Debris — Any surficial accumulation of loose material 

 detached from rock masses by chemical and mechan- 

 ical means, as by decay and disintegration, and occur- 

 ing in the place where it was formed, or transported by 

 water or ice and redeposited. It consists of rock frag- 

 ments, finer-grained earth material, and sometimes 

 organic matter. HP 



Debris flow (mudflow) — A mass movement process in- 

 volving rapid flow of highly viscous mixtures of debris, 

 water, and entrapped air. Water content may range up 

 to 60°7o. A mudflow is a type of debris flow with clastic 

 particles of sand size and finer. (See alluvial fan.) HP 



Delta — A body of alluvium, whose surface form is nearly 

 flat and fan shaped, deposited at or near the mouth of 

 a river or stream where it enters a body of relatively 

 quiet water, usually a sea or lake. HP 



Deposition — The laying down of potential rock-forming 

 or soil-forming materials; sedimentation. DGT 



Desert pavement — A layer of gravel or coarser fragments 

 on desert soil surfaces that (1) was emplaced by upward 

 movement of fragments from underlying sediments, or 

 (2) was formed as a lag concentrate after finer particles 

 were removed by running water or wind (for example, 

 a variety of erosion pavement). HP 



Detritus (geology) — Rock and mineral fragments occur- 

 ring in sediments that were derived from pre-existing 

 igneous, sedimentary, or metamorphic rocks. HP 



Dipslope — A slope of land surface, roughly determined 

 by and approximately conforming with the dip of 

 underlying bedded rocks; for example, the long, gently 

 inclined surface of a cuesta. A synonym for dipslope is 

 structural backslope. (See scarp slope.) HP 



Divide — The line of separation between two drainage 

 systems; the summit of an interfluve. The highest sum- 

 mit of a pass or gap. DGT 



Dome — A roughly symmetrical upfold, with beds dipping 

 in all directions, more or less equally, from a point. A 

 smoothly rounded landform or rock mass such as a 

 rock-capped mountain summit, roughly resembling the 

 dome of a building. GG 



Draw — A small stream valley, generally more open and 

 with broader floor than a ravine or gulch. HP 



Drift (glacial geology) — A general term applied to all 

 rock material (clay, silt, sand, gravel, boulders) trans- 

 ported by a glacier and deposited directly by or from 

 the ice, or by running water emanating from a glacier. 

 Drift includes unstratified material (till) that forms 

 moraines, and stratified glaciofluvial deposits that 

 form outwash plains, eskers, kames, varves, and gla- 

 ciolacustrine sediments. The term is generally applied 



to Pleistocene glacial deposits in areas (such as large 

 parts of North America and Europe) that no longer 

 contain glaciers. GG 



Drumlin — A low, smooth, elongated oval hill, mount, or 

 ridge of compact glacial till that may or may not have a 

 core of bedrock or stratified glacial drift. The longer 

 axis is parallel to the general direction of glacier flow. 

 Drumlins are products of streamline flow of glaciers 

 which molded the subglacial floor through a combina- 

 tion of erosion and deposition. HP 



Dune — A mound, ridge, or hill of loose, windblown 

 granular material (generally sand), either bare or cov- 

 ered with vegetation. HP 



Dune (barchan) — A crescent-shaped dune with tips ex- 

 tending to the leeward, making this side concave in 

 plan and the windward side convex. Barchan (para- 

 bolic) dunes tend to be arranged in chains extending in 

 the direction of the most effective wind. HP 



Dune (parna) — A dune largely composed of sand-size 

 aggregates of clay. HP 



Dune (seif) — A longitudinal dune about six times as wide 

 as it is high and oriented parallel, rather than trans- 

 verse, to the prevailing wind. HP 



Eolian — Pertaining to material transported and deposited 

 by the wind. Includes earth materials including dune 

 sands, silty loess deposits, and clay (parna). HP 



Ephemeral stream — A stream, or reach of a stream, that 

 flows only in direct response to precipitation. It re- 

 ceives no long-continued supply from melting snow or 

 other source, and its channel is at all times above the 

 water table. (See arroyo and intermittent stream.) HP 



Epiclastic — Pertaining to any clastic rock or sediment 

 other than pyroclastic. Constituent fragments are 

 derived by weathering and erosion rather than by direct 

 volcanic processes. (See volcaniclastic.) HP 



Erosion — The wearing away of the land surface by run- 

 ning water, waves, moving ice and wind, or by such 

 processes as mass wasting and corrosion (solution and 

 other chemical processes). The term "geologic 

 erosion" refers to natural processes occurring over 

 long (geologic) time spans. HP 



Erosional (geomorphology) — Owing its origin, form, 

 position, or general character to wearing-down (degra- 

 dational) processes, such as removal of weathered rock 

 debris by any mechanical or chemical processes to 

 form, for example, a pediment or valley-side slope. 

 (See constructional.) HP 



Erosion pavement — A concentration of gravel or coarser 

 fragments that remains on the soil surface as a lag after 



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