vium (Arroyo is a regional term of the Southwest; a 

 synonym for wash.) Note: Where arroyo reaches inter- 

 sect zones of ground-water discharge they are more 

 properly classed as intermittent stream channels. HP 



Ash (volcanic) — Fine pryoclastic material smaller than 

 4.0 mm diameter. HP 



Avalanche chute — The track or path formed by an ava- 

 lanche. (See avalanche track.) GG 



Avalanche track — The central channel-like corridor 

 along which an avalanche has moved; 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. GG 



Backslope — The geomorphic component that forms the 

 steepest inclined surface and principal element of many 

 hillslopes (for example, valley side, ridge side). Back- 

 slopes in profile are commonly steep, linear, and may 

 or may not include cliff segments, also called 'gravity 

 slopes" or "free faces." The term "mid-slope" maybe 

 used to designate an element without a cliff. In terms 

 of gradational process, backslopes are erosional forms 

 produced mainly by mass wasting and running water. 

 Note: Structural geomorphologists may use the term as 

 a synonym of dipslope in describing homoclinal ridges 

 (for example, cuesta "backslope"). (See footslope, 

 shoulder, and hillslope.) (fig. B-2) HP 



Backswamp (flood-plain landform) — Extensive marshy, 

 depressed areas of flood plains between the natural 

 levee borders of channel belts and valley sides or ter- 

 races. (See valley flat.) HP 



Badlands — Intricately stream-dissected topography char- 

 acterized by a very fine drainage network with high 

 drainage densities and short, steep slopes with narrow 

 interfluves. Badlands develop on surfaces with little or 

 no vegetative cover overlying consolidated or poorly 

 cemented clays or silts, sometimes with soluble min- 

 erals such as gypsum or halite. GG 



Bajada — A broad, gently-inclined, piedmont slope formed 

 by lateral coalescence of a series of alluvial fans, and 

 having a broadly undulating transverse profile (parallel 

 to the mountain front) resulting from the convexities 

 of component fans. The term is generally restricted to 

 constructional slopes of intermontane basins in the 

 southwest United States. (A synonym is coalescent fan 

 piedmont.) (fig. B-3) HP 



Ballena — A landform comprising distinctively round- 

 topped ridgeline remnants of fan alluvium. The ridge's 

 broadly rounded shoulders meet from either side to 

 form a narrow crest and merge smoothly with the con- 

 cave backslopes. In ideal examples, the slightly con- 



cave footslopes of adjacent ballenas merge to form a 

 smoothly rounded drainageway. (fig. B-4) FFP 



Bar — An elongated landform generated by waves and 

 currents and usually running parallel to the shore, 

 composed predominantly of unconsolidated sand, 

 gravel, cobbles, or stones and with water on two sides. 

 FWS 



Bar and channel— The microrelief common to flood 

 plains and relatively young alluvial terraces. With time, 

 the microrelief becomes subdued as the higher lying 

 bars erode into the channels. The ridge-like bars often 

 consist of accumulations of coarse sediment, while the 

 channels are finer textured. The relief between bar and 

 channel is largely related to the competence of the 

 stream. HP 



Barrier beach — A narrow, elongate sand ridge rising 

 slightly above the high-tide level and extending gener- 

 ally parallel with the shore, but separated from it by a 

 lagoon or marsh; it is rarely more than several kilome- 

 ters long. Synonyms are offshore barrier, offshore 

 beach, bar beach. (See barrier island.) GG 



Barrier flat — A relatively flat area, often occupied by 

 pools of water, separating the exposed or seaward edge 

 of a barrier from the lagoon behind it. GG 



Barrier island — A long, narrow, sandy, coastal island, 

 representing a broadened barrier beach that is above 

 high tide and parallel to the shore, and that commonly 

 has dunes, vegetated zones, and swampy terrains ex- 

 tending lagoonward from the beach. Also a long series 

 of barrier beaches. GG 



Basal till — Compact till deposited beneath a moving 

 glacier, commonly clay rich, but is loamy in many 

 places. (See till and lodgment till.) HP 



Base level — The theoretical limit or lowest level toward 

 which erosion of the earth's surface constantly pro- 

 gresses but seldom, if ever, reaches; especially the level 

 below which a stream cannot erode its bed. The ulti- 

 mate base level for the land surface is sea level, but 

 temporary base levels may exist locally. The base level 

 of eolian erosian may be above or below sea level; that 

 of marine erosion is the lowest level to which marine 

 agents can cut a bottom. HP 



Basin — A depressed area with no or limited surface outlet. 

 Examples are closed depressions in glacial till plain, 

 lake basin, river basin, or fault-bordered intermontane 

 structure such as the Bighorn Basin of Wyoming. GG 



Basin floor — A general term for the nearly level to gently 

 sloping, bottom surface of an intermontane basin (bol- 

 son). Component landforms include playas, broad 

 alluvial flats containing ephemeral drainageways, and 



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