GLOSSARY - Physical Geography - page 156

higher low tides. It usually occurs during the first and third quarters of
the Moon.
Occluded front A faster, steeper cold front overtakes a slower warm
front and lifts the mass of warm air upwards.
Organic farming Farming which prohibits or minimises the use of
human-made fertilisers and pesticides, and relies on natural biological
processes to maintain soil fertility and control pests.
Oxbow lake A crescent-shaped section of river channel cut off from
the main river channel by sediment deposition and the slow migration
of the channel location.
Peds Naturally occurring clumps of soil particles.
Periglacial Cold conditions with intense frost action but where there
are no glaciers.
Permafrost A zone in soil which has been frozen for more than two

years.
Photosynthesis The process by which organisms such as plants and
algae (autotrophs) create carbohydrates and release oxygen using light
energy, carbon dioxide and water.
Pingos An ice-cored mound of earth that can reach 60 metres high and
500 metres in length, which are only found in periglacial areas. They
are caused by the doming of the frozen ground beneath a former water
body.
Planform The shape of the river channel as viewed when looking
down from the air.
Plant functional types A way of classifying plant species based on their
traits and association with specific environmental variables. Plant func-
tional types are used by environmental models to predict how certain
groups of species might respond to climate change.
Plate tectonics A theory that concerns the large-scale movements of
the Earth's crust (lithosphere). The crust is split into several 'plates',
categorised as either oceanic or continental, which are moved by con-
vection within the liquid mantle upon which they float. The move-
ment of the plates is responsible for mountain building, trenches and
earthquakes.
Ploughing boulders Boulders found on periglacial slopes that move
slowly downslope pushing through the soil, leaving a trough behind
them and creating a bulge of sediment in front of the boulder. The
movement is thought to occur due to the difference in thermal con-
ditions beneath the boulder compared with its surroundings.
Podzolisation Rainwater in cool climates flows through a thick upper
litter layer to produce an organic rich leachate which then flows