A rock veneer is a geomorphic formation in which rock fragments (clasts) of gravel or cobble size form a thin cover over a surface or hillslope. Rock veneers are typically one or two clasts thick and may partially or fully cover the ground surface.[1] Veneers typically form in semiarid and arid regions where chemical weathering rates and the potential for mass wasting are low. Other names for a rock veneer are rock-fragment cover (RFC), stone pavement, desert pavement, stony mantle, hammada and reg.
Rock veneers commonly arise from the weathering of resistant rocks of quartzite, felsic granites, coarse granites, and dense basalts. As these larger rocks are deposited on a surface, smaller sands either are removed by wind or water erosion, or settle and form a fine-grained layer beneath the larger veneer rocks. The larger clasts then rearrange and settle to form the rock veneer.[2]
Rock veneers form a variety of ways, with two major types; in situ veneers form in place by means of weathering, fluvial erosion, deflation (the removal of loose, fine-grained particles) or accretion; other veneers are transported via soil creep, sheet flooding or gully gravure (described below).[3] Rock veneers are typically distinguished through six variables: particle size, particle density, stability characteristics, potential for storing mobile sediments, degree of abrasion, and varnishing.[4]
Gully Gravure
Gully gravure is a transportation process by which rock veneers can be formed. Valleys formed of gullies and rills are made and coarse rock fragments are deposited in side channels. As the side channels fill in, water forms new, less resistant channels down the borders of the coarse channels and finer gullies and rills. These channels build up, and the process happens again, resulting in a reversal of rills and gullies, with coarser clasts on the surface.
Root throw is the process that occurs when a tree topples, raising its rootwad and the rock fragments in it. Fine sediment falls back into the rootwad pit or travels downstream, but coarse sediments form a local rock veneer around the rootwad. This local rock veneer is larger than the pit of the rootwad, as falling clasts extend the area of veneer. In a study[which?] of rootwads in Westcliffe, Colorado, it was estimated that root throw could have created a rock veneer over 90% of the surface during the Holocene.
Root throw produces a rock veneer when there are a large number of clasts near the surface, where there are: a slow rate of chemical weathering, insufficient precipitation to move the large clasts, high velocity winds, conditions favorable to shallow-rooted trees, and trees large enough to embed coarse rock in their roots.
Hillslopes with a rock veneer are more stable than those without and have lower erosion rates. Rock veneers form a rocky armor on the hillslope which prevents the erosion of smaller sediments and stabilizes the slope. Rock veneers, forming when influx and outflux sediment rates are equal, indicate a hillslope in equilibrium, partly due to their protective cover on the slope. Erosion rates on veneered hillslopes are low to moderate.
↑ Lenart, M.T., Osterkamp, W.R., and Toy, T.J., "Development of partial rock veneers by root throw in a subalpine setting". Wiley InterScience, Sept 27, 2005. doi:10.1002/esp.1222
↑ Toy, Terrence, and Osterkamp, W.R., "The Stability of Rock-Veneered Hillslopes." International Journal of Sediment Research, Vol 14, No 3, 1999, pp. 63-73. doi:10.1002/esp.1222
Erosion is the action of surface processes that removes soil, rock, or dissolved material from one location on the Earth's crust and then transports it to another location where it is deposited. Erosion is distinct from weathering which involves no movement. Removal of rock or soil as clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by dissolution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.
A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from rock, such as sand, gravel, shingle, pebbles, etc., or biological sources, such as mollusc shells or coralline algae. Sediments settle in different densities and structures, depending on the local wave action and weather, creating different textures, colors and gradients or layers of material.
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. The geological detritus originated from weathering and erosion of existing rocks, or from the solidification of molten lava blobs erupted by volcanoes. The geological detritus is transported to the place of deposition by water, wind, ice or mass movement, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.
Till or glacial till is unsorted glacial sediment.
Soil erosion is the denudation or wearing away of the upper layer of soil. It is a form of soil degradation. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, and animals. In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion, snow erosion, wind (aeolean) erosion, zoogenic erosion and anthropogenic erosion such as tillage erosion. Soil erosion may be a slow process that continues relatively unnoticed, or it may occur at an alarming rate causing a serious loss of topsoil. The loss of soil from farmland may be reflected in reduced crop production potential, lower surface water quality and damaged drainage networks. Soil erosion could also cause sinkholes.
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation; if buried, they may eventually become sandstone and siltstone through lithification.
Geomorphology is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or near Earth's surface. Geomorphologists seek to understand why landscapes look the way they do, to understand landform and terrain history and dynamics and to predict changes through a combination of field observations, physical experiments and numerical modeling. Geomorphologists work within disciplines such as physical geography, geology, geodesy, engineering geology, archaeology, climatology, and geotechnical engineering. This broad base of interests contributes to many research styles and interests within the field.
In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used.
Conglomerate is a clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contains a matrix of finer-grained sediments, such as sand, silt, or clay, which fills the interstices between the clasts. The clasts and matrix are typically cemented by calcium carbonate, iron oxide, silica, or hardened clay.
Colluvium is a general name for loose, unconsolidated sediments that have been deposited at the base of hillslopes by either rainwash, sheetwash, slow continuous downslope creep, or a variable combination of these processes. Colluvium is typically composed of a heterogeneous range of rock types and sediments ranging from silt to rock fragments of various sizes. This term is also used to specifically refer to sediment deposited at the base of a hillslope by unconcentrated surface runoff or sheet erosion.
Detritus is particles of rock derived from pre-existing rock through weathering and erosion. A fragment of detritus is called a clast. Detrital particles can consist of lithic fragments, or of monomineralic fragments. These particles are often transported through sedimentary processes into depositional systems such as riverbeds, lakes or the ocean, forming sedimentary successions. Diagenetic processes can transform these sediments into rock through cementation and lithification, forming sedimentary rocks such as sandstone. These rocks can then in turn again be weathered and eroded to form a second generation of sediment. Detrital grains commonly weather at different rates, according to the Goldich dissolution series, which dictates that early crystallizing minerals are less stable at the earth's surface than late crystallizing minerals.
Plucking, also referred to as quarrying, is a glacial phenomenon that is responsible for the weathering and erosion of pieces of bedrock, especially large "joint blocks". This occurs in a type of glacier called a "valley glacier". As a glacier moves down a valley, friction causes the basal ice of the glacier to melt and infiltrate joints (cracks) in the bedrock. The freezing and thawing action of the ice enlarges, widens, or causes further cracks in the bedrock as it changes volume across the ice/water phase transition, gradually loosening the rock between the joints. This produces large pieces of rock called joint blocks. Eventually these joint blocks come loose and become trapped in the glacier.
Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks, and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic to refer to sedimentary rocks and particles in sediment transport, whether in suspension or as bed load, and in sediment deposits.
In hillslope geomorphology, a rill is a shallow channel cut into soil by the erosive action of flowing surface water. Similar but smaller incised channels are known as microrills; larger incised channels are known as gullies.
In geology, a graded bed is one characterized by a systematic change in grain or clast size from one side of the bed to the other. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into progressively finer ones. Such a bed is also described as fining upward. Normally graded beds generally represent depositional environments which decrease in transport energy as time passes, but these beds can also form during rapid depositional events. They are perhaps best represented in turbidite strata, where they indicate a sudden strong current that deposits heavy, coarse sediments first, with finer ones following as the current weakens. They can also form in terrestrial stream deposits.
Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems where the particles are clastic rocks, mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles along the sloping surface on which they are resting. Sediment transport due to fluid motion occurs in rivers, oceans, lakes, seas, and other bodies of water due to currents and tides. Transport is also caused by glaciers as they flow, and on terrestrial surfaces under the influence of wind. Sediment transport due only to gravity can occur on sloping surfaces in general, including hillslopes, scarps, cliffs, and the continental shelf—continental slope boundary.
Abrasion is a process of erosion which occurs when material being transported wears away at a surface over time. It is the process of friction caused by scuffing, scratching, wearing down, marring, and rubbing away of materials. The intensity of abrasion depends on the hardness, concentration, velocity and mass of the moving particles. Abrasion generally occurs in four ways: glaciation slowly grinds rocks picked up by ice against rock surfaces; solid objects transported in river channels make abrasive surface contact with the bed and walls; objects transported in waves breaking on coastlines; and by wind transporting sand or small stones against surface rocks.
Sheet erosion or sheet wash is the even erosion of substrate along a wide area. It occurs in a wide range of settings such as coastal plains, hillslopes, floodplains, beaches, savanna plains and semi-arid plains. Water moving fairly uniformly with a similar thickness over a surface is called sheet flow, and is the cause of sheet erosion. Sheet erosion implies that any flow of water that causes the erosion is not canalized. If a hillslope surface contains many irregularities, sheet erosion may give way to erosion along small channels called rills, which can then converge forming gullies. However, sheet erosion may occur despite some limited unevenness in the sheet flow arising from clods of earth, rock fragments, or vegetation.
This glossary of geology is a list of definitions of terms and concepts relevant to geology, its sub-disciplines, and related fields. For other terms related to the Earth sciences, see Glossary of geography terms.
Cape Town lies at the south-western corner of the continent of Africa. It is bounded to the south and west by the Atlantic Ocean, and to the north and east by various other municipalities in the Western Cape province of South Africa.
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