I’m world building, how ever I am not a geography guru (so please forgive me for any blatant errors or stupidities). I’m trying to severely isolate this tribe. I want them to have moved (back in pre-history) from a plain to a valley between two mountain ranges and then been closed in by something. I was thinking a canyon, but I have no idea how long it takes for canyons to form or if that’s even feasible (aren’t canyons a desert thing?). Maybe a cliff formed instead? Help me out. It’s a relatively lush area, full of forests. I had the idea that in spring time there’s a lot of temporary streams.
For that matter, how small of an area can I make this? They’re hunters and occasionally farmers (no domesticated animals). I figured they can clamber up the mountains or down the canyon / cliff to hunt and fish or otherwise take long trips to gather food.
Without a lot of time to flesh out an answer, and as a guide to further discussion in this thread, I suggest having a look at the gegography of Papua New Guinea.
Many tribes there were historically isolated in mountain valleys. Even tribes separated by a dozen miles (as the crow flies) spoke in languages unintelligible to one another.
For volcanoes, there are the Mexican ones - field one day, great honking volcano a year later - and then there’s the Deccan Traps. Nothing seals in a valley like having to cross a sea of lava.
An earthquake might shatter a mountain, causing a huge pile of rubble to seal the valley. Or it might cause land to shift upwards or downwards. Or it might open a gap through which the sea can flow (q.v. the Mediterranean). The sea can then evaporate leaving a vast and impassable salt flat. Or you can reverse it: think of the Great Rift Valley. Huge rift walls on either side, with the habitable area in the middle, the rift stretched every so often by earthquakes.
You’d need a year-round supply of fresh water, a big enough area for game to be harvested and have it replenish itself, a spot to grow some tubers, perhaps.
You could have a scenario where the tribe lives in the valley and the route they use to make their way up and down the cliff was suddenly destroyed by a landslide.
I suppose a glacier could seal off the entrance to a valley - in extreme conditions they can form reasonably quickly, but you’re still talking decades, probably, for anything substantial.
If an earthquake lowered the valley entrance by several feet, it could easily become a swamp if rivers backed up and couldn’t flow as before. Depending on all sorts od other factors, I could see it becoming a bit impassable at least!
Especially if man-eating crocodiles just happened to move in!
Ngorongoro Crater is a volcano caldera with pretty tall and steep walls that is a self-contained ecosystem. It’s damn spectacular, although probably not what you are looking for.
