Sarsen Stone 59 is the very prominent fallen one of the inner horseshoe that can be seen from the public path. It is noticeably unfinished with grooves and ridges. It is assumed that the ridges are still waiting to be pounded off to produce a smooth level finish.
The foot of the stone, which would have been underground and the top of the stone, including the two broken off pieces do not have the ridges, they have been "finished".
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So why not the middle section. After all the hard work in bringing, erecting and shaping the stone, why didn't they spend the few days to finish the job?
It is almost as though they left in a panic.
One theory is that the unfinished state is deliberate, it is significant of something.
Bluestone 71 is trapped underneath so the fall wasn't immediately after erection, unless the bluestone horseshoe was already in place. But I still wonder if the fall didn't happen as they were pounding the surface to finish it off. An historic "Oh ****!" moment.
The rabbits are digging underneath the stone at the moment so we can peek at the underside. It too is ridged and unfinished.