# What is Cayuse Basin? **URL:** \ **Category:** Factual Questions\ **Created:** [November 27, 2016, 5:15pm UTC](https://boards.straightdope.com/t/what-is-cayuse-basin/773182 "2016-11-27T17:15:18Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Strinka](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/strinka/32/512_2.png) [@Strinka](https://boards.straightdope.com/u/Strinka)\ **Post date:** [November 27, 2016, 5:15pm UTC](https://boards.straightdope.com/t/what-is-cayuse-basin/773182/1 "2016-11-27T17:15:18Z")
While flying home for Thanksgiving, I saw [this interesting geological formation](http://imgur.com/X0IwKk1) out the airplane window. I was able to find it on [Google Maps](https://goo.gl/maps/BKWNUuQkzkC2), which labels it as “Cayuse Basin”. The only information I’ve been able to find about it is [this blog post](http://blogs.agu.org/magmacumlaude/2008/02/25/sleepless-in-seattle/), which says it’s a “breached structural dome”, but it doesn’t explain what that means. Does anyone here know anything more about it? What is it, how did it form, how old is it? ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [November 27, 2016, 5:35pm UTC](https://boards.straightdope.com/t/what-is-cayuse-basin/773182/2 "2016-11-27T17:35:57Z")
Basically, the underlying rock was dissolved away and the roof collapsed. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [November 27, 2016, 8:32pm UTC](https://boards.straightdope.com/t/what-is-cayuse-basin/773182/3 "2016-11-27T20:32:08Z")
[https://en.wikipedia.org/wiki/Dome\_(geology)](https://en.wikipedia.org/wiki/Dome_(geology)) I really have nothing to add to this. I don’t think **bob++** is correct, though. ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [November 28, 2016, 2:54pm UTC](https://boards.straightdope.com/t/what-is-cayuse-basin/773182/4 "2016-11-28T14:54:54Z")
I would’ve pegged that for a volcanic caldera. Is that in a volcanic/tectonic active area? ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [November 29, 2016, 11:41am UTC](https://boards.straightdope.com/t/what-is-cayuse-basin/773182/5 "2016-11-29T11:41:31Z")
Bob is incomplete. Basically its a laccolith. Its listed here in this doc about lacoliths. > **[Laccoliths](https://books.google.com.au/books?id=w2AOJRZDMfQC&pg=PA86&lpg=PA86&dq=cayuse%2Bbasin&source=bl&ots=0BChOOr2Nr&sig=yNvwMI-YU6OQchq2ekQNaPLkjd8&hl=en&sa=X&ved=0ahUKEwiqzNGT8M3QAhWKuY8KHR5WBq8Q6AEIPzAG#v=onepage&q=cayuse%20basin&f=false)** laccolith is a dome formed by magma… so then this cayuse basin is where what bob said happened to the dome… the insides washed away and so it collapsed, leaving only the base/ring of the dome … a caldera is formed where the rock strata melts and sags into a lot of magma…kind of opposite… the dome is where the magma pushes the rock strata up. its only a bit north of yellowstone ffs. of course its volcanic in origin. ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [November 29, 2016, 12:49pm UTC](https://boards.straightdope.com/t/what-is-cayuse-basin/773182/6 "2016-11-29T12:49:08Z")
The basin itself isn’t itself a laccolith, it’s a dome of[sedimentary rock](https://books.google.co.za/books?id=aKcPAAAAIAAJ&pg=PA458&lpg=PA458&dq=cayuse+basin+geology&source=bl&ots=q__mskn4EE&sig=4Q5uSn9ZUB0kgUhw2sRtGxpuUX8&hl=en&sa=X&ved=0ahUKEwiwjYKC883QAhWnKMAKHXtzABUQ6AEIMTAH#v=onepage&q=cayuse%20basin%20geology&f=false) (most likely) formed by the intrusion of a laccolith underneath. This intrusion would have stressed and fractured the overlying rock, making it preferentially erode and leaving a ring. This would make it a breached dome but not a breached _structural_ dome (that’s from intersecting folds). IT’d only be a laccolith if there was an intrusive core but the [geological map of Montana](https://mrdata.usgs.gov/geology/state/map.html?x=-109.645565979702&y=47.0344825672421&z=6#)just shows sediments and a single dyke. As to how old the actual erosion of the landform or the intrusion is, I don’t know, but the actual sediments that make up the basin are roughly 325-330 million years old.