What is that round thing in the rock?

Kevin Middleton sent us a message with this photo and asked about the round feature in the top right of the cliff. Here's what our expert research geologist, Mark Hudson (USGS), had to say about it:

From the overhang and the knobby nature of the outcrop I would guess that this is a middle Bloyd sandstone cliff. In that context (i.e., porous sandstone) it is most likely that circular patterns are iron concentrations (Liesegang bands) formed from groundwater seepage that make the bands more resistant to weathering and stick out. I don't commonly see them quite this circular, so this is an interesting example. Of course one would like to get a closer look to verify the guess.

Send us your geoscience questions we'll do our best to answer! Thanks for sharing this with us, Kevin!