Stone 98 used to mark the southern entrance to the main henge. The figure below shows how this monolith incorporates a massive "block" face looking up into the sky. Interestingly I can now prove photographically that both this southern entrance shown here and the northern entrance to the henge displayed such heads. This other block head is on the Swindon Diamond stone at the north entrance, visible here.

I unexpectedly found other (really quite eerie) faces on this stone, I have set images of these below, the top two are carvings, the lower may be natural, or may be artificial: