What sorts sediment and is likely to make it well sorted?

Answer 1

Sediment is usually sorted by depositional processes, most common are via water and wind.

Well sorted sediments usually come from areas where energies only permit certain sediments to be transported. For example, in high energy mountain streams, the velocity of the water will easily pick up sand and clay particles, leaving the larger grains (gravel, boulders) to be deposited in the stream bed. This gives you a well sorted deposit, with large grain size.

If you want an example of well sorting with small grain sizes, take a look at a lakebed. Here, there is not enough force for large particles to be transported, only enough force for small clay and sand particles to be moved. These particles propagate through the water and will eventually settle at the bottom of the lake thanks to gravity and water pressure.