Urchins on Oaks?

 We took our usual walk the other day but we found something unusual.

These little pink growths were attached all over the leaves on the ground and we brought one home to identify it. I had an idea that it was gall of some sort but I thought it would be interesting to see if we could come up with an exact identification.

I Googled "pink gall oak" and here are a few of the things I found.

CalPhotos: Urchin Gall
Bingo!

Nature's Oddballs
Fascinating article with a great explanation of galls.

So there you have it.
Urchin Gall Wasp Antron quercusechinus

"There are many forms of these gall dwellings, and they may grow upon the root, branch, leaf, blossom, or fruit. The miraculous thing about each of them is that each kind of insect builds its magical house on a certain part of a certain species of tree or plant; and the house is always of a certain definite form on the outside and of a certain particular pattern within."
Handbook of Nature Study, page 335

You can read more about galls in the Handbook on pages 335-338.