Button Rock at Rio Viejo

Rio Viejo, Puriscal, Costa Rica
A few days ago I was practicing photography at the river with my new tripod. I wasn't doing anything serious other than learning the possibilities with the new tool. I took several shots of the rapids and then noticed a "button" on a rock. It looks like the same "button" that I found several months ago on the hard rock wall of the gorge. I was very surprised to find another one. Tomorrow I'll share close up photos and reveal what I think it is.