Folding

Folds result from the plastic deformation of rocks at low strain-rates, usually under elevated temperature and pressure conditions. Folds are braodly subdivided into anticlines (upwards convex) and synclines (downwards convex).

In synclines and anticlines, the axial plane is the plane of symmetry passing through the apex of the fold. The line of intersection of the fold apex and the horizontal plane is called the axis of the fold.

If the fold-axis is inclined to the horizontal, the "dip" of the axis is called the plunge. Plunging folds are the rule rather than the exception. Folds with a horizontal axis are a two-dimentional idealization. In nature, folds are symmetric or asymmetric plunging structures.

Symmetric plunging anticlines and synclines produce characteristic "bulls-eye" outcrop patterns. In synclinal folds, the beds at the centre of the pattern are the youngest and the beds get older in a radial direction. Such structures are called basins. In anticlinal plunging folds, the beds increase in age towards the centre of the pattern. Such structures are called domes.