Tuesday, August 14, 2012

Anybody recognize this famous outcrop/roadcut?

Many geologists -- mainly students and other field-trippers -- have pondered and chipped away at this exposure.  Locals may not know what it is, but they do know it's important, having seen how much attention it gets.  But how famous is it really?  Does anyone out there in the greater world recognize it?
Above -- gray dike cutting through equally interesting brownish-ochre host rock -- below.
Some closeups, dike and host rock.

If you need a few hints, continue on ...

Hint #1 -- Where on Google Earth? (click to view)
Hint #2 -- another view of the roadcut
Hint #3 -- near my current study area
Hint #4 -- I'm online courtesy of this very nice little public library:
Now have a look at the answer to this geo-challenge.


  1. Well, I didn't recognize it, but I sure did locate it in Google Earth. It's near the summit of Bear Lodge Mountain. I'd put your thumbtack location at 44.4747N, 104.4419W. I presume the dike is made of similar material to the Tertiary porphyritic phonolite that constitutes Devils Tower (they certainly share a similar porphyritic texture, in any case). Not certain what the country rock is since my Black Hills geologic map is still packed away in storage, but given the location I'd imagine its either one of the Paleozoic (or lowermost Mesozoic) units of the Black Hills uplift.

    How'd I do?

    1. Right on, Ron! I'm at the Sundance library again, throwing together an answer post ... should be up soon.

      Thanks for the Comment.

  2. Thanks! We spent the weekend over Mother's Day 2014 up there. I rode my bike near this outcrop. I knew it was a Tertiary phonolite porphyry. The phenocrysts are a feldspar and it is cutting through the Cretaceous Lakota Fm? signed Anonymous geo from Casper, WY

    1. Thanks for reading and glad you visited the dike! It crosscuts trachyte porphyry of the central part of the Bear Lodge intrusion as well as the huge granite xenolith mentioned above. I should have added the "answer" to this geo-challenge once it was up ... there's now a link at the end. I also included more explanation in that post:

  3. I've spent countless hours in the Bear Lodge Mountains, but have not been back since '93. One of my favorite geological areas by far. I have a few pics of the BLM posted on my website: www.12stoneconsulting.com including one of the very same outcrop back in the day.