roots

I'd second the idea that it is due to soil erosion. The roots probably grew around the rock and then the soil was washed away from the base of the tree exposing both the roots and the now suspended rock. (See one of my photos for an even more extreme example: http://www.projectnoah.org/spottings/9217606.) It would be great if you could ID this tree, but I also recognize that you may not have a chance to get back to it or remember enough details to ID it. You might want to look at the original photo and zoom in on the leaves laying at its base; that might be a big help. I like detective work...grin.

Hills of home.like Seema says it could be soil erosion due to rains ,wind etc. I found this spotting. http://www.projectnoah.org/spottings/7059028 In my case it was erosion due to burrowing activity of ground squirrels.

weird, fascinating shape...perhaps, a case of exposed root due to soil erosion

It doesn't really need to be very near, or the path of rushing water...It might have happened a very long time ago, the stone might be there for several years now...So I would probably guess that the rock wasn't raised from the ground, but that instead it rolled down the slope, and stuck in here, somehow...