I'm curious what you guys might think of the following.
I've always taken the story of Saint Brandon as an explanation for the presence of spirits, good and evil, from the old world, in the new. No doubt there's a lot going on in that story that I don't understand, but the bow of the boat is said to be in Boston Bay and at the end of the tale, when the cat and the king of rats are fighting, we're told that one is wickedness, and the other a fairy. As they fight, pieces of them run off into the woods.
I take this to mean that although Brandon brought Christianity to ancient America, he also brought...other things. Juxtaposed with this is Doherty's reference to his grandmother "the old Kate" which is another name for an evil spirit, mentioned later, called the Bell Witch. The banshee story that Den shares in chapter one is told by "the old Kate", or rather, by Hannah who knew her. Wolfe is showing us an oral tradition of stories from the old world being carried on in America. Is Doherty descended from the Bell Witch? Maybe.
The "Bell Witch" has a Wikipedia entry, and I'll hazard that Wolfe did not create her. You can visit Bell Witch Cave in Tennessee and, while it's not an exact match, I'm pretty sure that the cave Olivia, Den and Professor Peacock visit is meant to evoke it. Part of the legend is that disturbing the native bones buried there would awaken an evil spirit. Since I'm pretty sure Peacock carried away a skull, it's sad that he disappears from the story immediately after.