Playa — The usually dry and nearly level lake plain that 

 occupies the lowest parts of closed depressions, such as 

 those occurring on intermontane basinfloors. Temporary 

 flooding occurs primarily in response to precipitation- 

 runoff events. Playa deposits are fine grained and may 

 or may not be characterized by high water table and 

 saline conditions. HP 



Pleistocene — The first epoch of the Quarternary Period 

 of geologic time, fol'owing the Tertiary Pliocene Epoch 

 and preceding the Holocene (approximately from 

 2 million to 10 thousand years ago); also the corre- 

 sponding (time-stratigraphic) "series" of earth mate- 

 rials. Glacial-interglacial stage/age subdivisions in 

 North America include, in order of increasing age, 

 Wisonsinan-Sangamonian, Illinoian-Yarmouthian, 

 Kansan-Aftonian, and Nebraskan. (Synonyms for 

 Pleistocene are Glacial epoch and Ice Age.) HP 



Pliocene — The last epoch of the Tertiary Period of geo- 

 logic time, following the Miocene Epoch and preceding 

 the (Quarternary) Pleistocene Epoch (approximately 7 

 to 2 million years ago); also, the corresponding (time- 

 stratigraphic) "series" of earth materials. HP 



Plutonic — Pertaining primarily to igneous rocks formed 

 deep in the earth's crust, but also including associated 

 metamorphic rocks. 



Pluvial lake — A lake formed in a period of exceptionally 

 heavy rainfall; a lake formed in the Pleistocene epoch 

 during a time of glacial advance, and now either extinct 

 or existing as a remnant. (An example of a pluvial lake 

 is Lake Bonneville. See periglacial.) HP 



Pocosin — Indian term for swamp on a hill. A large wet 

 area on nearly level interstream divides in the coastal 

 plain. Soils may be mineral or organic with distinctive 

 shrub vegetation. RD 



Point bar — One of a series of low, arcuate ridges of sand 

 and gravel developed on the inside of a growing mean- 

 der by the slow addition of individual accretions ac- 

 companying migration of the channel toward the outer 

 bank. GG 



Pothole (geomorphology) — Any pot-shaped pit or hole. 

 GG 



Pothole (glacial geology)— (a) Giant's kettle, (b) A term 

 applied in Michigan to a small pit depression (1 to 15 m 

 deep), generally circular or elliptical, occurring in an 

 outwash plain, a recessional moraine, or a til plain. GG 



Pothole (lake) — A shallow depression, generally less than 

 10 acres in area, occurring between dunes on a prairie 

 (as in Minnesota and the Dakotas) often containing an 

 intermittent pond or marsh. GG 



Pyroclastic — Pertaining to fragmental materials produced 

 by usually explosive, aerial ejection of clastic particles 

 from a volcanic vent. Such materials may accumulate 

 on land or under water. (See epiclastic and volcaniclas- 

 tic.) HP 



Quarternary — The second period of the Cenozoic Era of 

 geologic time, extending from the end of the Tertiary 

 Period (about 2 million years ago) to the present and 

 comprising two epochs, the Pleistocene (Ice Age) and 

 Holocene (Recent); also, the corresponding (time- 

 stratigraphic) "system" of earth materials. HP 



Ravine — A small stream valley; narrow, steep-sided, and 

 commonly V-shaped in cross section; and larger than a 

 gully. (A general synonym is gulch. See draw.) HP 



Reef — (a) A ridgelike or moundlike structure, layered or 

 massive, built by sedentary calcareous organisms, espe- 

 cially corals, and consisting mostly of their remains; it 

 is wave-resistant and stands above the surrounding 

 contemporaneously deposited sediment. Also, such a 

 structure built in the geologic past and now enclosed in 

 rock, commonly of differing lithology. (b) A mass or 

 ridge of rocks, especially coral and sometimes sand, 

 gravel, or shells, rising above the surrounding sea or 

 lake bottom to or nearly to the surface, and dangerous 

 to navigation; specifically such a feature at 10 fathoms 

 (formerly 6) or less. GG 



Regolith — All unconsolidated earth materials above the 

 solid bedrock. It includes material weathered in place 

 from all kinds of bedrock and alluvial glacial, eolian, 

 lacustrine, and pyroclastic deposits. Soil scientists 

 regard soil as only that part of the regolith that is modi- 

 fied by organisms and other soil-forming forces. Most 

 engineers describe the whole regolith, even to a great 

 depth, as "soil." (See residuum.) HP 



Relict — Pertaining to surface landscape features (for 

 example, landforms, geomorphic surfaces, paleosols) 

 that have never been buried and are products of past 

 environments no longer operative in a given area. (See 

 exhumed) HP 



Relief — The differences in elevation of a land surface 

 considered collectively. GG 



Residuum (residual soil material) — Unconsolidated, 

 weathered, or partly weathered mineral material that 

 only accumulates by disintegration of bedrock in place. 

 (See saprolite and regolith.) HP 



Ridge — A long narrow elevation of the land surface, 

 usually sharp crested with steep sides and forming an 

 extended upland between valleys. The term is used in 

 areas of both hill and mountain relief (less and greater 

 than 300 m). HP 



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