How do I create a geologically realistic series of mountain valleys cutoff from the world?

Question

How do I create an absolutely isolated series of mountain alpine valleys cutoff from the world in a geologically realistic way?

I am trying to create an absolutely isolated series of alpine mountain valleys in a fantasy world with forests home to tribes that cannot travel and have been cutoff from the world. My problem is that the most isolated valleys in Bhutan that I have found appear to be rocky. Alpine valleys require a river or stream to pass through them generally or be in rainfall range. Thus the gorge of those rivers or river valley will allow passage out. Most passes through Bhutan appear to follow gorges, rivers, or the lay of the land.

I am presently researching Tibet and Bhutan for inspiration yet not sure I quiet understand how to place this realistically in my world yet. Even Tibet was populated by humans and passable.

The idea I am coming upon is that a river and streams pass through successive alpine valleys forming small gorges between the valleys before going subterranean once again. Thus travel within this small 'land' of valleys is possible. The valley is blocked north by massive 7,000 to 20,0000ft mountains with a pass that is also impassable. The only high elevation pass south beyond the lower range of mountains is blocked by a colder global climate age thus cutting the valleys off from the world. The area is still biodiverse as it has not always been cutoff from the world.

My other option is that an earthquake creates a pass out of the valley but that would seem more supernatural and perhaps less geologically realistic.

Answer 1

As always, New Guinea to the Rescue!

Turns out there are some isolated tribes on that island who have been isolated for a long, long time, 10-20 thousand years, in fact! Here's a reference: http://www.sciencemag.org/news/2017/09/islanders-new-guinea-are-some-most-diverse-people-world-here-s-why (Note the link between isolation and diversity; makes sense, right?)

Update: I'm emphasizing this genetic diversity point because the different highland types have been isolated from each other as well as the lowland guys on the same island, to hint that such a highland region could work out even if attached to a continent.

Their climate looks kind of like what you want, as the elevation mitigates the tropical nature of it. Check it out...

More details here: https://en.wikipedia.org/wiki/New_Guinea_Highlands#Central_Range_sub-alpine_grasslands but the money quote is:

Above 3,000 metres elevation, the high mountain forest yields to remote sub-alpine habitats including alpine meadows, conifer forest, tree-fern (Cyathea) grasslands, bogs, and shrubby heaths of Rhododendron, Vaccinium, Coprosma, Rapanea, and Saurauia all quite different from the tropical rain forest that covers most of New Guinea.

The alpine habitat above 4,000 metres consists of compact rosette and cushion herbs, such as Ranunculus, Potentilla, Gentiana, and Epilobium, grasses (Poa and Deschampsia), bryophytes, and lichens.[ 2 ]

It even has the word "alpine" in it! ;D I can't promise you edelweiss, but work with me here.

Okay I know PNG is an island, but it's a really big island. Does that count?

Answer 2

Use a very large mesa that has a concave profile, that is, the surrounding steep slopes are the highest points and the inside is like a bowl.

A mesa is usually made of sedimentary rocks, so it can have Karst characteristics, that is large cave systems underneath as well as internal valleys that have been formed by water. This would mean that you could have valleys with rivers that get drained through impassable subterranean cave rivers that appear as waterfalls at the sides of the mesa.

Climate-wise check out akaioi answer.

Answer 3

Put them on an island.

Iceland is pretty isolated because it is in the middle of the North Atlantic.

Iceland has the alpine terrain you are looking for.

http://www.onegeology.org/extra/kids/earthprocesses/alpineglaciers.html

https://www.123rf.com/photo_33927927_warm-summer-days-in-iceland-green-meadows-and-streams-of-meltwater.html

Could people on Iceland make boats and leave? Yes. It is a bit of a haul - 1200 km to Scotland. It is closer to Greenland. Here is what the east coast of Greenland looks like on Google maps.

So: your people are on a remote island, with the closest mainland presenting only forbidding glaciers. Bonus for using alpine Iceland clone: Iceland has volcanoes and geysers! Nothing better for a cold winter than a natural hot tub.

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One more thing to keep your people on their island: sea monsters galore.

https://upload.wikimedia.org/wikipedia/commons/e/ea/Carta_Marina.jpeg

Answer 4

Your people would have to have arrived at their location somehow.

Perhaps, in ages past, a pass existed which lead to or over the top of one of the mountains. Some intrepid explorers discovered a way into these isolated valleys, but returning was impossible or very difficult (I would say that they would have had to be able to leave in order to tell others about their wondrous find, and to bring them back).

Once the area became settled, a natural change of some sort blocked the already difficult passage, or perhaps the settlers wanted to isolate themselves from a tumultuous world and blocked the access themselves.

The river going subterranean is a good idea. If the descendants of the original settlers are unaware of the existence of life outside their valleys, they might consign the bodies of the dead, or those considered cursed to this path into the underworld.

Mountains are created when tectonic plates squish together and lift the land up. Mountains are also created due to volcanic activity. Either of these geologic events would work to isolate your society, or in combination.