The pic which I recently published -- with what Rocky refers to as "eight faceted faces" -- has raised some interesting issues -- the most interesting of which is the means by which we recognize rock surfaces as having been shaped in some way by human beings in prehistoric times -- before the advent of metal tools like wedges, chisels, hammers and crowbars. As far as I am concerned, the photo showed a piece of rock which appears to have fractured along a number of intersecting joints or planes of weakness -- which suggested to me that there had been no human intervention of any sort.
If you look at the pic above -- from one of the dolerite pinnacles close to Carn Meini -- it is very common for elongated blocks to have 5 or 6 facets. Look closely (click to enlarge) and you will see that there are lines of quartz crystals running across the blocks in places. These are also weakness lines, and cause transverse breakages. You can see that one big chunk or flake is about to break away from one of the pillars.......
If we contemplate the hypothesis of that other rock having been fashioned by Neolithic rock workers, how would they get surfaces as smooth as that by working just with stone axes and maybe wooden wedges? And why would they want to lever out a slab with that particular shape anyway, when there are thousands of other flattish (and elongated) slabs littering the landscape? I just don't see any logic in it........
Thanks to Rob for a reminder that the jointing pattern here is a hexagonal one -- caused by slow cooling in basalts and dolerites. The Giant's Causeway is a good example. Rob also suggests that the lines of white crystals are on joints, and that they are secondary minerals with some quartz but mostly other minerals.
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