Sedimentary Rocks
Geography › Rocks, Erosion and Landforms · Topic 5.2
Sedimentary rocks are formed from the accumulation, compaction and cementation of sediments, the fragments of older rocks and the remains of plants and animals, laid down in layers by water, wind or ice, in a process called lithification. They cover about three-quarters of the land surface, show bedding and often hold fossils, and include sandstone, limestone, shale, coal and rock salt.
5 min read · UPSC Prelims · Geography · Rocks, Erosion and Landforms
At a Glance
- Origin Deposition and compaction of sediments
- Process Lithification
- Types Clastic, chemical, organic
- Features Layers (strata), fossils, softer rock
- Share About 75 per cent of land surface, about 5 per cent of crust volume
- Exam link UPSC Prelims, Geography: Rocks, Erosion and Landforms
Where Does It Fit?
This is the second topic of the category on rocks, erosion and landforms. The previous topic described igneous rocks, which supply much of the raw material for sediments. The next topic covers metamorphic rocks and the rock cycle. Previous topic: Igneous Rocks. Next topic: Metamorphic Rocks and the Rock Cycle.
How Are Sedimentary Rocks Formed?
In short: loose sediments settle in layers, and over time they are pressed and cemented into rock.
Rocks at the surface are broken down by weathering and erosion, discussed in Exogenic Movements, Denudation and Weathering. The fragments, called sediments, are carried by rivers, wind, glaciers and waves and settle in low areas such as lakes, river valleys, deltas, shallow seas and deserts. Layer upon layer builds up, and the weight of the upper layers compresses the lower ones, squeezing out water, while minerals such as silica, calcite and iron oxide dissolved in groundwater bind the grains together, as cement. The whole process of change from loose sediment to rock is called lithification. Sedimentary rocks therefore form in layers, called strata or beds, with the oldest at the bottom. Sedimentary rocks cover about three-quarters of the land surface, though they make up only a small share of the volume of the crust.
What Are the Types of Sedimentary Rocks?
| Type | Origin | Examples |
|---|---|---|
| Mechanically formed (clastic) | Cemented fragments of other rocks, graded by grain size | Conglomerate and breccia (coarse), sandstone (sand), shale and clay (fine), loess (wind-blown silt) |
| Chemically formed | Minerals precipitated from solution or evaporation of water | Limestone, rock salt, gypsum, potash, flint |
| Organically formed | Remains of plants and animals | Coal, peat, chalk, some limestone |
Limestone is the most common chemical and organic sedimentary rock: it is made largely of calcium carbonate, from shells and skeletons of marine organisms or from precipitation, and it forms karst landscapes, as in Karst Landforms and the Cycle of Erosion. Coal is formed from plant matter buried and compressed in swamps, as described in Coal – Formation, Classification and Distribution. Rock salt and gypsum form when shallow seas or lakes dry up in arid climates.
What Are the Main Features of Sedimentary Rocks?
- Stratification: they are arranged in layers, separated by bedding planes, and the layers may be tilted or folded by later movements.
- Fossils: the remains or imprints of plants and animals are preserved between the layers and are used to date the rocks and to reconstruct past environments.
- Surface marks: ripple marks, mud cracks and cross-bedding show the conditions of deposition.
- Softer and porous: they are usually less hard than igneous rocks, and many are permeable and can hold water, which makes sandstone and limestone good aquifers.
- Fuels and minerals: coal, petroleum and natural gas occur in sedimentary rocks.
Where Are Sedimentary Rocks Found in India?
| System or region | Rocks and resources |
|---|---|
| Cuddapah and Vindhyan systems | Sandstone, limestone and shale; Vindhyan sandstone is used for monuments such as the Red Fort and Sanchi; building stone, diamonds in Panna |
| Gondwana system | Coal in Jharkhand, West Bengal, Odisha, Chhattisgarh and Madhya Pradesh; also sandstone |
| Marwar and Rajasthan region | Sandstone, limestone and gypsum |
| Tertiary rocks of the Himalayas and coasts | Petroleum bearing rocks of Assam, Gujarat and the Mumbai offshore; the Shiwalik sandstones and clays |
| Indo-Gangetic plain | Deep alluvium; the largest unconsolidated sediment body |
The Vindhyan and Gondwana systems are explained in Dravidian and Aryan Rock Systems of India. As an analytical point, most of India’s fuel and building-stone resources lie in sedimentary rocks, which makes sedimentary geology central to energy planning, as in Petroleum – Formation and Distribution.
How Do Sedimentary Rocks Fit the Rock Cycle?
Sedimentary rocks form when older rocks are worn down. In turn, if they are buried deep and heated or pressed, they become metamorphic rocks, and if they are melted they return to magma. If they are uplifted they are weathered again. This continuous recycling is described in Metamorphic Rocks and the Rock Cycle.
Why Is This Topic Important?
- Resources Coal, oil and building stone
- Fossils Dating and environment
- India Vindhyan and Gondwana systems
What Came Next?
The next topic describes metamorphic rocks and the rock cycle. Next topic: Metamorphic Rocks and the Rock Cycle.
Key People and Terms
- Sediment Fragments of rock or organisms
- Lithification Conversion of sediment into rock
- Strata Layers of sedimentary rock
- Clastic Made of rock fragments
- Evaporite Rock formed by evaporation
- Fossil Preserved remains of past life
Exam Corner
Points to Remember
- Lithification turns sediment into rock.
- Sedimentary rocks show strata and contain fossils.
- They cover about 75 per cent of the land surface.
- Coal and petroleum occur in sedimentary rocks.
- Limestone is mainly calcium carbonate.
Do Not Confuse With
- Clastic and chemical rocks Fragments versus precipitated minerals
- Sedimentary and igneous rocks Layered with fossils versus crystalline without
- Coal and graphite Organic sedimentary versus metamorphic
Memory Hook
Settle, squeeze, stick: layers hold fossils, fuels and stone.
Mains Angle
Questions ask for formation, types and resources. These points add depth.
- Process Weathering to lithification
- Types Clastic, chemical, organic
- Resources Fuels and aquifers
A Question You May Face
An original practice question, not a past paper question.
“Describe the formation and classification of sedimentary rocks and explain their economic importance.”
How to Answer
- Formation Lithification.
- Types Three classes.
- Features Strata and fossils.
- Use Fuels and stone.
GS Relevance
Frequently Asked Questions
How are sedimentary rocks formed?
Sedimentary rocks form when sediments, such as fragments of rocks and remains of organisms, are deposited in layers by water, wind or ice. The weight of overlying layers compacts them, and minerals dissolved in water cement them, a process called lithification.
What are the types of sedimentary rocks?
Sedimentary rocks are of three types: clastic or mechanically formed, such as sandstone, shale and conglomerate; chemically formed, such as limestone, rock salt and gypsum; and organically formed, such as coal, peat and some limestones and chalk.
Why are fossils found in sedimentary rocks?
Fossils are found in sedimentary rocks because the dead bodies of plants and animals can be buried in layers of sediment and preserved as the sediment hardens into rock. The cooling of molten rock and the heat and pressure of metamorphism would destroy such remains.
What is stratification?
Stratification is the arrangement of sedimentary rocks in distinct layers or beds, deposited one above another, with the oldest layer at the bottom. The layers are separated by bedding planes and may later be tilted or folded by earth movements.
Which fuels are found in sedimentary rocks?
Coal, petroleum and natural gas occur in sedimentary rocks. Coal forms from plant remains buried in swamps, as in the Gondwana system of India, and petroleum forms from marine organisms in sedimentary basins, as in Assam, Gujarat and the Mumbai offshore.
PYQ Practice — Statement Analysis
1 Sedimentary rocks are arranged in layers.
They are stratified.
2 Coal is a metamorphic rock.
It is an organic sedimentary rock.
3 Limestone is mainly composed of calcium carbonate.
It is a chemical or organic sedimentary rock.
4 Sedimentary rocks cover most of the volume of the Earth's crust.
They cover most of the surface but form a small share of the volume.
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