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Geologic Dictionary
USDA Geologic Dictionary Terms
| Term | Definition |
|---|---|
| a'a lava | a type of lava flow having a rough, jagged, clinkery surface |
| ablation till | a general term for loose, relatively permeable material deposited during the downwasting nearly static glacial ice, either contained within or accumulated on the surface of the glacier |
| accretion [sedimentology] | the gradual increase or extension of land by natural forces acting over a long period of time, as on a beach by the washing up of sand from the sea or on a flood plain by the accumulation of sediment deposited by a stream |
| active layer | the top layer of ground subject to annual thawing and freezing in areas underlain by permafrost |
| aaggradation | the building-up of the earth's surface by deposition; specifically, the accumulation of material by any process in order to establish or maintain uniformity of grade or slope |
| alas | a type of thermokarst depression with steep sides anda flat, grass-covered floor, found in thermokarst terrain, produced by thawing of extensive areas of very thick and exceedingly ice-rich permafrost |
| alluvial | pertaining to material or processes associated with transportation and/or subaerial deposition by concentrated running water (compare: colluvial) |
| alluvial fan | a low, outspread mass of loose materials and/or rock material, commonly with gentle slopes, shaped like an open fan or a segment of a cone, deposited by a stream (esp in a semiarid region), issuing from a narrow mountain segment |
| alluvial plain | a broad flood plain or low-gradient delta |
| alluvial terrace | refer to stream terrace |
| alpine | characteristic of, or resembling, the European Alps, or any lofty mountain or mountain system, especially one modified by intense glacial erosion as to contain cirques, horns, etc.; also an ecological community term |
| angle of repose | the maximum angle of slope (measured from a horizontal plane) at which loose, cohesional material will come to rest |
| anticline | a unit of folded strata that is convex upward |
| aquiclude | a layer of sediment or rock that may or may not be saturated, that is incapable of transmitting significant quantities of water under ordinary hydraulic gradients |
| aquifer | a saturated, permeable geologic unit of sediment or rock that can transmit significant quantities of water under hydraulic gradients |
| aquitard | a body of rock or sediment that retards but does not prevent the flow of water to or from an adjacent aquifer; does not readily yield water to wells or springs but may store groundwater |
| arete | a narrow, jagged mountain crest, often above the snowline, sculptured by alpine glaciers and formed by backward erosion of adjoining cirque walls |
| arroyo | (colloquial - US SW) the channel of a flat-floored ephemeral stream, commonly with very steep to vertical banks cut in unconsolodiated material, sometimes called a wash |
| ash (volcanic) | unconsolidated pyroclastic material less than 2 mm in all dimensions, AKA volcanic ash |
| ash flow | a highly heated mixture of volcanic gases and ash, traveling down the flank of a volcano or along the surface of the ground; produced by lava disintegration in a volcanic crater or emission of gas and ash through a fissure |
| aspect | the direction toward which a slope faces with respect to the compass or to the rays of the sun |
| atoll | a coral reef appearing in planar view as roughly circular, surmounted by a chain of closely spaces, low coral islets that encircle a shallow lagoon with no land or islands or non-coral reef; surrounded by open sea |
| avalanche | a large mass of snow, ice, soil, or rock falling, sliding, or flowing rapidly under force of gravity |
| avalanche chute | the central, channel-like corridor, scar, or depression along which an avalanche has moved; eroded surface marked by pits, scratches, and grooves |
| avulsion | a sudden cutting off or separation of land by a flood or by abrupt change in the course of a stream, as by a stream breaking through a meander or a sudden change in current whereby the stream deserts its old channel for a new one |
| axial stream | the main stream of an intermontane valley, flowing in the deepest part of the valley and parallel to its longest dimension; a stream along the axis of a syncline or anticline |
Created by:
eatherrell