Geology and Geologic Time through Photographs

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Oregon’s geologic history. A new cross-section and timeline –and some great places to see it.

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.

Folded Ribbon Chert on the Oregon Coast –click on the images to see them enlarged

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.

Schematic Cross-section across Oregon, from Oregon Rocks! –The red letters refer to places described here, the numbers refer to sites in my new book.

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.

Chief Joseph Mountain–made largely of the accreted Wallowa Terrane and Wallowa Lake, dammed by a glacial moraine.

Oregon might have the shortest geologic history, but it’s certainly one of the most eventful. Accretion of each terrane likely spawned a mountain-building event, and the Blue Mountains, Klamaths, and Franciscan Assemblage each consist of multiple terranes. And if you look at the cover rock, you can see just how variable it is. Rocks of the John Day Basin shown in green, for example, consist of a sequence of volcanic and sedimentary rocks that reflect changing landscapes and climates in central Oregon from about 48 million years to about 8 million years ago. So the cross-section is highly simplified and schematic: the actual story expands infinitely as you learn more about it.

Timeline of Oregon’s Main Geologic Events –from Oregon Rocks! –to be available in April

Using the cross-section, you can come up with a geologic timeline like the one above. The large letters on both the timeline and cross-section refer to the 7 locations described here while the many small numbers on the cross-section and time line refer to each of the 60 sites in my new book: “Oregon Rocks! A guide to 60 amazing geologic localities”. It just went to the printer and should be available sometime in mid-April. I’m beyond excited. It was a really fun project for the past three years, I visited a lot of places I’d never been before, and I learned a lot. And despite its cheesy title, I think it’s a great book and I’m really proud of it. I also had another wonderful experience working with the publisher, Mountain Press.

Here’s a copy of the book’s front cover!

So here are some sites that collectively, show much of Oregon’s geologic history. They’re in chronological order, beginning with A. terrane accretion and intrusion of stitching plutons; then B, post accretion sedimentary rock; C, the John Day Basin; D, Eruption of the Columbia River Basalt Group; E, High Cascades Volcanism; F, Basin and Range crustal extension; and finally G: modern-day coastal uplift. And the photos? All out-takes from my book. I simply had too many to use! So here are some that didn’t quite make it –and you can click on them to see them enlarged.

A. Terrane accretion and intrusion of stitching plutons: The View from Joseph
(site #54 in Oregon Rocks)

The Wallowa Mountains rise like skyscrapers along a fault zone just outside the town of Joseph –so from just about anywhere in town, you can see accreted rock of the Wallowa Terrane, as well as granitic rock of the Wallowa Batholith (light blue and pink on the cross-section). The granite intruded the Wallowa Terrane as a series of smaller plutons between 140 and 122 million years ago, probably at a depth of about 5km. 

Wallowa Mountains as seen from the town of Joseph. The peak on the right is capped by Columbia River Basalt Group that is overlying granite of the Wallowa Batholith. The stratified rocks to the left are accreted sedimentary rocks of the Wallowa Terrane.

From Joseph, you can also see bits of Columbia River Basalt perched high on the mountains. The basalts originated from fissures in the local area and once covered the entire region. With uplift of the range, however, the lavas mostly eroded. And if you go hiking in the Wallowas, you can see all the rocks up close, including those fissures, which are now preserved as basaltic dikes. Here’s an earlier blog post—with more detail– about this view.

B. Post-Accretion sedimentary rock: Cretaceous Conglomerate at Goose Rock, John Day Fossil Beds (site #49 in Oregon Rocks)

The sediment that gets deposited over the top of a terrane after it’s been accreted helps us determine just when the accretion took place. And because accretion of different terranes occurred at different times, their overlapping sedimentary rocks are of different ages. These conglomerates, beautifully exposed in the Sheep Rock Unit of John Day Fossil Beds National Monument, were deposited in streams during the Cretaceous Period about 115 million years ago, telling us that the underlying Baker Terrane was accreted no later than that. If you go westward to the coast, you can see the much younger (~45 million years) Tyee Formation, which was deposited on the accreted Siletzia Terrane (sites 9, 10 in the book).

Cretaceous Goose Rock Conglomerate, on both sides of the road in the Sheep Rock unit of John Day Fossil Beds National Monument. Inset shows the colorful pebbles that make up the conglomerate.

C. Rocks of the John Day Basin: Sheep Rock unit of John Day Fossil Beds.
(site #49 in Oregon Rocks)

Sheep Rock is the biggest –and arguably most instructive—part of the three units of the John Day Fossil Beds National Monument. Not only does it contain the Goose Rock Conglomerate of location B, but it also hosts the Thomas Condon Paleontology Center, and shows off all rock units of the rocks of the John Day Basin above the Clarno Formation. You can see beautiful exposures of volcanic mudflows of the Clarno Formation in a series of pinnacles at the Clarno unit of the national monument (site #48).

Sheep Rock itself displays the green-colored Turtle Cove Member of the John Day Formation and is capped by Picture Gorge Basalt

The John Day Basin rocks are important for a variety of reasons, none the least being they host a world-class treasure trove of mammal and plant fossils from the Eocene through the Miocene. They also preserve a clear record of climate change, from tropical to subtropical climates in the Clarno to temperate forests and grasslands in the John Day Formation, back to subtropical conditions in the middle Miocene during eruption of the Picture Gorge Basalt and lower part of the Mascall Formation, and then back to cooler and drier climates for the upper part of the Mascall and the Rattlesnake Formations. And the rocks are so pretty! They consist mostly of air fall tuffs that lingered long enough on the surface to turn into soils –and their colors make for a general –though not exact– proxy for the climate. The dark red soils reflect deep weathering in tropical climates, and lighter browns and tans formed in cooler climates. The green color of much of the Turtle Cove Member of the John Day Formation comes from the mineral celadonite, which formed from chemical alteration of the original rock– much of which formed from ash erupted from the giant Crooked River Caldera (site #33).

D. Eruption of the Columbia River Basalt: Silver Falls State Park.
(site #23 in Oregon Rocks)

Just west of Salem, Oregon, Silver Falls State Park hosts some 15 waterfalls that flow over the Grand Ronde and overlying Wanapum Basalts –members of the Columbia River Basalt Group. Here, you can actually walk behind many of the waterfalls and contemplate this amazing basalt, which erupted in northeastern Oregon and flowed some 200 miles to get here –and continued for another 100 miles to Oregon’s coastline!

Meg meets waterfall

The Columbia River Basalt Group so dominates the geology of northern Oregon and southern Washington that I’ve already blogged about it here –but suffice it to say that it erupted between about 17 and 6 million years ago and covers more than 70,000 square miles with a volume of more than 50,000 cubic miles. Some 94% of the lavas erupted by 14.5 million years ago—which means that some 46,000 cubic miles erupted in a period of less than 2.5 million years. Imagine!

South Falls at Silver Falls State Park. Notice how the trail goes into the large alcove behind the falls.

Still, there were quiet periods, lasting tens of thousands of years or longer –and if you go there, you’ll see stream-deposited sedimentary rocks between the Grande Ronde and Wanapum Basalts. The sedimentary rocks erode much more easily than the basalt and so make the large alcoves behind North and South falls, which drop 136 feet and 177 feet respectively. And the hiking trails, which are pretty awesome in themselves, actually pass through these alcoves behind a curtain of falling water.

E. High Cascades Volcanism: McKenzie Pass
(site #26 in Oregon Rocks)

If there is just one thing that Oregon’s geology is known for, it would have to be its range of potentially active volcanoes, the High Cascades. A direct product of subduction, these volcanoes make a line that continues south into northern California and north as far as southern B.C., Canada –but most of the volcanoes lie in Oregon.

And the range is so variable—with all types of volcanoes that have erupted all types of lavas and tephra over the past 6-8 million years. And before the High Cascades, the western Cascades (just to the west) were actively erupting back as far as 40 million years ago—even providing some of the ash in the John Day Formation. Today’s active volcanoes are all much younger than that, creating a landscape that’s mostly on the order of 10s of thousands of years old. Some of the volcanoes, like Mts Hood or Mazama/Crater Lake (sites 22, 28) have histories that go back about a half million years –but the landscape is strikingly fresh.

Mt. Washington and recently erupted basaltic lavas at McKenzie Pass

McKenzie Pass gives a wonderful sense for the High Cascades. You can walk a trail through basaltic lavas that erupted only two thousand years ago and take in spectacular views of nearby mountains, representing a variety of volcano types. These mountains include Belknap Crater, a basaltic shield, Mt. Washington, an eroded shield, Mt. Jefferson, an eroded andesite stratovolcano, Black Butte, an incredibly symmetrical but small stratovolcano of basaltic andesite –and just to the south, you can see Middle and North Sisters, made of basalt and basaltic andesite respectively. Phew!

North (left) and South (right) Sisters as seen looking southward from McKenzie Pass

F. Basin and Range Crustal Extension: Steens Mountain
(site #57 in Oregon Rocks)

The Alvord Desert as seen from near the summit of Steens Mountain. The vertical stripes in the snowy ridge in the middle ground are basaltic dikes.

When the snow’s gone, you can drive much of the way up Steens Mountain near the southeastern corner of the state—and from there, it’s a short hike to the summit. At an elevation of 9734’, it’s the highest spot for more than a hundred miles –and the view is fantastic.  Looking eastward, you look down the steep escarpment to the Alvord Desert, more than a vertical mile below; westward, you can look down long glacially carved canyons that descend at a much gentler grade. Like other mountains of the Basin and Range Province, Steens formed—and is still forming– as a tilted fault block, with a normal fault along one side that causes the mountain to rise steeply along the fault and tilt gently back in the opposite direction. Just like the cross section below.

Cross-Section across Steens Mountain, Oregon –from Miller, 2014, Roadside Geology of Oregon.

And the rocks! They consist mostly of the Steens Basalt –which is the oldest part of the Columbia River Basalt Group. There’s plenty of opportunity to see (and pet!) these rocks as you drive up the gravel road (High clearance) –and from the summit, you can look over hundreds of the Steens lava flows and see that they’re cut by dikes –the frozen conduits of later erupting lavas.  

G. Modern Day Coastal Uplift: Sunset Bay and Shore Acres State Parks
(site #13 in Oregon Rocks)

To see the effects of ongoing coastal uplift in Oregon, one of the best places is undoubtedly Sunset Bay and Shore Acres State Parks, just west of Coos Bay. At low tide, Sunset Bay displays an incredible wave-cut platform, a flat surface that formed by wave erosion of the local bedrock. Besides the wonderful tide-pooling, the surface creates a geologic marker; if it were uplifted, we would see it as just that: a flat surface on top the uplifted bedrock.

Wave-cut platform at Sunset Bay, Oregon as seen from the uplifted terrace on the north.

And that’s just what we see! On either side of the bay, the cliffs are capped by a marine terrace, a surface that, just like the one hosting today’s tidepools, formed at sea level. Only now, it sits some 20-30 feet above the waves. And if you go to Shore Acres a mile to the south, you can walk around on this terrace and see the incredible waves.

Wave breaking on tilted Coaledo Formation at Shore Acres State Park. The uplifted marine terrace makes the flat surface in the background. Those spherical things on the left-hand side are concretions.

And why are the waves so big at Shore Acres? I’m not entirely sure, but I think two things are at play. First and probably foremost, there is no wave-cut platform in front of Shore Acres like there is elsewhere along that part of the coast, so the waves collide with the cliffs with nothing to slow them down. Also, a cursory look at the sea floor bathymetry shows that the 120’ depth contour is just over 1.5 miles away from Shore Acres, whereas to the north and south, it’s closer to 2 miles away –or even greater. All that extra water has to go somewhere when it hits bottom –which is upward.

One of my favorite things about geology is how we can piece together a coherent picture of a region’s geologic history from a bunch of different places–places we might otherwise think are completely unrelated. And once you have an outline of the story, you can start filling in all sorts of amazing details the more you learn. Think how diverse and wonderful our landscape is today. Landscapes of the past were diverse and wonderful too –and the more we study the Earth, the more we can experience them.

For more geology photographs –of Oregon or elsewhere–please visit my site

Columbia River Basalt Group–outrageous!

I can’t stop thinking about the Columbia River Basalt Group–the series of basalt flows that blanketed so much of my state of Oregon about 15 million years ago. Abbreviated as “CRBG”, it covers a lot of Washington too, as well as parts of western Idaho and northern Nevada. If you’re driving across those parts, you’ll likely travel miles and miles and miles over basalt basalt basalt –and that causes some people to say (mistakenly) that it’s boring. Some geologists even get grumpy about it because it covers up all the older rock.  Outrageous!

Lava flows of the CRBG in northern Oregon and Mt. Adams of southern Washington.  With views like this, how can you say the CRBG is boring? (Location "F" on map below.)

Lava flows of the CRBG in northern Oregon and Mt. Adams of southern Washington. With views like this, how can you say the CRBG is boring? (Photo “F” on map below.)

But of course, the CRBG is outrageous for a whole host of other reasons. For one thing, it really is huge: it covers an area of more than 77,000 square miles with a volume of more than 52,000 cubic miles –that’s more than 50x the volume of air between the north and south rims of the Grand Canyon! Really—the National Park Service estimated the volume of “Grand Canyon Air” to be about 1,000 cubic miles. It also erupted over a fairly short period of time: from about 17 million years ago to 6 million –but 96% of it erupted between 17 and 14.5 million years ago.


And… most of it erupted from fissures in eastern Oregon and Washington –the roots of which are now preserved as dikes. And… many of the lavas made it all the way to the Pacific Ocean. And… (here’s the outrageous part), on reaching the Pacific, many of the flows re-intruded into the existing sediments and sedimentary rocks along the coastline to form their own magma chambers, some of which were thousands of feet thick! AND… some basaltic magma from those chambers then re-intruded the country rock to form dikes –and some even re-erupted on the seafloor!

All these outrageous details. Now think about them for a moment. They really happened. That’s what I find so wonderful and amazing about geology. We learn all these things and we put them in some part of our consciousness that doesn’t really let them soak in –but once in awhile they do.

Finally, the CRBG is beautiful and forms beautiful landscapes! Below are some photos to illustrate it, from feeder dikes in eastern Oregon to sea stacks eroded from a giant sill on the coast.

And I’ll save my snarky comments about young earth creationism for another post.

–and at the bottom, I’m adding a short glossary to explain some of the terms.

Ok… the photos!


Photo A. Steens Mountain and Alvord Desert.  The CRBG started with eruption of the Steens Basalt about 16.7 million years ago, which makes up the upper 3000′ or so of Steens Mountain, shown here.  Steens Mountain is one of our state treasures –it’s a fault-block mountain, uplifted by Basin-Range extension along a normal fault along its eastern side.

Fault-bounded east front of Steens Mountain and Alvord Desert.

Fault-bounded east front of Steens Mountain; mudcracked playa of Alvord Desert in foreground.


Photo B. Steens Basalt at Abert Rim. Like most of the CRBG, The Steens basalt covered outrageously huge areas.  It also makes up the cliffs above Lake Abert about 75 miles to the east.  Called Abert Rim, the cliffs are also uplifted by a big normal fault.  Lake Abert occupies the downdropped basin.  And much of the Steens basalt consists of this really distinctive porphyry with outrageously big plagioclase crystals!

Photo B.  Steens Basalt at Lake Abert; Abert Rim in background.

Photo B. Steens Basalt at Lake Abert; Abert Rim in background.


Photos C-1, C-2. CRBG dikes.  One reason we know that the CRBG erupted from fissures is that we can see their roots, as dikes cutting through older rock.  C-1 shows a dike cutting through previously erupted basalt flows in Grande Ronde Canyon, Washington; C-2 shows some narrow little dikes cutting accreted rock of the Triassic Martin Bridge Limestone. Photos 5 and 6 of my last post shows some aerial photos and describes this area in more detail.C. Feeder dikes


Photo D. Imnaha Canyon. The next major unit of the CRBG is the Imnaha Basalt, followed by the Grande Ronde Basalt.  Both these units erupted from sites in northeastern Oregon and southeastern Washington.  This view of Imnaha Canyon in Oregon shows the Imnaha Basalt near the bottom and the Grande Ronde Basalt at the top.

Photo D.  Imnaha Canyon, Oregon.

Photo D. Imnaha Canyon, Oregon.


Photo E. Picture Gorge Basalt at the Painted Hills.  And the next youngest unit of the CRBG was the Picture Gorge Basalt, shown capping the ridge in the background. Unlike most of the CRBG, the Picture Gorge Basalt originated in central Oregon, not too far from here–there’s a whole swarm of dikes near the town of Monument, Oregon.

The colorful hills in the foreground make up the Painted Hills of John Day Fossil Beds National Monument, another of our state treasures.  I like this photo because it gives a sense of what lies beneath the CRBG –and the John Day Fossil Beds are outrageous in their own way–but save that for another time.



Photo F. Lava Flows of the CRBG and Mt. Adams, a modern volcano of the High Cascades in Washington.  See the first picture at the top of the post!


Photo G. Wanapum Basalt near The Dalles. This exposure of the Wanapum Basalt, which overlies the Picture Gorge Basalt, tells the story of the CRBG as it flowed into and filled a lake along the Columbia River some 15 million years ago. At the bottom of the flow, pillow basalt formed as the lava poured into the lake, while the upper part of the flow shows the columnar jointing typical of basalt that flows across land.  What’s more, this exposure lies less than a mile off I-84 in The Dalles, Oregon.  See page 251 of the new Roadside Geology of Oregon for another photo and more description!

Photo G. Single flow of Wanapum Basalt near The Dalles, Oregon.

Photo G. Single flow of Wanapum Basalt near The Dalles, Oregon.


Photo H.  Upper North Falls, Silver Falls State Park, Oregon.  This wonderful state park hosts about a zillion waterfalls that spill over cliffs of CRBG, 14 of which lie in the main river channels of the north and south forks of Silver Creek.  The falls depicted in this photo are 136 feet high!

Upper North Falls at Silver Falls State Park, OR.  The roof of the alcove consists of Wanapum Basalt, the bedrock near the river channel consists of Grande Ronde Basalt.

Upper North Falls at Silver Falls State Park. The roof of the alcove consists of Wanapum Basalt, the bedrock near the river consists of Grande Ronde Basalt.

Notice that the picture’s taken from behind the water. The trail goes into a big alcove, so it’s easy and safe.  The alcove formed because this particular waterfall crosses the contact between the Wanapum Basalt and the underlying Grande Ronde Basalt –and there is a 10-20′ thick, easily eroded, sedimentary unit between the two.  Remember the Grande Ronde Basalt –from Photo D in northeastern Oregon? Here it is, just east of Salem!


Photo I.  Saddle Mountain, northern Coast Range.  Here starts the truly outrageous part of the CRBG story.  Saddle Mountain, the highest point in the northern Coast Ranges, consists almost entirely of the rock on the right: brecciated pillow basalt, full of the alteration mineral palagonite. Apparently, the basalt started to flow into the ocean at about here, formed pillows and fragmented like crazy in the water-lava explosions. I. Saddle Mtn

But!  These flows were likely confined too –such as in a submarine canyon–which allowed them to develop enough of a pressure gradient to intrude downward into bedding surfaces, faults, and fractures of the Astoria Formation.  The diagram below illustrates the process in cross-section.  The diagram also give a context for photos I-L.  intrusive CRBG diagram4


Photo J.  Sea stacks of intrusive Columbia River Basalt Group at Ecola State Park.  Some of the magma chambers were several thousand feet thick and are now exposed as gigantic sills along the coast.  One such sill is Tillamook Head, of which Ecola State Park is a part –and it’s eroding into the sea stacks you can see in the distance.J.130120-11lrs


Photo K.  Haystack Rock at Cannon Beach, OR.  Go figure, one of our iconic state landmarks is an undersea volcano?  You can actually walk out to this thing at low tide and see lots of pillow basalt and dikes intruding the Astoria Formation.  The smaller sea stacks are part of the same complex.K.130121-12


Photo L. Seal Rock, Oregon.  Seal Rock is the southernmost exposure of CRBG on the coast –and it too, is intrusive.  It’s a big dike that trends NNW for about a quarter mile out to sea.  And along its edges, there are smaller dikes that you can see intruding the Astoria Formation, such as in the smaller photo.  The arrow points to where you can see the small intrusion, at low to medium tides.

L. Seal Rock

Some Terms:
dike: a tabular-shaped intrusion that cuts across layering in the surrounding rock.  Imagine magma flowing along a crack and eventually cooling down and crystallizing.  That would form a dike.  A feeder dike is a dike that fed lava flows at the surface.

normal fault: a type of fault along which younger rocks from above slide down against older rocks below.  They typically form when the crust is being extended.

porphyry: an igneous rock with larger, easily visible crystals floating around in a matrix of much smaller ones.

sill: an intrusion that runs parallel to layering in the surrounding rock.


Some references:

Reidel, S.P., Camp, V.E., Tolan, T.L., Martin, B.S. 2013. The Columbia River flood basalt province: stratigraphy, areal extent, volume, and physical volcanology. In The Columbia River Basalt Province, Geological Society of America Special Paper 497, eds. S.P. Reidel, V.E. Camp, M.E. Ross, J.A. Wolff, B.S. Martin, T.L. Tolan, and R.E. Wells, p. 1-44.

Wells, R.E., Niem, A.R., Evarts, R.C., and Hagstrum, J.T. 2009. The Columbia River Basalt Group—From the gorge to the sea. In Volcanoes to Vineyards: Geologic Field Trips through the Dynamic Landscape of the Pacific Northwest, Geological Society of America Field Guide 15, eds. J.E. O’Connor, R.J. Dorsey, and I.P. Madin, p. 737-774.

Some links:

Roadside Geology of Oregon
Geology pictures for free download
Geologic map of Oregon

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