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A Landscape in a Hand Sample: To Settle

We began in fire. Let’s quench that fire with a little water. Sedimentary rocks don’t always form in water, mind, but many of them do.

Sedimentary

Cobble of the Astoria Formation from near Otter Crest and Devil’s Punchbowl, Oregon.

I’m cheating a little bit. This isn’t just a very nice piece of sandstone, it’s one with some apparent Liesegang banding. But that’s the charm of sedimentary rocks: while some of them can look quite plain, others have lovely patterns, either formed by the sediments themselves or later chemical and mechanical weathering processes.

So what’s a sedimentary rock? I’ve a bit of the philologist in me, so let’s look at its root: the Latin word sedere: to sit, to settle. (Don’t even talk to me about -ment, which comes from mentum, which means “chin.” How the word for chin ended up becoming a suffix that forms nouns is beyond me. Guess I should’ve gone to college for that philology degree after all.) Right, so we have something sitting, settling. Which is what sediment does best. Whether blown by the wind or carried by water, it eventually settles down: sand, silt, clay, mud, pebbles, gravel – even boulders – sediments all, come to rest. Given time, some pressure, and perhaps a nice bit of stuff like silica or calcite to cement it, sediment will become sedimentary rock.

Sedimentary rock happens in threes. We have three ways of forming it:

Compaction, in which sediments are squeezed down nice and tight by the weight of more sediment piled on top, eventually pressed so hard they turn to stone.

Cementation, in which sediments are basically glued together by the aforementioned silica, calcite, or some other binding stuff, which fills in the spaces around the clasts and turns them to stone. You can see cementation in action by watching people pour – drumroll please – cement.

Recrystallization, in which the original mineral grains of the sediments form new minerals. Unstable minerals change into something more stable, and those new minerals form up nice and tight.

And we have three basic kinds of sedimentary rock:

 

 

 

Clastic sedimentary rocks, which are formed of little bits of things like sand, silt, and so forth. They can include big bits, like cobbles and pebbles and, yes, even boulders sometimes. The minerals and other bits are clasts. Ergo, clastic sedimentary rocks. Your sandstones, claystones, conglomerates, and breccias belong in this group. Our hand sample is a clastic sedimentary rock.

Organic sedimentary rocks, which form from once-living things. Limestone and chert are in this group, formed from the calcite and silica shells of plankton and other sea critters. Coal’s also included in this group, all grades except for anthracite, which has been through enough in its long life to qualify as metamorphic. When you think organic sedimentary rocks, don’t leave plants out of it!

Chemical sedimentary rocks, which are precipitated from solution. If you dissolve limestone in water, for instance, say in a lake, and so much dissolves that the water can’t hold it anymore, it’ll start collecting in a sludge, which will eventually be reborn as (drumroll, please) limestone. Yep. Limestone can either be organic or chemical, depending on how it formed. Gypsum is another type of chemical sedimentary rock, and so is rock salt. Yes, seriously: rock salt is, in fact, a rock. You can have your rock and eat it, too! In small quantities, mind.

Now, I know what you’re thinking: sedimentary, sedentary, what a yawnfest, Dana. Or maybe you’re not thinking that, because you know that even the most dull-looking sedimentary rock can tell us all sorts of dramatic things. There are rocks in Arizona that talk about shallow seas and dinosaurs and oceans of wind-blown sand, that bear witness to hundreds of millions of years, and they form all the prettiest colors of that magnificent sliced-open layer cake of geology that is the Grand Canyon. I dare you to find sedimentary rocks boring after standing on the rim of that chasm. But the Grand Canyon gets all the press. We’re going to head to the Oregon coast, where it’s not just basalt that forms dramatic sea cliffs.

Oregon Coast at Devil’s Punchbowl and Otter Crest.

Right? This place, Devil’s Punchbowl and Otter Crest, is phenomenal. For one thing, you have these sharp cliffs carved out of the Astoria Formation (pdf), which is a sandstone and siltstone bit of yum that contains fossils and all sorts of other delights. 15.5 million years ago, it was chilling under the ocean close to shore, minding its own business, providing a home to all sorts of sea critters, with beautiful blue waves breaking overhead. Then the Columbia River Basalts invaded. Things got interesting. The Astoria Formation survived, perhaps even thrived – there’s some speculation that it forms such lovely cliffs here because it got baked nice and hard by all that warm basalt.

Now, after all the drama of 15.5 mya, and all the millions of years of uplift and weathering since, the cliffs are stark, wild, and wonderful.

Cliff and sea cave. You can see sediment of all sizes here, from sand and mud to boulders,

So much for boring ol’ sedentary sediments, eh? And just wait ’til you see what tops that.

 

Previously published at Scientific American/Rosetta Stones.

Comments

  1. says

    I was at the Devil’s Punchbowl a few years back. I just thought it was “pretty”. Who knew it was so interesting?

  2. thztds says

    I was just at the Devil’s Punchbowl last week. It’s great to have this additional info to go along with my pictures. Unfortunately, I hit the punchbowl at low tide, so it wasn’t quite as dramatic as it could be.

  3. says

    While similar to the word for “chin”, the suffix is actually a different word, admentum, which was used to create in Latin what in other languages would be the instrumental case. It adds a sense of “by means of” or “the result of.” The proper etymology for “sediment” is “because of settling.”

    It is found in a lot of other words:
    “fundament” = “by way of sitting”
    “firmament” = “by way of hardening” (based on the belief that the sky marked a point where air became solid)
    “government” = “the result of ruling”
    “movement” = “the result of setting into motion”
    “statement” = “by way of standing” (to speak in an assembly)
    “fragment” = “because of breaking”
    “enlightenment” = “the result of coming to the light”
    “parliament” = “by way of conversing”
    “clement” = “because of gentleness”

    (Sorry, I’m something of a language geek.)

  4. F [nucular nyandrothol] says

    Sedimentary rock is never boring, but it can seem that way when you live of thick layers of nearly-all-the-same stuff. Mostly, too familiar, I expect, and I want to know more about what’s going on in the basement. But boring, it is not.

  5. rq says

    I did not know coal was a sedimentary rock. Also, was F said @5.
    Not boring. In some ways, I think sedimentary rock is the most interesting kind of rock, since it forms out of supposedly slow and undramatic processes, unlike all that igneous stuff. Plus, in a sense it’s the most ‘dynamic’ type of rock, because it seems to weather most easily, forming into all kinds of delicious shapes and forms that are artistic, abstract, and totally random.
    Thanks for the ree-vyoo!

  6. grignon says

    The Atomic Caveman got it right. In northern Illinois, it’s silurian dolomite, all the way down. A marble of chert, a squish of proto-oil or -”GASP!!!” a hint of non re-crystallized fossil is the prairie equivalent of, I dunno, finding a previously undescribed terrane in the northwest. Shit like that seems to be just layin’ around.

    I almost wish you’d stop going on about all the amazing and unique and remarkable. But please don’t.

  7. Funny Diva says

    Late to this party, but

    Ohhhhh! If you ever get to visit Montana de Oro State park, near Morro Bay in California, you’ll have a whole ‘nother post about sedimentary coastline! It’s incredible. You’d spend HOURS just going a mile along the bluff and scrambling down at all the reasonably safe scramblingy-downey spots!

    I went to Uni near there. It was a fave place of mine. Avila Beach was just tooooo boring.