Imbrication and potholes in the Zebra River

I’m not feeling at my best today (I’ve been laid low over the last couple of days with a mild fever), but I thought that that a brief discussion of Friday’s puzzle was in order. As most of you correctly guessed, the photo was taken in a river channel, and many of the boulders in the photo show signs of being imbricated, which is a fabric generated by the effects of strong currents on flattened or disc-shaped clasts, with large flat surfaces. When such clasts are pointing into a current, then the water will strongly push on the flat face and flip them over so that they point downstream. After this, the current just pushes them down into the stream bed; so over time you tend to get all the flattened clasts pointing stacked on top of each other and pointing downstream.

clast imbrication


Thus Friday’s picture clearly shows a current flowing from left to right across the image. Chezjake astutely noted that the sand underneath the boulders has also been winnowed away on their left sides, which is another indication of the same current direction.
The photo is from the Zebra River in Western Namibia, which I visited last May. What impressed me was the sheer size of the boulders being stacked up in this way, which pointed to some very strong current flow.

clast imbrication, Zebra River

The raging torrent required to achieve all this was a marked contrast with the completely dry riverbed that we were walking down in search of stromatolites. The river is probably seasonal (like many in this part of the world) but I suspect that most of the larger rocks only get moved around in the extremely occasional flash flood. Perhaps we should have taken more note of this geological warning sign, and not camped in the river bed*. Either way, this illustrates one of the really cool things about geology: you might head off to look for one thing, and end up finding lots of other interesting, and yet completely unrelated, things as well.
Here was another cool modern sedimentary feature from the same part of the river, which again indicates strong currents: a pothole.

pothole.jpg

This is where a small stone gets trapped in an eddy on the river bottom and scours downwards into exposed bedrock.That may or may not be the actual scouring stone in the bottom, although I’d like to think that it might be.
*in our defence, it was the dry season.

Categories: basics, geology, geopuzzling

Comments (5)

  1. Lassi Hippel‰inen says:

    That kind of landscape is quite common here in Finland. The culprit is the last Ice Age. I’m not familiar with the the geology of Namibia, but it is a fairly high plateau, so I would expect that the Ice Age left some marks there as well.

  2. NJ says:

    What impressed me was the sheer size of the boulders being stacked up in this way, which pointed to some very strong current flow.

    When I was in grad school, some colleagues went on a photo shoot of the effects of a significant flood on the Roanoke River in Roanoke, VA. They got some excellent pics of imbricate cars on the floodplain.

  3. Richard Simons says:

    During the rainy season there can be extremely fast-flowing torrents of opaque, orange-brown water in Namibian river beds. The flows typically last for two or three days once or twice in a ‘rainy’ season (it is rainy only by comparison with the dry season). Many riverbeds consist of extremely loose coarse sand and gravel between patches of boulders similar to the photo. Incidentally, the only permanent waterways are on the borders of the country (plus one that cuts across the base of the Caprivi Strip, the panhandle) and the largest natural lake is a sinkhole about the size of a soccer field.
    Lassi, I do not know of any evidence of glaciation there, but I am not a geologist. I imagine it would be a fascinating place for any visiting geologist.
    OT: A person who lives by Zebra River told me he once came across an accident in which two cars had collided at a junction. No-one was hurt but neither car was drivable. They had been waiting 5 1/4 hours for someone else to come along.

  4. Chris Rowan says:

    Lassi – I don’t think that Namibia has been glaciated since the Permian; I could see how glacial till, with its mix of boulders and finer sediments dropped by melting glaciers, might look superficially similar, but I suspect the processes involved were very different (does anyone know if ice flow can cause imbrication? I don’t see why it shouldn’t, but whether it would survive the melting process is the question).
    NJ – Aha! Anthropocene deposits! Don’t suppose you could get your hands on those pictures?
    Richard – the wait really doesn’t surprise me. Having experienced African driving, I suppose that two cars managing to hit each other in the middle of a desert shouldn’t surprise me either…

  5. NJ says:

    My best lead on the pics would be Ed Simpson at Kutztown State in PA. If you’re seriously interested, I can drop him a line.