Mangetout, Your third link has a typo. Here it is corrected.
I’m not an expert, but I doubt the holes are for explosives. Certainly the clean drill marks we see didn’t also have explosions going off in them.
I also agree that nobody would use hand labor to split large rocks just to make bulk materials like rip rap (the US term for breakwater material).
I assume the quarries for architectural stone also produce waste; perhaps the chunks that are too small, too irregular, too many inclusions of debris, or whatever to make into slabs get sold to somebody for use as rip rap.
Ditto the process of stonecutting for construction, removing large rocks from foundation holes or from road cuts in hillsides.
They use drilling and splitting to make their rock walls where they want them, and the debris is carted off and further broken up as crudely & cheaply as possible. That would leave some facets carefully drilled and others just cleaved along natural faults.
I just did a google and found this - it’s a sort of hydraulic version of wedges and feathers. Even this looks a bit small-scale for the rocks I’ve seen.
It’s probably also worth noting that there are other bore holes on the rocks I observed - I didn’t get a picture of them, but they are typically solitary, a larger bore and they often go all the way through a piece of rock - I thnk these probably are the remains of part of a deeper hole that took the explosives for the initial quarrying.
The photos I took (on my camera phone, which is why they’re not particularly good quality) don’t give a very good impression of the size of the rocks; in this one, for example, the gravel is in fact composed of stones the size of walnuts; the rock itself is big enough for four people to sit on. (interestingly on this particular rock, the spacing of the bore hole remnants was wider, yet still quite uniform and very close to parallel. I reckon the drill must have been mounted on some sort of tracking arm.
Checked out that picture in your last post and I cannot imagine that those holes are related to drilling and quarrying. Finding room and placing a drill to attempt that many holes at those angles seems unlikely.
Instead it looks like the stone was being roughed into shape. Since it is now on a pile of other I must guess that it was rejected
As far as my original post (the wedge and shim) I was referring to the original post where the block was a small stone used in the patio. If you get the chance to check out the quarries in Rockport, Mass. you can see that they quarried some good size chunks of rock by hand.
Certainly it now seems very unlikely that the marks are due to holes drilled when the rock was in situ in the rock face, but they certainly are the remnants of drill holes, as is evidenced by the last photo (this one)- where the rock has cleaved along a different line an left some of the holes intact.
Just to clarify what we’re looking at – the grooves at the back of that rock connect up with the holes in the front, with a portion projecting above the plane they were (presumably) trying to create?
Yes; the holes connect with the grooves; all of the grooves, on close examination, are semicircular in profile - they are undoubtedly the remnants of cleavage along a line of deep/through, parallel drilled holes.
I should also mention (some of which is apropos of nothing) that:
This stone isn’t local - there’s no rock like this anywhere for several hundred miles.
The pieces are really very large - up to the approximate size of a (european) car - they also seem quite uniform in composition; they don’t seem like reject material to me (not that I’m any kind of expert, of course, but it looks like a load of rock that was quarried and cut up specifically for this purpose.
