Looks natural to me.

As far as is known all the "shapes" are natural rock basins. What relevance this had to ancient man I do not know. The area either side of the tor that King Arthurs bed is found upon is littered with settlements and cairns..if there were any ritual monuments they no longer exist. Saying that there is so much stone lying about that a circle that had fallen would soon be lost amongst all the other clitter.

I will add a couple more photos of the site for your viewing..including a nearby rock that I have nicknamed "jumbo"!