GRAVEL

aggregate of more or less rounded rock fragments coarser than sand (i.e., more than 2 mm [0.08 inch] in diameter). Gravel beds in some places contain accumulations of heavy metallic ore minerals, such as cassiterite (a major source of tin), or native metals, such as gold, in nuggets or flakes. Gravels are widely used building materials. Fragments in gravel range in size from pebbles (4-64 mm [0.16-2.52 inches] in diameter), through cobbles (64-256 mm [2.52-10.08 inches]), to boulders (larger than 256 mm). The rounding of gravel results from abrasion in the course of transport by streams or from milling by the sea. Gravel deposits accumulate in parts of stream channels or on beaches where the water moves too rapidly to permit sand to remain. Because of changing conditions, gravel formations generally are more limited and more variable in coarseness, thickness, and configuration than sand or clay deposits. Persistent accumulation of gravel or pebble beds may take place along an inner zone of breaking waves, on a beach that is otherwise sandy. Cobble and pebble beaches (shingle beaches) often originate from the points of rocky cliffs. In many regions marine gravels similar to those of the seashore exist tens or hundreds of metres above tide level; such gravel terraces (or raised beaches) may extend great distances and indicate that the sea at one time stood relatively higher. River gravels occur mostly in the middle and upper parts of streams where the currents are swiftest. Ancient gravel terraces found at levels much above those of the present rivers mark former streams or are evidence of uplift of the land or lowering of the sea. The prolonged weathering and extended transport of gravel by long rivers results in more complete rounding and sorting of the rock fragments by size and physical and chemical durability. Cemented gravels are called conglomerates.