‘At the Mountains of Madness’ consists of a main narrative into which eight shorter narratives are embedded. The narrator of the main story is a geologist, the leader of the Miskatonic University Expedition to Antartica. He is not explicitly named in the story, but astute readers can deduce from the embedded narratives that his surname is Dyer, and so he can be identified with the character “Professor William Dyer of the college’s geology department—leader of the Miskatonic Antarctic Expedition of 1930–31” who appears in Lovecraft’s later story ‘The Shadow Out of Time’ (Astounding Stories, June 1936, p. 136).
Dyer indicates that his narrative was composed subsequent to the events of the story, because in the first paragraph he explains that he is writing in consequence of the planning of a new expedition to the Antarctic, in the hope of warning its members:
I am forced into speech because men of science have refused to follow my advice without knowing why. It is altogether against my will that I tell my reasons for opposing this contemplated invasion of the antarctic—with its vast fossil hunt and its wholesale boring and melting of the ancient ice caps. And I am the more reluctant because my warning may be in vain.
H. P. Lovecraft (1936). ‘At the Mountains of Madness’, part 1. In Astounding Stories, February 1936, p. 8.
The story contains multiple other indications that Dyer is composing the text some time after the events. Here are three I spotted in a quick skim of the text:
Looking back to our sensations, and recalling our dazedness at viewing this monstrous survival from aeons we had thought pre-human, I can only wonder that we preserved the semblance of equilibrium which we did.
We cannot yet explain the engineering principles used in the anomalous balancing and adjustment of the vast rock masses, though the function of the arch was clearly much relied on.
I still wonder that we deduced so much in the short time at our disposal. Of course, we even now have only the barest outline; and much of that was obtained later on from a study of the photographs and sketches we made. It may be the effect of this later study—the revived memories and vague impressions acting in conjunction with his general sensitiveness and with that final supposed horror-glimpse whose essence he will not reveal even to me—which has been the immediate source of Danforth’s present breakdown.
So Dyer’s narrative is “ulterior” in the Rimmon-Kenan typology. Whereas the eight embedded narratives, records of radio messages from “Lake of the biology department”, each appear to have been composed shortly after the narrated events, and so probably count as “intercalated” in the Rimmon-Kenan typology:
When telling and acting are not simultaneous but follow each other in alternation, narration is of the fourth type, namely ‘intercalated’.
Shlomith Rimmon-Kenan (2003). Narrative Fiction: Contemporary Poetics, p. 90. New York: Routledge.