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Washington’s waterfalls–behind each one is a rock!

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.

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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).

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Palouse Falls in eastern Washington drops more than 180 feet over lava flows of the Grande Ronde and Wanapum members of the Columbia River Basalt Group.

You might also notice in the photo above that the waterfall is actually pretty small compared to its amphitheatre. That’s because Palouse Falls is part of another flood story –of the Ice Age Floods, described in rich detail on the Ice Age Floods Institute website. Basically, some 40 or 50 gigantic floods coursed through the area towards the end of the Ice Age, between about 15-18,000 years ago. and among other things, carved this canyon. Lobes of the continental ice sheet repeatedly dammed the Clark Fork River in northern Montana and then failed, repeatedly, after forming Glacial Lake Missoula. Imagine the flow volume in the above photo multiplied more than 100,000 times!

Mount Rainier and the Cascade Volcanoes
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At 14,410 feet above sea level, Mount Rainier is the highest volcano in the Cascade Range –and one of the highest spots in the conterminous United States. The volcano itself consists mostly of andesite flows that date back nearly a half million years.

Beneath those lava flows are older rocks that speak to a history of volcanic activity reaching back 70 times that of Rainier’s oldest lavas –to about 35 million years ago. At Christine Falls, you can inspect granitic rock of the Tatoosh Pluton, which is a crystallized magma chamber that formed beneath some early Cascade volcanoes. It was probably active at different times between 26-14 million years ago. At Narada Falls, you can see where Rainier andesite actually flowed over the top of the granite–which tells us that the granite was exposed at the surface 40,000 years ago when that flow erupted. Both these waterfalls are right along the road that winds its way from Longmire up towards Paradise Meadows.

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Christine Falls (left) cuts through granitic rock of Tatoosh Pluton; Narada Falls (right) flows over Rainier Andesite that itself flowed over Tatoosh granodiorite, exposed on the rocky hillside.

If you go to the south entrance of the national park, you can walk a quarter mile from the highway to Silver Falls and exposures of Rainier’s oldest rocks. The Ohanapecosh Formation, made mostly of tuffs and re-deposited volcanic particles, formed by explosive volcanic activity that stretches back 35 million years. The Ohanapecosh Formation forms cliffs throughout much of the national park –and shows up northward as far as Interstate 90.

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Silver Falls in Mount Rainier National Park, spills over outcrops of Ohanapecosh Formation, the park’s oldest rock.

Finding the oldest volcanic rock in the Cascade Volcanoes is important because this incredibly active volcanic chain is fueled by magma generated through the sinking of oceanic lithosphere at the Cascadia subduction zone –and the oldest rocks allow us to estimate when this process started. They get even older at Snoqualmie Falls, just north of I-90. There, rocks of the Mount Persis Volcanics reach ages of 38 million years. Most geologists agree that for Washington, these rocks mark the first volcanic activity after the formation of the Cascadia subduction zone.

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Snoqualmie Falls drops more than 250′ into a gorge of Mt. Persis Volcanics –rocks that mark the onset of volcanic activity related to today’s Cascadia subduction zone.


Early Volcanic Roots and Continental Accretion

Here it gets a little complicated, because subduction also drove much of Washington’s geologic history before the Cascade volcanoes started to form. This older subduction also formed volcanic chains and through the process of continental accretion, caused Washington to grow westward.

Intro-8. Accretion series-CS4This diagram, modified from my book Roadside Geology of Oregon, illustrates the process of accretion. Basically, some element of the subducting seafloor is unable to fully sink beneath the continent, probably because it’s topographically high– such as with a series of seamounts. This material jams up the subduction zone and causes the sinking to stop temporarily. Eventually, a new subduction zone forms farther offshore and the thing that jammed up the zone in the first place gets added, or accreted, to the edge of the continent. In Washington and Oregon, the younger Cascadia subduction zone is the one that formed the Cascade Volcanoes and the stuff that jammed the zone was a huge fragment of oceanic lithosphere called “Siletzia”. Siletzia now makes up the bottom of Washington and Oregon’s Coast Range. The older subduction zone that got jammed up is the one that’s responsible for the rocks described below.

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Gorge Creek cuts a slot through orthogneiss (inset) of the Skagit Gneiss Complex along State Highway 20 in Washington’s North Cascades.

Gorge Falls along State Highway 20 in the North Cascades cuts this narrow slot through rocks formed because of that older subduction zone. These rocks started as the granitic roots to volcanoes, much in the same way as the Tatoosh Pluton formed the roots to some Cascade volcanoes. Those roots then got squeezed and reheated to make a metamorphic rock called gneiss. In some places it even partially re-melted.

The inset gives a close-up view of the rock. It’s called “orthogneiss” because it started out as an igneous rock. It forms a big part of the Skagit Gneiss Complex, which makes up the core of the North Cascades.

It’s hard to say if the Skagit Gneiss Complex was actually added to the edge of North America from somewhere else, but a lot of other rocks in Washington were–and those episodes of accretion are what caused much of the metamorphism in the North Cascades.

 

For accreted rock, here’s probably my favorite waterfall: Nooksack Falls, along State Highway 542 between Bellingham and the Mt. Baker ski area. It’s made of conglomerate of the Nooksack Group, which accumulated in a submarine fan somewhere off the coast of North America during the Jurassic and Cretaceous Periods, maybe 140 million years ago.

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Nooksack Falls in the North Cascades. the horizontal lines across the falls mark traces of bedding in the rock that’s inclined directly upstream.

Ancient North America

If you go eastward towards Spokane, you eventually find yourself on the North America that existed before all this accretion. Of course, much of the area is now covered by the Columbia River Basalt, but in the northeast corner of the state, you encounter Paleozoic sedimentary rocks that formed along the continental margin of that older continent. Sweet Creek Falls is one place to see these rocks, right off State Highway 31. There, the beautiful stream spills over ledges of Ledbetter Slate, deposited as shale during the Ordovician Period. In the foreground are cobbles of Addy Quartzite, formed as beach-deposited sandstone in the Cambrian.

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Sweet Creek Falls spills over Ledbetter Slate. Cobbles of Addy Quartzite lie in the foreground.

 

Washington’s Geologic Timeline

The timeline below shows Washington’s main geologic events –and you can see where these 9 waterfalls fit. The red text and red-colored bars represent geologic events represented by individual waterfalls, shown in blue.  Kind of amazing… these 9 waterfalls show many of Washington’s most important elements: the Cascade Volcanoes, the Columbia River Basalt Group, continental accretion, and the old continental margin.

And they’re nice places to hang out!

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Timeline of Washington’s geology. Red text signifies events described in this post and represented by various waterfalls (in blue).

 


For more geology photos, please check out my website–it contains a searchable database of more than 2000 geology photos for free download.

Roadside Geology of Washington should be out and available in August, 2017.

Thanks for reading!

 

 

 

 

 

Landscape and Rock–4 favorite photos from 2015

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??

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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.

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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.

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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.

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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.

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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.

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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.

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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.

Here’s to 2015 –and to 2016.

To see or download these four images at higher resolutions, please visit my webpage: favorite 10 geology photos of 2015.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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