Desiree is sleeping in the shadow of a stone pillar when Armand first encounters her. He falls in love at first sight, and they eventually marry and have a child. In their race-conscious society, however, their marriage cannot last because at least one of them is not "white." That is, somewhere on their family tree is a person of the "slave race."
When Desiree notices that her baby looks like the little quadroon, or mixed-race, boy, she panics and writes to her mother, who asks her to come home. Armand wants her to go:
He thought Almighty God had dealt cruelly and unjustly with him; and felt, somehow, that he was paying Him back in kind when he stabbed thus into his wife's soul. Moreover he no longer loved her, because of the unconscious injury she had brought upon his home and his name.
So Desiree takes the baby and disappears into the bayou, never to be seen again.
Did she make it back to her mother's home? Probably not, and the stone pillar is our clue that she doesn't. The pillar itself is probably a commemorative statue. But with Desiree sleeping in its shadow, it becomes a foreshadow of her death. The pillar is her tombstone.
In our class discussions, the stone pillar always conjures up imagery of a tombstone, and what is more interesting is that she is found in the shadow of the grave, indicating that the "shadow" of her past, that is, the uncertainty of her past, that would ultimately lead her to her grave, which is what the pillar represents. The fact that the pillars are at the gates of the estate also may represent a fixed way of thinking, there is only one way in their society, and the pillars being made of stone could represent that.