Wut?

There was an interesting article in OregonLive last night on a successful campaign to clean up the mess and misuse along a 21-mile stretch of the Molalla River near Portland. It was such a success, in fact, that the run is being seriously considered as an addition to the streams recognized and protected under the National Wild and Scenic Rivers system.

What drew my attention, though, wasn't the story as much as the lead photo. Any geology types have an idea what we're looking at? I'm guessing it was a lava tube or flow; those radial columnar joints suggest a circular outline, at least on the bottom. The bedrock is somewhat obscured by moss and plants above the midline of the structure, and I can't tell from the picture whether the radiating joints continue to make a full circle. Whatever it is, I like.