Since oaks and, to a lesser extent, maples contain tannins, I'm going to guess that that is what stained the newish, probably not sealed concrete in front of the nursing home on Summer Street:
Oak, maple impressions. The brown leaf looks like it's from a tulip tree. The yellow bit, probably the same thing.
It's amazing what one will find (aside from spare change. Found $.36 this afternoon.) when one remembers to look down.