Yellow crustose ascomycete lichen
A crustose yellow ascomycete lichen growing on a rock. Multiple apothecia are visible. I believe this may be _Acarospora_ (_Acarospora socialis_?) or _Pleopsidium_, but I'm not sure. This lichen was found in Vasquez Rocks County Park in Los Angeles County. The scale bar is 5mm (contact me if you want a version of the image without the scale bar).