gravel

Aggregate of more or less rounded rock fragments coarser than sand (i.e., more than 0.08 in., or 2 mm, in diameter). Gravel beds in some places contain heavy metallic ore minerals, such as cassiterite (a major source of tin), or native metals, such as gold, in nuggets or flakes. 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. In many regions gravel terraces (or raised beaches) extend great distances inland, indicating that the sea at one time stood higher than it does today. Gravels are widely used building materials.