# Which of the following would be deposited by a river last? sand silt small rocks large rocks?

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• David
Lv 7
3 months ago

Sorting describes the distribution of grain size of sediments. Very poorly sorted indicates that the sediment sizes are mixed (large variance); whereas well sorted indicates that the sediment sizes are similar (low variance).

Sediment is deposited when the energy of the transporting medium becomes too low to continue the transport process. In other words, if the velocity of the transporting medium becomes too low to transport sediment, the sediment will fall out and become deposited. The final sediment thus reflects the energy of the transporting medium.

In general, the coarser sediment gets left behind by the transportation process. Thus, coarse sediment is usually found closer to its source and fine grained sediment is found farther from the source. We can use the texture of the resulting deposits to give us clues to the mode of transport and deposition.

Particles become sorted on the basis of density, because of the energy of the transporting medium. High energy currents can carry larger rocks and fragments. As the energy decreases, heavier particles are deposited and lighter fragments continue to be transported. This results in sorting due to density.

If the particles have the same density, then the heavier particles will also be larger, so the sorting will take place on the basis of size. We can classify this size sorting on a relative basis - well sorted to poorly sorted. Sorting gives clues to the energy conditions of the transporting medium from which the sediment was deposited.

In a river setting the smaller rocks and silt grains are deposited along the edges where the water slows, the larger grains and rocks are found toward the deeper center where water velocity is fastest.

At the end of the river, again, where the river slows, sediment will begin to fall out into an estuary or river delta. The formation of a delta consists of three main forms: the topset, foreset/frontset, and bottomset.

The bottomset beds are created from the suspended sediment that settles out of the water as the river flows into the body of water and loses energy. The suspended load is carried out the furthest into the body of water than all other types of sediment creating a turbidite. These beds are laid down in horizontal layers and consist of smaller grains.

The foreset beds in turn build over the bottomset beds as the main delta form advances. The foreset beds consist of the bed load that the river is moving along which consists of larger sediments that roll along the main channel. In cross section, one would see the foresets lying in angled, parallel bands, showing each stage of the creation of the delta.

The topset beds in turn overlay the foresets, and are horizontal layers of smaller sediment size that form as the main channel of the river shifts elsewhere and the larger particles of the bed load no longer are deposited.

So the answer to your question is: The larger rocks are deposited last...

• Ron
Lv 7
3 months ago

The answer is likely to be silt, but the question is flawed.  Small rocks include clays, which are smaller than silt.  So, technically, small rocks is the right answer, but I doubt that was what was intended.