Gravel is rock that is of a specific particle size range. In geology, gravel 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 63 millimeters (about 2.5 inches). Sometimes gravel is restricted to rock in the 2-4 millimeter range, with pebble being reserved for rock >4-63 millimeters. The next smaller size class in geology is sand, which is >0.063 mm to 2 mm in size. The next larger size is cobble, which is >63 millimeters to 256 millimeters (about 2.5 to 10 inches). One cubic foot (28.32 dm3) of gravel typically weighs about 100 pounds (45 kg).
Gravel is an important commercial product, used in many 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.
