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Unformatted text preview: The resulting curve
shows which particle sizes may be enriched or deficient, and allows comparison with other
particle-size distribution curves.
30 Pebble is a general term for a small rock fragment, typically from 4 to 64 mm in diameter, that
has been rounded through the process of stream transport.
Pebble count is a systematic method of sampling and measuring the diameters (b-axis) of a
sufficient number of pebbles (and possibly other rounded rock fragments of smaller and larger
size) to attain a significant representation of the range of sizes and median size of a deposit of
Pediment is a low-angle sloping surface that is typically developed on bedrock or older, partially
consolidated alluvial deposits in an arid or semiarid region; pediments encroach largely by
headward fluvial erosion into the bases of hills, mountains, or plateaus, forming and
maintaining abrupt and slowly receding fronts or escarpments. Thus, processes of
pedimentation yield low-relief surfaces of uniformly gentle slope, or ones that are slightly
concave upward, on which erosion is minimal and the erosion products from the high-relief
bedrock exposures move downslope with little or no permanent storage of sediment. Channel
incision of a pediment may occur following renewed uplift of the bedrock area or incision of the
principal stream at the downslope end of the pediment
Pedogenesis, or soil genesis, refers to the mode of origin of a soil, with emphasis on the
processes of soil-forming factors responsible for the development of the solum (true soil) from
unconsolidated parent material. Among the processes that contribute to pedogenesis are
additions from above of precipitation and contaminants; changes of organic matter to humus
and of rock-forming minerals to hydrous oxides, clays, ions, and H4SiO4; downward transfer of
humus compounds, clays, ions, and H4SiO4; upward transfer of ions and H4SiO4; and removal
(normally by ground-water movement beneath the soil profile) of ions and H4SiO4.
Pedology, a composite science and a synonym of soil science, includes the study of soil
processes and treats or considers the formation, properties, classification, and mapping of soils;
some soil scientists regard pedology to be one of two main sub-disciplines of soil science, the
Pendant bar is a narrow, sharp-crested accumulation of relatively coarse bed sediment
deposited at the downstream (lee) side of a resistant protrusion during a large flood; pendant
bars, which typically are separated from the protrusion (typically bedrock) by a depression
caused by heightened flow velocity and scour around the protrusion, parallel the flow direction
and thus aid in the reconstruction of flood dynamics. The term pendant bar generally is applied
to large-scale landforms of high-magnitude floods, whereas the term sand splay often refers to
flood-plain or terrace features of smaller scale caused by similar processes during floods of high
Perennial stream, as a hydrologic term, is continuously flowing water in a natural stream
channel; the surface of a perennial stream fluctuates at or near the upper level of the zone of
saturation in the adjacent water-bearing alluvium or rocks. 31 Periglacial refers to the conditions, processes, features, and landforms of those areas at and
near the margins of present or former glaciers or glacial conditions; snow and other forms of
precipitation in periglacial areas generally are insufficient to result in the occurrence of glacial
processes, but frost action and related types of physical weathering may be important as
determinants of surface processes and landforms.
Phenology, as a general concept, is the branch of science that treats of relations between
short-term climatic variations and periodic biological phenomena; as a part of ecology,
phenology concerns principally short-term changes in biological processes, such as bird
migration, plant flowering and seed dispersal, and tree growth, that vary with season and
seasonal variations such as air and water temperatures and water availability.
Phreatophyte is a plant dependent on water in the zone of saturation (ground water of the
saturated zone), either directly or through the capillary fringe.
Phytogeomorphology, the blending of the Greek word for plant with the study of landforms,
refers to the study of interactions among processes of geomorphology and plants and to how
landforms and plant occurrences are mutually affected.
Phytolith, from the Greek words for plant and stone, is a mineral body of microscopic size that
precipitates in or around cell walls of plants; the size, shape, and composition of phytoliths
often are diagnostic of the plant species from which they precipitated. Most phytoliths are
opaline silica but many are formed of calcium oxalate. Because phytoliths are inorganic
products of plant growth, they do not decay readily after death of the plant or plant cell and if
extracted from the soil in which they were deposited can be used to recognize previous
vegetation patterns and climate.
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