Masonic gravestones depict an Eye of Providence (all-seeing eye of God) at the top, sometimes encompassed by a triangle. It demonstrates that the deceased was a member of the Freemasonry organisation. The square and compasses is the most prominent of Freemasonry symbols and also signify a Masonic grave.
This is the memorial dedicated to Captain Elisha Halsey, who was murdered at sea in 1844.