Classification of Sedimentary Rocks
Our classification of sedimentary rocks is a bit of a hodge-podge. We first consider whether a rock is clastic (made from pieces or clasts of older rocks) or chemically precipitated (deposited from chemicals dissolved in water). This subdivision is not always satisfactory—a sea shell is a chemical precipitate because the animal pulled the material in its shell from the water, but a limestone made up of sea shells might be called clastic because the sea shells are chunks. Usually, people consider limestones and evaporites (rocks left by the evaporation of water containing salts) to be chemical precipitates, and all others to be clastics.
Clastics are classified further based primarily on grain size. The very smallest particles of clay make claystone, also called shale. Slightly coarser pieces are silt and make siltstone. Coarser still is sand, which makes sandstone. Going to still-bigger clasts, cobbles, and boulders produce cobblestone and boulder-stone, but we also call both of these conglomerates.
Conglomerate within a Conglomerate: Sevier Fault near Bryce Canyon National Park
Geologists read rocks, and the stories are fascinating--historical novels full of intrigue. In this next GeoClip, Dave Janesko and Dr. Alley perch high up in Red Canyon just west of Bryce and read one of those stories of deep time.
A conglomerate is a sedimentary rock in which many of the clasts are bigger than sand. Dave and Dr. Alley are looking at a conglomerate that includes many different clast types, including one that is itself a finer-grained conglomerate. The clasts in that conglomerate-within-a-conglomerate include several types of sedimentary rocks, including sandstones that are themselves made from older pieces.
Video: Conglomerates, Sevier Fault, Dixie National Forest (3:05)
Um, maybe. I can kind of break it out.
No, that's a conglomerate, isn't it?
Yeah. It is. Look at that, babe. That's the conglomerate we were looking for. Just look at that! Isn't that a beauty?
It is.
A conglomerate in a conglomerate. What do you make of the class in the little one?
It looks like sandstone and limestone to me.
Deep time. Deep time!
Very deep.
We got deep time! This is just beautiful.
A conglomerate classed in a conglomerate?
A conglomerate, you have to have sand grains glued together to make sandstone, broken and rolled to round them off, because this is rounded. And you have to have an ocean or a lake that has things living in it, precipitating calcium carbonate to make a limestone, broken loose and rounded off. And then those have to get together in a beach or in a stream channel. They have to be deposited with sand around them, then they have to be cemented together with hard water deposits. Then that has to be broken out, rolled in a stream and rounded, put together with all these other rocks coming from every which way. All of those then have to be glued together with hard water deposits. And then we have to get from the stream or the lake or whatever it was that was making these, up here in the middle of the desert on a cliff. And it's all in that one rock right there. And there's really no other way to explain that rock.
In geology, we often meet people who tell us that we do not know what we're talking about, that the world looks like it is very, very young, that there's nothing in the world older than written history. We never tell those people that they're wrong, that they're false, that they've been led astray. But we can tell these people, it looks old, that the scientific interpretation, the best way to look at these rocks, is that they tell of vast, deep time. So when one looks at a rock like this one that has little tiny remnants of old rocks glued together into a rock which is bounced around in the stream and rounded, that then is glued in with a whole bunch of other rocks into another rock that has been bounced around in a stream and rounded, that is sitting here glued with a bunch of these things on a cliff in the desert-- it is very hard to imagine how this could be something that is only 6,000 years old. And so all we do in science is, what can we observe? What can we test? And what we can observe, what we can test says, really, really old.