Editing Mountain chain (section)

== Formation of parallel mountain chains ==
[[File:Satellitenaufnahme der Alpen.jpg|thumb|upright=1.2|In this satellite image of the [[Alps]], the snow limit picks out the individual mountain chains]]
[[File:Централен Балкан.jpg|thumb|260px|A view of the [[Balkan Mountains]] chain]]

The chain-like arrangement of summits and the formation of long, jagged [[mountain crest]]s – known in Spanish as [[wikt:sierra#Spanish|sierras]] ("saws") – is a consequence of their collective formation by [[orogeny|mountain building forces]]. The often linear structure is linked to the direction of these thrust forces and the resulting [[folding (geology)|mountain folding]] which in turn relates to the [[fault line]]s in the upper part of the [[Earth's crust]], that run between the individual mountain chains. In these [[fault zone]]s, the rock, which has sometimes been pulverised, is easily eroded, so that large [[river]] valleys are carved out. These, so called [[longitudinal valley]]s reinforce the trend, during the early mountain building phase, towards the formation of parallel chains of mountains.

The tendency, especially of fold mountains (e.&nbsp;g. the [[Cordilleras]]) to produce roughly parallel chains is due to their rock structure and the propulsive forces of [[plate tectonics]]. The uplifted rock masses are either magmatic [[plutonic rock]]s, easily shaped because of their higher temperature, or [[sediment]]s or [[metamorphic rock]]s, which have a less robust structure, that are deposited in the [[syncline]]s. As a result of orogenic movements, strata of folded rock are formed that are crumpled out of their original horizontal plane and thrust against one another. The longitudinal stretching of the folds takes place at right angles to the direction of the lateral thrusting. The [[overthrust]] folds of a [[nappe]] belt (e.g. the [[Central Alps]]) are formed in a similar way.

Although the fold mountains, chain mountains and nappe belts around the world were formed at different times in the Earth's history, all during their initial mountain building phases, they are nevertheless morphologically similar. Harder rock forms continuous [[arête]]s or ridges that follow the [[strike (geology)|strike]] of the beds and folds. The mountain chains or ridges therefore run approximately parallel to one another. They are only interrupted by short, usually narrow, [[transverse valley]]s, which often form [[water gap]]s. During the course of Earth history, [[erosion (geology)|erosion]] by water, ice and wind carried away the highest points of the mountain crests and carved out individual summits or [[double summit|summit chains]]. Between them, notches were formed that, depending on [[altitudinal zonation|altitude]] and rock-type, form knife-edged [[col]]s or gentler [[mountain pass]]es and [[saddle (geology)|saddles]].