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.