a'a lava
a type of basaltic lava (material) having a rough, jagged, clinkery surface and a vesicular interior. compare - block lava, pahoehoe lava, pillow lava. GG & MA

a'a lava flow
a type of basaltic lava flow dominated by a'a lava and a characteristically rough, jagged, clinkery surface. compare - pahoehoe lava flow. compare - block lava flow, pahoehoe lava flow, pillow lava flow. GG & MA

ablation till
a general term for loose, relatively permeable, earthy material deposited during the downwasting of nearly static glacial ice, either contained within or accumulated on the surface of the glacier. compare - till, ground moraine. GG

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. synonym: aggradation. GG

active layer
the top layer of ground subject to annual thawing and freezing in areas underlain by permafrost. NRC

active slope
(not recommended)

aeolian
(not recommended; obsolete) use eolian.

aggradation
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. compare - accretion, degradation. GG

alas
a type of thermokarst depression with steep sides and a flat, grass-covered floor, found in thermokarst terrain, produced by thawing of extensive areas of very thick and exceedingly ice-rich permafrost. compare - thermokarst depression. NRC and GG

alluvial
pertaining to material or processes associated with transportation and/or subaerial deposition by concentrated running water. compare - colluvial. GSST

alluvial cone
a semi-conical type of alluvial fan with very steep slopes; it is higher, narrower, and steeper (e.g., > 40% slopes) than a fan, and composed of coarser, and thicker layers of material deposited by a combination of alluvial episodes and to a much lesser degree, landslides (e.g., debris flow). compare - alluvial fan, talus cone. SW

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 (best expressed in semiarid regions) at the place where it issues from a narrow mountain or upland valley; or where a tributary stream is near or at its junction with the main stream. it is steepest near its apex which points upstream and slopes gently and convexly outward (downstream) with a gradual decrease in gradient. GG

alluvial flat
a) (colloquial: western U.S.A.) a nearly level, graded, alluvial surface in bolsons and semi-bolsons which commonly does not manifest traceable channels, terraces or floodplain levels. compare - flood-plain step, terrace, valley flat. FFP, GG, & SW.
b) (not preferred) a general term for a small flood plain bordering a river, on which alluvium is deposited during floods. GG

alluvial plain
a) a large assemblage of fluvial landforms (braided streams, terraces, etc.,) that form low gradient, regional ramps along the flanks of mountains and extend great distances from their sources (e.g., High Plains of North America. SW
b) (not recommended, use flood plain.) an general, informal term for a broad flood plain or a low-gradient delta. compare - alluvial flat. FFP

alluvial plain remnant
an erosional remnant of an alluvial plain which retains the surface form and alluvial deposits of its origin but was not emplaced by, and commonly does not grade to a present-day stream or drainage network. compare - alluvial plain, erosional remnant, paleoterrace. SW

alluvial terrace
(not preferred) refer to stream terrace.

alluvium
unconsolidated, clastic material subaerially deposited by running water, including gravel, sand, silt, clay, and various mixtures of these. compare - colluvium, slope alluvium. HP

alpine
a) [geomorphology] - (adjective) characteristic of, or resembling the European Alps, or any lofty mountain or mountain system, especially one so modified by intense glacial erosion as to contain cirques, horns, etc. (ex. alpine lake) GG;
b) (not recommended as a landform term). an ecological community term for high-elevation plant communities. GG & SW

alpine glacier
a) any glacier in a mountain range except an ice cap or ice sheet. It usually originates in a cirque and may flow down into a valley previously carved by a stream. compare - continental glacier. GG
b) (not preferred - refer to U-shaped valley): (relict) - landforms or sediments formed, modified or deposited by a glacier in or on mountains or high hills that has since melted away. compare - glacial-valley floor, glacial-valley wall. SW

angle of repose
the maximum angle of slope (measured from a horizontal plane) at which loose, cohesionless material will come to rest. GG

anthropogenic feature
an artificial feature on the land surface, having a characteristic shape and range in composition, composed of unconsolidated earthy, organic materials, artificial materials, or rock, that is the direct result of human manipulation or activities; can be either constructional (e.g., artificial levee) or destructional (quarry). SW

anticline
a) [landform] a unit of folded strata that is convex upward and whose core contains the stratigraphically oldest rocks, and occurs at the earth's surface. in a single anticline, beds forming the opposing limbs of the fold dip away from its axial plane. compare - monocline, syncline, fold. SW & HP
b) [structural geology] a fold, at any depth, generally convex upward whose core contains the stratigraphically older rocks. GG

aquiclude
a layer of sediment or rock that may or may be saturated, that is incapable of transmitting significant quantities of water under ordinary hydraulic gradients. FC

aquifer
a saturated, permeable geologic unit of sediment or rock that can transmit significant quantities of water under hydraulic gradients. FC

aquitard
a body of rock or sediment that retards but does not prevent the flow of water to or from an adjacent aquifer. it does not readily yield water to wells or springs but may serve as a storage unit for groundwater. GG

arete
a narrow, jagged mountain crest, often above the snowline, sculptured by alpine glaciers and formed by backward erosion of adjoining cirque walls. HP

arroyo
(colloquial: southwest U.S.A.) the channel of a flat-floored, ephemeral stream, commonly with very steep to vertical banks cut in unconsolidated material; sometimes called a wash. it is usually dry but can be transformed into a temporary watercourse or short-lived torrent after heavy rain within the watershed. where arroyos intersect zones of ground-water discharge, they are more properly classed as intermittent stream channels. HP

artificial collapsed depression
a collapse basin, commonly a closed depression, which is the direct result of surficial subsidence associated with subsurface mining (e.g., long-wall mining). SW

artificial levee
an artificial embankment constructed along the bank of a watercourse or an arm of the sea, to protect land from inundation or to confine streamflow to its channel. GG

ash [volcanic]
unconsolidated, pyroclastic material less than 2 mm in all dimensions. commonly called "volcanic ash" compare - block [volcanic], cinders, lapilli, tephra. SW & KST

ash flow
(not preferred - see pyroclastic flow, pyroclastic surge) 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 the explosive disintegration of viscous lava in a volcanic crater, or by the explosive emission of gas-charged ash from a fissure or group of fissures. the solid materials contained in a typical ash flow are generally unsorted and ordinarily include volcanic dust, pumice, scoria, and blocks in addition to ash. (also called a pyroclastic flow.) compare - nueé ardente, lahar, pyroclastic. GG

aspect
the direction toward which a slope faces with respect to the compass or to the rays of the sun; also called slope aspect. GSST

atoll
a coral reef appearing in plan view as roughly circular, and surmounted by a chain of closely spaced, low coral islets that encircle or nearly encircle a shallow lagoon in which there is no land or islands of non-coral origin; the reef is surrounded by open sea. GG

avalanche
a large mass of snow, ice, soil, or rock, or mixtures of these materials, falling, sliding, or flowing very rapidly under the force of gravity. velocities may sometimes exceed 500 km/hr. GG

avalanche chute
[preferred term] the central, channel-like corridor, scar, or depression along which an avalanche has moved. an eroded surface marked by pits, scratches, and grooves. GG

avalanche track
(not recommended as a landform term - use avalanche chute). the path formed by an avalanche. it may take the form of an open path in a forest, with bent and broken trees, or an eroded surface marked by pits, scratches, and grooves. compare - avalanche chute. GG

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 by a sudden change in current whereby the stream deserts its old channel for a new one. compare - crevasse, flood-plain splay. GG

axial stream
a) the main stream of an intermontane valley, flowing in the deepest part of the valley and parallel to its longest dimension.
b) a stream that follows the axis of a syncline or anticline. GG