accretion

The gradual accumulation of material — in geology, the slow addition of sediment to a landmass by wave action, river deposit, or wind; in coastal geography, the building of a beach or shoreline by the delivery of sand faster than it is removed. Accretion is how land grows at the edges, grain by grain. It also applies at planetary scale: The earth itself was built by accretion, dust and rock gradually collecting into a sphere under the pull of gravity.