B

BACKSHORE: The upper, generally dry, zone of the shore, extending landward from the upper limit of wave wash at high tide to the upper limit of shore-zone processes.

BACKWASH: The return flow of water down a beach after a wave has broken.

BADLAND: Topography characterized by intricate patterns of stream erosion developed on surfaces with little or no vegetative cover overlying unconsolidated or poorly cemented clays, silts, or sands.

BARCHAN: A crescent-shaped eolian sand dune that moves across a flat surface with its convex face upwind and its concave slip face downwind.

BARRIER ISLAND: A long, narrow island parallel to the shore, composed of sand and built by wave action.

BASAL SLIP: The sliding of a glacier along its base.

BASALT: A fine-grained, dark, mafic igneous rock composed largely of plagioclase feldspar and pyroxene: the extrusive equivalent of gabbro.

BASE LEVEL: The level below which a stream cannot erode: usually sea level, sometimes locally the level of a lake or resistant formation.

BASEMENT: The oldest rocks recognized in a given area; a complex of metamorphic and igneous rocks that underlies all the sedimentary formations. Usually Precambrian or Paleozoic in age.

BASIN (TECTONIC): A circular, synclinelike depression of strata that dips radically toward a central point.

BATHOLITH: A great, irregular mass of rock cutting across the country rock, with an exposed surface of at least 100 km²; usually an intrusive igneous rock, but sometimes derived from the country rock through very high temperature and pressure metamorphism. (See also Discordant intrusion.)

BAUXITE: A rock composed primarily of hydrous aluminum oxides and formed by intense chemical weathering in tropical areas with good drainage; a major ore of aluminum.

BEDDING: A characteristic of sedimentary rocks in which parallel planar surfaces separate layers of different grain sizes or compositions deposited at different times.

BEDDING SEQUENCE: A pattern of interbedding of different sedimentary rock types or sedimentary rocks with different sedimentary structures that is characteristic of a certain sedimentary environment.

BED LOAD: The sediment that a stream moves along the bottom of its channel by rolling and bouncing (saltation).

BEDROCK: The solid rock underlying unconsolidated surface materials, such as soil.

B-HORIZON: The intermediate layer in a soil, below the A-horizon and above the C-horizon, consisting of clays and oxide materials.

BIOCHEMICAL SEDIMENT, ROCK: A sediment or rock containing the mineral remains of organisms, such as shells, or minerals precipitated as a result of biological processes, such as in iron formations.

BIOMASS: Organic carbon-containing material of biological origin, including living and dead animals and plants.

BIOSPHERE: The parts of the atmosphere, hydrosphere, and lithosphere occupied by living organisms.

BIOTURBATION: The reworking of existing sediments by organisms.

BLOWOUT: (1) A parabola-shaped eolian sand dune, typically one blown back from a beach, that has its convex slip face oriented downwind. (2) A shallow circular or elliptical depression in sand or dry soil formed by wind erosion. (See also Deflation.)

BLUESCHIST: A metamorphic rock formed under conditions of high pressure (in excess of 5000 bars) and relatively low temperature, often containing the blue minerals glaucophane (an amphibole) and kyanite.

BOLIDE-IMPACT HYPOTHESIS: The proposal that an extraterrestrial object slammed into Earth 65 million years ago, causing a global climate change that wiped out half of Earth's species, including the dinosaurs.

BOTTOMSET BED: A flat-lying bed of fine sediment deposited seaward of a delta and then buried by continued delta growth.

BOWEN REACTION SERIES: A simple schematic description of the order in which different minerals crystallize during the cooling and progressive crystallization of a magma.

BRAIDED STREAM: A stream so choked with sediment that it divides and recombines numerous times, forming many small and meandering channels.

BRECCIA: See Sedimentary breccia; Volcanic breccia.

BRITTLE MATERIAL: A material that breaks abruptly when its elastic limit is reached; the opposite of a ductile material.

BURIAL METAMORPHISM: A low-grade metamorphism in which buried sedimentary rocks are metamorphosed by the heat and pressure exerted by overlying sediments and sedimentary rocks; bedding and other sedimentary structures are preserved.