Gravel // is a loose aggregation of rock fragments. Gravel is classified by particle size range and includes size classes from granule- to boulder-sized fragments. In the Udden-Wentworth scale gravel is categorized into granular gravel (2 to 4 mm or 0.079 to 0.157 in) and pebble gravel (4 to 64 mm or 0.2 to 2.5 in). ISO 14688 grades gravels as fine, medium, and coarse with ranges 2 mm to 6.3 mm to 20 mm to 63 mm. One cubic metre of gravel typically weighs about 1,800 kg (or a cubic yard weighs about 3,000 pounds).
Gravel is an important commercial product, with a number of applications. Many roadways are surfaced with gravel, especially in rural areas where there is little traffic. Globally, far more roads are surfaced with gravel than with concrete or asphalt; Russia alone has over 400,000 km (250,000 mi) of gravel roads. Both sand and small gravel are also important for the manufacture of concrete. Natural gravel has a high hydraulic conductivity, sometimes reaching above 1 cm/s.
Geologists define gravel as rounded rock fragments with a particle size over 2 millimeters (0.079 in), with the upper size limit ranging from 10 millimeters (0.4 in) to no upper limit in size. Civil engineers in the U.S. define gravel as particles from 4 to 76 millimeters (0.16 to 3.0 in) in diameter, such that they pass through a number 4 wire mesh.
When sediments containing over 30% gravel become lithified into solid rock, the result is a conglomerate. Conglomerates are widely distributed in sedimentary rock of all ages, but usually as a minor component, making up less than 1% of all sedimentary rock. Alluvial fans likely contain the largest accumulations of gravel in the geologic record. These include conglomerates of the Triassic basins of eastern North America and the New Red Sandstone of south Devon.
Gravel often has the meaning a mixture of different size pieces of stone mixed with sand and possibly some clay. In American English, rocks broken into small pieces by a crusher are known as crushed stone.
Types of gravel include:
- Bank gravel: naturally deposited gravel intermixed with sand or clay found in and next to rivers and streams. Also known as "bank run" or "river run".
- Bench gravel: a bed of gravel located on the side of a valley above the present stream bottom, indicating the former location of the stream bed when it was at a higher level.
- Creek rock or river rock: this is generally rounded, semi-polished stones, potentially of a wide range of types, that are dredged or scooped from stream beds. It is also often used as concrete aggregate and less often as a paving surface.
- Crushed stone: rock crushed and graded by screens and then mixed to a blend of stones and fines. It is widely used as a surfacing for roads and driveways, sometimes with tar applied over it. Crushed stone may be made from granite, limestone, dolomite, and other rocks. Also known as "crusher run", DGA (dense grade aggregate) QP (quarry process), and shoulder stone.
- Fine gravel: gravel consisting of particles with a diameter of 2 to 8 mm.
- Stone dust: fine, crushed, gravel from the final stage of screen separation, such that the gravel is not separated out from fine dust particles.
- Lag gravel: a surface accumulation of coarse gravel produced by the removal of finer particles.
- Pay gravel: also known as "pay dirt"; a nickname for gravel with a high concentration of gold and other precious metals. The metals are recovered through gold panning.
- Pea gravel: also known as "pea shingle" is gravel that consists of small, rounded stones used in concrete surfaces. Also used for walkways, driveways and as a substrate in home aquariums.
- Piedmont gravel: a coarse gravel carried down from high places by mountain streams and deposited on relatively flat ground, where the water runs more slowly.
- Plateau gravel: a layer of gravel on a plateau or other region above the height at which stream-terrace gravel is usually found.
Relationship to plant life
In locales where gravelly soil is predominant, plant life is generally more sparse. This outcome derives from the inferior ability of gravels to retain moisture, as well as the corresponding paucity of mineral nutrients, since finer soils that contain such minerals are present in smaller amounts.
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