The Altar Stone lies at 80° to the main solstitial axis beneath the collapsed upright of the Great Trilithon (Stone 55b) and its lintel (Stone 156), sunk into the grass. The stone itself was broken by the fall of the Great Trilithon's upright and is in two pieces.
In the first photo the eastern end of the Altar Stone is seen to the left of Stone 55b, the middle section is visible between Stone 55b and Stone 156 and the western end is out of shot to the right of Stone 156.
The eastern end:
The middle section between Stone 55b and Stone 156:
The western end beneath Stone 156: