Under the failed roof section between
stones 50 & 51.
The roof here was originally comprised of two long slabs of diorite that spanned
the gap
between the roof stone in the upper centre of the shot and the roof stone out of
shot
behind this location. These stones failed and caused a roof fall here, blocking
the passage.
When the fall was discovered by the 1920's excavators, they dug a shaft down
from above
and removed the fallen material. The concrete visible in the above shot is a
plug
that was inserted to stabilise the roof here.