I have recently been looking again at Carn Briw, a Bronze Age (?) cairn on the highest point on the upland of Carningli in North Pembrokeshire. It's a smallish conical mound in quite an isolated position, and the assumption is that it might have contained a cist grave or maybe several burials; maybe it was the key burial site for the Bronze age community that lived in these parts. There are at least a dozen round houses, ring cairns and other features in the vicinity. The mound has been substantially rearranged over the centuries, probably by Dad's Army in WW2 (this was a lookout position) and by large numbers of small boys before and after WW2.
The mound might be classified as a round barrow if it was somewhere else -- but it has close relations in the three gigantic cairns on the summit of Foel Drigarn in eastern Preseli.
The land surface hereabouts is covered with a layer of broken rock debris -- this is essentially a periglacial blockfield with some glacial erratic material added for good measure. The soil is thin and stony -- and between Carn Briw and the summit of Carningli literally thousands of angular boulders and stones break the land surface.
Above is a satellite image of the cairn. As we can see, its northern section has been damaged. But the most interesting thing about this feature is that it is made of stones up to 1m in length -- nearly all capable of being moved by one or two men working together. Because the heath is dry here, and because there has been no peat growth to cover things over, we can see in striking detail exactly where the stones in the cairn have come from. For a radius of about 50m from the cairn the land surface is pock-marked with pits and hollows, some of them 2-3m across and some up to a metre deep. Some of them contain standing water after wet weather. These are the Carn Briw Quarries -- the builders of the cairn were concerned above all else with economy of effort. They took what they needed from as close as possible, probably spreading out further and further from the centre of their little circle as more and more stones were needed.
There is no evidence at all that certain stone types or shapes were preferentially used.