This is a shot taken standing at the sharp end of the eye-shaped arrangement of stones known as Bedd Arthur. No stone in the picture stands more than 0.6 of a metre tall, one or two have fallen and are now almost completely covered with turf.
There have been a couple of suggestions that if the arrangement is shaped like a boat it resembles (albeit on a smaller scale) the Swedish burial monuments from the viking age (quite modern at circa 700 AD). Bedd Arthur is mentioned in several historical source books including Archaelogica Cambriensis (1898) p.74 and (1984) p.52 and plan p.62 showing 15 stones. Barber & Williams, The Ancient Stones of Wales (Blorenge, Abergavenny, 1989) p.22, p.45 (followed by some spiral forces stuff...), and Richard Sales, Best Walks in Southern Wales (Constable 1990) in a section about the Preselis with some background about the Twrch Trywyth. But nothing useful about its origins. (thanks to Theo for that background)
The mystery remains...... Please e-mail me if you know anything about this curious little site.