I'm not sure this image is what you're thinking of---Terry so often uses/re-uses with modifications, images throughout a series. But, based on your thought that it is late in the book, is it the conversation that Tiffany has with the Hiver (in HFOS) who is becoming Arthur? Specifically (see -p. 241 American edition Hardback), Tiffany tells the hiver that it is not alone, and that they are actually quite alike. She uses a fully developed image of realizing that she is part of everything that went before her (from physical characteristics to psychological). And she knows (p. 243) that what had once been living creatures is now the chalk of the hills."
But Terry uses that same image in ISWM when Tiffany &the Kelda are talking in the chalk pit. There Tiffany sees a "ghost" of one of the fossils fishes swimming briefly through the air is a reference to an ancient creature swimming along actually through the air. And to further complicate matters, when Granny brings Tiffany out from the control of the Hiver, in Hat Full of Sky, Tiffany finds herself swimming through a white soup that she realizes is the chalk, the bones of her body and her body is the chalk. Remember when the Feegles are inside her head fighting for her soul -- the green land (they think) turns out to be Tiffany herself.
Don't know if these ideas helped at all, but as Terry is fond of pointing out, everything is related to everything else.