# Geometry of a fold

• An upward fold is called an anticline, while a downward fold is called a syncline.
• In many areas it’s common to find a series of anticlines and synclines , although some sequences of rocks are folded into a single anticline or syncline.
• A plane drawn through the crest of a fold in a series of beds is called the axial planeof the fold.
• The sloping beds on either side of an axial plane are limbs.
• An anticline or syncline is described as symmetricalif the angles between each of limb and the axial plane are generally similar, and asymmetrical if they are not.
• If the axial plane is sufficiently tilted that the beds on one side have been tilted past vertical, the fold is known as an overturned anticline or syncline.
• A very tight fold, in which the limbs are parallel or nearly parallel to one another is called an isoclinalfold
• Isoclinal folds that have been overturned to the extent that their limbs are nearly horizontal are called recumbent folds.
• Folds can be of any size, and it’s very common to have smaller folds within larger folds .
• Large folds can have wavelengths of tens of kilometres, and very small ones might be visible only under a microscope.
• Anticlines are not necessarily, or even typically, expressed as ridges in the terrain, nor synclines as valleys.
• Folded rocks get eroded just like all other rocks and the topography that results is typically controlled mostly by the resistance of different layers to erosion.