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So I'm working on a home-brew D&D setting and I have this idea for the my campaign to take place in a large isolated Valley completely surrounded by mountains. I also had the idea to have these mountains be at the edge of the known world in a cold otherwise inhospitable environment. The valley in question however would be temperate which would spark an initiative to settle the region, causing many brave individuals to make a hard trek for weeks to reach said location motivated by the promise of a land filled with many natural resources.

I am currently trying to come up with a geological explanation as to why the valley would be temperate. I think it would also be cool for said explanation to be a mystery that the players have the opportunity to uncover. Any thoughts would be appreciated. (Also, my world is Loosely Forgotten Realms based)

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    – JBH
    Commented Mar 8, 2019 at 18:00
  • $\begingroup$ You might consider the Okanaga Valley in Canada as a starting point: en.wikipedia.org/wiki/Okanagan Though if it's completely surrounded by mountains, it's going to be a lake :-) *Or desert if the rainfall is low enough, e.g. US Great Basin.) $\endgroup$
    – jamesqf
    Commented Mar 8, 2019 at 18:03

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It may look like a valley, but it's actually the caldera of a volcano

The caldera of a volcano is the crater at the top of the mountain (as a very specific example). So, what you have is a tightly clustered mountain range created over time through volcanic action and your valley happens to be the caldera of one of the volcanoes, but...

  • Your caldera was once a lake, but erosion wore away a side such that the lake drained, leaving a valley.

  • The volcano, bless its heart, is still active. Beneath the caldera is an active magma pocket warming the valley.

Because volcanic soil is often very rich, your valley would be justifiably lush. Also, casting any spell having to do with earthquakes would be bad. Very bad. As in "Oh, Cra..." bad.

Edit: Note that a caldera need not be at the top of a high mountain. Yellowstone National Park is, basically, one huge caldera. What the caldera buys you is justification for the mountains/hills surrounding your valley and its unusual warmth.

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  • $\begingroup$ Yellowstone still freezes in the winter. Only the actual geothermal basins and the rivers fed by them remain snow-free. $\endgroup$
    – Mark
    Commented Mar 8, 2019 at 22:14
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    $\begingroup$ @Mark, I didn't suggest otherwise. I've only used Yellowstone as an example of a caldera that isn't at the peak of a tall mountain. $\endgroup$
    – JBH
    Commented Mar 8, 2019 at 22:28
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    $\begingroup$ Considering how thin the crust over this volcano would need to be for it to make the temperate environment, this can also explain why nobody's living here already... $\endgroup$
    – Ed Grimm
    Commented Mar 9, 2019 at 14:45
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I like JBH's suggestion. If that should be unsuitable for your campaign:

  • The mountain ranges and prevailing winds bring air from more temperate areas. The wind gets "funneled" just to that valley.
  • It is a coastal area surrounded by mountains on three sides and the ocean on the remaining side. A warm current raises the average temperature a couple of degrees.
  • It is the only real valley in the area, but there are plenty of highlands, a thousand metres or so higher. The altitude difference affects temperature and rainfall. That's not immediately obvious because the passes between the mountains are yet higher.
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  • $\begingroup$ I upvoted this answer because it's a great idea. What o.m. is talking about is making the valley a narrow banana belt. In other words, the mountain range is such that warm weather from the southern oceans either fall at the coast or are funneled through the valley. All other areas (other than the cost itself) are cold. Nice idea! $\endgroup$
    – JBH
    Commented Mar 8, 2019 at 23:12
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One possible explanation is a combination of a Rain Shadow and a Foehn Wind. Long story short, is that the height of mountain range will interfere with normal precipitation patterns, and create a warm dry spot on the leeward side of the mountain.

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