What is Sediment Sorting?
Sediment sorting refers to the degree of uniformity in grain size within a sediment sample.
* Well-sorted sediments: Have grains that are all roughly the same size (e.g., all fine sand or all coarse gravel).
* Poorly sorted sediments: Contain a mix of grain sizes (e.g., boulders mixed with sand and clay).
Factors that Influence Sediment Sorting
1. Transportation:
* Wind: Wind is a highly effective sorting agent. It carries smaller grains (like sand and silt) much farther than larger particles (like gravel). This results in well-sorted sand dunes and windblown deposits.
* Water: Water can also sort sediments, but the sorting process is less efficient than wind. Rivers and streams typically deposit well-sorted sediments in their lower courses (where the water velocity is slower).
* Glaciers: Glaciers are very poor sorters. They move and deposit a wide range of grain sizes in a chaotic mixture.
2. Depositional Environment:
* Beaches: Beaches often have well-sorted sand because the waves and currents selectively remove smaller particles and leave behind coarser sand grains.
* River Deltas: Deltas (where rivers meet the ocean) can have well-sorted sediments in some areas, especially where there is a gradual transition in water velocity and sediment deposition.
* Sand Dunes: As mentioned before, wind-blown sand dunes are typically well-sorted.
3. Sediment Source:
* Rock Weathering: The initial breakdown of rocks into sediments plays a role. If the source rock is relatively homogeneous, it may produce a more uniform starting material, making sorting easier.
Examples of Well-Sorted Sediments
* Dune Sand: Wind-blown sand dunes are classic examples of well-sorted sediments.
* Beach Sand: Beaches, especially those with a significant wave action, can have well-sorted sand.
* Some River Channel Deposits: The bottom of a river channel, where the water velocity is consistently high, can deposit well-sorted sediments.
In Summary
Well-sorted sediments are usually created by agents like wind or water that have a strong ability to transport and selectively deposit particles of similar sizes. The depositional environment and the nature of the sediment source also contribute to the degree of sorting.
