The two photos above show assorted boulders (mostly made of dolerite and rhyolite) in Pembrokeshire moraines. The top photo shows the boulders projecting through the ground surface on the Pont Ceunant Moraine, near Newport, and the lower photo shows boulders excavated recently from the Cilgwyn Moraine, on the south side of Carningli, during building work. As in all moraines, we see some well rounded boulders, some sub-rounded, some sub-angular and some angular -- with shape determined above all else by the distance a boulder has been transported by the ice.
Now let's take a look at some of the stones found in megalithic structures in Pembrokeshire:
Some of the stones used in these structures are quite sharp-edged or angular, and could have been taken from nearby rocky outcrops, but in general the stones used by the megalith builders have rounded or sub-angular edges and have clearly been picked up from the morainic / erratic materials readily available across the Pembrokeshire landscape. One day I'd like to do a detailed analysis. Some of the stones used are quite interesting -- for example, the capstone at Pentre Ifan has a glaciated upper surface and a fresh or broken underside. Before being used as a capstone, was it embedded in the ground and smoothed by over-riding ice?
As suggested by many authors, the key determinant of megalith location has nothing to do with ley lines, solar or lunar alignments or astronomical observations and everything to do with the availability of convenient stones.
What was true for Wales was no doubt also true for Stonehenge........