gravel - Types of gravel

Unconsolidated deposits of rock in the form of pebbles (2–60 mm/0·08–2·5 in in size) laid down by rivers or along seashores. It may be mined for alluvial mineral deposits, but is most commonly used as an aggregate in concrete.

Sometimes gravel is restricted to rock in the 2-4 millimeter range, with pebble being reserved for rock 4-75 millimeters (some say 64 millimeters).

Gravel is an important commercial product, used in many applications. Some important types of gravel include:

Crushed stone: This is generally limestone or dolomite that has been crushed and graded by screens to certain size classes.

In Britain, gravel always refers to smooth, rounded, river-worn material, never to angular stones or crushed rock. British gravel ranges in size from 4 mm to about 30 mm, the smaller sizes up to 8 mm are usually called 'pea gravel'.

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;

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.

Types of gravel

Multiple types of gravel have been recognised by geologists. They include:

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. Fine gravel: gravel consisting of particles with a diameter of 1 to 2 mm. Lag gravel: a surface accumulation of coarse gravel produced by the removal of finer particles. a nickname for gravel with a high concentration of gold and other precious metals. 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. River run gravel: naturally deposited gravel found in and next to rivers and streams.