If you’re wandering the country on I-90, say to or from Sturgis, SD, a stop at the Devil’s Tower north of Sundance, WY, is very worthwhile.
The identity of this geological oddity is the subject of some disagreement. Three theories of its origin are in play: 1) an igneous stock, 2) a volcanic neck, and 3) the remnants of a laccolith. Whatever the case, it is plain that the sedimentary rock surrounding it has long since eroded away to reveal the more weather resistant igneous rock.
Close up, the columnar structure of the formation is evident. This feature speaks to a slow cooling process, one made possible for a magmatic body deep underground insulated by the surrounding formation.