The Moat Boulder can be found at the far end of the boulderfield. You will notice a house-sized boulder perched in the middle of a marsh--that's it. The water level is dependant on the season, but the side facing the road usually offers the dryest starts. It goes without saying (as I say it right now) that you should avoid this area during the spring, as billions of sadistic insects find their home at the base of the Moat Boulder.
Getting There
Follow the main trail through the Black Jacks. The Moat Boulder is the westmost huge boulder.
There is more big stuff further West Jay. If you keep going, then head back from the road and crawl around you will find a bunch of cool hidden overhangs
I have to say that the boulder field is an amazing place to explore. It is much bigger than it appears and it seems that there are an infinite number of little caves and passageways. It's a mysterious place for sure.