Gravel is rock that is of a specific particle size range. Specifically, it is any loose rock that is larger than two millimeters (2mm) in its largest dimension (about 1/12 of an inch) and no more than 64 millimeters (about 2.5 inches). The next smaller size class in geology is sand, which is >0.0625 mm to 2 mm in size. The next larger size is cobble, which is >64 millimeters to 256 millimeters (about 2.5 to 10 inches). Gravel can be sub-categorized into granule (>2-4 mm) and pebble (>4-64 mm). One cubic foot (28.32 dm³) of gravel typically weighs about 100 pounds (45 kg).

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 tarmac; Russia alone has over 400,000 km of gravel-surfaced roads. Both sand and small gravel are also important for the manufacture of concrete.

Geological formation

Large gravel deposits are a common geological feature, being formed as a result of the weathering and erosion of rocks. The action of rivers and waves tends to pile up gravel in large accumulations. This can sometimes result in gravel becoming compacted and concreted into the sedimentary rock called conglomerate. Where natural gravel deposits are insufficient for human purposes, gravel is often produced by quarrying and crushing hard-wearing rocks, such as sandstone, limestone, or basalt. Quarries where gravel is extracted are known as gravel pits. Southern England possesses particularly large concentrations of them due to the widespread deposition of gravel in the region during the Ice Ages.

Modern production

China is currently the world's leading producer of gravel, displacing the USA as the world's chief supplier.

Etymology

The word comes from the French gravelle, meaning 'coarse sand'.

Types of gravel

• Bank gravel: gravel intermixed with sand or clay.

• 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: This is generally rounded, semi-polished stones, potentially of a wide range of types, that are dredged or scooped from river beds and creek beds. It is also often used as concrete aggregate and less often as a paving surface.