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Geology and Geologic Time through Photographs

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Countertop Geology: Desperate for rocks? Visit a “granite” countertop store!

Where can you see some rocks? It’s winter and everything’s covered in snow –or you’re visiting family in some place where there’s virtually no bedrock exposed anywhere –or you’re simply stranded far from any good rocks in the center of a big city.IP18-0957c

Take yourself on a field trip to a granite countertop store! You might not see very much real granite, but you will see some other types: folded gneiss, pegmatite, amphibolite, quartzite, maybe even some granite… and a lot of amazing metamorphic and igneous features and faults –and they’re all polished and none are covered by vegetation.

I needed a rock fix the other day while visiting my mother in SW Florida –so I drove to a granite countertop store. And wow— I saw all sorts of great stuff, a lot of which related to faulting and fracturing, and a lot of it could go right into a geology textbook. In Florida!

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Red garnet along with quartz and feldspar in gneiss -a metamorphic rock.

But first of all, the term “granite”. Countertop places call just about everything made of silicate minerals to be granite –and the other day, I didn’t see a bit of granite. Being an intrusive igneous rock, granite is generally pretty homogeneous in appearance, unlike metamorphic or sedimentary rocks, which tend to be banded or layered. (see this primer on rock types)  Granite also has a specific chemistry, which translates to a pretty specific set of minerals: mostly orthoclase and plagioclase feldspars and quartz, with some white or black mica and maybe some amphibole thrown into the mix. As a result, granitic rocks tend to be pretty light-colored so there’s no such thing as a black granite –or a charcoal granite.

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Some real granite –with quartz (Q), Orthoclase feldspar (O), Plagioclase feldspar (P), and the black mica Biotite (B).

 

The first rock I saw on my Florida field trip was this gently folded quartzite (notice how the two opposing slabs make it look more folded than it is). It’s chock full of dark zig-zaggy things called stylolites roughly parallel to bedding, many of which are offset along small fault zones. Stylolites form because some rocks partially dissolve when they’re under compressive stress; the actual stylolite consists of insoluble residue left behind after the dissolution occurs.  You mostly see them in limestone because limestone’s pretty soluble. They’re actually pretty unusual in quartzites.IP18-0975

But what I found so instructive with this rock was the faulting. Look! With the 100% exposure, you can see how the apparent offset along the fault just below the penny diminishes as you go to the left. That’s an important feature about faults: their slip tends to die out towards their tips.

Stylolites offset by faults in quartzite

Stylolites and bedding offset by faults in quartzite

In this next photo of a granitic gneiss, you can see an igneous-filled fault offsetting the rock near the top, but it dies out completely as you go downward.
Migmatite gneiss

 

Dismembered pegmatitte in gneiss

Biotite schist with pegmatite

Then there’s the positively swirly biotite schist to the left with white blobs made of the rock pegmatite. It looks to me like the pegmatite once cut through the darker rock as a dike, and then got pulled apart and folded during some later time. Biotite-rich rocks tend to deform very easily whereas pegmatite tends to be pretty stiff, so the pegmatite retained some of its shape as it broke up while the rest of the rock flowed around it.

 

Alteration along fractures in serpentinite

Alteration along fractures in serpentinite

And serpentinite! Serpentinite forms by metamorphism of rocks from Earth’s mantle, so they tend to be comparatively poor in silica and rich in iron and magnesium. This serpentinite is highly altered to a pretty brown color (which makes this a prized countertop rock) –and you can see that alteration’s taken place along fractures –right where you’d expect high temperature fluids to circulate. And if you’re the type of geologist who really gets into fractures… well here you go: 100% exposure!

For me, this last example of a ductile shear zone might be the most helpful. I always have a difficult time describing these features to students because ductile shear zones are conceptually difficult; they’re basically faults without any breakage. As you can see in the photo, material got displaced in a sense roughly parallel to the arrows –but nothing got broken. The metamorphic layers simply bend into the zone and thin out and don’t really break. THAT is a ductile shear zone!

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Ductile shear zone in gneiss (annotated on right)

Of course, you could also wander into the downtown of a big city and see amazing facing stones too –or you could look at the awesome countertop in somebody’s kitchen –or see incredible polished rocks in bathroom sinks or as floor tiles –and those places are great too. But countertop “granite” places have many many more samples to ogle –the they also have a refuse bin. You might be able to take home some samples—and some of those might even be granite!

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on an earlier trip to a “granite” store, I found an amazing metaconglomerate –and blogged about the metaconglomerate and geologic time. Please take a look!

Also, most of these pics are available for free download from the search function at my website: geologypics.com.

Shaping of Landscape: A primer on weathering and erosion

Most of us love landscapes –and many of us find ourselves wondering how they came to look the way they do. In most cases, landscapes take their shape through the combined processes of weathering and erosion. While weathering and erosion constitute entire fields of study unto themselves, this primer outlines some of the basics—which pretty much underlie all the further details of how natural processes shape landscapes.

Incised meanders on the Green River, Utah

Aerial view of incised meanders of Green River, Utah.

Two definitions: weathering describes the in-place breakdown of rock material whereas erosion is the removal of that material. Basically, weathering turns solid rock into crud while erosion allows that crud to move away.

Weathering
Weathering processes fall into two categories: physical and chemical.  Physical weathering consists of the actual breakage of rock; any process that promotes breakage, be it enlargement of cracks, splitting, spalling, or fracturing, is a type of physical weathering.  Common examples include enlargement of cracks through freezing and thawing, enlargement of cracks during root growth, and splitting or spalling of rock from thermal expansion during fires.

Spalling of volcanic rock

Spalling of volcanic rock–likely from thermal expansion during a fire.

Chemical weathering alters the composition of the rock and is critical for soil development.  The most prevalent processes are oxidation of iron-rich minerals, dissolution of material, most notably of the calcium carbonate that makes up limestone or the cement of many sandstones, and hydrolysis, which turns feldspars and micas to clay.  As chemical processes all require water to proceed, they are most active in wet, warm climates, and least active in dry cold ones.

Strongly chemically weathered granitic rock above less weathered granitic rock.

Grus, disintegrated granitic rock, Montana

Disintegration of granitic rock through hydrolysis of feldspars

 

Physical and chemical weathering processes help each other degrade rocks. By breaking rock into smaller pieces, physical weathering processes greatly increase the surface area over which chemical processes can attack.  At the same time, chemical processes greatly weaken a rock’s strength and make it more susceptible to breakage.  Chemical weathering of individual mineral grains also increases their volume, which itself leads to fracture.

Groundwater staining along fractures in granite, Sierra Nevada,

Chemical weathering (oxidation and hydrolysis) concentrated along fractures in granitic rock.

 

Erosion
In contrast to breaking things down in-place, erosion removes material, typically through gravity, water, or wind.  The single most important influence is gravity, which drives processes such as rock falls, avalanches, and debris flows.  Most erosion, however, is a product of gravity and water acting together: water facilitates gravity-driven processes like debris flows, and gravity ultimately lies behind the power of water.   Wind can also be important, but is a relatively minor contributor overall because it can transport only the finer-grained particles.

Recent rock avalanche

Rock avalanche deposit, Utah

 

Weathering and Erosion together
Weathering and erosion work hand-in-hand in their creation of landscapes. Weathering processes break exposed bedrock into smaller and weaker fragments, which allows erosion to proceed.  By removing that material, erosion then exposes new bedrock to weathering processes.

Landscape and Bedrock

Besides telling us about Earth history, the bedrock of a particular place helps determine how it looks.  The bedrock provides the initial block of clay, so to speak, that is carved by weathering and erosion to create the landforms at the surface.  As a result, the shape of the landscape depends on the actual type of rock and its structure– that is, its orientation, relationship to other rock types, and presence or absence of fracture or fault zones.  For example, some rock types are easily weathered and removed by erosion while others are extremely resistant, and these may show through as topographic lows and highs respectively. 

Differential erosion: a hogback, Utah

Resistant sandstone forms a “hogback ridge” whereas less resistant shale and mudstone form gullies, Utah.

While a number of factors affect a rock’s resistance to weathering and erosion, the single most important one is its ability to repel water.  As a result, prevalence of bedding in sedimentary rock or foliation in metamorphic rock, both of which allow water penetration, decrease the resistance.  By the same reasoning, increased grain size tends to increase resistance because coarser grains offer less surface area for a given volume.  Similarly, stronger cementation in sedimentary rocks increases resistance, as does the crystalline nature of metamorphic rocks.  Carbonate rocks, such as limestone and dolomite, provide interesting exceptions to these rules.  In arid environments, they are resistant, but in wetter climates where they can dissolve, they are not resistant.

Differential erosion of sandstone vs shale, Utah.

Sandstone beds, being coarser grained and thicker bedded than the red-colored shale and siltstone, stand out in relief because they are more resistant to erosion. Utah.

As the single most important factor in weathering and erosion is water, it stands to reason that the presence of fractures or fault zones in rock, which tend to localize water flow, greatly influences the landscape. Additionally, movement along fault zones tends to crush some of the adjacent rock, which makes them even easier to erode.  As a result, fractures and faults frequently form canyons or valleys.  For example, at Arches and Canyonlands national park in Utah, vertical fractures erode into slot canyons to leave the intervening, non-fractured rock standing upright as narrow ridges called fins.  Some of these fins erode in from their sides to form arches.  At Pt. Reyes National Seashore in California, Tomales Bay protrudes inland as a long narrow bay eroded along the San Andreas fault. Click here to see a post about the San Andreas fault!

Vertical joints in sandstone, SE Utah.

Vertical fractures in sandstone eroding into fins, Arches National Park, Utah.

The vertical changes in a sequence of sedimentary rocks show the most predictable, yet dramatic, impacts on landscape, especially in arid landscapes. In flat-lying rocks, such as in many parts of the Colorado Plateau, the resistant rock units form cliffs whereas more easily erodable units form slopes.  The easily eroded slopes are typically littered with large blocks from the cliffs above. These blocks fall when erosion of the slopes undercuts the cliffs and gravity takes over.

Talus cones and Cedar Mesa, Utah

Edge of Cedar Mesa, Utah. The cliffs consist of resistant sandstone whereas the slopes are made of less resistant shale and siltstone.

Below is another example of how the resistant cliffs tend to erode by rock fall.

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Sandstone blocks, fallen from the cliffs above, onto slopes of less resistant shale and siltstone, Utah.

 

In places where the rocks are tilted, the more resistant rocks form ridges whereas the less resistant ones form valleys.  In places where the rock is folded, these ridges and valleys curve in the same way as the rocks.   Importantly, bedding takes on an important role in erosion of tilted rocks, as it encourages sliding of rock in the direction of tilt.  As a result, the ridges are typically asymmetric: they slope parallel to bedding on one side to create a “dip slope”, and form ledges and cliffs on the opposite side.

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Tilted resistant sandstone forms asymmetric ridge with dip-slope (on the left). Colorado.

The photo below shows numerous ridges held up by resistant sedimentary rock with intervening “strike” valleys eroded into less-resistant rock. The rocks dip steeply to the right (east).

Hogback Ridges near Mora, New Mexico

Aerial view of resistant hogback ridges and strike valleys near Mora, New Mexico.

 

Lava flows and metamorphic rocks exhibit layering that can influence topography in a similar way, although in these rocks, the extent of the layering tends to be more local and less predictable.  The widespread lava flows of late-Cenozoic age in the western US have probably the biggest effect on the landscape.  Many of these flows are relatively undeformed and so remain approximately flat-lying.  Similar to sedimentary sequences, they form large plateaus that in places are incised by deep canyons.  Examples of these flows include the basalt flows of the Snake River Plain in Idaho and the basalt flows of the Columbia Plateau in Oregon and Washington.

Lava Flows of Columbia River Basalt Group, Washington

Flat-lying Lava Flows of Columbia River Basalt Group, Washington

So landscapes are shaped by erosion, but the erosion depends on the weathering processes at hand and the rock type and structure. The weathering processes allow erosion to proceed and the rock type and structure guide both the weathering and erosion. The lead-off photo in this post shows canyons in the meandering Green River. It looks that way because the rocks are flat-lying. The stair-stepped topography down to the river, of cliffs and slopes, simply results from alternating resistant and less-resistant rock types. And the river? It’s entrenched in the canyons, probably because it continued to downcut as the region uplifted.

Here’s another example –another aerial from the Colorado Plateau –you can see how flat-lying rocks create the stair-stepped topography down to the river –and the uppermost resistant rock forms a nearly flat upland.

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Stair-stepped topography and entrenched meanders, northern Arizona.

Yay!

For more images of weathering and erosion, all freely downloadable, please check out my gallery of weathering and erosion photos –or go to the keyword search on my website and type in “weathering” or “erosion” –or both!

 

Iceland –where you can walk a mid-Atlantic rift –and some other geology photos

While Iceland hosts an amazing variety of awesome landscapes, what stands out to me most are its incredible exposures of the Mid-Atlantic ridge. To the north and south, the ridge lies beneath some 2500m of water, forming a rift that separates the North American plate from the Eurasian plate. The rift spreads apart at a rate of some 2.5 cm/year, forming new oceanic lithosphere in the process. But in Iceland, you can actually walk around in it!

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Geologic map of Iceland as compiled from references listed below.

Read more…

Hug Point State Park, Oregon, USA –sea cliffs expose a Miocene delta invaded by lava flows

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Alcove and tidepool at Hug Point

Imagine, some 15 million years ago, basaltic lava flows pouring down a river valley to the coast –and then somehow invading downwards into the sandy sediments of its delta. Today, you can see evidence for these events in the sea cliffs near Hug Point in Oregon. There, numerous basalt dikes and sills invade awesome sandstone exposures of the Astoria Formation, some of which exhibit highly contorted bedding, likely caused by the invading lava. It’s also really beautiful, with numerous alcoves and small sea caves to explore. And at low to medium-low tides, you can walk miles along the sandy beach!

(Click on any of the images to see them at a larger size)

Read more…

Devil’s Punchbowl –Awesome geology on a beautiful Oregon beach

You could teach a geology course at Devil’s Punchbowl, a state park just north of Newport, Oregon. Along this half-mile stretch of beach and rocky tidepools, you see tilted sedimentary rocks, normal faults, an angular unconformity beneath an uplifted marine terrace, invasive lava flows, and of course amazing erosional features typical of Oregon’s spectacular coastline. And every one of these features tells a story. You can click on any of the images below to see them at a larger size.

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View southward from Cape Foulweather to the Devil’s Punchbowl.

 

180629-58ceThe rocks. They’re mostly shallow marine sandstones of the Astoria Formation, deposited in the early part of the Miocene, between about 16.5 to 22 million years ago. The rocks are tilted so you can walk horizontally into younger ones, which tend to be finer grained and more thinly bedded than the rocks below. This change in grain size suggests a gradual deepening of the water level through time. In many places, you can find small deposits of broken clam shells, likely stirred up and scattered during storms –and on the southern edge of the first headland north of the Punchbowl, you can find some spectacular soft-sediment deformation, probably brought on by submarine slumping. Later rock alteration from circulating hot groundwater caused iron sulfide minerals to crystallize within some of the sandstone. Read more…

Grand Canyon Unconformities –and a Cambrian Island

A prominent ledge punctuates the landscape towards the bottom of the Grand Canyon. It’s the Tapeats Sandstone, deposited during the Cambrian Period about 520 million years ago, when the ocean was beginning to encroach on the North American continent, an event called the Cambrian Transgression. Above the ledge, you can see more than 3000 feet of near-horizontal sedimentary rocks, eroded into cliffs and slopes depending on their ability to withstand weathering and erosion. These rocks, deposited during the rest of the Paleozoic Era, are often used to demonstrate the vastness of geologic time–some 300 million years of it.

View of the Grand Canyon from the South Rim trail. Arrows point to the Cambrian Tapeats Sandstone.

View of the Grand Canyon from the South Rim trail. Arrows point to the Tapeats Sandstone.

But the razor-thin surface between the Tapeats and the underlying Proterozoic-age rock reflects the passage of far more geologic time  –about 600 million years where the Tapeats sits on top of the sedimentary rocks of the Grand Canyon Supergroup. Those rocks are easy to spot on the photo above because they contain the bright red rock called the Hakatai Shale. Even more time passed across the surface where the Tapeats sits on top of the 1.7 billion year old metamorphic basement rock. You can put your thumb on the basement and a finger on the Tapeats –and your hand will span 1.2 billion years! Read more…

Sampling New Zealand’s (Amazing) Geology

New Zealand’s landscape can make just about anybody appreciate geology. Its glaciated peaks, its coastline –that ranges from ragged cliffs to sandy beaches to glacial fjords– its active volcanoes… they all work together to shout “Earth Science!” With that in mind, here’s some basics of New Zealand’s amazing geology, followed by some geological highlights of my trip of January and early February, 2018.

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Map of New Zealand, showing accreted terranes in colors and cover assemblage in gray.

North and South Island Bedrock  The different colors on this map show New Zealand’s basement rock, named so because it forms the lowest known bedrock foundation of any given area. The basement tells stories of New Zealand’s deep past, from about 500-100 million years ago. Individual colors signify different terranes, accreted (added) one-by-one through plate motions to the edge of what was then the supercontinent Gondwana. They mostly consist of sedimentary and metamorphosed sedimentary rock, although the narrow belt of purple-colored Dun Mountain Ophiolite formed as oceanic lithosphere, and the red-colored areas consist of granitic igneous rock, some of which has been metamorphosed to gneiss.

Gray indicates the younger cover rock, formed after accretion of the terranes. Consisting of a wide range of sedimentary and volcanic rocks, as well as recently deposited sediment, it’s just as interesting and variable as the terranes. Because it includes volcanoes, it’s largely the cover that gives the North Island its distinctive flair. By contrast, the South Island consists largely of uplifted basement rock, much of which has been –and still is—glaciated. All those long deep lakes, such as Lakes Wanaka and Tekapo, were carved by glaciers and are now floored with their deposits of till.

Andesite stratovolcano, New Zealand

Mt. Ngauruhoe, a 7000 year-old andesite stratocone near Ruapehu on the North Island

Those differences exist largely because the North and South Islands occupy different plate tectonic settings. The North Island sits over a subduction zone, so it hosts an active Read more…

Cove Palisades, Oregon: a tidy short story in the vastness of time

If I were a water skier, I’d go to Lake Billy Chinook at Cove Palisades where I could ski and see amazing geology at the same time. On the other hand, I’d probably keep crashing because the geology is so dramatic! Maybe a canoe would be better.

Lake Billy Chinook, Oregon

View across the Crooked River Arm of Lake Billy Chinook to some of the 1.2 million year old canyon-filling basalt (right) and Deschutes Fm (left). The cliff on the far left of the photo is also part of the 1.2 million year basalt.

The lake itself fills canyons of the Crooked, Deschutes and Metolius Rivers. It backs up behind Round Butte Dam, which blocks the river channel just down from where the rivers merge. The rocks here tell a story of earlier river canyons that occupied the same places as today’s Crooked and Deschutes Rivers. These older canyons were filled by basaltic lava flows that now line some of the walls of today’s canyons.

CovePalisades2From the geologic map, modified from Bishop and Smith, 1990, you can see how the brown-colored canyon-filling basalt, (called the “Intracanyon Basalt”) forms narrow outcrops within today’s Crooked and Deschutes canyon areas. It erupted about 1.2 million years ago and flowed from a vent about 60 miles to the south. You can also see that most of the bedrock (in shades of green) consists of the Deschutes Formation, and that there are a lot of landslides along the canyon sides.

The cross-section at the bottom of the map shows the view along a west-to-east line. Multiple flows of the intracanyon basalt filled the canyon 1.2 million years ago –and since then the river has re-established its channel pretty much in the old canyon. While the map and cross-section views suggest the flows moved down narrow valleys or canyons, you can actually see the canyon edges, several of which are visible right from the road.

Read more…

Geologypics.com– A new (and free) resource for geological photographs

What better way to kick off my new website than to write about it on my blog? To see it, you just need to click on the word “home” in the space above. Or you can click the link: geologypics.com.

Here’s part of the front page:
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As it says, the site offers free downloads for instructors –and for anybody who’s craving a good geology photograph. It’s my way of contributing to geology education –showing off some of our landscape’s amazing stories and providing resources for other folks who want to do the same.

I think the best part of the whole site is that red button in the middle of the home page. It says “Image Search by Keyword”.

Right now, there are more than 2200 images you can search for — all of which are downloadable at resolutions that generally work for powerpoint. If you search for “sea stack” for example, you’ll get 38 hits –and the page will look like this:

Sea Stack search

First page of sea stacks when you search on the term.

 

Notice that ALL the photos are presented as squares–which works for most photos, but not all. To help mitigate that, the photos with vertical or panorama formats say so in their title, so you know to click on them to see the whole image. Take the photo in the upper center, for example –it’s got a  vertical format. Here it is:vertial image

 

A more detailed caption below the photo, along with its ID number appears at the bottom of the pic. This particular image is the chapter opener to the Coast Range in my new book “Roadside Geology of Washington“, which I wrote with Darrel Cowan of University of Washington.

There are also galleries –a chance to browse a variety of images without having to think of keywords. Similar to the search, they’re presented as squares so you need to click on the photo to see the whole thing.

 

Here’s what the photo gallery page looks like (on the left), followed by part of the “glaciation” page you’d see if you clicked on “glaciation”.  Woohoo!

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part of Galleries page (left) and part of Glacial page (right)

 

Then there’s the “About” page, which gives some information about me and details my policies regarding use of the images (basically, you can download freely for your personal, non-commercial use if you give me credit; if you want to use the image in a commercial publication you need to contact me to negotiate fees). There’s also a “News” page, that gives updates on the website. There’s a contact page from which you can send me emails. And the blog? It goes right back to here!

And finally, if you’re looking for a great web designer? Try Kathleen Istudor at Wildwood SEO –she created the site and spent hours coaching me on how to manage it.

Enjoy the site!

 

Summarizing Washington State’s Geology –in 19 photo out-takes

Washington State displays such an incredible array of geologic processes and features that it makes me gasp –which is one reason why writing “Roadside Geology of Washington” was such a wonderful experience. I also got to do it with my long-time friend and colleague (and former thesis advisor at the University of Washington) Darrel Cowan. The book should be on bookshelves in mid-September –and I can’t think of a better way to celebrate than by summarizing Washington’s amazing geology with a bunch of out-take photos –ones that didn’t made it into the book or even to my editor. Like the photo below:

Mount Baker, Washington (150916-4)

Mt. Baker, a glaciated stratovolcano in northern Washington State.

Mount Baker’s a stratovolcano that erupted its way through the metamorphic rock of the North Cascades. I took the photo from the parking lot at a spot called Artist’s Point –at the end of WA 542 –and my editor nixed it because I already had enough snow-capped volcanoes in the book.

On the cross-section below–which includes elements of Oregon as well as Washington, Mt. Baker is represented by the pink volcano-shaped thing labelled “High Cascades”. The following 15 or so photos illustrate most of the other features on the cross-section –so together, they illustrate much of the geology and geologic history of the state!

Cross-section across PNW

Generalized cross-section across Washington and Oregon.

Washington State and geologic provinces

Washington State and geologic provinces.

A quick note about organization: I’m separating the images according to their  physiographic province. There are six in Washington: Coast Range, Puget Lowland, North Cascades, South Cascades, Okanogan Highlands, and Columbia Basin.

 

Coast Range:
As you can see in the cross-section, the Coast Range borders the Cascadia Subduction Zone and consists of three main elements: the Hoh Accretion Assemblage in yellow, Siletzia (called the “Crescent Formation” in Washington) in purple, and the post-accretion sedimentary rock in brown. Siletzia is the oldest. It was thrust over the Hoh Accretion Assemblage, which is still being accreted at the subduction zone. The post-Accretion sedimentary rocks were deposited over the top of Siletzia after it was accreted about 50 million years ago.

And here are some photos! Siletzia formed as an oceanic plateau and so is characterized Read more…

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