Quantcast

«

»

Oct 03 2012

Geopuzzle: One of These Three Postulations is Right Out

Those informational signs at various attractions can sometimes be more aptly described as mis-informational. This tends to frustrate the geotraveler: responses may include groans, gripes, and rolling eyes. One severely-annoyed geologist at Summer Lake, Oregon took matters (and a Sharpie) into their own hands, and engaged in a little correcting-their-fieldwork.

Summer Lake Sign

Close up of the correction.

If it’s educational, is it still vandalism – or a public service?

One sign that’s just begging for the Sharpie treatment is this one at Deception Falls.

Mis-Informational sign at Deception Falls

The relevant bits. It reads: No one knows for sure why the river turns.1. Was the river diverted by the massive stack of logs piled during a flood?

2. Could the water be following a layer of soft rock that runs at right angles to the stream’s original course?

3. Does the stream turn right to flow along a fault line? Fault rocks erode easily since they are cracked and damaged when the earth moves.

It’s displayed at a fascinating part of the Tye River, where it goes from a broadish (if rocky) channel

The Tye River upstream from the falls.

to a rather vigorous falls

The falls (which actually aren’t Deception Falls. There are lots of falls at Deception Falls).

and then makes a razor-sharp 90° turn and flows through a narrow, straight grandiorite channel for a bit.

Hairpin turn. Rivers don’t normally cut angles like that…

Wow, right? That’s some dramatic geology, that is. Rivers don’t usually do that.

The sign gives three speculations as to how this odd feature formed. See if you can spot the laughably ridiculous one:

1. Log Jam: Did logs carried down and crammed in during a maclargehuge flood dam the river long enough to divert it 90°?

Logs tossed about by the river and left stranded in the dry season.

2. Dike. Did the river erode away a dike of softer rock?

Small andesite dikes cutting through bedrock. There are several large dikes around the area as well.

3. Fault. Did the river attack the crushed and broken rock of a fault, finding it easier to remove than unbroken bedrock?

Flowing through a fault, perhaps?

Bonus geoblogosphere points shall be awarded to those who make a case for one or both of the remaining options.

Overview of the whole puzzle.

And, for those who like falling water, you can haz some video I shot.

Lovely, isn’t it? Shame about the sign. Who’s bringing the Sharpie for our next visit?

9 comments

Skip to comment form

  1. 1
    rq

    I can bring the Sharpie, but it’ll be a while before I make it out there, so you’ll have to get someone else. Not an issue, I think.
    Anyway, those logs just don’t seem to be sitting just right to tilt that river the full 90 degrees. Also, I doubt there’s a species of lignum plants out there that can outlast the pressure of a river, long enough to divert it. So, my guess is that Option 1 (“Log Jam”) is out.

    As for the others, I’m going to cheat a bit. If I recall correctly, you had a post up about these falls not-so-long-ago, which I cannot find at the moment, but it had information on both items, I think. I remember information about the dikes, which seems like a plausible one, except that’s one huge dike, in that case (not impossible, right?). :)
    I’m going to try to find that post and see what you actually said.

    (Oh but for the record, the Fault Option also seems plausible, even a bit more so, because 90 degrees is a lot, and it seems difficult for a river to be diverted at that angle within such narrow confines over a very long period of time – a fault might appear more suddenly, hence create a sharper angle by creating an easier path for the river to follow… Also, from the photos, it appears that the two sides of the stream’s rocky walls could fit together – or is that my imagination?)

  2. 2
    Trebuchet

    Does the “old channel” depicted in the fault option exist?

  3. 3
    lochaber

    I’m guessing that there was a sudden fault slip, creating a noticeable scarp at some point.
    Due to gravity and all, instead of climbing the scarp, the water just flowed along it instead.

    I don’t think the log jam makes much sense, and I can’t see how a river can erode softer bedrock without flowing over it in the first place

  4. 4
    Lithified Detritus

    The logjam option is pretty clearly someone just babbling.

    I’m going to go with the eroded dike.I looked through the pictures before I had read the post carefully, and when I got to picture #10 I thought “Wow, looks almost like there was a dike that ran through there.” This because the sides are parallel and equidistant from each other. There are also visible intrusions on the channel wall.

    If the dike was less resistant to weathering than the surrounding rock, it would eventually form a lower path for the water, and the flow would follow it.

    This would obviously be falsified if offset could be shown on the opposite sides of the stream, or if it makes another 90° turn to the left farther on – something I can’t tell from the pictures.

  5. 5
    F [i'm not here, i'm gone]

    An andesite intrusion into granodiorite country rock just makes my brain itch. If the granodiorite was a prior intrusion, it reduces the itching somewhat.

    Then the thought that a dike will, you know, form in a fault decidedly did not help. I don’t think I expect this potential nuance is relevant to the question, though.

    I have no idea about the relative ages and other bits of geological lore for this area, but you could also have a fault or old erosional feature or fold that was filled with soft or unconsolidated sediment that a younger river excavated and eventually diverted itself in a hyooj flud wit teh loggz. Scaryfast erosion can happen at all scales.

    So I choose gremlins. They did it. You’d think it was dwarves, but no.

    1. 5.1
      rq

      Yeah, gremlins. Dwarves usually do their work underground, if previous experience is anything to go by.

  6. 6
    Ann

    The logs is out, too much rock has been eroded down for that to be believable since the wood would weather away first.
    The fault idea is more plausible, but the trouble with that there should be more evidence of a river bed on the left side of the outcrop. Were you able to get to the other side and look for rounded rocks and grading in them? Or did you see other evidence of a channel in the area?
    I’m going with the water eroded a softer dike in the area. I see white streaks in some of the rocks which shows its been fractured and has intrusions in it.
    I’m surprised they didn’t come up with a glaciation theory- like a glacier was once there and left a big boulder blocking the path forcing the water to go around it. The boulder has since eroded or been moved down stream but the river continues to flow in the channel created. More believable than logs.
    Or maybe it was the dwarfs tunneling in the area and their shaft collapsed creating that channel to flow into.

    1. 6.1
      F [i'm not here, i'm gone]

      The fault idea is more plausible, but the trouble with that there should be more evidence of a river bed on the left side of the outcrop.

      Yeah, it’s there. I can’t find anything that shows just how far it goes, or how old it is. For all I know, the other bed could be caused by occasional flooding, and the current channel was always the main.

  7. 7
    Brian Lang

    This reminds me of strongly of the stream that follows a fault line in Glacier National Park in Montana. Sunrift Gorge is where Baring Creek cuts a virtually straight line for 150m or more. If I recall correctly, the signage reveals that the creek is following a fault line.
    I did a quick Google search and found this photo of Sunrift Gorge: http://travels-with-nancy.blogspot.ca/2010/09/glacier-national-park.html

Comments have been disabled.