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One possibility is that bracing materials to hold up the roof are valuable trade goods in the Ork Underground, and there are orkish experts (trained in a tradition started by goblinized geology professors) who occasionally get extracted when someone needs a tunnel building expert.
I should reread Parageology and come up with a name for sedimentary rock created through spirit action...
Looks like Greek roots are in order, so I’ll go with “eidolonite”.
Another is that “turn this compacted sediment into sedimentary rock” is a straightforward physical service for earth spirits, and shamans in the Ork Underground often do that to make the tunnels and chambers more secure.