Geology And Geophysics

Classification of Sedimentary Rocks

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There are basically two ways of classifying sedimentary rocks, either by how they originated or their composition. The latter takes into account such matters as whether they contain primarily coarse-textured sands or fine clays, or whether they have a high carbon content due to being composed largely or entirely of plant or animal matter. However, it is generally more convenient to combine the two methods into a single classification, as below:

Mechanical Formation

This group comprises rocks that have been formed after material has been moved in fragments from one or several places to another (by the action of wind, water, ice or gravity), where it has become consolidated, either by pressure from later deposits, or by cementation, or both. The original material may have been very fine in nature, such as river-borne silt, or much coarser, such as rounded or angular pebbles or rock fragments.

The material that enables fragments to cement together may be a solution containing minerals of various kinds, such that sandstones may contain quartz, calcium carbonate or iron, the proportions of these determining its colour.

Very fine material will form clays or mudstones, less fine deposits lead to grits forming, and much coarser material results in a conglomerate or brecchia (in the former the pebbles are rounded, whereas they are angular in the latter).

Terms used to distinguish rocks by the size of their particles are Argillaceous (e.g. clay, mudstone, shale); Arenaceous (e.g. sandstone, grit); and Rudaceous (e.g. brecchia, conglomerate, boulder clay).

Organic Formation

These rocks were created from the remains of once living organisms which built up over very long periods of time. These can be further classified according to the nature of the plants or animals that comprised the deposits.

Calcareous rocks (chalks and limestones) consist mainly of calcium carbonate, formed from the skeletons of marine organisms, and are distinguished by the size and nature of the particles that comprise them. The finest particles are seen in pure white chalk. Limestone is more varied, including crinoidal, coral, oolitic and shelly, the terms denoting the type of primitive organism that is mainly represented in its formation. Fossils of much larger organisms are often found embedded in limestone.

Ferruginous is a term that denotes the presence of iron, usually from the precipitation of hydrated iron oxide in the water of ancient lakes and marshes. Decomposing vegetable matter formed the basis of ironstone and “bog iron-ore”.

Siliceous rocks can be formed from the remains of sponges and minute organisms such as diatoms (single-celled plants rich in silica). These include nodules of chert and flint found in other rocks, and beds of diatomite.

Carbonaceous rocks are formed from plant accumulations and are high in carbon content. Depending on the age of the deposits and the pressure they have been put under, they can take the form of peat, lignite or coal.

Chemically Formed

These come about from the precipitation or evaporation of solutions of salts. All water that falls as rain will acquire salt in some form as it runs across the surface or finds its way underground, and these salts are often partially or totally released before the water cycle is completed. Rock formation can occur when sufficient salts accumulate in the same place. Five types of chemical formation of rock types can be distinguished.

Carbonates.  Stalactites and stalagmites in limestone caves, or travertine around hot springs, are examples of carbonate deposition. Dolomite is a chemically formed compound of calcium and magnesium carbonate.

Sulphates. Hydrated calcium sulphate, in the form of gypsum or alabaster, is formed by evaporation in inland drainage basins.

Chlorides. These produce rock-salt, either on the surface or at depth.

Silicates. As well as flint and chert (mentioned above), sinter is a silicate rock, formed around the vents of hot springs.

Ironstones. Most iron ores have accumulated from chemical precipitation within sediments, although some are the result of igneous activity.

Sedimentary rocks are typically laid down in strata of varying thicknesses, and the process can continue at the same place for extremely long periods of time (millions of years in some cases). It is sometimes possible, for example, to detect annual depositions made by ancient rivers, and use these to determine the age of a particular formation.


Monkhouse, F J, Principles of physical geography. 6th ed. University of London Press, 1965.

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