Gravel is a mixture of loose pieces of rock and particles of sand and clay. At least 30 percent of gravel consists of pieces of rock that are coarser, or larger, than sand–that is, they measure more than 1/12 inch (2 millimeters) across. Sand and clay make up the rest of gravel, often forming 50 to 70 percent of the gravel’s total mass.
Most of the useful deposits of gravel were laid down by rivers and glaciers or were formed in lakes and oceans. In preparation for industrial use, gravel is dug up from pits and washed. Screens are then used to sort the chunks of rock according to size.
Taken together, gravel and sand are one of the largest products by volume in the construction industry. Gravel is used to build roads, railroad embankments, and airfield landing strips. Paved surfaces are made with gravel mixed with cement or asphalt, a dark tarlike substance. Gravel mixed with sand, water, and cement makes concrete.
