Doorway to...where?

At first glance, the simple carving on the gravestone for Phebe Jane Kellam in Butter Cemetery appears to be an obelisk on a pedestal.

But why is there a doorway in it?

PHEBE JANE,
Daughter of
George and
Sarah M. Kellam,

The door could be interpreted as a doorway to the afterlife, especially if it were ajar. Or does the carving represent Phebe’s tomb? Is this simply a door such as one you would see on a mausoleum?

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Butter Cemetery, Morrow County, Ohio