Glossary - Fundamentals of the Physical Environment - page 890

Grey box A partially understood system, in which interest is centred on a restricted
number of subsystems and the remainder are ignored.
Grey dune zone A stage in a psammosere on sand dunes when mosses and lichens
colonize the ground surface, and darken the colour of the yellow sand.
Gross primary productivity ( GPP ) The total amount of solar energy fixed in
photosynthesis by autotrophs per unit area per unit time.
Ground heave Small-scale ground expansion and uplift in unconsolidated materials
through hydration, ice formation and influent water seepage, with consequent
disturbance of any incipient structure and its strength.
Ground ice Any form of frozen water below the land surface, irrespective of its origin
and whether it is interstitial or segregated ice .
Ground moraine A surface veneer of glacial till which is not differentiated by individual
glacial depositional landforms.
Grounding line The water depth at which an ice shelf or tidewater glacier begins to float
by virtue of its lower density than water.
Groundwater The portion of all subsurface water stored in saturated rock below the
water table; sometimes extended to include water in the overlying unsaturated layer.
Gulf Stream The warm, north-east-flowing current of the clockwise gyre in the North
Atlantic Ocean, flowing from the Florida coast into the Arctic basin past Britain and
Norway; also known as the North Atlantic Drift.
Gully A modest, steep-sided channel eroded by intermittent stream flow with a frequency
and vigour capable of keeping the channel open.
Guyot A flat-topped submarine mountain or seamount .
Gyre A system of surface, wind-driven ocean currents forming a closed or partially
closed circulation which transfers heat from warmer to colder surface waters.
Halocline A zone of marked change in salinity with ocean depth.
Halophyte A plant adapted to growth in saline environments.
Halosere The sequence of plant communities which, successively, occupy a salt marsh.
Hamada An upland desert land surface of wind-scoured bare rock with patches of lag
gravels .
Hardware model A model of a system composed of real objects. A flume is a hardware
model of a river; a wind tunnel is a hardware model of air flow near the ground.
Headward retreat The upslope migration of the point of initiation of channel flow as it
continues to attract ever more surface or saturated overland flow .
Heinrich event A surge of marine-based portions of the former Laurentide ice sheet in
Canada which sent a pulse of icebergs, ice-rafted debris and cold fresh water into the
north-west Atlantic Ocean.
Helical flow A spiral motion superimposed on the general direction of stream flow or air
flow which causes lateral transport of energy and entrained materials.
Heliophyte A sun-loving plant, adapted to high exposure to sunlight in its mature form.
Hemicryptophytes Tussock plants whose buds are located at or just below the surface of
the soil.
Herbivores Organisms which eat plants and therefore occupy the second trophic level in
ecosystems.
Holocene The second epoch of the Quaternary period, which started 10,000 years ago
and in which we now live.