Does anybody know anything more about this feature? It looks like a rather crude Neolithic burial chamber. Apparently there are a number of other burial mounds and later features (Bronze Age?) in the area.
Update: According to the GCR for the Quaternary of Wales, there are abundant Namurian "quartzite" erratics in and beyond the Paviland Moraine -- the key source is assumed to be Mynydd-y-garreg in Carmarthenshire. Some boulders of this same rock type are found on the foreshore at Western Slade.