Here's a weird, great geological feature I spotted yesterday while out hiking in rural Oklahoma. We were out in a flat, flat plan that was dotted with a few tall, angular sandstone mounds and narrow sandstone canyons carved out by erosion. This rock was sticking out of the side of one of the mounds. It was the only place we saw anything like these vertical, tube-like structures, which stretched from the ground up to probably about my shoulder.
When I posted this image on Twitter yesterday, several people suggested that the tubes might be skolithos — tube-shaped fossils that were probably made by some kind of ancient worm creature and turn up sometimes in sandstones. While the pictures on Wikipedia don't look very similar to what I saw, there are apparently lots of different forms these things (and similar tube fossils) can take.