Lichen body composed of small flecks of greenish to orangish yellow on surface between the orange fruiting bodies (called apothecia) which are flat disks with paler rims.

In the wild, grows on basic (alkaline) rocks, such as limestone. With the advent of civilization it found the artificial alkaline rocks that humans make, such as concrete, cement and mortar, to be quite suitable. Occasional in Wildwood, on cement structures (the one shown in the top row was on a sewer access structure near the South Bridge), and on limestone rocks (the one shown in the lower two rows). Outside the Park, very common on sidewalks, and on the mortar of rock walls.

Look for a bright orange (not yellow) stain on cement or rock and examine it for the yellow flecks and round orange apothecia. A similar paint-like smear that is more yellow is likely Lemon Lichen (Candelaria concolor). An actual rosette of orange lobes would be Weber's Sunburst (Xanthomendoza weberi). Both are common in the Park. 

This lichen was previously misidentified as Sidewalk firedot, Caloplaca feracissima. That species can be distinguished from sulfphur firedot by the lack of any noticeable thallus in between the orange apothecia. It grows in similar habits and could occur in the Park, although we are at the southern edge of its known range.