Sediment is solid material that is transported by water, wind, or ice and that eventually settles out of the fluid in which it was suspended. Most sediments consist of mineral grains that are deposited by gravity after having been carried along by moving water or air. The term sediment also includes organic debris—such as shells, leaves, and twigs—that is transported by water and eventually deposited.
The study of sediments is called sedimentology. Sedimentologists use a variety of techniques to determine the origin and history of sediments. These techniques include examination of the physical properties of the sediments (such as size, shape, and composition), analysis of the chemical composition of the minerals in the sediment, dating of fossils found in the sediment, and study of deposits laid down under similar conditions in other parts of the world.
Sedimentology is important for many reasons. It can provide information about past climates and environments, help us to understand how landscapes have changed over time, aid in the search for fossil fuels such as oil and gas, and be used to monitor present-day environmental conditions.
Most sediments are composed of particles that were originally deposited by running water. The energy from flowing water helps to move these particles along until they eventually settle out in areas where there is little or no flow. The rate at which this happens depends on a number of factors including: grain size—larger grains settle out more slowly than smaller ones; density—heavier minerals settle out more quickly than lighter ones; shape—rounded grains settle out more slowly than angular ones; surface area—grains with a large surface area (i.e., those that are flaky) settle out more slowly than those with a small surface area; solubility—insoluble minerals (such as quartz) settle out more slowly than soluble ones (such as calcite); and temperature—cooler waters allow settlement to occur more quickly than warmer waters.
When all these factors are taken into account, it can be seen that even though some minerals might be very dense or very insoluble, they might still remain suspended in water for long periods if they are small or have a large surface area relative to their size. For example, clay particles can remain suspended indefinitely because they satisfy all four conditions: they are small (<2 µm across), have a large surface area (~100 m²/g), are insoluble at typical groundwater concentrations (~10-3 mol/L), and occur in cool groundwater (~10°C). In contrast, sand-sized quartz particles will quickly settle out because they do not satisfy any one condition completely: they range from 2-500 µm across (so some are small but many are not), have a moderate surface area (15-30 m²/g), are only slightly soluble (~10-9 mol/L), but often occur in warm groundwater (~20°C). It should also be noted that even though most sediments originate from running water deposits, there are other important sources too including windblown dusts (loess) , glacial till , ash falls from volcanic eruptions , organic matter settling from lakes & ponds , beach sands & dunes .