fold

In geology, a deformation (bend) in beds or layers of rock. Folds are caused by pressures within the Earth's crust resulting from plate-tectonic activity. Rocks are slowly pushed and compressed together, forming folds. Such deformation usually occurs in sedimentary layers that are softer and more flexible. If the force is more sudden, and the rock more brittle, then a fault forms instead of a fold.

Folding can result in gentle slopes or mountain chains such as the Rocky Mountains and the Alps. They can be eroded to form escarpments, giving rise to an undulating topography.

If the bend of the fold is arched up in the middle it is called an anticline; if it sags downwards in the middle it is called a syncline. The line along which a bed of rock folds is called its axis. The axial plane is the plane joining the axes of successive beds.