The rocks which are formed due to aggregation and compactness of sediments are known as “sedimentary rocks.” According to their origin and composition, sedimentary rocks can be divided into two main categories. They are:
1. Inorganic rocks: Inorganic rocks are rocks formed from mineral matter which has been obtained by disintegration or decomposition of rocks. Inorganic rocks can be divided into two types. They are
a. Mechanically-formed rocks: The rock which have been formed form the accumulation of rock materials, derived form other rocks and have been cemented together are known as “ Mechanically formed rocks.” The mechanically formed rocks consist of sediments which have been carried and deposited by rivers, glaciers, winds or waves and cemented together with clay or line. On the basis of rock materials, these rocks can be divided into three main categories. They are
1. 1.Rudaceous rocks
2. Aremaceous rocks
3. Argilious rocks
1. Rudaceous rocks: Rudaceous rocks are formed due to cementation and consolidation of pebbles and boulders. Rudaceous rocks consist of rock pieces and grave, cemented together, by clay or line. They are porous and can hold water. If the rock pieces are angular in shape and cemented together, they are called breccias. If the rock pieces are rounded and cemented tighter, they are called conglomerate.
2. Arenaceous rocks: These are formed mainly due to deposition, commentating and consolidation of sane grains e.g. Sand stone.
3. argillaceous rocks: These rocks are formed largely by fine clay particles e..g. Shale or mudstone these are impermeable.
b. Chemically formed rocks: The chemical often settle down and hardened to form rocks known as chemically formed rocks. For example: Gypsum and rock salt, running water dissolves and carries chemicals and where evaporation takes place, these chemicals are deposited at the mouth of springs, caves or caverns or in lakes. Rock salt and gypsum are formed form deposit of salt in strata on the beds of lakes.
2. Organic Rocks: Organic rocks are formed form the remains of organisms, i.e. of animals and plants. Examples: coal, limestone etc. On the basis of lime and carbon content, organically formed rocks can be divided into two kinds, namely
- Calcareous rocks: Calcareous rocks are formed mostly from the remains of living organisms. These rocks contain calcium carbonate or lime. They include limestone and chalk. They are porous and soluble.
- Carbonaceous rocks: These are formed due to the transformation of vegetative matter. Under the impact of heat and pressure the remains of plants are turned into hard layers. E.g. coal.
