A deposit of sediments formed at the mouth of a river where it enters a lake or the sea. It is normally built up only where there is no tidal or current action capable of removing the sediment as fast as it is deposited, and hence the delta builds forward from the coastline. This process of building-up is complex, and leads to the formation of a number of separate channels (distributaries), isolated lagoons, levees, marshy ground, and a network of small creeks. Most deltas are complicated and multiple, but in a simple delta the main types of bed may be distinguished:
The bottomset beds - forming on the sea-bottom beyond the seaward face of the delta.
The foreset beds - building outward from the seaward face of the delta over the bottomset beds.
The topset beds - which are deposited above the already deposited foreset beds. This triple structure may also be seen on a small scale in cross-bedding.
Deltaic sediments can generally be recognised by this pattern of major sedimentary feature. Sediments found are, in general, sands of various sorts, clay material and silts, together with a certain amount of organic debris. A good deal of deposition takes place by the flocculation of colloidal material in the river water when it comes into contact with the sea. Occasionally conglomerates are deposited during extreme floods. (see also Meandering river, Levee, Lagoon, Swamp, Coal, Crevasse splay)