This quirky monolith has been extensively worked to a very specific shape. As with many Avebury monoliths it has more than one visage, one is the left profiled face looking horizontally down the Avenue - you can find details of that face by clicking here. There is yet another face on this monolith, this is shown in figure 1 (with facial parts labelled) and figure 2 (where the purple arrow shows where in the southern sky the face is gazing).  I call this class of faces equator-seekers.

Interestingly, I suspect that the Avenue's Stone 29a equator-seeker here has a close analogue carved on the upper left hand side of Stone 25 up at West Kennet Long Barrow. This barrow equator-seeker also appears to gaze up to the southern celestial equator (figure 3).

Figure 1