Regolith is any solid but unconsolidated material laying on the top surface of the bedrock. Regolith includes soil, alluvium, and rock fragments weathered from the bedrock. The thickness of this mantle varies from nil over rock exposures to very deep in areas protected from erosion.
Soil is the top layer of the regolith. The distinctive characteristic of the soil is, it supports the growth of the plants. Thus, the uppermost layer of the regolith in which plants grow is called soil. Soil is a combination of minerals, organic matter, water, and air.
If we examine the walls of a trench, they are found to contain a series of horizontal layers. These layers are called “horizons”. All horizons together form the “soil profile”. There are five basic horizons of regolith from to bottom are O, A, B, C, and R. Among these horizons A, B and C are included in the Soil horizons. The top level horizon O, and the lowest horizon R are not included in the Soil. Soil consists of only the three horizons, A, B, and C. Detail of all these horizons in as under:
- O-Horizon (Organic Surface): It is also called the organic surface of the soil. The top surface starting from 0 to 2 centimeters is known as the O-Horizon. The material of soil on O-Horizon is the moveable particles of the soil, and they are not struck to the surface. It is also called Over-surface soil.
- A-Horizon (Soil Layer): It lies beneath the O-Horizon, It is also called Surface soil. This layer contains organic matter and micro-organisms. In this layer the greatest biological activity takes place.
- B-Horizon (Soil Layer): The B-Horizon is also called the Subsoil layer. This intermediate zone is also called the “Zone of Accumulation” as much of the material which is leached out from A-horizon, is deposited here.
- C-Horizon (Soil Layer): The lower horizon starts from the depth of 30 inches and ends at a maximum of 50 inches. It mainly consists of the partly altered parent rock material.
- R-Horizon (Bedrock): This layer is not included in soil layers. It is the top portion of the bedrock. A mass of rock such as granite, basalt, quartzite, limestone or sandstone that forms the parent material for some soils – if the bedrock is close enough to the surface to weather. This is not soil and is located under the C horizon.
Depending upon their mode of formation, the soil deposits have been broadly grouped into two classes: (i). Residual soil deposits, and (ii). Transported Soil Deposits.
- Residual Soil Deposits: In plain areas the products of rock weathering continue to accumulate in place over the parent rock masses and give rise to a “residual soil deposits”. As the action of weathering decreases with depth, such soil deposits gradually change from soil at the surface to broken rock fragments and merge with fresh rock underneath. The common examples of residual soils are Laterite, Terra-rosa, and peat bogs.
- Transported Soil: The weathered and broken rock materials are eroded and transported from one place to another by natural agencies such as wind, water, ice or gravity. The deposits of soil formed I n this manner are called “transported soil deposits”. Such soils generally have no relations with the underlying rock mass. The transported soils have been classified according to the nature of the transporting agency responsible for their formation. This classification has been given in the following table.
|Transporting Agency||Nature of Soil|
The sizes of soil particles are extremely variable. It ranges from a big boulder to fine clays. The nomenclature of soil particles according to size is given in following table. The soil containing mixture of clay and sand is called “loam”, and the clay soil having appreciable lime content is called “marl”.
|Sediment Name||Size Range|
|Boulder||60 mm or more|
|Coarse Sand||2.0 to 0.6 mm|
|Medium Sand||0.6 to 0.2mm|
|Fine Sand||0.2 to 0.06 mm|
|Silt||0.06 to 0.002 mm|
|Clay||Less than 0.002 mm|