冲积层是冲积物在河床上的堆积，主要含有卵石、砂粒或粘土，有时候也会含有贵金属或宝石。贵重的金属可以通过淘砂的方法收集。历史上许多宝石（特别是钻石）都发现于印度、非洲西岸等地的卵石中，人们仍在缅甸、斯里兰卡、纳米比亚等地的冲积层中寻找机会。冰川也会携带冲积物。

Alluvium (from the Latin, alluvius, from alluere, "to wash against") is loose, unconsolidated (not cemented together into a solid rock) soil or sediments, which has been eroded, reshaped by water in some form, and redeposited in a non-marine setting. Alluvium is typically made up of a variety of materials, including fine particles of silt and clay and larger particles of sand and gravel. When this loose alluvial material is deposited or cemented into a lithological unit, or lithified, it is called an alluvial deposit.