Glossary - Fundamentals of the Physical Environment - page 901

unvegetated surfaces before infiltrating or concentrating in channels, whereas
saturated overland flow emerges towards valleys floors on vegetated slopes.
Oxidation A chemical weathering process involving the combination of oxygen with a
mineral accompanied by a positive shift in its valency.
Palaeocurrent A historic current, aspects of whose former direction, form and energy
level may be inferred from sedimentary structures and textures now preserved in rocks
deposited at that time.
Palsa A small mound of peat and ice formed by segregated ice growth in a peat bog.
Palynology The science of pollen analysis .
Pangaea Earth's most recent supercontinent, formed by the coalescence of most
continental plates c . 300 million years ago and rifted apart c . 200 million years ago.
Parabolic dune A tight crescent-shaped dune with elongated arms pointing upwind,
often developing from a blow-out .
Particle sorting The segregation of debris particles according to their size and the
competence or power of a moving medium; also, the range of particle sizes in a
particular sediment sample expressed by its standard deviation.
Passive margin A tectonically passive continental margin associated with divergent plate
boundaries and marking the zone of initial rifting.
Patterned ground A collective term for a variety of plan-form patterns on a permafrost
land surface formed by turbulent heaving, sifting and collapse in the active layer ;
symmetrical patterns develop in more homogeneous earth materials and flat surfaces,
becoming irregular elsewhere.
Peak discharge value The highest water discharge in a stream channel stimulated by a
precipitation event and appearing as a peak on its hydrograph .
Peat Dark organic material composed of plant residues accumulating under wet or
waterlogged conditions.
Ped A natural soil aggregate consisting of primary particles and colloidal material.
Pediment A concave erosion surface sloping gently down to a lowland plain from rather
more abrupt contact with a mountain front.
Pediplain The coalescence of one or more pediments to create a more extensive lowland,
considered by proponents of denudation cycles to develop through parallel slope
retreat in semi-arid climates.
Pedological process Any process associated with the formation and development of soil.
Pegmatitic An igneous rock texture characterized by very large crystals representing the
final magma fraction.
Pelagic Of the open ocean environment, as opposed to the ocean margin and coastline.
Pelagic sediments Sediments associated with the pelagic zone and excluding terrigenous
material; they consist of the remains of marine organisms and red clays .
Peneplain A lowland plain on which erosion of whatever nature has progressively
obliterated structural and morphological features; considered to be the final stage of a
humid fluvial denudation cycle .
Percolation Water transfer through the voids of unsaturated soil or rock.
Peridotite The coarse-grained, olivine-rich ultramafic rock which forms the
asthenosphere and is the raw material of oceanic crust.
Periglacial A term formerly used to describe the environment and processes around the
margins of a glacier or ice sheet and strongly influenced by its proximity; this use is