(a) Sedimentary rocks are rocks formed from accumulation of sediments or particles of rock over long periods usually by water, wind or ice, or derived from the remains of plants and animals or chemical reaction. The sediments are accumulated in layers hence, they are called statified or layered rocks.
(b) The three classes of sedimentary rocks according to their mode of formation:
- Mechanically formed sedimentary rocks
- Organically formed sedimentary rocks
- Chemically formed sedimentary rocks
(c) (i) Mechanically formed sedimentary rocks;
(1) Accumulation of materials derived from other rocks
(2) Cemented together over a long period
(3) Agents of erosion such as wind, water and glaciers are involved.
(4) Examples are sandstone, clay, gravel, shale
(5) Example are found in Lake Chad Basin, Sotoko Basin, Niger-Benue trough.
(ii) Organically formed sedimentary rocks;
(1) Formed from remains of dead plants and animals
(2) When formed from dead animals, they are known as calcareous rocks e.g. limestone, chalk.
(3) When formed from dead plants, they are known as carbonaceous rocks.
(4) Examples include limestone, chalk, peat, lignite or coal
(iii) Chemically formed sedimentary rocks;
(1) Formed from solutions of one kind or another
(2)Chemically precipitated from other rocks before solidification.
(3) Gypsum is precipitated from salt lakes
(4) Examples include rock salt, potash, gypsum, nitrates.
(d) Importance
(1) Source of materials for construction
(2) Source of minerals e.g. petroleum, coal, gold
(3) Determines the availability and supply of underground water
(4) Rocks provide tourists attractions e.g. Ogbunike cave.
(5) Source of raw materials e.g. limestones for cement
(6) Source of food e.g. rock salt
(7) Used as medicine e.g. kaolin
(8) Used for scientific research e.g. fossils.