Geology photo quiz #1

Take a look at the photo below, which I took it on the way up to McBride Peak (in McBride, British Columbia). It is a view up the Sunbeam Creek, part of an Ecological Reserve. Question: why would (only) part of the creek be so white? My father-in-law and I had been wondering since a previous hike to the top of the Peak, and speculations were running rampant. Finally, yesterday, we decided to hike up to the top again, then go down to the creek to find out. I think we did, and it was a great hike and a lot of fun. I am in the process of writing a nice post on this geo-adventure, but I though in the meantime I’d post the photo and make it a quiz.


I will give readers two clues:

1) the mysterious white “stuff” sits in a creek where water is actually running;

2) this is a south face, exposed to the sun all day long, so it couldn’t be snow or ice.

Below is a close-up photo. So, what do you think it is?
Or at least, what do you think it could be?


6 thoughts on “Geology photo quiz #1

  1. I want to say “soap suds”, but maybe some sort of runoff from the vegetation? Unless, it being a glaciated terrain, there is some kind of local rock flour that actually floats….?

  2. The change is stream and bank colouration is likely due to changes in stream water pH. The uphill part of the drainage is red-orange – likely due to oxidation of pyritic rocks. Although it is not evident, the creek water and bed are red, brown or orange. The water and bed colour probably changes abruptly to a milky-white when the stream crosses a carbonate bed. The acidic water is buffered by the carbonate and calcium is then precipitated.
    In some cases, the whitish colour can be caused by the preciptaion of zinc oxides and carbonates. It makes for a good prospecting tool in areas that do not have a lot of carbonates.

  3. Hi there,
    I found your blog via a google search trying to answer the very same question: Why the heck is the bed of Sunbeam Creek so white? Looks like this thread is long dead, but I’ll fling my questions into the ether nonetheless.

    I came across this creek this week when a thunderstorm forced my hiking partner and I off the ridge and into the treeline. We’d seen it from afar and assumed the colour was just due to whitewater or residual snow, and it wasn’t until we were much closer that we realized how odd it was. I confess to poking the white coating on the rocks with a finger and discovering it to be slimy and easily removed to expose the smooth black rocks beneath it. In the name of science I also tasted the water; it was sour like an acid.

    True to our respective backgrounds (geology and biology) my hiking partner and I each had different theories as to what might cause the phenomena, both revolving around dissolved calcium precipitating out of acidic water. We guessed that it was precipitating either upon contact with a more basic or buffering rock, or, that it was being biologically precipitated by an extremophile algae. Both of us admitted that we were completely grasping at straws. I’d be very interested in what conclusion you came to!

  4. I don’t know that threads are ever really dead these days. 🙂
    I can’t believe it’s seven years since I left my comment…
    I have a bit of a background in geology as well and the answer escaped me. I have seen milt water in waterways in karst terrain but nothing actually white. I assumed it was some sort of cryptocrystalline sediment load. That there might be a biological answer is really interesting.
    The sour taste is most interesting. Almost certainly biological in origin.
    I’ll ask an algae specialist I know and report back.

  5. Hello Steve and Caitlin: thanks for your comments. This really is one of those projects that does not see the end. I had pictures and some maps and some notes, but it all got lost once I moved back to Calgary. At the time I had come to similar conclusion to what David commented, that these were carbonate deposits due to interaction of water due to an abrupt change in pH.

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