A trip to Death Valley over Thanksgiving two weeks ago reignited all sorts of things in my brain, one of which being my love of conglomerate. Honestly, conglomerate HAS to be the coolest rock!
Just look at this stuff! Just like any good clastic sedimentary rock, it consists of particles of older rock–but with conglomerate, you can easily see those particles. Each of those particles opens a different door to experiencing deep geologic time.
As an example, look at the conglomerate below, from the Kootenai Formation of SW Montana. It contains many different cobbles of light gray and dark gray quartzite and pebbles of black chert. The quartzite came the Quadrant Formation and chert from the Phosphoria Formation. So just at first glance, you can see that this conglomerate in the Kootenai contains actual pieces of two other older rock units.
But consider this: The Quadrant formed as coastal sand dunes during the Pennsylvanian Period, between about 320-300 million years ago and the Phosphoria chert accumulated in a deep marine environment during the Permian, from about 300-250 million years ago. The Kootenai formed as river deposits during the early part of the Cretaceous Period, about 120 million years ago. All those are now together as one.
Similar to the modern river below (except for the glaciers), the Kootenai rivers transported gravel away from highlands –the highlands being made of much older rock that was uplifted and exposed to erosion. That older rock speaks to long gone periods of Earth history while the gravel speaks to the day it’s deposited.
But this is where my head starts to spin: the modern gravel is made of rounded fragments of old rock –so when you look at a conglomerate, you glimpse at least two time periods at once: you see the conglomerate, which reflects a river or alluvial fan –or any environment near a bedrock source– and you also see the particles, which formed in even older environments.
And it gets worse –or better. What happens when you see a conglomerate eroding? The conglomerate is breaking up into modern sediment, which consists of pieces of older sediment –that at one time was modern sediment that used to be older sediment? Look at the pebbles below. I keep them in a rusty metal camping cup on a table in my office.
These stream pebbles eroded out of the Kootenai conglomerate. So… they’re simultaneously modern stream pebbles and ancient ones –AND… they originated as the Quadrant and Phosphoria Formations. Four periods of time, spanning 300 million years, all come together at once.
And if that’s not enough, those conglomerates in Death Valley? They contain particles of… conglomerate! Look! The arrow in the left photo points to the boulder of conglomerate on the right. If you click on the photos, you can see them enlarged.
All those particles, which are now eroding and becoming modern sediment, were yesterday’s sediment. And the conglomerate boulder? It too is becoming “modern sediment” and it too was “yesterday’s sediment” when it was deposited on an alluvial fan with the rest of the material. However, it goes a step further: its pebbles and cobbles were both “modern” and “yesterday’s” sediment at a still older time. And before that? Those pebbles and cobbles eroded from even older rock units, some of which date from the Cambrian, about 500 million years ago.
For fun, here’s a photo of another conglomerate boulder.
I can’t help but wonder how Young Earth Creationists would deal with these rocks. Given their story of the Grand Canyon, in which the Paleozoic section was deposited during early stages of “The Flood” and the canyon was carved during the later stages (they really do say that too!), they’d probably roll out that same blanket answer: The Flood. End of discussion. No questioning, no wondering.
In my opinion, one of the beautiful things about geology is that we’re always questioning and wondering.
for more geology photos, please visit my website.
Reblogged this on Primate's Progress and commented:
Compare the same author’s “Rockin’ Countertops”, which I reblogged earlier as “The amazing geology of a ‘granite’ kitchen countertop”. In both cases, generation after generation of rock formation; time made visible.
Wow… thanks for re-posting!
“I can’t help but wonder how Young Earth Creationists would deal with these rocks. Given their story of the Grand Canyon, in which the Paleozoic section was deposited during early stages of “The Flood” . . . “
Ha! I used to be one . . . I still own (ironically) a textbook from my theology degree coursework, “The Genesis Flood” by Henry Morris. Of course now I think it’s all gibberish, but at the time in 1971 I thought it was a pretty sane explanation. Now that I know a bit of REAL geology, with hindsight it was (and still is) ridiculous how YEC’ers try to twist and cram the evidence into their narrative, created by bronze-age sheepherders.
For a geology novice like myself, conglomerates are among the most fascinating, but most confusing parts of geology. Thanks for this wonderful post! I understand a fair amount more now as a result . . .
Thanks for the thoughtful comment! It always amazes me how the YEC “scientists” can come off as so convincing if you don’t already understand some of the basics of geology.
Marli, Word Press wasn’t able to deliver my reply; they suggested I try here.
It is always a delight to read and “see” your work. And educational as well!
A rock near hwy. 140 has long captivated the imagination; perhaps now I can better understand its “story”? I’ll give it a try. Thank you.
LikeLiked by 1 person
Thanks for another great post.
Conglomerates: yes. I used the several geological processes evident in conglomerates to try to impress upon my Geo101 students the profundity of deep time. I also once used carefully chosen hand samples of conglomerate as the basis for the entire final exam: “Describe the five [or six or seven or whatever] processes that went in to making this rock….” Students hated the exam, I really hated grading it, so I didn’t repeat.
Anyhow, thanks again.
Matthew von der Ahe
Thanks Matthew! Sounds like an interesting exam actually! 🙂
I was introduced to your work by Harley Gordon, one of your former Structural Geo students. We met on a mapping project of the Tieton Andesite Flows. Harley is the son-in-law of Daryl Gussey, who leads the work. I volunteer on the project as well. I have no formal schooling in Geology, but I have an unbridled love for the subject. Please keep up the excellent work because I find your blog a most helpful aid to my learning. Cheers! John Lasher
Thanks for the nice comment John! And please say hi to Harley!