Where Did Stonehenge's Sarsens Come From?

I came across THIS ARTICLE and then a correspondent sent me the link to the ORIGINAL SOURCE. Stonehenge is a never ending subject. These articles are well worth looking at.

Sarsen stones are post-glacial silcretes - quartz sandstones with a quartz cement and are - almost - 100% silica. The authors discuss the place of origin of the sarsens. Almost all of them have an identical trace element composition and they use this to identify a place with an identical chemical signature. 

This is West Wood, 25 km to the north of Stonehenge. As some of the sarsens weigh about 30 tonnes, this would not have been a trivial undertaking!