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.
UPDATE - Added April 2018
The excavation sketch included in Cleal et al. shows that there is a stump of the stone left in the stonehole, 1' 4" (410mm) down.
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