| Sibelius Eos Owm |
The Narlmarches run immediately alongside the hilly terrain known as the Kamelands (or at least this is how the two main regions were labelled in 1e... the specific regional names may have changed but the point stands that this is hill country. When I first played Kingmaker I ran with the idea that among the hills of the forest was a sort of sheer rock face from a hill eroded by time into a small escarpment.
The original map of the Temple showed an empty cavern just past the stairs with several wide chambers which presumably once acted as part of the temple but of which there was nothing to say.
This means if you want to make up something for the cave, you can go nuts. Personally I'd just come up with a description for the interior of a humble Temple to Erastil maintained by priests who had probably limited means and no real desire to work the stone, but who might have considered the formation sacred enough to carry out their rites in there.