Mathematics—especially geometry—is instrumental in understanding rock layers. In a branch of geology called stratigraphy, scientists measure angles and planes in rock in order to know the location of certain rock layers and the possible geologic events that affected the layers over time. In particular, geologists measure strike and dip. Strike is the angle between true north and a horizontal line contained in any planar feature, such as a fault (usually caused by an earthquake) or inclined bed (often caused by the uplift of hot molten rock around a volcano). Dip is the angle at which a bed or rock vein is inclined to the horizontal; it is measured perpendicular to the strike and in the vertical plane (as opposed to the strike’s horizontal line).