Facies: The set of all characteristics of a sedimentary rock that indicates its particular environment of deposition and which distinguish it from other facies in the same rock.
Fault: A planar or gently curved fracture in the Earth's crust across which there has been relative displacement.
Fault-block mountain: A mountain or range formed as a horst when it was elevated between parallel normal faults.
Fault plane: The plane that best approximates the fracture surface of a fault.
Faunal succession: The evolutionary sequence of life forms, especially as recorded by the fossil remains in a stratigraphic sequence.
Felsic: An adjective used to describe a light-colored igneous rock poor in iron and magnesium content, abundant in feldspars and quartz.
Fiord: A former glacial valley with steep walls and a U-shaped profile now occupied by the sea.
Fissure: An extensive crack, break, or fracture in the rocks.
Fissure vein: A cleft or crack in the rock material of the earth's crust, filled with mineral matter different from the walls and precipitated therin from aqueous solution.
Flood basalt: A plateau basalt extending many kilometers in flat, layered flows originating in fissure eruptions.
Flood plain: A level plain of stratified alluvium on either side of a stream; submerged during floods and built up silt and sand carried out of the main channel.
Flood tide: The part of the tide cycle during which the water is rising or leveling off at high water
Flow cleavage: In a metamorphic rock, the parallel arrangement of all planar or linear crystals as a result of rock flowage during metamorphism.
Fluid inclusion: A small body of fluid that is entrapped in a crystal and has the same composition as the fluid from which the crystal formed.
Flume: A laboratory model of stream flow and sedimentation consisting of a rectangular channel filled with sediment and running water.
Focus (earthquake): The point at which the rupture occurs; synonymous with hypocenter.
Fold: A planar feature, such as a bedding plane, that has been strongly warped, presumably by deformation.
Foliation: Any planar set of minerals or banding of mineral concentrations including cleavage, found in a metamorphic rock.
Foraminifera: A class of oceanic protozoa most of which have shells composed of calcite.
Foraminiferal ooze: A calcareous sediment composed of the shells of dead Foraminifera.
FORCE:An influence that if applied to a free body results chiefly in an acceleration of the body and sometimes in elastic deformation and other effects.
Forset bed: One of the inclined beds found in crossbedding; also an inclined bed deposited on the outer front of a delta.
Formation: The basic unit for the naming of rocks in stratigraphy: a set of rocks that are or once were horizontally continuous, that share some distinctive feature of lithology, and are large enough to be mapped.
Fossil: An impression, cast, outline, or track of any animal or plant that is preserved in rock after the original organic material is transformed or removed.
Fossil fuel: A general term for combustible geologic deposits of carbon in reduced (organic) form and of biological origin, including coal, oil, natural gas, oil shales, and tar sands.
Friction breccia: A breccia formed in a fault zone or volcanic pipe by the relative motion of two rock bodies.
Fringing reef: A coral reef that is directly attached to a landmass not made of coral.
Fumarole: A small vent in the ground from which volcanic gases and heated groundwater emerge, but not lava.