There were numerous galls on the underside of bur oak leaves. The wasp that makes these galls is tiny, so tiny that it probably goes unnoticed, certainly far, far smaller than the galls. I have always found these on bur oaks, not so far, on red oaks, though I have seen photos of them on white oaks, so they be on more oak species than I know. I find them quite interesting in their shape, like small drinking vessels.