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terrace,a level field built on top of a hillslope into the floor of a deep valley to improve cultivation of crops. Terracing uses the runoff from the hill to increase soil retentiveness and arability and is often part of a larger irrigation system that includes canals. Although widespread in areas of high population pressure, such as Japan and the Philippines, it has been abandoned in some regions, such as the Mediterranean, because of its high maintenance costs.
(1) A horizontal or slightly sloping area that forms a step in the slope of the local terrain. Terraces may be natural or artificial. Artificial terraces are made for the construction of buildings and in the creation of terrace parks, as well as in road-building and for agricultural and other purposes. A vertical wall or embankment is usually made along the lower edges of artificial terraces.
(2) An unheated summer addition to a building that is open on three sides and covered by a roof on columns, with a door leading to the building. (Since the 19th century, terraces have usually been glassed in.)
(3) Any of the levels of terraced buildings, which descend in steps along a slope.
in geology and geography, a natural horizontal or gently inclined surface bounded above and below by steeper slopes. Terraces can have various origins and are found on mountain slopes, on the sides of stream valleys, along the shores of lakes, seas, and oceans, and on the ocean floor. They may occur singly or in steplike series.
The most common type of terrace is the stream, or river, terrace. Such terraces appear on the sides of most stream valleys and represent the remnants of former valley floors. Stream terraces are formed most often as a result of the periodic cutting, owing to oscillatory movements of the earth’s crust, by the stream into the floor and sides of the valley; indeed, stream terraces are used as a criterion in the study of such movements. The downcutting of the stream into the valley floor may also result from a lowering of the level of the body of water into which the stream flows, from an increase in discharge owing to climatic changes, or from other local causes. In the series of terraces rising above the floodplain in a stream valley, the highest terrace is the most ancient, and the lowest terrace is the youngest. Depending on the depth of downcutting by the river and the thickness of the alluvium, three basic types of stream terraces may be distinguished: alluvial terraces; compound terraces, where bedrock is exposed beneath the alluvium; and rock terraces, which are carved out of the bedrock by fluvial erosion (Figure 1).
Marine and lake terraces are surfaces produced by wave action along the shores of seas and large lakes. The rear seam of such a surface indicates the former elevation of the level of the body of water. Marine terraces are found along the coasts of all seas and oceans, including the coasts of oceanic islands. The levels of bodies of water undergo variations associated with periodic changes in climate. Marine terraces are made use of in studying the history of such variations, as well as the history of the vertical tectonic movements of the coasts.
No less widespread are terraces that are formed in the course of various slope denudation processes. Such terraces are located above the level of a basin or the level of the present valley floor and have variable areas, inclinations, and relative and absolute elevations. The number of such terraces may be unlimited and depends on the characteristics of the geologic structure. Several types of these terraces are distinguished. What is called a structural terrace, for example, occurs on slopes composed of alternating, gently dipping beds of strong and weak rocks. The surfaces of such terraces are related to the surfaces of beds of rocks that are resistant to weathering and to washing by water flowing down the slopes. Landslide terraces are the surfaces of rock masses that are arranged in a steplike manner as a result of landsliding (seeLANDSLIDE). Solifluction terraces are formed as a result of the nonuniform flow of the water-saturated surface layer of the soil, especially in cases where soils and unconsolidated rock materials overlie permanently frozen ground (seeSOLIFLUCTION). Bald-mountain terraces develop within the mountain tundra (the bald-mountain zone) as a result of frost weathering and solifluction.
Stream, marine, and lake terraces are convenient surfaces for building and are often the sites of cities, towns, industrial plants, and highways. Terrace surfaces are also used for agriculture, especially in mountain regions. Placers are often associated with terraces. The study of terraces is of great theoretical importance for determining the paleogeographic circumstances of a region.
REFERENCESShchukin, I. S. Obshchaia geomorfologiia, vol. 1. Moscow, 1960.
Gorshkov, G. P., and A. F. Iakushova. Obshchaia geologiia, 3rd ed. Moscow, 1973.