A profile of Silbury Hill as would be seen from the top of Silbaby might be useful. If it was an observation platform then the phenomenon being observed - ritually - will possibly have been this far northern moonset. Might it be possible that this setting moon could be eclipsed by Silbury just as it's about to set - or something similar. I named a pair of small stone cairns 'David's Cobs', but it's the landowner's name. It looks as though Silbaby's stuck.