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Sandstone is an arenaceous sedimentary rock composed mainly of feldspar and quartz and varies in colour (in a similar way to sand), through grey, yellow, red, and white. Since sandstones often form highly visible cliffs and other rock formations, certain colors of sandstone may be strongly identified with certain regions. For instance, much of the American West is well-known for its red sandstones.Sandstones are often relatively soft and easy to work which therefore make them a common building and paving material.
Rock formations that are primarily sandstone usually allow percolation of water, and are porous enough to store large quantities, making them valuable aquifers. Fine grained aquifers, such as sandstones, are more apt to filter out pollutants from the surface than are rocks with cracks and crevices such as limestones or other rocks fractured from seismic activity.
1 Origins
Sandstones are clastic in origin (as opposed to organic, like chalk or coal). They are formed from the cemented grains that may be fragments of a pre-existing rock, or else just mono-minerallic crystals. The cements binding these grains together are typically calcite, clays and silica. Grain sizes in sands are in the range of 0.1mm to 2mm. (Rocks with smaller grainsizes include siltstones and shales and are typically called argillaceous sediments, as are also clays. Rocks with larger grainsizes include both breccias and conglomerates and are termed rudaceous sediments.).
The principle mechanism for the formation of sandstone is by the sedimentation of grains out of a fluid, such as a river, lake or sea. The environment of deposition is crucial in determining the characteristics of the resulting sandstone, which on a finer scale include its grainsize, sorting, composition and on a larger scale include the rock geometry. Principal environments of deposition may be split between terrestrial and marine, as illustrated by the following broad groupings:
- Rivers ( levees, point bars, channel sands)
- Lakes
- Shoreface sands
- DeltasA delta is the mouth of a river where it flows into an ocean, sea, or lake, building outwards (as a deltaic deposit from sediment carried by the river and deposited as the water current is dissipated. Deltaic deposits of larger, heavily-laden rivers are c
- TurbiditeTurbidites form as a result of turbidity currents, a form of turbulent, sediment bearing gravity current. They are comparable to avalanches, moving due to the density excess provided by the suspended material, but travel under water. They are responsibles (submarine channels)
2 Types of sandstone
Once the geological characteristics of a sandstone have been established, it can then be assigned to one of three broad groups:
- arkosic sandstones, which have a high (>25%) feldspar content
- quartzose sandstones, such as quartziteQuartzite is a hard, metamorphic rock which was originally sandstone. Through heating and pressure usually related to tectonic compression within orogenic belts, the original quartz sand grains and quartz silica cement were fused into one. Pure quartzite, which have a high (>90%) quartz content. Sometimes these sandstones are termed "quartzites", e.g., the Tuscarora Quartzite of the Ridge-and-valley AppalachiansThe Ridge-and-valley Appalachians are a belt within the Appalachian Mountains extending from northern New Jersey westward into Pennsylvania and southward into Maryland, West Virginia, and Virginia. They form a broad arc between the Piedmont and the Allegh
- argillaceous sandstones, such as greywackeGreywacke ( German grauwacke signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness, dark color, and poorly-sorted, angular grains of quartz, feldspar, and small rock fragments set in a compact, clay-fine matrix., which have a significant fine-grained element
