Galls are some of the most fascinating things found on the planet. At least in my opinion. And this is one of the most spectacular. This is created by a tiny gall wasp, Callirhytis seminator, the Wool Sower Gall (-maker). But of course that is a mis-leading statement. The gall is actually created by the plant, in this case an oak, in response to irritation/agitation/chemistry of the wasp. The wasp is warping the plant’s defenses for its own uses, protecting its eggs and feeding its larvae.
These were discovered at Fort Nonsense Park, site of a Confederate earthworks to defend slavery, in Mathews Co., Virginia. But we also have them up here in Brooklyn.