There's unironically a bunch of Appalachian cosmic horror stuff out there. In fact iirc Savage Worlds has a setting for it called Holler and Monte Cook games published a ttrpg for the Old Gods of Appalachia podcast.
By the end of the Mesozoic Era, the Appalachian Mountains had been eroded to an almost flat plain.[27] It was not until the region was uplifted during the Cenozoic Era that the distinctive topography of the present formed.
Most of the Appalachians is now located within the eastern part of the United States as runoff. Imagine how long it took for huge mountains to erode down and wash outwards into the ocean that distance.
And the Appalachians are still young compared to a few other mountain areas around the world.
Yup. Makhanjwa range in the north west of SA is three times as old as the Appalachians at 3.5 billion years. Days were only twelve hours long back then….
Also thematically related is The Twisted Ones by T Kingfisher, which itself is a reinterpretation of The White Ones by Arthur Machen (written in the late 1890s). Appalachia has been creeping people out for a long time!
As I mentioned in another comment, but elaborating further here, there's a Savage World's setting that revolves around eldritch horror and rampant corporate industry called Holler.
the appalachian mountains are older than saturn's rings. the appalachian mountains are older than dinosaurs. the appalachian mountains are older than trees. the appalachian mountains are literally older than BONES. the appalachian mountains should be regarded with pure terror.
The Rockies are actually relatively recent, 55M to 80M years old. The Appalachians are much older and part of the reason they aren't anywhere near as big as (for example) the Rockies, is because they have been eroded for so much longer. That said, they are still definitely not the oldest mountain range. It looks like the Makhonjwa Mountains win that one.