The site is a very beautiful one -- with a small series of crags in a rough paddock which is surrounded by green cultivated fields. The outcrop -- or series of outcrops -- called Garn Turne is made of the volcanic rocks of the Sealyham Volcanic Formation, intruded into or resting above dark grey silty shales and mudstones of Arenig (Ordovician) age. The rocks are older than the Fishguard Volcanic Formation -- green-grey in colour and with a rough knobbly surface texture. The rocks are classified as andesite and dacite lava and hyaloclastite, with some interbedded tuffaceous mudstones.
The crags consist of a series of vast detached and tumbledown blocks resting on bedrock outcrops with small cliff faces. Right across the paddock there are hundreds of other boulders and detached blocks, many of which are in approximately their "original" positions and others which have been moved downslope by slope processes over hundreds of thousands of years and possibly by Anglian ice action as well.
The famous earthfast dolmen excavated by Colin Richards and others a couple of years ago is just a huge slab of bedrock weighing more than 60 tonnes with other stones moved into position around it. Colin thought originally that the big "capstone" (if that is the right word) was quarried from the rock outcrops 50 - 100m upslope and moved to its lower position by the cromlech builders; but he seems now to have abandoned that idea. I agree that it was used where it was found, with a burial "chamber" simply excavated beneath it. The forecourt faces NE -- for the simple reason that that was the natural direction in which the stone "faced" when it was embedded in the ground.
The smaller stones do not appear to have been shaped at all by the men who placed them into position-- they have sub-rounded edges but rough and irregular outlines. Some of them have probably been moved a short distance by ice -- so they have again been used more or less where found.