Rock deformation is broadly classified as brittle (involving fracturing and discrete discontinuities, such as faults, fractures, and joints) or ductile (involving continuous, permanent flow/bending of rock without loss of cohesion, such as the folding of rock layers); buckle folds, formed by layer-parallel shortening causing rock layers to bend and warp continuously, are a classic example of ductile deformation, whereas faults, fractures, and joints all involve brittle failure along discrete break surfaces.