Gorge Button - Rio Viejo

white circular lump on rock wall, Rio Viejo
Yesterday I walked through the river's gorge looking for interesting things to photograph and I found this. In this area the rock walls were mostly an orange/rust color except for this unusual white "button". There was nothing like this anywhere else on the gorge walls. The spot was less than an inch wide and hard like the surrounding rock. I'm not sure what to make of it...