This might be a mission to nothing and may just be a spot of rambling on my part but given a bit of spare time i had a good look around the bottom of the tor to maybe try to identify the largest of the toppled rocking stones, though to be honest i have no idea if it's still there or been broken up and carted off at some point.
Initially thrown by Rhiannon's post quoting -

"Two of them were small, not perhaps a yard high, but one, nearly spherical, was about ten feet high; and could be made to vibrate by continued though easy pushes."

The "spherical" description was a great clue but ultimately disappointing as there are no spherical 50 tonners below the tor, i then read another of Rhiannon's posts stating

"an irregular shaped block estimated to weigh about fifty tons, that could be shook by the pressure of the hand . . . was forced from its equilibrium by the mischievous efforts of fourteen young men, who were assembled for the purpose on Whit-Sunday in the year 1799."

Now this is where it gets all air brained. !
I identified a possible rock, it looks like a 50 tonner and does lay strangely compared to the others which got me more intrigued.
Now i know that's not enough to go on so thinking a little more i reckoned a 50 ton stone is gonna cause a little damage on it's way down so that was my next project.
The stone i identified lays resting on another angular boulder coming in contact with the ridged top of it and unlike any of the other large boulders does appear to have caused damage to the smaller one beneath it.
I'll be the first to admit though, i'm no geologist and have no experience in the field at all besides being obsessed with all things rocky and this is where you lot come in.
In these pics there's obvious damage where the rocks come into contact, does anyone here have any idea how this could have been caused other than by an impact.?
All the other large rocks below the tor do have signs of damage but it all looks like natural fissures and cracks and none of them sport the same signs of impact damage though many of them lay resting on other boulders.

Of course there could be something i don't know and this might all be complete bollox, i don't even have any evidence that the stone was toppled down the tor, only from it's "equilibrium" but i'd be interested in your views.
Feel free to ask questions and rip my theory to bits, indeed i welcome such.

http://i378.photobucket.com/albums/oo226/Aveburywhs/18022010380.jpg

http://i378.photobucket.com/albums/oo226/Aveburywhs/18022010381.jpg

http://i378.photobucket.com/albums/oo226/Aveburywhs/18022010378.jpg

http://i378.photobucket.com/albums/oo226/Aveburywhs/18022010374.jpg

http://i378.photobucket.com/albums/oo226/Aveburywhs/18022010375.jpg