ing, rolling, or sliding. (See colluvium, mass wasting, 

 and scree.) HP 



Tephra — A collective term for all clastic volcanic materials 

 which are ejected from a vent during an eruption and 

 transported through the air, including volcanic ash, 

 cinders, lapilli, scoria, pumice, bombs, and blocks. (A 

 synonym for tephra is volcanic ejecta.) HP 



Terrace (geomorphic) — A step-like surface, bordering a 

 valley floor or shoreline, that represents the former 

 position of an alluvial plain, or lake or sea shore. The 

 term is usually applied to both the relatively flat sum- 

 mit surface (platform, tread), cut or built by stream or 

 wave action, and the steeper descending slope (scarp, 

 riser), graded to a lower base level of erosion. (See 

 stream terrace.) HP 



Terracettes — Small, irregular, step-like forms on steep 

 hillslopes, especially in pasture, formed by creep or 

 erosion of surficial materials that may or may not be 

 induced by trampling of livestock such as sheep or cat- 

 tle. (Synonyms for terracettes are catsteps, sheep or 

 cattle tracks, and soil ripples.) HP 



Tertiary — The first period of the Cenozoic Era of geologic 

 time, following the Mesozoic Era preceding the Quar- 

 ternary (approximately from 65 to 2 million years ago); 

 also the corresponding time-stratigraphic subdivision 

 (system) of earth materials. Epoch/series subdivisions 

 comprise, in order of increasing age, Pliocene, Mio- 

 cene, Oligocene, Eocene, and Paleocene. HP 



Thermokarst — Karstlike topographic features produced 

 in a permafrost region by local melting of ground ice 

 and subsequent settling of the ground. GG 



Tidal flat — An extensive, nearly horizontal, marshy or 

 barren tract of land that is alternately covered and 

 uncovered by the tide, and consisting of unconsoli- 

 dated sediment (mostly mud and sand). It may form 

 the top surface of a deltaic deposit. Tide flat is a syno- 

 nym for tidal flat. GG 



Till (glacial) — Dominantly unsorted and unstratified 

 drift, deposited by and underneath a glacier, and con- 

 sisting of a heterogeneous mixture of clay, silt, sand, 

 gravel, stones and boulders. (See ablation till, lodg- 

 ment till, glacial drift, and moraine.) A synonym is gla- 

 cial till. GG 



Till plain— An extensive flat to undulating area underlain 

 by glacial till. (See till and moraine ground.) HP 



Toeslope — The geomorphic component that forms the 

 outermost, gently, inclined surface at the base of a hill- 

 slope. Toeslopes in profile are commonly gentle and 

 linear; and in terms of gradational processes, they are 

 constructional surfaces forming the distal part of a 



hillslope continuum that grades to valley or closed- 

 depression floors. (See footslope and valley floor.) 

 (fig. B-2) HP 



Topography — The relative position and elevations of the 

 natural or human made features of an area that de- 

 scribe the configuration of its surface. HP 



Trough (geomorphology)— (a) Any long, narrow depres- 

 sion in the earth's surface, such as one between hills or 

 with no surface outlet for drainage; esp. a broad, elon- 

 gate U-shaped valley, such as a glacial trough or a 

 trench, (b) The channel in which a stream flows. GG 



Trough end — The steep, semicircular rock wall forming 

 the abrupt head or end of a glacial trough. See also: 

 oversteepened wall. A synonym for trough end is 

 trough wall. GG 



Trough valley— U-shaped valley. GG 



Trough wall— (See trough end.) GG 



Tuff — A compacted deposit that is 50% or more volcanic 

 ash and dust. 



Upland (geomorphology) — Land at a higher elevation, in 

 general, than the alluvial plain or low stream terrace; 

 land above the footslope zone of the hillslope con- 

 tinuum. HP 



Uplift (tectonic) — A structurally high area in the crust, 

 produced by positive movements that raise or upthrust 

 the rocks, as in a dome or arch. GG 



Upthrust — (a) An upheaval of rock; said preferably of a 

 violent upheaval, (b) A high angle gravity or thrust 

 fault in which the relatively upthrown side was the 

 active (moving) element. GG 



Valley — An elongate, relatively large, externally-drained 

 depression of the earth's surface that is primarily de- 

 veloped by stream erosion. (See basin and intermon- 

 tane basin.) HP 



Valley-border surfaces — A general grouping of valley-side 

 geomorphic surfaces that occur in a stepped sequence 

 graded to successively lower stream base levels pro- 

 duced by episodic valley entrenchment. HP 



Valley fill — The unconsolidated sediment deposited by 

 any agent (water, wind, ice, mass wasting) so as to fill 

 or partly fill a valley. HP 



Valley flat (flood-plain landform)— A general term for 

 broad, nearly level flood-plain surfaces that are not 

 subject to frequent inundation. (See backswamp and 

 meander belt.) HP 



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

 sloping, bottom surface of a valley. Component land- 

 forms include axial stream channels, the flood-plain, 

 and in some areas, low terrace surfaces that may be 

 subject to flooding from tributary streams. (See flood- 



47 



