There was some earlier discussion about this site, with suggestions that it is a site used for sharpening implements, or that the marks are tally-marks of some sort. what we have is a string of notches cut along the sides of a natural fissure, with the notches cut perpendicular to the fissure itself and on both sides of it. I have put a picture on the Stonehenge Thoughts blog.

I now think that this looks like very early Ogham script, dating maybe from the period before 400 AD when Irish / Celtic groups were wandering about in north Pembrokeshire and maybe competing for territory. The site is now out in the open air, but I'm convinced that when the notches were cut there was a roof over it, supported by the natural sloping rock face on one side and by a semi-circular stone wall on the other. Imagine some old fellow, sitting in there one day, nice and dry, while a storm raged outside, wanting to do something just to pass the time......