[Crex]

Amory is pretty much right on.
You could have some fun and tell folks it's the kind of anvil early natives used to forge their stone points. You would be amazed how many government educated people would believe you. Make a great door stop as well.

I've seen a few out in the south western desert that looked very similar that were actually wind/sand formed. One is quite famous down in (I think) south Arizona, called "Anvil Rock", but it's quite a bit bigger.
Help me out Chuck, it's over on your side of Big Muddy.