Glossary - Fundamentals of the Physical Environment - page 738

Laccolith A lens-shaped igneous rock body of moderate
size (10 3-4 m long and 10 1-2 m thick) formed by accordant
( sill -like) intrusion into existing rocks.
Lag deposit A residual accumulation of coarse rock
fragments, too large to move in a particular force field,
after the removal of fines.
Lag gravel A surface layer of loose, coarse granular
debris left after the deflation or surface wash of fines.
Lag time The time lapse between a stimulus and its
effect such as that between the peak of a precipitation
event and the peak discharge response of a stream.
Lahar A volcanic mud flow of liquefied volcanic and
other debris, exacerbated by the melt and disruption of
summit glaciers or crater lakes.
Laminar flow Fluid flow in which the direction of each
individual flow strand remains discrete and unidirec-
tional, although strands may shear past each other as the
channel walls are approached.
Land degradation The reduction, or loss, of land surface
stability and its bioproductivity as a result of adverse
human activity.
Lapilli

Leaf drip The concentration and onward transfer, as
large drops, of precipitation intercepted by a leaf.
Leaf leaching The removal by rainfall of chemicals,
including nutrients, from the surface and interior of a
plant leaf.
Leaf uptake The uptake of nutrients via stomata on

leaves.
Lee wave A lens-shaped cloud forming in a standing
wave of turbulent air in the lee of a mountain barrier, with
continuous condensation as air rises and cools at its
leading edge and evaporation as air falls and warms at its
trailing edge.
Levée A bank of coarse debris flanking a floodplain
river, formed by the concentration of suspended sediment
during overbank discharge ; boulder levées also flank debris
flows as a result of collision and ejection during their
turbulent flow.
Limiting equilibrium A state of balance between shear
stress and shear strength - or eroding and resisting forces
- in Earth materials, defined by Mohr-Coulomb criteria .
Lineament A large-scale, linear feature of tectonic or
other structural origin, visible at the land surface; it may
be (or represent the trace of) a fault , suture , fracture zone,
etc.
Liquid limit The critical water content of a granular
solid beyond which it develops liquid behaviour.
Lithification The transformation of unconsolidated
sediments into a cohesive rock mass through syngenetic
and diagenetic dewatering and cementation, compaction
and crystallization.
Lithology The macroscopic character of rock mass
determined by its geochemical (mineral) and mechanical
(particulate) components and related structures.
Lithosere The sequence of plant communities which,
successively, occupy a bare rock surface.
Lithosphere The rigid, outermost solid layer of Earth
and its upper mantle, which supports crustal plates .
Little Ice Age The period from about AD 1500 to AD
1800 when climatic conditions in Europe were much
colder than before or since. Many glaciers advanced in the
Alps and Scandinavia.
Load The total mass of mineral and organic sediment
transported by a stream by bed traction, suspension and
solution.
Load casts A protrusion from the base of one sedi-
mentary lamina into the underlying surface of another by
syngenetic deformation due to unequal settling, water
content or mass.
Loch Lomond stadial The period between 10,800 and
10,000 BP when glaciers reappeared in many of the
mountain areas of the British Isles. Other parts of the

Unconsolidated coarse-grained pyroclastic

material 2-64 mm in diameter, ejected into the

atmosphere by volcanic eruption.
Latent heat The quantity of heat absorbed or emitted
during a change of state of a substance. In climatology it
usually refers to the change of state of water from solid to
liquid to vapour or vice versa.
Lateral convection Convectional processes in the oceans
brought about by horizontal differences in density.
Laterite A reddish tropical clay composed of the
sesquioxides of iron and aluminium, and kaolinite; it
hardens irreversibly on drying, often with a concretionary
structure.
Lava An extrusive flow of molten magma and the rock
into which it solidifies.
Law of the minimum The law which states that the
productivity of an ecosystem is controlled by, or is
proportional to, the growth factor which is operating at
a minimum (i.e. in shortest supply).
Leaching The washing out of materials in solution or
suspension from a soil horizon or profile.
Leading edge The advancing edge of a continental plate,
marked by a coastal orogen, narrow continental shelf and
deep offshore trench which influence the nature of
coastline development (cf. trailing edge ).
Leaf area index A measure of the density of vegetation
surfaces capable of intercepting precipitation or insolation ,
given as the total area of leaves in the layered canopy
covering a unit area of ground.