Patterned ground — A term for the more or less symmet- 

 rical forms such as circles, polygons, nets, stripes, gar- 

 lands, and steps that are characteristic of, but not 

 confined to, mantles subjected to intense frost action 

 as in periglacial environments. It is classified according 

 to type of pattern and presence or absence of sorting. 

 Patterned ground occurs principally in polar, subpolar, 

 and artic regions, but also includes features in tropical 

 and subtropical areas. Stone polygons generally form 

 on slopes of less than 8 percent, while garlands and 

 stripes occur on slopes 8 to 15 percent and more than 

 15 percent, respectively (Parsons 1976). (fig. B-8) HP 



Peak — Sharp or rugged upward extension of a ridge chain, 

 usually at the junction of two or more ridges; the promi- 

 nent highest point of a summit area. HP 



Peat — Unconsolidated soil material consisting largely of 

 undecomposed, or only slightly decomposed, organic 

 matter accumulated under conditions of excessive 

 moisture. GSST 



Pediment — A gently sloping erosional surface developed 

 at the foot of a receding hill or mountain slope. The 

 surface may be essentially bare, exposing earth mate- 

 rial that extends beneath adjacent uplands; or it may be 

 thinly mantled with alluvium and colluvium, ultimately 

 in transit from upland front to basin or valley lowland. 

 (In hill-footslope terrain the mantle is designated "pedi- 

 sediment" by Ruhe). The term has been used in several 

 geomorphic contexts: Pediments may be classed with 

 respect to (1) landscape position, for example, inter- 

 montane-basin piedmont or valley-border footslope 

 surfaces (respectively, apron and terrace pediments of 

 Cooke and Warren), (2) type of material eroded, bed- 

 rock or regolith, or (3) combinations of the above. HP 



Pedisediment — A layer of sediment eroded from the 

 shoulder and backslope of an erosional slope, that lies 

 on and is, or was, being transported across a pediment 

 (footslope). FFP 



Peneplain — A low nearly featureless, gently undulating 

 land surface of considerable area, which presumably 

 has been produced by the processes of long-continued 

 subaerial erosion (primarily a mass wasting of and 

 sheetwash on interstream areas of a mature landscape, 

 assisted by stream erosion) almost to base level in the 

 penultimate stage of a humid, fluvial geomorphic 

 cycle; also such a surface uplifted to form a plateau 

 and subjected to dissection. A peneplain may be char- 

 acterized by gently graded and broadly convex inter- 

 fluves sloping down to broad valley floors, by truncation 

 of strata of varying resistance and structure, by accor- 



dant levels, and by isolated erosion remnants rising 

 above it. GG 



Periglacial — Pertaining to processes, conditions, areas, 

 climates, and topographic features occurring at the 

 immediate margins of former and existing glaciers and 

 ice sheets, and influenced by cold temperature of the 

 ice. The term was originally introduced to designate the 

 climate and related geologic features peripheral to ice 

 sheets of the Pleistocene. It has been loosely defined to 

 include frost-action effects and loess deposits that may 

 or may not be related to glaciers. HP 



Permafrost— (a) Permanently frozen material underlying 

 the solum, (b) A perennially frozen soil horizon. GSST 



Piedmont— adj. Lying or formed at the base of a moun- 

 tain range; for example, a piedmont terrace or a pied- 

 mont pediment. — n. An area, plain, slope, glacier, or 

 other feature at the base of a mountain; for example, a 

 foothill or a bajada. In the United States, the Pied- 

 mont is a plateau extending from New Jersey to Ala- 

 bama and lying east of the Appalachian Mountains. GG 



Piedmont slope — The dominant gentle slope at the foot 

 of a mountain; generally used in terms of intermontane- 

 basin terrain in arid to subhumid regions. Main com- 

 ponents include: (1) an erosional surface on bedrock 

 adjacent to the receding mountain front (pediment); 

 (2) a constructional surface comprising individual allu- 

 vial fans and interfan valleys, also near the mountain 

 front; and (3) a distal complex of coalescent fans (ba- 

 jada), and alluvial slopes without fan form. Piedmont 

 slopes grade to either a basin-floor depressions with 

 alluvial and temporary lake plains, or surfaces of 

 through drainage. (See bolson.) (fig. B-3) HP 



Pingo — A large frost mound; especially a relatively large 

 conical mound of soil covered ice (commonly 30 to 

 50 m high and up to 400 m in diameter) raised in part 

 by hydrostatic pressure of water within and below the 

 permafrost of Arctic regions, and of more than one 

 year's duration. GG 



Plain — An extensive lowland area that ranges from level 

 to gently sloping or undulating. A plain has few or no 

 prominent hills or valleys, and occurs at low elevation 

 with reference to surrounding areas (local relief gener- 

 ally less than 100 m. (See plateau.) HP 



Pleateau — An extensive upland mass with relatively flat 

 summit area that is considerably elevated (more than 

 100 m) above adjacent lowlands, and is separated from 

 them on one or more sides by escarpments. A compar- 

 atively large part of a plateau surface is near summit 

 level. (See mesa and plain.) HP 



43 



