Stone 31a is a diffcult monolith to interpret, I've puzzled over it for a long time.   When you stand in the Avenue, and look at its western face, the overall impression is of a small, truncated, squarish and dark stone*, really quite obscure when compared to some of its larger neighbours:

After repeated viewings under different lighting conditions I began to suspect that this stone incorporated an upturned face, that was in fact possibly a zenith-seeking statue (click here for details of others), but at first I couldn't see an eye. Possibly, I thought, the eye was omitted as being "understood" (see Stone 24b here for an example of this), but eventually I spotted the small but distinct eye, convex and created by the gouging of a circle out of the stone, quite reminiscent of that on Stone 25b, visible here.

Looking harder, the face here on Stone 31a appears to be framed by a long curvaceous the straight line, which might just possibly represent the edge of a leather hood or cap - a broadly similar feature may well exist on the great right looking head of Stone 26a, visible here. Below I attempt to make the comparison, both stones are strong right facing profiles, and both have areas that could represent head coverings, and both live on the eastern row of the West Kennet Avenue:

[Stone 26a is show immediately left above and Stone 31a to the right]

*Because the Stone 31a zenith-seeker gives the impression of being hidden away on top of a dark and obscure stone I have decided to photograph it as if brightly by a spotlight.   My intention is to give the carving the prominence it deserves, but ensuring that the viewer of this webpage must still work a little at finding the eye, just as a visitor to West Kennet Avenue must.