Soils as they are found in different regions can be classified into two broad categories:
(1) Residual soils
(2) Transported soils
Residual soils are found at the same location where they have been formed. Generally, the depth of residual soils varies from 5 to 20 m.
Chemical weathering rate is greater in warm, humid regions than in cold, dry regions causing a faster breakdown of rocks. Accumulation of residual soils takes place as the rate of rock decomposition exceeds the rate of erosion or transportation of the weathered material. In humid regions, the presence of surface vegetation reduces the possibility of soil transportation.
As leaching action due to percolating surface water decreases with depth, there is a corresponding decrease in the degree of chemical weathering from the ground surface downwards. This results in a gradual reduction of residual soil formation with depth, until unaltered rock is found.
Residual soils comprise of a wide range of particle sizes, shapes and composition.
Weathered rock materials can be moved from their original site to new locations by one or more of the transportation agencies to form transported soils. Tranported soils are classified based on the mode of transportation and the finaldeposition environment.
(a) Soils that are carried and deposited by rivers are called alluvial deposits.
(b) Soils that are deposited by flowing water or surface runoff while entering a lake are called lacustrine deposits.Atlernate layers are formed in different seasons depending on flow rate.
(c) If the deposits are made by rivers in sea water, they are called marine deposits. Marine deposits contain both particulate material brought from the shore as well as organic remnants of marine life forms.
(d) Melting of a glacier causes the deposition of all the materials scoured by it leading to formation of glacial deposits.
(e) Soil particles carried by wind and subsequently deposited are known as aeolian deposits.