U-shaped valley A valley having a pronounced parabolic cross profile suggesting the form of a broad letter “U”, with steep walls and a broad, nearly flat floor; specifically a valley carved by glacial erosion ( Jackson, 1997). Compare V-shaped valley.

Udalfs [soil taxonomy] A suborder of Alfisols of hu-mid climates that are more or less freely drained. They commonly have brownish colors throughout. Udalfs have frigid or warmer soil temperature regimes and a udic soil moisture regime. Udalfs are not saturated with water for periods long enough to limit their use for most crops (USDA, 1999).

Udands [soil taxonomy] A suborder of Andisols of humid climates that are more or less freely drained. They characterisitcally have an ochric or umbric epipedon and a cambic horizon; some have a duripan. Udands have a frigid or warmer soil temperature regime, and a udic soil moisture regime (USDA, 1999).

Udepts [soil taxonomy] A suborder of Inceptisols of humid climates that are more or less freely drained. They commonly have an ochric or umbric epipedon and a cambic horizon; some also have a sulfuric horizon, a fragipan, or a duripan. Udepts have a udic or perudic soil moisture regime (USDA, 1999; Appendix I).

Uderts [soil taxonomy] A suborder of Vertisols of humid climates that are more or less freely drained. They have wide, deep cracks that open and close, depending upon the amount of precipitation. In some years the cracks may not open completely. Uderts have a frigid or warmer temperature regime and a udic or perudic soil moisture regime (USDA, 1999; Appendix 1).

udic soil moisture regime [soil taxonomy] A soil moisture regime that is neither dry for as long as 90 cumulative days in normal years. If the mean annual soil temperature is lower than 22°C and if the mean winter and mean summer soil temperatures at a depth of 50 cm from the soil surface differ by 6°C or more, the soil moisture control section, in normal years, is dry in all parts for less than 45 consecutive days in the 4 months following the summer solstice Additionally, the udic moisture regime requires (except for short periods), a three-phase system (solid-liquid-gas), in part or all of the soil moisture control section when the soil temperature is above 5°C. It commonly has well distributed rainfall annually, has an amount of stored moisture plus rainfall that is equal to, or exceeds, the amount of evapotranspiration, or has adequate winter rains to recharge the soils and cool, foggy summers (e.g., as in coastal areas). Water moves downward through the soils at some time in normal years (USDA, 1999). Refer to perudic soil moisture regime.

Udolls [soil taxonomy] A suborder of Mollisols of humid climates that are more or less freely drained. They have a mollic epipedon, and may have a cambic, calcic, natric, or argillic horizon. Udolls have a frigid or warmer soil temperature regime. They have a udic soil moisture regime and are not saturated with water for periods long enough to limit their use for most crops (USDA, 1999; Appendix 1).

Udox [soil taxonomy] A suborder of Oxisols of humid climates that are well drained. They have a udic soil moisture regime, and there are fewer than 90 d that are so dry that crops are not planted (USDA, 1999; Appendix 1).

Udults [soil taxonomy] A suborder of Ultisols of humid climates that are more or less freely drained, and humus-poor (low or moderate amounts of organic carbon). Characteristically they have light colored upper horizons, and commonly a grayish horizon that rests on a yellowish brown to reddish argillic or kandic horizons; some have a fragipan, plinthite, or both, in or below the argillic or kandic horizon. Ultisols have a udic soil moisture regime and are not saturated with water for periods long enough to limit their use for most crops (USDA, 1999; Appendix 1).

Ultisols [soil taxonomy] An order of low-base mineral soils that have an argillic or kandic horizon with a base saturation (<35% when measured at pH 8.2). Ultisols may have any soil temperature regime and any soil moisture regime (except aridic; USDA, 1999; Appendix 1).

Umbrepts [soil taxonomy] (obsolete; not used in current soil taxonomy) Prior to 1999, this was a suborder of Inceptisols formed in cold or temperate climates that commonly have an umbric epipedon, but they may have a mollic or an anthropic epipedon 25 cm or more thick under certain conditions. These soils are not dominated by amorphous materials and are not saturated with water for periods long enough to limit their use for most crops (USDA, 1999; Appendix 1).

umbric epipedon [soil taxonomy] A surface layer of mineral soil that has the same requirements as the mollic epipedon with respect to color, thickness, organic carbon content, consistence, structure, and phosphorus content but that has a base saturation <50% when measured at pH 7 (USDA, 1999).

unaccommodated Applied to peds. Virtually none of the faces of adjoining peds are molds of each other.

unconformity A substantial break or gap in the geologic record where a unit is overlain by another that is not in stratigraphic succession ( Jackson, 1997). Compare conformity, discontinuity.

underfit stream A stream that appears to be too small to have eroded the valley in which it flows; a stream whose volume is greatly reduced or whose meanders show a pronounced shrinkage in radius. It is a common result of drainage changes effected by capture, glaciers, or climatic variations ( Jackson, 1997).

underground runoff (seepage) Water that seeps toward stream channels after infiltration into the ground.

undifferentiated group A kind of map unit used in soil surveys comprised of two or more taxa components that are not consistently associated geographically. Delineations show the size, shape, and location of a landscape unit composed of one or the others, or all of two or more component soils that have the same or very similar use and management for specified common uses. Inclusions may occur up to some allowable limit. Compare component soil, soil consociation, soil complex, soil association, miscellaneous areas.

unit structure Refer to phyllosilicate mineral terminology.

Universal Soil Loss Equation (USLE) Refer to erosion, Universal Soil Loss Equation (USLE)and Revised Universal Soil Loss Equation (RSULE).

unsaturated flow The movement of water in soil in which the pores are not filled to capacity with water.

upland [geomorphology] An informal, general term for (a) the higher ground of a region, in contrast with a low-lying, adjacent land such as a valley or plain. (b) Land at a higher elevation than the flood plain or low stream terrace; land above the footslope zone of the hillslope continuum. Compare lowland (Hawley and Parsons, 1980; Jackson, 1997).

uplift [tectonic] A structurally high area in the earth’s crust, produced by positive movements that raise or upthrust the rocks, as in a dome or arch ( Jackson, 1997).

upper plastic limit Refer to liquid limit.

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. (Hawley and Parsons, 1980).

urban land [soil survey] A miscellaneous area (map unit) applied to land so altered or obstructed by urban works or structures that identification of soils is not feasible (USDA, 1993). Refer to anthroscape.

urban soil Soil altered by human activity related to land develpment and road construction (Effland and Pouyat, 1997).

Ustalfs [soil taxonomy] A suborder of Alfisols of subhumid to semiarid climates that are more or less freely drained. Commonly they are brownish or reddish throughout the profile and generally lack the combination of aquic conditions and low chroma redox features near the soil surface. If there are carbonates in the soil system, Ustalfs tend to have a Bk or calcic horizon below or in an argillic or kandic horizon. Ustalfs have a a frigid or warmer soil temperature regime, an ustic soil moisture regime and are not saturated with water for periods long enough to limit their use for most crops (USDA, 1999; Appendix 1).

Ustands [soil taxonomy] A suborder of Andisols of subhumid to semiarid climates that are more or less well drained. Characteristically, they have an ochric or mollic epipedon and a cambic horizon; some have a duripan. Ustands have a frigid or warmer soil temperature regime and an ustic soil moisture regime (USDA, 1999; Appendix 1).

Ustepts [soil taxonomy] A suborder of Inceptisols of subhumid to semiarid climates that are more or less freely drained. Commonly they have an ochric epipedon and a cambic horizon. Many are calcareous at a shallow depth and have a Bk or calcic horizon; some have a duripan or an umbric epipedon. Ustepts have an isomesic, hyperthermic, or warmer soil temperature regime or recieve dominantly summer precipitation, and have an ustic moisture regime (USDA, 1999; Appendix 1).

Usterts [soil taxonomy] A suborder of Vertisols of subhumid to semiarid climates that in of temperate areas do not receive high amounts of rainfall during the summer and have a monsoonal climate, or in tropical and subtropical areas have two rainy and two dry seasons, and are more or less freely drained. They have wide, deep cracks that open and close once or twice during the year. Usterts have a frigid or warmer soil temperature regime and an ustic soil moisture regime (USDA, 1999; Appendix 1).

ustic soil moisture regime [soil taxonomy] A soil moisture regime that is intermediate between the aridic and udic regimes and common in temperate subhumid or semiarid regions, or in tropical and subtropical regions with a monsoon climate. The amount of water available for plants is limited but is present when the soil temperature is suitable for plant growth. The ustic moisture regime is not applied to soils that have permafrost or a cryic soil temperature regime (USDA, 1999).

Ustolls [soil taxonomy] A suborder of Mollisols of subhumid to semiarid climates that are more or less freely drained. They have a mollic epipedon, most Ustolls have a Bk horizon that has identifiable secondary carbonates or a calcic horizon. Ustolls may also have a cambic, argillic, kandic, petrocalcic, or natric horizon. If there is a natric horizon, there may be an albic horizon overlying it, or if there is a cambic or argillic horizon, there may be a duripan below it. Ustolls have a frigid or warmer soil temperature regime and an ustic soil moisture regime. Ustolls are not saturated with water for periods long enough to limit their use for most crops (USDA, 1999; Appendix 1).

Ustox [soil taxonomy] A suborder of Oxisols of subhumid to semiarid climates. They commonly have an oxic or a kandic horizon. Ustox that have an ustic moisture regime; due to natural rainfall, they are moist in normal years for at least 90 d (a period that usually is long enough for one rain-fed crop) but not for more than 270 d (USDA, 1999; Appendix 1).

Ustults [soil taxonomy] A suborder of Ultisols of subhumid to semiarid climates that are the more or less freely drained and have relatively low content of organic carbon. They are brownish or reddish throughout and characteristically have an ochric epipedon that rests on an argillic or kandic horizon, which may or may not contain plinthite. A petroferric contact is common in some parts of the world. Ustults can have any soil temperature regime but generally are isomesic, thermic, or warmer. Ultisols have an ustic soil moisture regime and occur in regions where rainfall is moderately low to high but evapotranspiration exceeds precipitation. (USDA, 1999; Appendix 1).

uvala (not preferred) use karst valley.