Something else that's maybe worth considering... today we have the notion of schedules, getting things done in one go and on time (well sort of :-), but perhaps when the big stones were moved people were more in tune with the right conditions. Maybe one year, or one season, was good and things were moved so far until conditions were good again. Maybe the stones sat midway for a season, or a year or two, before conditions were optimum again for moving them.