Which one among the following is a mechanically formed sedimentary rock?

1. Composition and Layers of the Earth's Crust (basic)

Welcome to your first step in understanding the Earth's "bones"—the rocks and minerals that make up our world. Before we dive into specific rock types, we must understand the stage they are built upon: the Earth's Crust. The crust is the extremely thin, outermost "skin" of our planet, representing less than 1% of the Earth's total volume and mass Physical Geography by PMF IAS, Earths Interior, p.52. Even though it seems deep to us, it is a mere fraction of the Earth's total radius.

The crust is not a uniform shell; it is divided into two distinct types: Continental and Oceanic. The continental crust is like a thick, light "cork" floating on the denser mantle below, while the oceanic crust is thinner but heavier (denser). For instance, while the crust under the oceans might only be 5–30 km thick, it can reach up to 70–100 km under massive mountain ranges like the Himalayas Physical Geography by PMF IAS, Earths Interior, p.52.

Historically, geologists categorized these layers based on their dominant minerals:

• SIAL: Standing for Silica (Si) and Alumina (Al). This makes up the lighter, granitic rocks of the continental crust.
• SIMA: Standing for Silica (Si) and Magnesium (Mg). These are the denser, basaltic rocks that form the ocean floors Certificate Physical and Human Geography, The Earth's Crust, p.17.

Generally, as we go deeper into the Earth, the density increases. While the average density of the crust is about 2.7 g/cm³, the Earth as a whole has a much higher average density of 5.51 g/cm³ because of its heavy metallic core Physical Geography by PMF IAS, Earths Interior, p.52.

Feature	Continental Crust (SIAL)	Oceanic Crust (SIMA)
Thickness	Thick (50-100 km)	Thin (5-30 km)
Major Minerals	Silica & Aluminium	Silica & Magnesium
Density	Lighter (~2.7 g/cm³)	Heavier/Denser
Common Rock	Granite-like (Felsic)	Basalt-like (Mafic)

Sources: Physical Geography by PMF IAS, Earths Interior, p.52; Certificate Physical and Human Geography, The Earth's Crust, p.17

2. The Three Major Rock Families (basic)

To understand the crust of our Earth, we first need to look at rocks, which are essentially aggregates of one or more minerals held together by chemical bonds. The scientific study of these rocks is known as petrology. Based on how they are formed, geologists classify all rocks into three distinct families: Igneous, Sedimentary, and Metamorphic. This classification is not just about what the rock looks like today, but the story of its birth and transformation over millions of years Physical Geography by PMF IAS, Chapter 13, p.169.

The first family, Igneous rocks, are often called primary rocks because they are the starting point of the rock cycle. They form from the cooling and solidification of molten matter — magma when it is below the surface and lava when it reaches the surface. Because they originate under conditions of extreme heat, these rocks are unfossiliferous (they contain no fossils). Examples include the granite found in mountain ranges or the basalt of the Deccan Plateau Physical Geography by PMF IAS, Chapter 13, p.174.

When any rock — igneous or metamorphic — is exposed to the elements, it undergoes weathering and erosion. The resulting fragments are transported by wind, water, or ice and deposited in layers. Over time, these layers are compacted and cemented together to form Sedimentary rocks. These are easily identified by their stratified (layered) appearance, which is why they are often called stratified rocks. Unlike igneous rocks, these frequently house the remains of plants and animals (fossils) Certificate Physical and Human Geography, Chapter 2, p.18.

Finally, the Metamorphic rocks are formed when existing rocks (either igneous or sedimentary) are subjected to intense heat and pressure, often during earth movements. This doesn't melt the rock, but it causes recrystallization, changing the rock's original character and appearance entirely. For instance, soft limestone can be transformed into hard marble, and sandstone can be metamorphosed into quartzite Certificate Physical and Human Geography, Chapter 2, p.19.

Rock Family	Mode of Formation	Key Characteristic
Igneous	Solidification of Magma/Lava	Primary rocks; No fossils.
Sedimentary	Deposition and Lithification of fragments	Layered/Stratified; Often contains fossils.
Metamorphic	Recrystallization under Heat/Pressure	Original character is altered (e.g., Clay to Slate).

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169; Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.174; Certificate Physical and Human Geography, The Earth's Crust, p.18; Certificate Physical and Human Geography, The Earth's Crust, p.19

3. Exogenic Processes: Weathering and Erosion (intermediate)

To understand how our landscape evolves, we must look at Exogenic Processes—forces that derive their energy from the atmosphere (the sun) and the internal temperature of the earth's surface. The umbrella term for these processes is Denudation, which literally means 'to strip bare.' It encompasses weathering, mass wasting, erosion, and transportation Environment and Ecology by Majid Hussain, Major Crops and Cropping Patterns in India, p.106.

The most critical distinction you must master is between Weathering and Erosion. Weathering is an in-situ (on-site) process. It refers to the mechanical disintegration or chemical decomposition of rocks right where they stand, without any significant movement. This happens through three main lenses: Physical (like frost wedging), Chemical (like oxidation or carbonation), and Biological (like plant roots prying rocks apart) Physical Geography by PMF IAS, Geomorphic Movements, p.83. In contrast, Erosion is a dynamic, mobile process. It involves the acquisition and transportation of rock debris by geomorphic agents such as running water, wind, glaciers, or waves. While weathering 'prepares' the rock by weakening it, erosion 'carries it away' using kinetic energy Fundamentals of Physical Geography NCERT Class XI, Geomorphic Processes, p.43.

Feature	Weathering	Erosion
Nature	Static / In-situ (On-site)	Dynamic / Mobile
Process	Disintegration & Decomposition	Acquisition & Transportation
Energy Source	Solar energy, chemical reactions	Kinetic energy (movement)
Agents	Temperature, moisture, organisms	Water, Wind, Glaciers, Waves

In regions with high humidity and temperature, Chemical Weathering is dominant because heat speeds up chemical reactions like solution and hydration Physical Geography by PMF IAS, Geomorphic Movements, p.90. Conversely, in arid or high-altitude regions, Mechanical Weathering (physical) takes the lead through processes like frost action, where water freezes in rock crevices and expands, eventually shattering the rock Certificate Physical and Human Geography by GC Leong, Weathering, Mass Movement and Groundwater, p.46.

Sources: Environment and Ecology by Majid Hussain, Major Crops and Cropping Patterns in India, p.106; Physical Geography by PMF IAS, Geomorphic Movements, p.83, 90; Fundamentals of Physical Geography NCERT Class XI, Geomorphic Processes, p.43; Certificate Physical and Human Geography by GC Leong, Weathering, Mass Movement and Groundwater, p.46

4. The Rock Cycle and Transformation (intermediate)

Imagine the Earth’s crust not as a static floor, but as a massive, slow-motion recycling plant. This is the essence of the Rock Cycle—a continuous, never-ending process where old rocks are broken down, altered, or melted to give birth to new ones. There is no 'final' state for a rock; it is simply in a temporary stage of its long geological journey. Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.174

The cycle typically begins with Igneous rocks, often called 'primary rocks' because they are formed directly from the cooling of molten magma or lava. However, once these rocks reach the surface, they are attacked by weathering and erosion. The resulting fragments (sediments) accumulate and harden over millions of years to form Sedimentary rocks. But the journey doesn't stop there. If these sedimentary or even the original igneous rocks are buried deep underground, they encounter intense heat and pressure (often due to mountain-building or orogenic movements), transforming into Metamorphic rocks. Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.173

One of the most fascinating aspects of this cycle is the metamorphic transition, where the physical and chemical character of a rock changes completely without it ever melting. For example:

Original Rock (Parent)	Metamorphosed Rock
Granite (Igneous)	Gneiss
Limestone (Sedimentary)	Marble
Sandstone (Sedimentary)	Quartzite
Shale (Sedimentary)	Schist / Slate

Certificate Physical and Human Geography, The Earth's Crust, p.19

The cycle is completed through subduction. As tectonic plates move, crustal rocks (whether igneous, sedimentary, or metamorphic) can be pushed down into the Earth's mantle. Here, they melt back into molten magma due to extreme temperatures, ready to erupt or cool again as new igneous rocks. Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.174

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.169-174; Certificate Physical and Human Geography, The Earth's Crust, p.19

5. Lithification and Sedimentary Features (intermediate)

To understand sedimentary rocks, we must first look at the word itself — derived from the Latin sedimentum, meaning 'settling.' Unlike igneous rocks which crystallize from molten magma, sedimentary rocks are formed from the debris of pre-existing rocks and organic matter. This journey begins with denudation (weathering and erosion), where rocks are broken down into fragments. These fragments are transported by agents like water, wind, or ice and deposited in layers. Over vast periods, these loose deposits undergo Lithification — the process of turning sediments into solid rock through compaction (the weight of overlying layers squeezing out air and water) and cementation (minerals like silica or calcite acting as 'glue' between grains) Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.171.

Sedimentary rocks are the Earth's 'storytellers' because they are stratified (layered). While they cover nearly 75% of the Earth's land surface, they are relatively thin, accounting for only about 5% of the total volume of the Earth's crust Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.171. This is because they are primarily a surface phenomenon. Depending on how they are formed, we classify them into three distinct categories:

• Mechanically Formed (Clastic): These are made of fragments of other rocks. Sandstone is the classic example, often formed from quartz grains. Other examples include Shale (fine clay), Conglomerate (rounded pebbles), and Loess (wind-deposited silt).
• Organically Formed: These arise from the accumulation of organic remains. Coal comes from buried vegetation, while Chalk and certain Limestones form from the skeletal remains of marine organisms.
• Chemically Formed: These occur when minerals precipitate out of a saturated solution (like evaporating seawater). Common examples include Halite (Rock Salt), Gypsum, and Potash.

Type	Formation Process	Key Examples
Mechanical	Accumulation of weathered rock fragments	Sandstone, Shale, Conglomerate
Organic	Lithification of plant/animal remains	Coal, Chalk, Limestone
Chemical	Precipitation from mineral-rich water	Halite, Gypsum, Potash

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.171

6. Organic and Chemical Sedimentary Rocks (intermediate)

While many sedimentary rocks are formed by the physical breakdown of other rocks, a significant portion of the Earth’s crust is composed of rocks formed through biological activity or chemical reactions. Unlike mechanically formed rocks (like sandstone), these do not rely on the transportation of solid grains but rather on the accumulation of organic remains or the precipitation of minerals from water.

Organically Formed Sedimentary Rocks are born from the remains of living organisms. These are broadly divided into two types based on their chemical composition:

• Calcareous rocks: These are rich in calcium carbonate (CaCO₃) and are formed from the accumulation of skeletons and shells of marine organisms like corals and mollusks. Limestone and chalk are the most prominent examples Certificate Physical and Human Geography, Chapter 2, p.19.
• Carbonaceous rocks: These originate from vegetative matter. When forests or swamps are buried under sediments, the resulting pressure and heat compress the plant remains into compact masses of carbon. This process creates peat, lignite, and ultimately coal Physical Geography by PMF IAS, Chapter 13, p.172.

Chemically Formed Sedimentary Rocks, on the other hand, occur when minerals precipitate out of a solution. This often happens in arid regions where high rates of evaporation cause minerals to reach a state of supersaturation in water bodies like salt lakes or shallow seas. These are often referred to as evaporites. For example, Rock salt (Halite) is derived from ancient sea beds, while Gypsum (CaSO₄·2H₂O) is formed from the evaporation of highly saline waters like the Dead Sea NCERT Class X Geography, Chapter 4, p.43. Interestingly, some rocks like limestone can be formed either organically (from shells) or chemically (from direct precipitation in warm, shallow waters).

Category	Primary Process	Examples
Organic (Calcareous)	Accumulation of shells/skeletons	Limestone, Chalk
Organic (Carbonaceous)	Compression of vegetative matter	Coal, Lignite, Peat
Chemical (Evaporites)	Precipitation due to evaporation	Rock salt, Gypsum, Potash

Sources: Certificate Physical and Human Geography, Chapter 2: The Earth's Crust, p.19; Physical Geography by PMF IAS, Chapter 13: Types of Rocks & Rock Cycle, p.172; NCERT Class X Geography, Chapter 4: Minerals and Energy Resources, p.43

7. Mechanically Formed (Clastic) Rocks (exam-level)

Often called Clastic sedimentary rocks (from the Greek word klastos, meaning 'broken'), mechanically formed rocks are the 'recycled' products of the geological world. They originate from the physical disintegration of pre-existing rocks—whether igneous, metamorphic, or even older sedimentary ones. When rocks are exposed to the elements, they undergo denudation (weathering and erosion), breaking down into smaller fragments like sand, silt, or clay. These fragments, known as detritus, are then transported by natural agents like water, wind, or ice and deposited in layers Physical Geography by PMF IAS, Chapter 13, p.171.

The transition from loose sediment to solid rock happens through a process called lithification. As layers of sediment accumulate over millions of years, the weight of the upper layers exerts intense pressure on the lower ones, squeezing out water and air. This compaction, combined with cementation (where minerals like silica or calcite act as a natural glue), binds the particles together. For example, sandstone is formed when sand grains—often quartz derived from granites—are cemented together. If the particles are larger and rounded, we call the rock a conglomerate; if they are fine-grained like clay, they form shale Certificate Physical and Human Geography, Chapter 2, p.19.

It is fascinating to note that the specific agent of transport often gives the rock its unique character. While water is the most common agent, others play a significant role as shown below:

Transporting Agent	Rock/Sediment Name	Characteristics
Ice (Glaciers)	Tillite	Unsorted, jagged fragments dropped by melting ice.
Wind	Loess	Fine, fertile dust-like particles accumulated over large areas.
Water (Rivers/Seas)	Sandstone / Shale	Typically well-layered or stratified due to settling in water.

Sources: Physical Geography by PMF IAS, Types of Rocks & Rock Cycle, p.171; Certificate Physical and Human Geography, The Earth's Crust, p.19

8. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of the rock cycle and the processes of weathering and erosion, this question tests your ability to categorize sedimentary rocks based on their specific mode of formation. As you recall, mechanically formed (or clastic) rocks are the result of physical disintegration where pre-existing rocks are broken down into fragments, transported, and eventually undergo lithification. This process is distinct from chemical precipitation or organic accumulation, focusing entirely on the mechanical accumulation of mineral grains, a concept thoroughly detailed in Certificate Physical and Human Geography, GC Leong.

To arrive at the correct answer, you must identify which rock is composed of these physical fragments. Sandstone is the definitive choice because it consists of sand grains—typically quartz—that have been cemented together after being deposited by wind or water. When you see the term "mechanically formed," you should immediately think of detrital processes—the physical stacking of "rubble" into solid rock. This distinguishes it from the other options which rely on different transformative triggers.

UPSC often uses Limestone as a distractor because while it can occasionally have a mechanical origin, it is primarily categorized as organically formed (from skeletal remains) or chemically formed. Similarly, Salt rock and Gypsum are classic chemical sedimentary rocks (evaporites) that form through the precipitation of minerals from saturated solutions, not physical accumulation. As explained in Physical Geography by PMF IAS, distinguishing between physical deposition and chemical precipitation is key to avoiding these common classification traps.