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Chris Rowan is a geologist specialising in the dark arts of paleomagnetism, and getting people to pay him to travel to exotic destinations for fieldwork. Having drilled up New Zealand during his PhD, and South Africa in his first post-doc, he now works at the University of Edinburgh.
Anne Jefferson has a love of all things water-related and blends hydrology, geomorphology, geology, and climate change in her work. She has a Ph.D. from Oregon State University and is now an assistant professor at the University of North Carolina at Charlotte.
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« To BPR3, or not to BPR3? | Main | Imbrication and potholes in the Zebra River »
Category: geology • geopuzzling
Posted on: February 8, 2008 10:55 AM, by Chris Rowan
These boulders are telling us something. What?
For this one, I had the choice of a photo where the answer is quite subtle, and one where it's pretty obvious. Being evil, I chose the former, and am holding the latter in reserve.
Update: Click through for the answer.
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Comments
Current flow (flash flood?) from left to right?
Posted by: cope | February 8, 2008 11:25 AM
Judging by their orientation, I'd say they're telling us something about flow direction - probably in water, since this looks like it could be a dry stream/riverbed.
Hey, where's the object for scale? :)
Posted by: Tuff Cookie | February 8, 2008 11:25 AM
It looks a lot like debris flow/flash flood deposits that I've seen, too. It looks like it's in a channel, too - the foreground is in shadow, but there's bright sunshine in the background, as if there's an embankment behind the photographer.
Posted by: Kim | February 8, 2008 11:34 AM
Saw that in my feed reader before seeing the comments and immediately thought it was a dried riverbed, upstream to the left hand side of the picture. Coming on here and looking at the other comments it appears I am not alone!
Posted by: Julia | February 8, 2008 11:41 AM
They're telling us that Noah's Deluge happened! Just like the Bible says! THE ROCKS CRY OUT! REPENT OF YOUR*whack*
Sorry. Had a momentary talk.origins flashback. I'll take my meds now....
Posted by: Eamon Knight | February 8, 2008 11:43 AM
Alright, you're all very good. But perhaps someone would care to explain to the non-geologists exactly how you know about the current direction?
Eamon: Shhhhh!!
Posted by: Chris Rowan | February 8, 2008 11:46 AM
I'd guess that this also shows the results of a fairly recent erosion event into a stream/river bed. The smaller rocks and gravel in the foreground look to be more embedded and more smoothed by water flow, while the boulders further back have more sharp edges and show some signs/coloration of having been more recently covered with earth.
Just an amateur's guess.
Posted by: chezjake | February 8, 2008 11:47 AM
But perhaps someone would care to explain to the non-geologists exactly how you know about the current direction?
More sand deposition on the downstream side of rocks. Current would tend to erode under the upstream side and deposit in eddies on the downstream side.
Also, forgot to mention above that further evidence of a recent erosion event into an established watercourse is the lack of gravel in the sand to the rear of the photo.
Posted by: chezjake | February 8, 2008 11:53 AM
Imbrication of flatter (more oblate) cobbles -- they're tilted up on one another ("shingled" would be another way of putting it). Because of this pattern of imbrication, it appears that the current which arranged these cobbles was flowing from left to right.
Posted by: Callan Bentley | February 8, 2008 11:56 AM
The larger boulders are midway between angular, and rounded. The angular part means it wasn't terribly far back in time since they broke off their parent rocks. The rounded nature, that they have been involved in one or more flooding events. Its not clear if there is some size sorting verticaly, i.e. are large boulders more common on the bank, or in the stream bed. If they are sorted it might indicate that a lot of small/medium flood events have occurred since the larger boulders were rounded off.
I can't tell about the background boulders (overexposed in the sunlight). There is a hint that they may be rounded as well. That would be interesting if it is true, for it would indicate that a thick layer of rounded boulders is being eroded by the stream. The question would then arise as to the origin of the thick layer of rounded boulders (ancient superflood?).
Posted by: bigTom | February 8, 2008 12:01 PM
In addition to the sand on one side, I would point to the imbrication of some of the flatter slabs of rock ...they are laying somewhat diagonally, sloping in the direction of current flow. I seem to remember something like that from classes thirty years ago.
I am being influenced by peer pressure here. The VERY first thing I thought of was a fresh fault scarp running through an alluvial fan, as there are exposures that look like this in the Alabama Hills of California where a huge earthquake struck in 1872.
Oh, and yes, Noah's Flood too.
Posted by: Garry Hayes | February 8, 2008 12:03 PM
I also see that I write too slow. Callan mentioned imbrication before I could finish my post just now.
Posted by: Garry Hayes | February 8, 2008 12:08 PM
The right side shows evidence of turbulence?
Posted by: rpsms | February 8, 2008 1:03 PM
I've never seen boulders that big imbricated before. Nice photo!
Posted by: Mel | February 8, 2008 4:23 PM
And as a side note on imbrication, for the non-geologists out there: imagine water wooshing from right to left. (Lots and lots of fast-moving water, and/or mud, because the boulders are so big...) If the water came from the right, it would catch the side of the rocks that are sticking out, and the rocks would flip over.
At least, I think that's how imbrication happens.
Posted by: Kim | February 9, 2008 11:15 PM
[snark mode on] You people are so easy. The creationists stacked these rocks outside Boulder to help God confound the unbelievers. Or maybe ranchers created a diversion dam wherever it is; stacking rocks like this makes them more stable. But only if they don't have a tractor and use illegal immigrants much as Pharaoh did. [end snark]
Posted by: Old Bogus | February 10, 2008 9:13 PM