A large growth of lichen on old cement retaining wall at the head of a drainage basin, worn to the point that much of the coarse rock aggregate is exposed. I believe there are several species growing here, but I am in way over my head when it comes to separating and identifying them all. Maybe some Xanthoria candelaria (image # 4101) and possibly Caloplaca citrina (image # 4091). I can't even venture a guess on the others, which may or may not represent a variety of species.