|This article is about an older version of DF.|
A mountain is a naturally high elevation usually culminating in a peak. They are usually formed by the collision of tectonic plates or through volcanic activity. Volcanoes are usually mountains, but mountains are not necessarily volcanoes.
As part of a starting site, mountains provide jagged terrain and impassable cliffs. They usually have very limited water resources, i.e., few ponds or streams, although many streams and rivers originate from the base of mountain ranges. They often have sparse vegetation, and depending on the elevation may lack any sort of usable soil. It's impossible to cross a Mountain tile in Travel mode, even if you have flying capabilities.
The stratification listed for mountains in Dwarf Fortress usually consists of one lighter rock type, generally a sedimentary stone (such as limestone, mudstone or dolomite), and then two further layers of heavy intrusive igneous (such as Granite, Rhyolite, or Gabbro) or metamorphic rock (such as Gneiss, Quartzite, or Phyllite). Stone, and subsequently ores, are always in abundance in mountain biomes.
It is not possible to start a fortress on an exclusively mountain biome; however, fortress sites can straddle mountains on one side and more (or less) hospitable biome on the other, which will provide more valuable diversity of wildlife and resources.
Mountains imply high elevation in the surrounding area. This can result in a "forest" biome that does not actually have any trees, since above a certain elevation there may be no soil and therefore no place for trees to actually grow. When choosing a starting fortress site, be sure to check the elevation map (using the tab key), and try to get a mix of low and medium or high elevations if possible.