[Fr.] Loose, rounded, water-worn fragments of rock in which the pebbles range in size from a pea to a walnut. When smaller, they form sand ; and when larger, shingle. Gravel is formed by the action of rivers and of the sea ; and, since the pieces of the harder species of rocks are best able to withstand the action of water, gravel is found to consist chiefly of fragments of quartz and other silicious materials.