These acorns, on oaks in Bantock Park, are contorted because a wasp has laid an egg in them. The wasp was an oak knapper wasp, and the acorns have become oak knapper galls. There’s twenty or more different species of wasps which lay their eggs in oaks, each of which produces a different type of gall. The most familiar is the so-called oak-apple.
I’ve visited different sections of the park a couple of times since the knapper galls were pointed out to me. Every single oak I’ve checked has had every single acorn (at least those visible from ground level) visited by a wasp.