What is Fold (geology)?
[[File:Folded Rock.jpg|230px|right|thumb|Very tight folds in [[New South Wales]], [[Australia]]]] [[Image:Kielce-slichowice2.jpg|thumb|230px|A fold in Slichowice nature reserve in [[Kielce]], [[Poland]] (Variscan [[orogeny]])]] [[File:Rainbow_Basin.JPG|thumb|210px|Rainbow Basin Syncline near Barstow, [[California]], [[United States]]]] Rock often deforms in such a way that it bends instead of breaking. This is called a '''fold'''. The term '''fold''' is used in [[geology]] when one or a stack of originally flat, level surfaces, such as [[sediment]]ary [[strata]], are bent or curved as a result of [[pressure]] and high temperature. The basic cause is likely to be some aspect of [[plate tectonics]]. When two forces act towards each other from opposite sides, rock layers are bent into ''folds''. How folds are formed due to [[compression]] is known as '''folding'''. Folding is one of the [[endogenetic process]]es; it takes place within the Earth's [[Crust (geology)|crust]]. Folds in rocks vary in size from microscopic crinkles to mountain-sized folds. They occur singly as isolated folds and in extensive fold trains of different sizes, on a variety of scales. A set of folds distributed on a regional scale constitutes a ''fold belt'', a common feature of [[orogeny|orogenic zones]]. There are large-scale and small-scale folds. Large-scale folds are found mainly along a [[collision boundary]] between two [[tectonic plate]]s. == Structure of a fold == The upfold is called an anticline. The downfold is called a syncline. The imaginary line joining the highest points along the upfold is called the crest line. The [[flanks]] of a fold are known as the limbs. The central line from which the rock strata dip away in opposing directions is called the axis of fold. According to the degree of folding of the layers, folds can be classified into five main types. * symmetrical fold: two limbs are of equal steepness * Asymmetrical fold: one limb is steeper than the other * Overfold: one limb is pushed over the other * Re