Geo-Mystery on the Duke Ellington Bridge
To walk from the Woodley Park neighborhood of DC to my neighborhood (Adams-Morgan), you have to cross the deep gorge of the Rock Creek Valley. To do this, walk east on Calvert Street over the Duke Ellington Bridge.
My question for you: What's up with the coloration of these exposures? Why are they black on top and white on bottom? It's the same rock (Indiana limestone), so why the difference in color?
First person to post the correct answer in the comments section below gets a "GEOLOGY ROCKS" bumper sticker as a reward. Full answers tomorrow in a separate post...
Here's something you might notice as you walk over the bridge:
My question for you: What's up with the coloration of these exposures? Why are they black on top and white on bottom? It's the same rock (Indiana limestone), so why the difference in color?
First person to post the correct answer in the comments section below gets a "GEOLOGY ROCKS" bumper sticker as a reward. Full answers tomorrow in a separate post...
5 Comments:
You said that it was because the cleaning person could not reach (bleach) the upper level of that rock. Other possibilities: age/time, temperature, pressure, weathering/moisture, etc.
I might guess that a major flood scoured the lower parts, but one of the photos shows some blackened limestone below the clean, white stuff.
Hmmmm.
Is it because of graffiti cleanup? No one ever tags higher than the top of the what part, so the rock above that point never gets sanded clean?
I think it's some kind of algal or fungal growth that causes the black staining - with the lower parts having been cleaned or bleached somehow - an incomplete cleaning.
Erik gets it!
Anonymous apparently attended my "History before history" walking tour of DC's geology, so I'm going to nix that answer on account of prior specific knowledge. (But also because they are anonymous, and where would I send the bumper sticker?)
And Erik specifically mentioned graffiti, so he gets it. Congrats!
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