Check out the woods north and east of Volksville--listen for the music to change to a mysterious tune. That means you're getting close. When you're on the right screen, the narrator will say something about it--then you just need to interact a bit with the scenery there.
Remember, you're looking for a burial cairn, which is typically made of stone--in this case, it was also meant to blend into the landscape. It's very likely on a screen you've passed through many times before...