It's a laccolith.

Thanks to That One Guy, we now know that the object below is actually a micro version of the whole Black Hills formation. And that it's also know as "Green Mountain" or "Little Sundance Dome." I like the latter, so as not to confuse it with the Green Mountain Coffee file folder in my mental hard drive.

Here's some more info and a picture of the formation at ground level from an old geology book*. I'm guessing

we're looking roughly south to north.

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*Geology, physical and historical, Volume 2

By Herdman Fitzgerald Cleland