Formed through the agency of water. Also called as aqueous rocks. Formed from
sediments brought by water. The sediment may contain various types of substances and
sizes of particles. The particles are cemented by silica, iron oxide or lime to give a
consolidated form. The rocks are mostly deposited in layers or strata – so called as
stratified rocks. Sometimes they are formed by cooling, evaporation or by direct chemical
precipitation.
Anyway they are of secondary origin.Sedimentary rocks divided into six groups as follows
1. Arenaceous: Formed of the deposits of coarse grained particles. They are
composed of siliceous material derived from the disintegration of older rocks. The
fragmental material so derived is deposited in beds of varying thickness through the agency of water. Depending upon the nature of cementing material present, some arenaceous rocks are hard and refractory, but most are loose and fall away very easily. E.g. Sandstone, grit, conglomerate and breccia.
2. Argillaceous rocks: Consist of small sized particles known as clay. They are
composed of hydrated silica of alumina in admixture with sand, various other silicates and
calcareous matter. When clay is deposited mainly of silicate of alumina, it is known as
kaolin or China clay. E.g. clay, mudstone, shale and fuller’s earth.
3. Calcareous rocks: Consists of carbonate of lime or lime and magnesia. They may
be of sedimentary origin or formed by chemical precipitation or by organic agency. When,
they are of organic agency, they are composed mainly of debris from plant and animal life.
They are formed either by growth and decay of organisms in situ or by the transport and
subsequent accumulation of their remains. The rocks so formed are found in layers, which
vary considerably in depth of thickness. When formed by chemical precipitation, the calcareous material is deposited in the form of layers/sheets from waters containing calcium carbonate in solution. The precipitate when first formed is usually soft and chalky, but soon acquires a hard, compact structure and crystalline texture. The important calcareous rocks of aqueous origin are limestone, chalk, magnesian, ferruginous limestones, dolomite, marks of various varieties and coral.
4. Carbonaceous rocks: Formed from decomposing vegetation under anaerobic
conditions. When plants undergo decomposition under restricted air supply, is greater
portion of the carbonaceous matter is retained and the material is slowly converted into
coal. E.g. peat, lignite, coal, anthracite.
5. Siliceous rocks: Siliceous rocks of organic origin formed from parts of minute
plants and animals like diatoms, radiolaria etc, some are soft and friable and crumble to
powder very easily. Others like flint and chert are hard and compact.
6. Precipitated salts: Consist mainly of deposits formed as rock masses either by
cooling, evaporation or by chemical precipitation. Water charged with acid or alkaline
material, acting under pressure as it does under subterranean regions, dissolves various
mineral substances from rocks with which it comes in contact. The salts thus formed
deposit as rocks and such rocks vary in composition. They are
i. Oxides: e.g. hematite, limonite, bauxite and quartz.
ii. Carbonates: e.g. stalactite, stalagmite, magnetite and limestone.
iii. Sulphates: e.g. gypsum and anhydrite
iv. Phosphates: e.g. phosphorite
v. Chlorides: e.g. rock salt.
