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The formation of sandstone involves two principal stages. First, a layer or layers of sand accumulates as the result of sedimentation, either from water (as in a stream, lake, or sea) or from air (as in a desert). Typically, sedimentation occurs by the sand settling out from suspension; i.e., ceasing to be rolled or bounced along the bottom of a body of water or ground surface (e.g., in a desert or erg). Finally, once it has accumulated, the sand becomes sandstone when it is compacted by the pressure of overlying deposits and cemented by the precipitation of minerals within the pore spaces between sand grains.
The most common cementing materials are silica and calcium carbonate, which are often derived either from dissolution or from alteration of the sand after it was buried. Colors will usually be tan or yellow (from a blend of the clear quartz with the dark amber feldspar content of the sand). A predominant additional colourant in the southwestern United States is iron oxide, which imparts reddish tints ranging from pink to dark red (terracotta), with additional manganese imparting a purplish hue. Red sandstones, both Old Red Sandstone and New Red Sandstone, are also seen in the Southwest and West of Britain, as well as central Europe and Mongolia. The regularity of the latter favours use as a source for masonry, either as a primary building material or as a facing stone, over other forms of construction.
Weathering
Weathering is the breaking down of rocks, soil, and minerals as well as wood and artificial materials through contact with the Earth's atmosphere, water, and biological organisms. Weathering occurs in situ (on site), that is, in the same place, with little or no movement, and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, snow, wind, waves and gravity and then being transported and deposited in other locations.
Two important classifications of weathering processes exist – physical and chemical weathering; each sometimes involves a biological component. Mechanical or physical weathering involves the breakdown of rocks and soils through direct contact with atmospheric conditions, such as heat, water, ice and pressure. The second classification, chemical weathering, involves the direct effect of atmospheric chemicals or biologically produced chemicals also known as biological weathering in the breakdown of rocks, soils and minerals.[1] While physical weathering is accentuated in very cold or very dry environments, chemical reactions are most intense where the climate is wet and hot. However, both types of weathering occur together, and each tends to accelerate the other. For example, physical abrasion (rubbing together) decreases the size of particles and therefore increases their surface area, making them more susceptible to chemical reactions. The various agents act in concert to convert primary minerals (feldspars and micas) to secondary minerals (clays and carbonates) and release plant nutrient elements in soluble forms.
The materials left over after the rock breaks down combined with organic material creates soil. The mineral content of the soil is determined by the parent material; thus, a soil derived from a single rock type can often be deficient in one or more minerals needed for good fertility, while a soil weathered from a mix of rock types (as in glacial, aeolian or alluvial sediments) often makes more fertile soil. In addition, many of Earth's landforms and landscapes are the result of weathering processes combined with erosion and re-deposition.
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