Folds form under varied conditions of stress, pore pressure, and temperature gradient, as evidenced by their presence in soft sediments, the full spectrum of metamorphic rocks, and even as primary flow structures in some igneous rocks. A set of folds distributed on a regional scale constitutes a fold belt, a common feature of orogenic zones. Folds are commonly formed by shortening of existing layers, but may also be formed as a result of displacement on a non-planar fault ("fault bend fold"), at the tip of a propagating fault ("fault propagation fold"), by differential compaction or due to the effects of a high-level igneous intrusion e.g. above a laccolith.