The glacial pothole was filled in because it was discovered only in the
early 20th century; the natural fill in it was at that time removed, and a
fence was put up around it so people could view it.  In recent years the
fence was removed and the hole was filled in, because it was not open
historically at the time the fort was occupied, and people assumed or
concluded it was a feature dating from the time of the fort.

As for the cannon, it is still in the collections.  I believe additional,
matching cannons have been found.  The one now on display may be a different
one from the one in the picture.  But it may look different only because it
has been cleaned and conserved.  Also, I believe the one in the old photo has
a breech block.  Unfortunately, the breech block was lost or stolen when the
cannon was on display at the State Fair in 1967 or 68.  Does anyone have any
clue where the breech block might be now?

Paul Huey