I wonder if there was anywhere nearby to Silbury Hill that would, or was able to be, flooded in December ? I think the centres of the two inner circles, at Avebury, point toward north (+36 declination) and supposed indicates the setting point of Deneb - a near circumpolar star, then, in the constellation of Cygnus, the Swan. There's speculation about one of the Irish winter solstice sites that the festival, then, was marked by flocks of migratory swans arriving. (And presumably being added to the feast. Sadly Deneb doesn't go so far north nowadays - precession of the equinoxes - but this +36 line is present at several sites (and is 'awaiting an explanation').