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37-750: The Ouled Naïl mountain range is formed by parallel ridges rising between the level 850 m high terrain of the Hodna region of the Hautes Plaines in the north and the 600 m of the southern plain of the Dayas . It is located in the eastern zone of the Saharan Atlas, with the Amour Range in the western and the Zab Range at the eastern end. The range is formed by mountains created by folds of

74-794: A major generator of silt, which accumulated to form the fertile soils of north India and Bangladesh, and the loess of central Asia and north China. Loess has long been thought to be absent or rare in deserts lacking nearby mountains (Sahara, Australia). However, laboratory experiments show eolian and fluvial processes can be quite efficient at producing silt, as can weathering in tropical climates. Silt seems to be produced in great quantities in dust storms, and silt deposits found in Israel, Tunisia, Nigeria, and Saudi Arabia cannot be attributed to glaciation. Furthermore, desert source areas in Asia may be more important for loess formation than previously thought. Part of

111-440: A number of mechanisms. However, the main process is likely abrasion through transport, including fluvial comminution , aeolian attrition and glacial grinding. Because silt deposits (such as loess , a soil composed mostly of silt ) seem to be associated with glaciated or mountainous regions in Asia and North America, much emphasis has been placed on glacial grinding as a source of silt. High Asia has been identified as

148-482: A platy or bladed shape. This may be characteristic of how larger grains abrade, or reflect the shape of small quartz grains in foliated metamorphic rock , or arise from authigenic growth of quartz grains parallel to bedding in sedimentary rock . Theoretically, particles formed by random fracturing of an isotropic material, such as quartz, naturally tend to be blade-shaped. The size of silt grains produced by abrasion or shattering of larger grains may reflect defects in

185-687: A pollutant in water the phenomenon is known as siltation . Silt deposited by the Mississippi River throughout the 20th century has decreased due to a system of levees , contributing to the disappearance of protective wetlands and barrier islands in the delta region surrounding New Orleans . In southeast Bangladesh, in the Noakhali district , cross dams were built in the 1960s whereby silt gradually started forming new land called "chars". The district of Noakhali has gained more than 73 square kilometres (28 sq mi) of land in

222-527: Is detritus (fragments of weathered and eroded rock) with properties intermediate between sand and clay . A more precise definition of silt used by geologists is that it is detrital particles with sizes between 1/256 and 1/16 mm (about 4 to 63 microns). This corresponds to particles between 8 and 4 phi units on the Krumbein phi scale . Other geologists define silt as detrital particles between 2 and 63 microns or 9 to 4 phi units. A third definition

259-418: Is granular material of a size between sand and clay and composed mostly of broken grains of quartz . Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension with water. Silt usually has a floury feel when dry, and lacks plasticity when wet. Silt can also be felt by the tongue as granular when placed on the front teeth (even when mixed with clay particles). Silt

296-401: Is a common material, making up 45% of average modern mud . It is found in many river deltas and as wind-deposited accumulations, particularly in central Asia, north China, and North America. It is produced in both very hot climates (through such processes as collisions of quartz grains in dust storms ) and very cold climates (through such processes as glacial grinding of quartz grains.) Loess

333-626: Is a particular challenge for civil engineering . The failure of the Teton Dam has been attributed to the use of loess from the Snake River floodplain in the core of the dam. Loess lacks the necessary plasticity for use in a dam core, but its properties were poorly understood, even by the U.S. Bureau of Reclamation , with its wealth of experience building earthen dams . Silt is susceptible to liquefaction during strong earthquakes due to its lack of plasticity. This has raised concerns about

370-528: Is a steppe where the climate is harsh, with very dry summers, cold winters and very irregular precipitation. Formerly all the inhabitants of the Hodna were nomads . The soil is poor and local agriculture is limited to the grazing of sheep and goats on grassy high plateau areas and to some cereal cultivation. There are uncultivated areas of djerr , small hillocks, to the north and to the east. The mountains have good reserves of water and there are freshwater springs in

407-710: Is carried through the vadose zone to be deposited in pore space. ASTM American Standard of Testing Materials: 200 sieve – 0.005 mm. USDA United States Department of Agriculture 0.05–0.002 mm. ISSS International Society of Soil Science 0.02–0.002 mm. Civil engineers in the United States define silt as material made of particles that pass a number 200 sieve (0.074 mm or less) but show little plasticity when wet and little cohesion when air-dried. The International Society of Soil Science (ISSS) defines silt as soil containing 80% or more of particles between 0.002 mm to 0.02 mm in size while

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444-601: Is common throughout the geologic record , but it seems to be particularly common in Quaternary formations. This may be because deposition of silt is favored by the glaciation and arctic conditions characteristic of the Quaternary. Silt is sometimes known as rock flour or glacier meal , especially when produced by glacial action. Silt suspended in water draining from glaciers is sometimes known as rock milk or moonmilk . A simple explanation for silt formation

481-495: Is marked by the Ouled Nail Range . There are no mountains in its western limit, where the plateau stretches further westwards. The northern part of the Hodna is composed of silt and the southern part is mainly sandy. The lowest area of the basin is covered by the sebkha , the salt lake, with a surface of 76 000 ha of naked salty clay that is seldom covered with water and is completely devoid of vegetation. The Hodna

518-413: Is often found in mudrock as thin laminae , as clumps, or dispersed throughout the rock. Laminae suggest deposition in a weak current that winnows the silt of clay, while clumps suggest an origin as fecal pellets . Where silt is dispersed throughout the mudrock, it likely was deposited by rapid processes, such as flocculation . Sedimentary rock composed mainly of silt is known as siltstone . Silt

555-528: Is soil rich in silt which makes up some of the most fertile agricultural land on Earth. However, silt is very vulnerable to erosion, and it has poor mechanical properties, making construction on silty soil problematic. The failure of the Teton Dam in 1976 has been attributed to the use of unsuitable loess in the dam core, and liquefication of silty soil is a significant earthquake hazard. Windblown and waterborne silt are significant forms of environmental pollution, often exacerbated by poor farming practices. Silt

592-634: Is that it is a straightforward continuation to a smaller scale of the disintegration of rock into gravel and sand. However, the presence of a Tanner gap between sand and silt (a scarcity of particles with sizes between 30 and 120 microns) suggests that different physical processes produce sand and silt. The mechanisms of silt formation have been studied extensively in the laboratory and compared with field observations. These show that silt formation requires high-energy processes acting over long periods of time, but such processes are present in diverse geologic settings. Quartz silt grains are usually found to have

629-443: Is that silt is fine-grained detrital material composed of quartz rather than clay minerals . Since most clay mineral particles are smaller than 2 microns, while most detrital particles between 2 and 63 microns in size are composed of broken quartz grains, there is good agreement between these definitions in practice. The upper size limit of 1/16 mm or 63 microns corresponds to the smallest particles that can be discerned with

666-591: The Nile and Niger River deltas. Bangladesh is largely underlain by silt deposits of the Ganges delta. Silt is also abundant in northern China, central Asia, and North America. However, silt is relatively uncommon in the tropical regions of the world. Silt is commonly found in suspension in river water, and it makes up over 0.2% of river sand. It is abundant in the matrix between the larger sand grains of graywackes . Modern mud has an average silt content of 45%. Silt

703-461: The Sahara has a less encumbered terrain with smooth mountaintops separated by water gaps . The climate is dry in all the surrounding areas, but the rainfall is slightly better on the mountains. Therefore, cereal cultivation is possible in the heights, although with rather irregular results. Date palms are mostly absent throughout the range owing to the prevailing cold temperatures. They only grow in

740-430: The U.S. Department of Agriculture puts the cutoff at 0.05mm. The term silt is also used informally for material containing much sand and clay as well as silt-sized particles, or for mud suspended in water. Silt is a very common material, and it has been estimated that there are a billion trillion trillion (10 ) silt grains worldwide. Silt is abundant in eolian and alluvial deposits, including river deltas , such as

777-469: The area, but historically they have not been properly exploited. 35°30′N 4°50′E  /  35.500°N 4.833°E  / 35.500; 4.833 This article about a location in M'Sila Province is a stub . You can help Misplaced Pages by expanding it . This article about a location in Batna Province is a stub . You can help Misplaced Pages by expanding it . Silt Silt

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814-513: The crystal structure of the quartz, known as Moss defects. Such defects are produced by tectonic deformation of the parent rock, and also arise from the high-low transition of quartz: Quartz experiences a sharp decrease in volume when it cools below a temperature of about 573 °C (1,063 °F), which creates strain and crystal defects in the quartz grains in a cooling body of granite. Mechanisms for silt production include: Laboratory experiments have produced contradictory results regarding

851-401: The early Tertiary which remained relatively undisturbed after formation but have been heavily eroded. Their altitude is relatively moderate, the highest point of the range, Djebel Lazrag (جبل الأزرق), not even reaching 1,500 m. However, towards the south the look of the highest ridges is quite spectacular, with jagged sandstone or limestone outcrops and almost vertical walls. The part facing

888-558: The earthquake damage potential in the silty soil of the central United States in the event of a major earthquake in the New Madrid Seismic Zone . Silt is easily transported in water and is fine enough to be carried long distances by air in the form of dust . While the coarsest silt particles (60 micron) settle out of a meter of still water in just five minutes, the finest silt grains (2 microns) can take several days to settle out of still water. When silt appears as

925-470: The effectiveness of various silt production mechanisms. This may be due to the use of vein or pegmatite quartz in some of the experiments. Both materials form under conditions promoting ideal crystal growth, and may lack the Moss defects of quartz grains in granites. Thus production of silt from vein quartz is very difficult by any mechanism, whereas production of silt from granite quartz proceeds readily by any of

962-556: The few places at the feet of the Ouled Naïl mountains that are at an altitude below 600 m, such as near Bou Saada and the Oued Djedi. Two important towns are located in the immediate area of the range, Bou-Saada to the north at the feet of the mountains and Djelfa in the middle of the range at an elevation of 1,208 m. North of Djelfa town there is an imposing physical feature known as Rocher de Sel (English: Salt Rock) that

999-443: The fine silt produced in dust storms and the coarse silt fraction possibly representing the fine particle tail of sand production. Loess underlies some of the most productive agricultural land worldwide. However, it is very susceptible to erosion. The quartz particles in silt do not themselves provide nutrients, but they promote excellent soil structure , and silt-sized particles of other minerals, present in smaller amounts, provide

1036-401: The laboratory using the pipette method, which is based on settling rate via Stokes' law and gives the particle size distribution accordingly. The mineral composition of silt particles can be determined with a petrographic microscope for grain sizes as low as 10 microns. Vadose silt is silt-sized calcite crystals found in pore spaces and vugs in limestone . This is emplaced as sediment

1073-478: The level terrain of the large lake basin, which becomes a salt flat when it dries. The elevation of the lake's surface is slightly below 400 m, while the surrounding steppe -like plain, characterized by wadis and oases , averages between 500 and 600 meters in elevation in the communes near the lake. The Hodna natural region gives its name to the Hodna Mountains located to the north. Its southern limit

1110-547: The mountain grasslands. The years of drought and years with prolonged, cold winters are hard times for the Ouled Nail; in 1944 and 1947 about 50% of their livestock died. Hodna The Hodna ( Arabic : حضنة ) is a natural region of Algeria located between the Tell and Saharan Atlas ranges at the eastern end of the Hautes Plaines . It is a vast depression lying in the northeastern section of M'Sila Province and

1147-787: The necessary nutrients. Silt, deposited by annual floods along the Nile River , created the rich, fertile soil that sustained the Ancient Egyptian civilization. The closure of the Aswan High Dam has cut off this source of silt, and the fertility of the Nile delta is deteriorating. Loess tends to lose strength when wetted, and this can lead to failure of building foundations. The silty material has an open structure that collapses when wet. Quick clay (a combination of very fine silt and clay-sized particles from glacial grinding)

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1184-513: The past 50 years. With Dutch funding, the Bangladeshi government began to help develop older chars in the late 1970s, and the effort has since become a multi-agency operation building roads, culverts , embankments, cyclone shelters, toilets and ponds, as well as distributing land to settlers. By fall 2010, the program will have allotted some 100 square kilometres (20,000 acres) to 21,000 families. A main source of silt in urban rivers

1221-440: The problem may be the conflation of high rates of production with environments conducive to deposition and preservation, which favors glacial climates more than deserts. Loess associated with glaciation and cold weathering may be distinguishable from loess associated with hot regions by the size distribution. Glacial loess has a typical particle size of about 25 microns. Desert loess contains either larger or smaller particles, with

1258-699: The unaided eye. It also corresponds to a Tanner gap in the distribution of particle sizes in sediments : Particles between 120 and 30 microns in size are scarce in most sediments, suggesting that the distinction between sand and silt has physical significance. As noted above, the lower limit of 2 to 4 microns corresponds to the transition from particles that are predominantly broken quartz grains to particles that are predominantly clay mineral particles. Assallay and coinvestigators further divide silt into three size ranges: C (2–5 microns), which represents post-glacial clays and desert dust; D1 (20–30 microns) representing "traditional" loess ; and D2 (60 microns) representing

1295-413: The very coarse North African loess. Silt can be distinguished from clay in the field by its lack of plasticity or cohesiveness and by its grain size. Silt grains are large enough to give silt a gritty feel, particularly if a sample is placed between the teeth. Clay-size particles feel smooth between the teeth. The proportions of coarse and fine silt in a sediment sample are determined more precisely in

1332-650: The western end of Batna Province . The Hodna region was home to the Kingdom of Hodna , born out of the Mauro-Roman Kingdom in the post-classical era . The Hodna region has a length of about 120 km in an NW - SE direction and a width of about 80 km. It is characterized by the Chott El Hodna , an endorheic salt lake , in its centre with seasonal brackish and saline pools and marshes. A thin film of water collects during wet seasons on

1369-558: Was formed from the erosion of rock salts and marls by rain. To the west of the town Megalithic funerary structures are found. Neolithic art is widespread in different caves and walls throughout the range. The range is well known for the Ouled Naïl people living in its cold highlands. Djelfa is an important market centre for the Ouled Naïl who live in black-and-red striped tents and claim ancient Arab descent. These semi-nomadic or nomadic people have traditionally reared cattle in

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