phenomenon “differential erosion”. You can find more detail on the processes in one of my earlier posts, called “Shaping of Landscape“.
Ok… the minerals:
Quartz –most people have come to know quartz because of its pretty crystals –especially the purple variety called amethyst—but they might not realize that it’s a hugely important component of many igneous, metamorphic, and sedimentary rocks, and it forms the cement of many sedimentary rocks. When it comes to weathering, quartz is practically inert chemically, so a strongly quartz-cemented sandstone will be more resistant to weathering and erosion than most other sedimentary rocks. The sandstone will stand out in relief, likely as cliffs or even overhangs. By contrast, weakly cemented, often finer-grained rocks tend to form slopes or even valleys.
If you look closely at a sandstone like the one below, chance are you’ll see mostly quartz grains. Quartz grains might break up into little pieces as they’re being transported by rivers, for example, but they won’t turn into clay like most other minerals. The result? Sediment gets increasingly rich in quartz the farther it’s transported from its source –and so if it travels a great distance, quartz is just about the only thing left!
And if you’re one of those people who really love this mineral, check out this post by Roseanne Chambers, which is all about quartz!
Micas are those shiny sheet-like minerals we see catching the light in a whole host of rocks. The best known micas are muscovite (white mica) and biotite (black mica). In metamorphism, they grow larger with increases in temperature, providing a handy way to distinguish between slate, phyllite, and schist — a nice thing to know in a geology class. The diagram below illustrates the process.
When it comes to weathering and erosion, those little mica sheets allow infiltration of water. Metamorphic rocks overall are pretty resistant because of their crystallinity –but slates or schist, for example, degrade much more quickly than gneisses because of their prevalence of mica minerals, which together create a fine-scale layering in the rock. Gneisses are coarse-grained, and their layering tends to be thicker and less permeable.
Collectively, feldspars are the most common minerals of the Earth’s crust –and there are numerous varieties, the most common being orthoclase and plagioclase, which often occur together. For the purposes of landscapes, I grouped them together as one: they’re all kind of chunky looking and are generally light-colored. Feldspars are the main components of igneous rocks, making up 60-70% (or even more) of most granitic rocks, which tend to form distinctive landscapes marked by cliffs and large rounded boulders.
The “mafic minerals” are named so because they’re generally rich in iron and magnesium and poor in silica when compared to a lot of other common minerals –and they’re typically dark green to black in color, so “mafic rocks” like basaltic lavas also tend to be darker colored. The main mafic minerals are olivine, pyroxene, and amphibole. Olivine, as it turns out, is our planet’s most common mineral! It’s the main stuff of the mantle, which depending on where you are, lies some 10-40km below the surface.
The physical properties of these mafic minerals explain all sorts of things like why oceanic plates subduct beneath continental ones (being made of mafic minerals, they’re denser so they sink) to the shapes of volcanoes (mafic lavas, being less viscous than silicic ones, tend to form broad, low-relief shield volcanoes). For surface weathering and erosion, the mafic minerals tend to break down into clay more quickly than most other silicate minerals –which means that all else being equal, rocks with more mafic minerals will weather and erode faster than rocks with the other silicate minerals described here.
Finally, calcite, the sole non-silicate on this list, is hugely important because it’s what makes up the very common sedimentary rock limestone. Limestones occupy a separate class of sedimentary rock than sandstones: they’re “biogenic”, having formed through the precipitation of calcite through biological processes, as opposed to “clastic”, which are just broken particles. As a result, they’re important to understanding sedimentary rocks, and because of the biology connection, Earth history and evolution. For landscapes, limestones are also really important because the calcite will dissolve in slightly acidic water to form caves and sinkholes–there’s a whole class of landscape called “Karst”, which results from the dissolution of limestone.
And re-precipitation of calcite within caves forms the beautiful speleothems we so love –stalactites, stalagmites, flowstone… the list goes on. That rock, called travertine, occupies a third class of sedimentary rock called “chemical sedimentary rocks”.
Phew! My apologies to those mineralogy people who might read this and think, “but she missed that important idea! And that one too! And what about that mineral?” Maybe that’s the point though –this is just a start. There is so much these five minerals can teach us –and there are so many other wonderful minerals I didn’t even mention. So here’s to reading more info that somebody might’ve put into the comments section. Here also to all the geology majors out there who take upper level geology courses that delve into all the amazing detail and make connections that I wasn’t able to in this little space!
And if you want to download any of these photos for your own –just type in the photo id into the search function of my geology photo website.
Also, I posted a primer on rock types –if you’re interested, please have a look –and thanks for reading!
Published in 1974 by the US Geological Survey, the geologic map of the United States beautifully lays out our country’s geology. If you’re stuck at home these days –as most of us are—you can gaze at this masterpiece and go anywhere!
Southern Willamette Valley. Eugene’s in the center of the map
At their simplest, geologic maps read like road maps: they tell you what rock unit or recent deposit forms the ground at a given place. So right here where I am in Eugene, Oregon, I can see that I live on “Q” –which stretches north up the Willamette Valley. “Q” stands for Quaternary-age material (2.5 million years to present), which is typically alluvial material –or sediment deposited by rivers and streams. Just to the south of me lie a variety of older volcanic and sedimentary rocks. You can look them up in the map’s legend using their symbols. Here’s Rule #0: white areas, being mostly alluvium, mark low areas; colorful areas indicate bedrock, so are typically higher in elevation.
The beauty of this map is that you can see the whole country at once, and using just a few rules, can immediately glean the underlying structure of a region.For any of the maps, photos, and diagrams here, you can see a larger size if you click on the image.
Rule 1: Rock bodies without internal contacts appear as color swatches (or “blobs”) as seen on the block diagram below. These rocks include intrusive igneous, undifferentiated metamorphic, and flat-lying sedimentary rock. (If you’re not sure what geologic contacts are, please see my recent post on the nature of geologic contacts.) Read more…
I always try for window seats when flying and I always try to shoot photos out the window –with varying results! So often, the window’s badly scratched, there are clouds, it’s hazy, the sun angle’s wrong –there are myriad factors that can make good photography almost impossible from a commercial jet. Last year though, I had a few amazing flights with clear skies and a great window seat –and I’ve now loaded nearly 100 images onto my website for free download. Here are 10 of my favorites, in no particular order. You can click on them to see them at a larger size. They’re even bigger on my website.
Mt. Shasta at sunset. Volumetrically, the biggest of the Cascade Volcanoes, Mt. Shasta last erupted between 2-300 years ago –and it’s spawned over 70 mudflows in the past 1000 years. From the photo, you can see how the volcano’s actually a combination of at least 3 volcanoes, including Shastina, which erupted about 11,000 years ago.
Aerial view of Mt. Shasta, a Cascades stratovolcano in northern California.
If you want to see more aerials of Mt. Shasta (shot during the day) –and from a small plane, go to the search page on my website and type in “Shasta”.
Meteor Crater, Arizona. Wow –I’ve ALWAYS wanted to get a photo of Meteor Crater from the air –and suddenly, on a flight from Phoenix to Denver, there it was!
Aerial view of Meteor Crater, Arizona
Meteor Crater, also called Barringer Crater, formed by the impact of a meteorite some 50,000 years ago. It measures 3900 feet in diameter and about 560 feet deep. The meteorite, called the Canyon Diablo Meteorite, was about 50 meters across.
Dakota Hogback and Colorado Front Range, near Morrison, Colorado. Same flight as Meteor Crater –and another photo I’d longed to take. It really isn’t the prettiest photo, BUT, it shows the Cretaceous Dakota Hogback angling from the bottom left of the photo northwards along the range and Red Rocks Amphitheater in the center –then everything behind Red Rocks, including the peaks of Rocky Mountain National Park in the background, consist of Proterozoic basement rock.
Aerial view of hogback of Cretaceous Dakota Formation and Colorado Front Range.
Distributary channels on delta, Texas Gulf Coast. I just thought this one was really pretty. Geologically, it shows how rivers divide into many distributary channels when they encounter the super low gradients of deltas. And whoever thought that flying into Houston could be so exciting!
Distributary channels on delta, Texas Gulf Coast
Meander bends on the Mississippi River. My mother lives in Florida, so I always fly over the Mississippi River when I go visit –but I was never able to take a decent photo until my return trip last October, when the air was clear, and our flight path passed just north of New Orleans. Those sweeping arms of each meander are about 5 miles long!
Meander bends on the Mississippi River floodplain, Louisiana
Salt Evaporators, San Francisco Bay. Flying into San Francisco is always great because you get to see the incredible evaporation ponds near the south end of the bay. I always love the colors, caused by differing concentrations of algae –which respond to differences in salinity. And for some reason, salt deposits always spark my imagination. Salt covers the floor of Death Valley, a place where I do most of my research, and Permian salt deposits play a big role in the geology of much of southeastern Utah, another place I know and love.
Salt evaporators, San Francisco Bay, California
Bonneville Salt Flats and Newfoundland Mountains, Utah. And then there are the Bonneville Salt Flats! They’re so vast –how I’d love the time to explore them. They formed by evaporation of Pleistocene Lake Bonneville, the ancestor of today’s Great Salt Lake. When the climate was wetter during the Ice Age, Lake Bonneville was practically an inland sea –and this photo shows just a small part of it.
Aerial view of Bonneville Salt Flats and Newfoundland Mountains
Stranded meander loop on the Colorado River. I like this photo because it speaks to the evolution of this stretch of the Colorado River. Just left of center, you can see an old meander loop –and it’s at a much higher elevation than today’s channel. At one time, the Colorado River flowed around that loop, but after breaching the divide and stranding it as an oxbow, it proceeded to cut its channel deeper and left the oxbow at a higher elevation.
Stranded meander loop (oxbow) on the Colorado River, eastern Utah
San Andreas fault zone and San Francisco. See those skinny lakes running diagonally through the center of the photo? They’re the Upper and Lower Crystal Springs Reservoirs –and they’re right on the San Andreas Fault. And you can see just how close San Francisco is to the fault. As the boundary between the Pacific and North American Plates, its total displacement is about 200 miles. See this previous post for more photos of the San Andreas fault.
San Andreas fault zone and San Francisco
And my favorite: Aerial view of the Green River flowing through the Split Mountain Anticline –at Dinosaur National Monument, Utah-Colorado. Another photo I’ve so longed to shoot –but didn’t have the opportunity until last year.
The Green River cuts right across the anticline rather than flowing around it. It’s either an antecedent river, which cut down across the fold as it grew –or a superposed one, having established its channel in younger, more homogeneous rock before cutting down into the harder, folded rock. You can also see how the anticline plunges westward (left) because that’s the direction of its “nose” –or the direction the fold limbs come together. The quarry, for Dinosaur National Monument, which you can visit and see dinosaur bones in the original Jurassic bedrock, is in the hills at the far lower left corner of the photo.
Split Mountain anticline and Green River, Utah-Colorado
So these are my ten favorites from 2019. Thanks for looking! There are 88 more on my website, at slightly higher resolutions and for free download. They include aerials of the Sierra Nevada and Owens Valley, the Colorado Rockies, including the San Juan Volcanic Field, incised rivers on the Colorado Plateau, and even the Book Cliffs in eastern Colorado. Just go to my geology photo website, and in the search function type “aerial, 2019” –and 98 photos will pop up. Boom!
Along with a zillion other people in the US, I witnessed the total solar eclipse today. Yes, it was amazing and yes, I feel somewhat addicted. The quality of light just before totality was something I’d never before experienced –and the sun’s flash just as it reappeared was something I’ll never forget. Apparently the next one will be in South America on July 2, 2019–and the next one in the US will be April 8, 2024. Oooh!
Sun’s corona as seen during the total solar eclipse, August 21, 2017 from Salem, Oregon.
Amazing that us humans can accurately predict these phenomena –to the exact place and time –to the second. Seems like our predictions work! These predictions, of course, are grounded in the physical sciences.
At the same time, many people insist that scientists are mistaken or misguided when they predict global climate change. I wonder if any of those people saw the eclipse. If so, they might want to reflect on their contradiction.
This monument marks the position of the front of the Athabasca Glacier of Alberta, Canada in the year 2000. Photo taken in 2012.
Of all the many reasons why waterfalls are great, here’s another: they expose bedrock! And that bedrock tells a story extending back in time long long before the waterfall. This posting describes 9 waterfalls that together paint a partial picture of Washington’s geologic history. The photos and diagrams will all appear in my forthcoming book Roadside Geology of Washington (Mountain Press) that I wrote with Darrel Cowan of the University of Washington.
Rainbow falls along WA 6 in the Coast Range
And waterfalls in heavily forested areas are especially great because they may give the only view of bedrock for miles around! Take Rainbow Falls, for example–the small waterfall on the left. It’s in Washington’s Coast Range along State Highway 6–a place where a roadside geologist could otherwise fall into total despair for lack of good rock exposure. But this beautiful waterfall exposes a lava flow of the Grande Ronde Basalt, which belongs to the Columbia River Basalt Group. Significant? Yes!
This lava erupted in southeastern Washington and northeastern Oregon between about 16 and 15.6 million years ago and completely flooded the landscape of northern Oregon and southern Washington. We know how extensive these flows are because we can see them–and they cover the whole region. The photo below shows them at Palouse Falls in the eastern part of Washington. Take a look at my earlier blog post about the Columbia River Basalt Group? (includes 15 photos and a map).
Landscape and bedrock… seems we seldom connect the two. We all like beautiful landscapes, but most of us don’t ask how they formed –and even fewer of us think about the story told by the rocks that lie beneath it all. Those make two time scales, the faster one of landscape evolution and the much slower one of the rock record. Considering that we live in our present-day human time scale, it’s no wonder there’s a disconnect!
Take this photo of Mt. Shuksan in northern Washington. My daughter Meg and I drove up to the parking lot at Heather Meadows and went for a quick hike to stretch our legs and take some pictures just before sunset.We had about a half hour before the light faded –and all I could think about was taking a photo of this amazing mountain. But the geology? What??
1. Mt. Shuksan and moonrise, northern Washington Cascades.
Thankfully, I’d been there in September scoping out a possible field project with a new grad student, and had the time to reflect… on time. From the ridge we hiked, shown as the dark area in the lower left corner of the left-hand photo below, we could almost feel Shuksan’s glaciers sculpting the mountain into its present shape. Certainly, that process is imperceptibly slow by human standards.
Mt. Shuksan: its glaciated NW side, summit, and outcrop of the Bell Pass Melange.
But the glaciers are sculpting bedrock –and that bedrock reveals its own story, grounded in a much longer time scale.
It turns out that the rock of Mt. Shuksan formed over tens of millions of years on three separate fragments of Earth’s lithosphere, called terranes. These terranes came together along faults that were then accreted to North America sometime during the Cretaceous. At the top of the peak you can find rock of the Easton Terrane. The Easton Terrane contains blueschist, a metamorphic rock that forms under conditions of high pressures and relatively low temperatures, such as deep in a subduction zone. Below that lies the Bell Pass Melange (right photo) –unmetamorphosed rock that is wonderfully messed up. And below that lies volcanic and sedimentary rock of the Chilliwack Group.
Here’s another of my favorites from 2015: the Keystone Thrust! It’s an easy picture to take –you just need to fly into the Las Vegas airport from the north or south, and you fly right over it. It’s the contact between the gray ledgey (ledgy? ledgeee?) rock on the left and the tan cliffs that go up the middle of the photo.
2. Keystone Thrust fault, Nevada–gray Cambrian ridges over tan Jurassic cliffs.
The gray rock is part of the Cambrian Bonanza King Formation, which is mostly limestone, and the tan cliffs consist of Jurassic Aztec Sandstone. Cambrian, being the time period from about 540-485 million years, is a lot older than the Jurassic, which spanned the time 200-145 million years ago. Older rock over younger rock like that requires a thrust fault.
Talk about geologic history… the thrust fault formed during a period of mountain building during the Cretaceous Period, some 100-70 million years ago, long before the present mountains. And the rocks? The limestone formed in a shallow marine environment and the sandstone in a sand “sea” of the same scale as today’s Sahara Desert. We know it was that large because the Aztec Sandstone is the same rock as the Navajo Sandstone in Zion and Arches national parks.
left: Limestone of the Cambrian Bonanza King Formation near Death Valley; right: Cross-bedded sandstone of the Jurassic Navajo Sandstone in Zion NP, Utah.
So… the photo shows cliffs and ledges made of rocks that tell a story of different landscapes that spans 100s of millions of years. But today’s cliffs and ledges are young, having formed by erosion of the much older rock. Then I flew over it in about 30 seconds.
At Beach 2 near Shi Shi Beach in Washington State are some incredible sea stacks, left standing (temporarily) as the sea erodes the headlands. The sea stack and arch in the photo below illustrates the continuous nature of this erosion. Once the arch fails, the seaward side of the headland will be isolated as another sea stack, larger, but really no different than the sea stack to its left. And so it goes.
3. Sea arch and headland at Beach 2, Olympic Coast, Washington.
And of course, the headland’s made of rock that tells its own story –of deposition offshore and getting scrunched up while getting added to the edge of the continent.
Bedrock at Beach 2 consists mostly of sandstone and breccia. The white fragment is limestone mixed with sandstone fragments.
And finally, my last “favorite”. It’s of an unnamed glacial valley in SE Alaska. My daughter and I flew by it in a small plane en route to Haines, Alaska to visit my cousin and his wife. More amazing landscape–carved by glaciers a long time ago. But as you can expect, the rock that makes it up is even older and tells it’s own story.
4. Glacial Valley cutting into Chilkat Mountains, SE Alaska.
Of course, this message of three time scales, the human, the landscape, and the rock-record time scale applies everywhere we go. Ironically, we’re usually in a hurry. I wish I kept it in mind more often, as it might slow me down a little.