FOLDS, a term applied to bends or flexures in the rocks of the earth's crust. They vary from a few inches across to structures many miles in extent. Up-arches from which rocks dip outward in both directions are known as anticlines. Down folds, or toward the centre of which beds dip from both direc tions are known as synclines. A flexure in the rocks with dip in only one direction is a monocline. Anticlines and synclines may be broad gentle folds of simple character, or they may be composite, with many minor crumplings on the limbs of the major fold. Such com posite folds are known as anticlinoria and synclinoria. Very broad, gentle, simple folds are sometimes spoken of as geanticlines and goesynclines. Nearly circular anticlines, from which the beds dip outward equally in all directions from the central point, are known as domes or quaquaversal folds. Nearly cir cular synclines are called centroclinal folds. Sometimes folds are so closely compressed that the two sides become nearly parallel, in which case they are called isoclines. Of course there may be Isoclinal synclines and isoclinal anticlines. If folding has gone so far that one side of the fold has passed the vertical and the beds are upside down, the fold is said to be overturned. If overturned so far that the
reversed beds are nearly horizontal the fold is recumbent. Overturned and even recumbent folds are common in closely folded mountain regions, such as the southern Appalachians. The trough or crest of the fold is called the axis. It is rarely horizontal over any consider able distance. The angle of inclination of the axis is called the pitch of the fold.
After folds have been made (see Mome TAINS), erosion starts cutting them down, and if given a sufficiently long time will plane the topographic irregularities away completely. They are still folds, however. In such a case any given bed' will outcrop in two narrow bands, one on each flank of the fold. If the fold is without pitch the bands will be parallel, but if the fold pitches the beds will meet at one end or the other in the shape of a long narrow Closely folded regions are also often cut by large faults (q.v.) and are frequently areas of intense metamorphism. (q.v.). For a discussion of the origin of folds, see MOUNTAINS.