Searching Sand: About Sand

Sand, along with gravel, silt and clay are collectively known as sediment, and are produced by the mechanical and chemical breakdown of rocks. Once disaggregated from the original source rock, this material is then eroded and transported by either wind, water or ice, often ending up at the deposits of rivers or lakes, as sand dunes, or ultimately as sediment in the sea. Eventually this material may be buried to sufficient depth within the earth to harden and form sedimentary rock.

The composition of sand is largely dependent on the source material. For example, the sand around volcanic islands is often composed of volcanic rock fragments, volcanic glass, and other minerals associated with volcanic rocks. In contrast, sediment found on the beaches of southern California are largely composed of quartz (the most durable common mineral), possibly some feldspar (also durable, but more easily chemically weathered to clay), and other minerals associated with the plutonic igneous rocks which form the bulk of the mountain ranges nearby. In areas where there is no good source of sedimentary material from mountains or volcanoes, sand is often entirely composed of organic material i.e. shell fragments, coral, and the tests (skeletons) of small planktonic organisms.

The texture of sediment is largely determined by the transportation process. The three important parameters used to assess the texture of sediment are size, rounding and sorting. Grain Size - The terms gravel, sand, silt and clay carry with them a size connotation. Gravel is any material greater than 2 millimeters in its largest dimensions. This includes boulders, cobbles, pebbles and granules (in decreasing size order). Sand is any material between 2mm and 0.06 mm in size. We usually sub-divide this catagory into very coarse, coarse, medium, fine etc... In practical terms, very fine sand is about the smallest grain size you can still see with the naked eye. Silt is material which is finer than sand, but still feels gritty when rubbed on your teeth. Clay is the finest material of all, and pure clay will feel smooth on your teeth, and will form a sticky ball when wet. As a general rule, material gets smaller the more it has been transported. Therefore very coarse material usually indicates a short distance of transport and visa versa.

Rounding - As material is transported, it is subject to abrasion and impact with other particles which tends to "round-off" the sharp edges or corners. Therefore a well rounded sand grain has probably traveled a great distance from its original source area, while an angular grain has probably only been transported locally. Be careful not to confuse rounding with sphericity. A well-rounded grain may or may not resemble a sphere. Rounding is also related to the size of the grains, i.e. boulders tend to round much more quickly than sand grains because they strike each other with much greater force.

Sorting - The sorting of a sediment is simply how well the sedimentary material is separated out by size. For example, if all the grains in a sediment sample are very nearly the same size, then we say the sample is "well-sorted." If a sediment sample were to contain pieces of gravel, as well as sand and silt, it would be a "poorly sorted" sample. Sorting is somewhat dependent on the distance of transport, but it is primarily effected by the medium of transport. Water is an excellent medium for sorting of particles by size (and density). Wind is probably the best sorting mechanism of all, but only on the finer grain sized (not much gravel is moved by wind transport). Ice is the poorest sorting mechanism, transporting and depositing all sizes of sediment with equal ease.

We can learn a lot by looking closely at sand. By carefully examining the composition, size, rounding and sorting of sand, along with other clues such as the surface texture of the grains and the kind of organic material present, we can make an interpretation as to depositional environment of the sand, how far it has traveled, and its ultimate source area.

updated February 6, 2002