I have posted on Carn Briw before. It's the Bronze Age (?) burial mound on the common to the south-west of Carningli. As I have pointed out before, it's a classic example of a mound built of locally sourced materials -- gathered up from a scatter of "stone takes" or pits from a radius of about 50m of the burial site itself. I came across this excellent photo taken for RCAHMW by Toby Driver on one of his winter flights. The pock-marked moorland surface is beautifully shown. The biggest of the pits is almost a metre deep and almost 2m across, but most are a lot smaller than that. So availability of stone may well have been one of the prime reasons why this burial site is where it is -- this is essentially a periglacial block field covered in till and frost-shattered bedrock debris, partly masked by soil and vegetation.