The stress that results from the weight of overlying materials (overburden). Also called overburden stress and lithostatic pressure. Rock failure prevents formation pressure from exceeding this value; see Figure P-12. Often specified by a pressure gradient, about 1 psi/ft (2.26×104 Pa/m or 20 lb/gallon mud); the value depends on rock density and tends to increase with depth. Overburden pressure gradient is often expressed as EMW, equivalent mud weight (q.v.).