Anyhow, here are my guesses:
1. It may have looked like the nearest known ones, in the Avebury area http://www.english-nature.org.uk/ImageLibrary/web/11/11622.jpg
http://www.walkweb.org.uk/Stone_209.jpg
If so, it contained an awful lot of stones for them to have found enough suitable ones since, as you see, they are in a tiny minority at Avebury.
And if that's the case we need to know why that huge number of rejects aren't there now.
2. Alternatively it might have been only a small scatter. But that would mean there was a high instance of usable stones, else they wouldn't have enough. By usable, I mean about the right size and shape. I guess that's possible (but needs checking with a geologist) but there's a further factor - this tendency to homogeneity would have to be in two forms - suitable upright shape and size AND suitable lintel shape and size. That would be a very strange happenstance, and rather a speculative stretch in the absence of other examples or geologists' confirmation that such things happen.
3. My favourite guess - there warn't no scatter at Stonehenge and they lugged them there.
Incidentally, just for fun, on the subject of pulling big loads and whether it's conceivable, take a look at this, it's amazing what CAN be done that one would probably assume couldn't.... http://www.lakesideinn.net/lakesideinnstory/media/image005.jpg
