A Tentative Suggestion That The Carvings Were Made At Night
In my "Photographer's Statement" note I refer to the academic work proving the existence of faces and designs among the Avebury stones. These artifacts were typically created so that they would become visible to worshippers by catching the light at particular times of day and seasons of the year. This would have caused a problem for their sculptors - a problem of consistent lighting while they carved.
Sarsen is notoriously hard to work, and I imagine that even a small amount of carving would have taken days, if not weeks, to create. During this extended period the sun would move across the sky repeatedly every day, casting light from continually changing directions all the time. How would this moving daylight enable the sculptors to create work intended only to be read when illuminated from one specific direction? To put it another way, any carvings intended to be concealed/revealed in this way could only have been made under the same lighting conditions they were intended to become activated by - and sunlight cannot consistently provide the sculptor with light from the right direction.
This leads me to suggest that the most precise carvings may have in fact been made at night, by the light of open fires or burning torches/lamps mounted on wooden stands. These nocturnal lights would have been carefully placed at the specific position and/or height needed to cast light onto the stone being sculpted as if coming from the sun at the required time of day/season of the year. The sculptors would have then taken as many nights as they needed to complete their carving. Such work would have been particularly suited to the winter half of the year - lots of darkness allowing the artificial lights to show up the stones adequately, and the absence of farming activity potentially freeing up workers to shape the stones.
I can imagine that the flickering light would have made the emerging carvings appear alive as they appeared under the sculptor's hand, an awe inspiring sight for anyone reverently watching the process.