Spinning Ice Circle
This is a 100% natural phenomenon. From Wikipedia:
Ice discs form on the outer bends in a river where the accelerating water creates a force called ‘rotational shear’, which breaks off a chunk of ice and twists it around. As the disc rotates, it grinds against surrounding ice — smoothing into a circle.
But imagine randomly encountering one of these in the forest. Bricks would be shat.