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