Since both of Pip's parents died before he could remember them, he derived his first impressions of them from their tombstones. The shape of the lettering of his father's tombstone caused Pip to imagine him as a "square, stout, dark man" with black hair. Because of the way “Also Georgiana Wife of the Above,” was carved on the gravestone, Pip thought of his mother as having freckles and being sickly. This shows how a child's mind, in the absence of information, will use whatever is at hand to fashion its ideas.
We learn too that Pip's parents died before photographs were taken, so he was not able to see what they looked like that way. Apparently his older sister, with whom he lived along with her husband Joe, never described his parents to him, leaving him to wander the marshy graveyard and conjure what ideas he could.