The Stonehenge Neolithic stone collectors

OK -- so the people who collected the Stonehenge stones (both sarsens and bluestones) had to range across the countryside in order to collect everything that was available to them.  So -- assuming that the climate was favourable and that terrain conditions were also suitable -- how far did they go?

The two maps above are helpful.  Both the maps are courtesy Wiltshire County Council.  The one shows the terrain to the W and NW of Stonehenge / Amesbury, and the other shows the geology, with two circles superimposed.  The inner circle has a radius of 20 km and the other 30 km.  I'm increasingly coming to the view that the stone collectors first collected their stones from quite close to the monument, and pushed further and further afield -- maybe up to 30 km away in some instances.  The key factor was the smooth and undulating grassy surface of the chalk downland.  So what stopped them in the end?  I've suggested many times that maybe they ran out of stones, ran out of energy, and ran out of motivation -- or that priorities simply moved on.  But maybe one key factor also was the chalk scarp.  It's a pretty impressive barrier, full of indentations but generally facing west.  Hauling stones up that scarp, or even up the valleys that broke into it, would have been very daunting indeed.

So sarsens and bluestones that were above the scarp were maybe quite easy to collect, whereas anything below the scarp was not.  Worth thinking about.......