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
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.
© John Welford
