Glossary - Fundamentals of the Physical Environment - page 749

ment; this may be so, or the structures may reflect post-
depositional removal or settling of fines or platy debris.
Stratigraphy The study of the organization and sequence
of geological strata, with various specialist subsets covering
minerogenic materials (lithostratigraphy), biogenic
materials (biostratigraphy) and their age relationships
(chronostratigraphy).
Stratosphere Layer of atmosphere above troposphere up
to about 50 km. Temperatures increase upwards as a result
of warming by ultra-violet absorption.
Stratovolcano A composite volcano built up by the
accumulation of successive, stratified layers of lava and
pyroclasts during its eruption history.
Stream competence A measure of the ability of stream
flow to maintain particles in motion, in terms of their size
and current velocity.
Stream gauging The manual or automatic measurement
of water velocity, depth and wetted channel cross-sectional
area from which discharge is calculated, in cubic metres
per second.
Stream order The designation of the position of a
stream channel, by a value from 1 to n , in a network,
indicative directly or indirectly of the number of tributary
channels contributing to a channel of a particular number.
Stream power The rate of energy supply available for
work at the stream bed, measured in W m -2 .
Striation A scratch on a rock surface made by the
abrasive contact of a moving material of greater hardness
and aligned in the direction of motion.
Strike The orientation of a geological structure (e.g.
fold or fault) in the horizontal plane.
Strike slip That part of the displacement of rocks on
opposite sides of a fault which is parallel to its orientation
and thus in the horizontal plane.
Stromatolite A dome-shaped calcareous mat of algae
and trapped, fine-grained sediment accumulating in
shallow-water lagoons.
Structural basin A crustal depression defined by large-
scale geological structures rather than the product of
surface erosion.
Structural control The influence of structures or planar
discontinuities on the denudation and geomorphic
development of the land surface through the defects which
they create in rock-mass strength and their geometric
arrangement.
Structureless soil A soil showing no aggregation into
peds , i.e. single grain or massive.
Subduction zone A linear boundary between convergent
plates at which one plate is drawn or forced down under
the other, usually by virtue of its greater density; the zone

Specific susceptibility The susceptibility of rock to a
specific weathering process determined by its specific
lithological (chemical and structural) characteristics.
Specific yield The volume of water released from an
aquifer by gravity flow, sometimes measured as a ratio of
that volume to the volume of rock.
Spit A narrow, coarse-grained sediment bar extended
across a bay or estuary by longshore drift from a headland
and often curved at its free end in response to estuarine
cross-currents.
Spring sapping The undercutting or headward retreat of
the slope immediately above a spring or point of initiation
of a stream channel by the concentration of erosive power.
Spring tide The twice-monthly tidal period when the
gravitational pull of the sun and that of the moon are in
line and maximize tidal range.
Stability of soil structure The ability of the soil structural
units to remain coherent under an applied stress, e.g.
raindrop impact, waterlogging or ploughing.
Stack A residual rock pinnacle which marks coastal cliff
retreat and/or the landward advance of a rock platform .
Stage The height of the water surface above a specific
location in a fluvial channel, usually the deepest point.
Standard deviation A measure of the variability within

a data set.
Standing crop

The total weight of living organic

material per unit of area at any one time.
Standing wave A water wave which oscillates vertically
between two points without propagating horizontally.
Steady-state equilibrium The state of a system where the
steady output of matter and energy is equal to the input
over a particular period of time.
Stem flow That portion of intercepted precipitation
which is concentrated and transferred towards the ground
by plant stems and trunks.
Stern layer Inner layer of adsorbed ions in the double
layer of ions which surround soil colloids.
Stomata Microscopic pores on plant leaves through
which most water vapour and other gaseous exchanges
take place.
Storage Locations where energy and matter may be
stored for certain periods of time.
Storage capacity

The amount of available water which

a soil can hold.
Storm surge An abnormal rise in sea level driven against
the coast by extreme weather events, most severe and
liable to cause coastal damage when coinciding with high
spring tides .
Stratified scree Scree material showing apparent
bedding planes dipping downslope, suggesting episodic
solifluction or sheet flow typical of a permafrost environ-