CoachA pebble is a small piece of rock that has been rounded and smoothed by water rolling it along, in a river or on a beach. Gravel is a mass of small stones and pebbles lying together, with grains bigger than grains of sand.
示例
Example 5.6 (Where do we find each size?)
Fast water can carry big stones; slow water drops them and carries only small grains. So boulders and sharp stones lie in the steep mountain streams; pebbles and gravel lie on the banks of the river in the valley; sand gathers on beaches and in quiet bends of the river; and mud settles where the water is almost still, in ponds, lakes and the mouths of rivers.