Distorted acorns produced when a tiny wasp lays her egg in them.
Growing on one of the oak trees by the Smestow Valley Railway Walk, right by Castlecroft Road. The view from the bridge put me on the level with the acorns.
According to sources elsewhere on the web, there are occasional years where these galls are very common. I don’t remember ever having seen them before. As far as I could see, every acorn on that oak was affected. I’ve been looking round at other local oak trees since, any not seen any more of these galls – though I haven’t seen many normal-looking acorns either.