Sandal straps trap sweat, add dust and you get mud. Kick off your sandals and you get that appearance.
I am pretty sure I see the dirt outline of the thong of a flip-flop. Amazing that he would be allowed to wear such flimsy foot protection at a dig.
Asomeone who hates shoes — I am barefoot at this moment, AAMOF — I sympathize with the guy’s desire for comfort, but I think I would wear real shoes at a dig.