I was thinking about these excellent questions from @ariadne-mouse, and in particular those about what this upper Un megafauna eats most of the time, in relation not to the crew, but the Sloop:
Consider also, @utilitycaster's suggestion of a toroidal cosmos, wherein the limits do not necessarily end, but instead circle around to meet each other. We do see that the mica described at this altitude is "fresher" and "cleaner", in a sense suggesting that this mica is newly formed:
We cannot yet know what lies beyond the tunnels of mica, but if the cosmos do in fact circle back upon themselves, and we take as foundation the gravitational direction of the cosmos—i.e., that the the Un is up, and the Fold is down, and overall, everything flows from top to bottom—then the biggest question is: where does the Fold at the deepest depths go?
But when we get to the description of the Fold of the dark mica being siphoned, the idea of mica forming begins to be called into question:
Contrary to this image of inorganic, dead matter, we do know that Fold is, technically, alive, that it is not liquid but innumerable living micro-organisms, described as algal or bacterial. We also know that mica can be excreted as by-product by other beings, notably mirrorhawks:
And, as we've learned, people contain Fold, which can be siphoned from themselves, and it is fundamental to their being and life. This could explain the chemical origin of this mica secretion, if Fold is a fundamental basis of life in the cosmos:
So perhaps mica, far from being any sort of originating matter, is in fact the by-product of dead, processed, consumed Fold.
Certainly, we don't entirely know how more Fold is created, though we know it has an origin at the Font, so it must have ways of generating more, and the cosmos must have ways of balancing the amount of Fold in the sense that the universe has not become purely Fold. Of course, we do not know where Fold goes beyond the Delta, but based on this evidence, I would hypothesize that one method of keeping this balance is that the megafauna of the upper Un and the megafauna of the extreme depths of the Fold are one in the same—evolved within a toroidal cosmos to consume and process Fold where it is most abundant, and maintaining an equilibrium of Fold in the universe.