Foliation in geology refers to repetitive layering in
metamorphic rocks. Each layer may be as thin as a sheet of paper, or over a meter in thickness. The word comes from the Latin
folium, meaning "leaf", and refers to the sheet-like planar structure. It is caused by
shearing forces (pressures pushing different sections of the rock in different directions), or differential pressure (higher pressure from one direction than in others). The layers form parallel to the direction of the shear, or perpendicular to the direction of higher pressure. Nonfoliated
metamorphic rocks are typically formed in the absence of significant differential pressure or sheer. Foliation is common in rocks affected by the regional
metamorphic compression typical of areas of mountain belt formation (
orogenic belts).