Glossary - Fundamentals of the Physical Environment - page 725

deposition of sediment and is therefore representative of
fluid velocity, flow conditions and sediment particle size.
Bedding plane A planar boundary separating two layers
of sediment and marking a break in the continuity of
sedimentation or materials; this discontinuity is likely to
be exploited during subsequent weathering and erosion.
Benioff subduction or B-subduction The downward
displacement of oceanic crust into the mantle beneath
continental (or other oceanic) crust by virtue of its greater
density, leading to metamorphism and melting; named
after geologists Wadati and Benioff and less commonly
known as Wadati-Benioff subduction .
Benthic Of the sea- or lake-bed environment.
Berm A sand or shingle bank with a steep seaward face
and flat top marking the upper limit of the swash zone on
a beach.
Beta diversity The diversity within a defined area which
reflects changes between habitats; measured by the degree
of change in a community index along a transect in the
field.
Bifurcation ratio The ratio of the number of streams of
one order to those of the next highest order, providing a
measure of the connectivity of the stream network; see
also stream order .
Biocomplexity Biodiversity studies which include social
and economic considerations.
Biodiversity 'The variability among living organisms
and the ecological complexes of which they are a part; this
includes diversity within species, between species and of
ecosystems' (UN Conference on the Environment and
Development, 1992).
Biogenic sediment Sediment produced by the biological
activity of living organisms and consisting wholly or partly
of their remains or derivatives.
Bioherm An alternative name for a reef which stresses
its biogenic origin.
Biomass The total weight of living biological organisms
within a specified unit (area, community, population).
Biome A major ecological community extending over
large areas;

Bank caving The slumping, sliding or toppling of fluvial
sediments into an active river channel by current
turbulence or at low flow stages when lateral support is
absent.
Bankfull discharge

River discharge through a channel at

maximum capacity.
Bar (1) A ridge of coarse, granular fluvial sediment
deposited where and when stream velocity falls, especially
in mid-stream and on the inside of meanders. (2) The unit
of pressure under the metric (cgs) system; one bar =
0·987 atmosphere.
Barchan An individual sand dune with a crescentic plan
form, pointing downwind, with a gentle windward slope
and a steep leeward slope.
Barchanoid Said of the coalescence of individual barchan
dunes into a transverse dune.
Barrier island A coast-parallel low ridge of coarse
granular debris (sand, gravel) sheltering a lagoon on its
landward side; common on wide-shelf coasts and may
eventually accrete on to the land through storm washover
and migration.
Basal shear stress The shear stress exerted by Earth
materials, particularly glacier ice, moving over their bed.
Basal sliding The sliding of a glacier past its bed and
sides, greatly facilitated by the presence of subglacial water.
Basalt A basic fine-grained extrusive igneous rock.
Base cations Metallic cations (e.g. potassium, calcium,
magnesium, sodium) that are plant nutrients and take
part in cation exchange reactions.
Base exchange The process whereby a basic ion in the
soil solution exchanges with a basic cation adsorbed on a
soil colloid.
Base flow A more enduring component of stream flow
contributed by groundwater transfers.
Base saturation The condition when the entire cation
exchange capacity is occupied by base cations.
Basin and range A zone of continental crustal extension
marked by elevated mountain blocks ( horsts ) separated by
graben fault basins.
Batholith A large mass of intrusive (usually granitic)
igneous rock emplaced at depth in the core of an orogen ;
eventual exposure at Earth's surface indicates the removal
of many kilometres of overlying rock.
Beach A gently sloping, concave sand and gravel
accumulation occupying the foreshore and backshore , the
product of net onshore sediment movement.
Beach cusp The crescentic component of a transient,
sinuous line cut in a beach by breaking waves and caused
by turbulence in the swash zone.
Bed form A feature developed in soft sediment by fluid
motion across its surface; it involves the entrainment or

the dominant plants have a similar

physiognomy.
Biosphere The zone occupied by living organisms at
the common boundary of Earth's lithosphere, hydros-
phere and atmosphere and dependent for its raw materials
on geological fractionation and photosynthesis .
Biotic climax The interacting complex of plants, soils
and animals which develops in a specified region in
response to climate, environmental factors and time.
Bioturbation structure A sedimentary structure, such as
a burrow or cast, produced by the motion or behaviour
of living organisms.