Marine deltas
The mouths of rivers fall into two general categories. A river with a low sediment load may reach the sea in an estuary, where the effect of current, waves, and tides keeps sediment from accumulating. A heavily laden river, however, usually creates a marine delta, a seaward extension of land at or near sea level caused by the accumulation of sediments at the river's mouth. A delta is composed mostly of sediments brought down by the river, but mixed with them are fine wave-borne sediments brought in by currents and tides.
A growing (prograding) delta is a complex structure of interbedded sediments with three more or less distinct zones of deposition. The zone nearest to the shore is occupied by topset beds, complexes of heavier, coarser particles. Depending upon flow energy and sediment load, these are typically overlapping sand and gravel bodies forming seaward extensions of the stream's natural channel and levees. They often show ripple marks, erosion surfaces, and other evidence of flow. Shallow bays between diverging (distributary) channels contain finer sediments from both the river and the sea, and often shelter abundant plant and animal life.
Some of the finer sediments carried beyond the topset beds settle out in foreset beds on the steep seaward face of the delta. In the upper foreset zone, where wave action suspends the finer sediments, the foreset beds may consist of clean sand; below the wave base, muddy sand and silt; and near the toe of the delta, thinner beds of silt and mud. As the delta grows seaward, the foreset beds are overlain by extensions of the topset beds.
Bottomset beds are tapering layers of silt and clay extending seaward from the face of the delta. Beyond reach of river and waves, these sediments accumulate slowly and sometimes support considerable seafloor life.
The delta described above is an idealized model that is rarely found in nature, where conditions vary widely. Distinct topset, foreset, and bottomset beds are more often seen in lacustrine deltas in sheltered continental environments.
Beaches
The transition zone also includes beaches, coastal depositional environments unrelated to the mouths of rivers. Sand and finer sediments are redistributed by longshore currents (the movement of seawater parallel to the coast). The growth and shrinkage of non-delta coastal deposits are related to the energy of longshore currents, waves, and tides.
A cross section of a typical beach shows how the sediments brought in by long shore currents are sorted and redistributed by wave and tidal energy. Differences in energy levels divide the beach profile into the shoreface (the wave action zone up to the low-tide mark), the foreshore (between low- and high-tide levels), the backshore (from high tide to storm-flood level), and the dunefield (above storm-
flood level). Because these zones are continually affected by wind and waves, their sediments are usually clean, well-sorted mineral grains and shell fragments. A cross section of a beach dune may show the same excellent sorting and cross-bedding found in desert dunes. Coarser gravels are deposited in deeper water, and finer sediments are carried seaward by wave action or inland by wind.
As the shoreface slopes upward toward the land, ocean waves begin to "feel" the bottom-that is, friction retards the movement of water near the bottom of a wave. The top of the wave goes ahead of the bottom, causing the wave to topple, or "break." While at the seashore you may have noticed a line of breakers some distance offshore. A breaker line marks an underwater bar where the concentration of wave and back- wash energy causes an accumulation of coarser sediments, such as gravel and seashells. One or more bars may form on the shoreface, depending upon the energy of incoming waves and the availability of sediments.
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Beach Profile Another high-energy zone occurs where the incoming wave laps up onto the foreshore. This action sorts the beach sediments, washing the sand clean of filler sediments and leaving particles of similar size together in distinct zones. Sediments fine enough to be suspended in the turbulent waves are carried offshore and deposited in quiet water beyond the breaker line. If the beach is on an offshore barrier (spit or island), the transition zone may also include a shallow lagoon where sediments accumulate in a backbarrier complex. Such depositional environments are highly variable and may include tidal channels, salt marshes, shell reefs, and mangrove swamps, among other features. Deposition in the backshore zone is intermittent. Coarser sediments may be deposited here when storm waves surge over the highest beach bars and run back into the sea in shallow channels parallel to the shore. Dry sand in the backshore zone may migrate inland in desert-like dunes driven by onshore winds. I If the lagoon behind the barrier is relatively isolated from the sea, the W backbarrier complex may include such disparate features as sebkhas, ¡F thin-bedded deposits of salt or other evaporites formed where rat; intermittent, landlocked pools dry out; peat, formed from heavy ¡¡3 deposition of plant debris (as in mangrove swamps), which may, if Ik buried deeply, become coal; and organic muds rich in carbonates and other skeletal debris. On the other hand, strong currents behind the barrier island or spit may flush out these deposits, leaving sediments in configurations much like those in stream deposits. 
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