Theory question 203
Rocks(a) What are sedimentary rocks? (b) Name the three classes of sedimentary rocks according to their mode of formation. (c) Describe one of the classes of sedimentary rocks in 3(b) above. (d) Outline four ways in which sedimentary rocks are of importance to man.
Show the model answer
(a) Sedimentary rocks are rocks formed from layers of sediment (rock particles, plant and animal remains, or chemical deposits) laid down mostly in water, then compacted and cemented.
(b) Mechanically formed; organically formed; chemically formed.
(c) e.g. Mechanically formed: particles from weathered rocks carried by rivers, wind, ice or waves, deposited in layers and cemented; e.g. sandstone, shale, clay, conglomerate. Organically formed: from remains of plants (coal) or animals (limestone, chalk, coral). Chemically formed: from minerals precipitated from solution by evaporation, e.g. rock salt, gypsum, potash.
(d) Building materials (sandstone, limestone for cement); source of fuel (coal, petroleum, natural gas); clay for pottery and bricks; salt and potash; good aquifers for water; fertile soils; fossils for study; tourist attraction (caves).
For (c) give how the class forms plus two examples.