Chapter 2 Structure And Physiography

The Earth has a long and complex history, and the landforms we observe today are the result of processes that have shaped the planet over millions of years. The Earth is estimated to be approximately 4,600 million years old. Throughout this immense timescale, both internal (endogenic) and external (exogenic) forces have constantly interacted to create the diverse surface and subsurface features we see.

Understanding plate tectonics is key to comprehending India's geological history. Millions of years ago, the Indian Plate was situated south of the equator and was a much larger landmass, joined with the Australian Plate. Over geological time, this supercontinent fragmented, with the Australian Plate moving southeastward and the Indian Plate embarking on a remarkable northward journey. This northward movement is ongoing and has had profound consequences for the physical geography of the Indian subcontinent, most notably the formation of the Himalayas.

The current geological structure and the ongoing geomorphic processes across the Indian subcontinent are primarily a result of the interplay between these endogenic and exogenic forces and the lateral movement and collision of tectonic plates. Based on its distinct geological structure and formations, India can be broadly divided into three major geological divisions:

1. The Peninsular Block
2. The Himalayas and other Peninsular Mountains
3. Indo-Ganga-Brahmaputra Plain

The Peninsular Block

The **Peninsular Block** constitutes the oldest and most stable landmass of India. Its northern boundary is generally defined by an irregular line starting from the region of Kachchh, extending along the western side of the Aravalli Range near Delhi, and then following roughly parallel to the course of the Yamuna and Ganga rivers eastward to the Rajmahal Hills and the Ganga delta. Extensions of this block are also found in the northeast (Karbi Anglong and Meghalaya Plateau) and in the west (underlying the Rajasthan desert). The northeastern parts are separated from the Chotanagpur plateau by a geological feature known as the Malda fault in West Bengal. In the west, the ancient rocks of the Peninsular Block are largely covered by the sands of the Thar desert.

Geologically, the Peninsula is primarily composed of very ancient, hard rocks, including **gneisses** and **granites**. These crystalline rocks form the core of this block. Since the Cambrian geological period, approximately 500 million years ago, the Peninsular Block has largely behaved as a rigid and stable landmass. While parts of its western coast have submerged, and other areas have experienced tectonic changes, these events have not fundamentally altered the underlying ancient basement structure.

As part of the larger Indo-Australian Plate, the Peninsular Block has been subjected to various internal stresses, leading to vertical movements and fracturing, particularly **block faulting**. Examples of features formed by this activity include the rift valleys of major rivers like the Narmada, Tapi, and Mahanadi, as well as associated block mountains like the Satpura range.

The mountain ranges found within the Peninsula, such as the Aravalli, Nallamala, Javadi, Veliconda, Palkonda, and Mahendragiri hills, are typically **relict** or **residual mountains**. This means they are not young, actively building mountains but are remnants of much older mountain systems that have been extensively eroded over vast timescales.

The rivers of the Peninsular Block, especially those flowing eastward, generally have **shallow valleys** and **low gradients** compared to the steep, rapidly eroding Himalayan rivers. This indicates a mature or old stage of fluvial erosion in the Peninsula. Most of the significant east-flowing rivers (Mahanadi, Krishna, Godavari, Kaveri) form well-developed **deltas** at their mouths before entering the Bay of Bengal, a characteristic feature of rivers with low gradients carrying large sediment loads.

The Himalayas And Other Peninsular Mountains

In contrast to the ancient, rigid Peninsular Block, the **Himalayas** are geologically **young, weak, and flexible mountain ranges**. Formed much more recently due to the collision of tectonic plates, they are still undergoing deformation. Consequently, the Himalayas are actively shaped by both internal (endogenic) and external (exogenic) forces, leading to frequent seismic activity, faulting, folding, and the formation of thrust plains (where rock layers are pushed over one another). These mountains are fundamentally **tectonic in origin**, a result of the immense forces from the continental collision.

The rivers originating in the Himalayas are typically in their **youthful stage** of development. They are fast-flowing and actively erode the mountain landscape, carving out distinct landforms such as deep **gorges**, steep-sided **V-shaped valleys** (Figure 2.1), rapids, and waterfalls. These features are characteristic indicators of intense downward erosion by young, powerful rivers.

While the subheading mentions "other Peninsular Mountains," the text under this specific heading focuses solely on the contrast between the Himalayas and the Peninsular Block's characteristics. Details about the Peninsular Mountains are provided in the later section on the Peninsular Plateau.

Indo-Ganga-Brahmaputra Plain

The third major geological division of India is the vast **Indo-Ganga-Brahmaputra Plain**. This extensive lowland was formed much more recently than the Peninsula or the initial stages of Himalayan uplift. It is believed to have originated as a large **geo-synclinal depression**, a long, narrow, subsiding trough, located between the rising Himalayas to the north and the stable Peninsular Block to the south.

This depression reached its maximum development around 64 million years ago, during the third phase of the Himalayan mountain-building process. Since then, this trough has been continuously filled by the vast amounts of sediment (alluvium) transported and deposited by the major river systems originating in the Himalayas (Indus, Ganga, Brahmaputra) and, to some extent, by rivers from the northern parts of the Peninsula. The accumulation of these alluvial deposits is substantial, reaching an average depth of 1,000 to 2,000 meters in places.

The discussion above highlights the fundamental geological differences across India's major regions – the ancient, stable Peninsula; the young, active Himalayas; and the recently formed, sediment-filled plains. These variations in geological structure and history have a profound influence on other aspects of India's geography, particularly its diverse physical features and relief patterns (physiography), which are the outcome of ongoing geological and geomorphological processes.

Physiography

**Physiography** refers to the physical features of a region, shaped by its underlying geological structure, the geomorphic processes acting upon it, and the stage of landscape development. India's land is marked by significant diversity in its physical features.

The country can be broadly characterized by:

• A vast, rugged northern region dominated by a series of high mountain ranges with varied peaks, valleys, and deep gorges.
• A stable southern region consisting of an elevated tableland (plateau) that is often highly dissected by rivers, featuring eroded rocky areas and prominent scarps (steep slopes or cliffs).
• A vast, relatively flat plain stretching across northern India, situated between the northern mountains and the southern plateau.

Based on these large-scale variations in physical features, India can be divided into the following six major physiographic divisions (Figure 2.2 provides a physical map):

1. The Northern and North-eastern Mountains
2. The Northern Plain
3. The Peninsular Plateau
4. The Indian Desert
5. The Coastal Plains
6. The Islands

The North And Northeastern Mountains

This physiographic division includes the extensive mountain systems in the north and northeast of India, primarily consisting of the **Himalayas** and the associated **Northeastern hills**. The Himalayas are not a single mountain range but a series of parallel ranges.

Important Himalayan ranges include the Greater Himalayas (also known as the Himadri or the central axial range), the Middle Himalayas (Himachal), and the Outer Himalayas (Shiwalik). The general alignment of these ranges varies across the region:

• In the northwestern part of India, the ranges are oriented from **northwest to southeast**.
• In the Darjiling and Sikkim regions, the orientation is primarily **east-west**.
• In Arunachal Pradesh, the ranges run from **southwest to northwest**.
• In Nagaland, Manipur, and Mizoram (part of the Northeastern hills), the orientation is predominantly **north-south**.

The Great Himalayan range stretches for an approximate length of **2,500 km** from west to east. The width of the Himalayan mountain belt varies from about 160 km to 400 km from north to south.

Geographically, the Himalayas stand as a significant physical barrier, forming a high wall that separates the Indian subcontinent from the vast expanses of Central and East Asia. Beyond being a physical obstruction, the Himalayas act as a major **climatic divide** (influencing rainfall patterns and temperature distribution), a **drainage divide** (determining the flow direction of major rivers), and a **cultural divide** (influencing human migration and cultural interactions throughout history).

The impact of the Himalayas on the geoenvironment of South Asian countries is profound, affecting everything from water resources and agriculture to climate and biodiversity. Similar geographical divides exist in other parts of the world, like the Andes in South America or the Rocky Mountains in North America, which also exert significant influences on regional climates and environments.

The Northern Plains

The **Northern Plain** of India is a vast, flat region formed by the extensive alluvial deposits laid down by the three major Himalayan river systems: the **Indus**, the **Ganga**, and the **Brahmaputra**, and their tributaries. This plain stretches for approximately **3,200 km** from the east to the west. Its average width ranges between 150 and 300 km, and the alluvial deposits can reach depths of 1,000 to 2,000 meters in some areas.

Based on relief and characteristics from north to south, the Northern Plain can be divided into three main zones:

1. **The Bhabar:** A narrow belt (8-10 km wide) located parallel to the foothills of the Shiwalik range. Here, rivers descending from the mountains deposit coarse sediments, including pebbles and boulders, in an alluvial fan-like formation. Smaller streams often disappear underground within this porous belt.
2. **The Tarai:** Located south of the Bhabar belt (10-20 km wide). In the Tarai, the underground streams from the Bhabar re-emerge, often without well-defined channels, creating marshy and swampy conditions. This zone traditionally supports dense forests and a rich variety of wildlife, although much has been cleared for agriculture.
3. **The Alluvial Plains:** The vast area south of the Tarai, consisting of older and newer alluvial deposits.
   • **Bhangar:** Represents the older alluvium, forming slightly elevated terraces above the current flood level. These areas may contain calcareous deposits called 'kankar'.
   • **Khadar:** Represents the newer alluvium deposited by rivers during recent floods. This material is renewed almost annually, making the Khadar plains highly fertile.

The alluvial plains exhibit characteristic landforms associated with the mature stage of river development and deposition, such as sand bars, meanders (loop-like river bends), oxbow lakes (cut-off meander loops), and braided channels (where a river divides into multiple smaller channels separated by sandbars). The Brahmaputra plains are particularly known for their dynamic river channels, frequent flooding, riverine islands (like Majuli, one of the largest river islands in the world), and extensive sand bars, often forming braided streams (Figure 2.4). (This refers to Figure 2.4).

The major rivers of the Northern Plain, like the Ganga and Brahmaputra, form some of the world's largest deltas at their mouths, such as the famous Sunderbans delta, a complex network of waterways and mangrove forests. The Northern Plain is generally a featureless expanse with a gentle slope, ranging from 50 to 150 meters above mean sea level. The states of Haryana and Delhi are located on a subtle elevation that acts as a **water divide** between the Indus and Ganga river systems. The Brahmaputra river flows eastward in the northeast before turning southwards sharply near Dhubri to enter Bangladesh. This plain, with its deep layer of fertile alluvial soil, is agriculturally very productive, supporting crops like wheat, rice, sugarcane, and jute, and consequently sustains a very large population.

The Peninsular Plateau

The **Peninsular Plateau** is a large, elevated tableland rising above the surrounding plains, with an average elevation ranging from 600 to 900 meters. It has an irregular triangular shape. Its outer boundaries are generally defined by the Delhi ridge (an extension of the Aravallis) in the northwest, the Rajmahal hills in the east, the Gir range in the west, and the Cardamom hills in the south. A significant extension of the Peninsular plateau is also found in the northeast, comprising the Shillong and Karbi-Anglong plateaus, separated from the main block by the Malda fault.

The Peninsular Plateau is not a single, continuous landmass but is composed of a series of smaller plateaus or elevated regions, often described as "patland plateaus" in some areas (e.g., Hazaribagh, Palamu, Ranchi, Malwa, Coimbatore, Karnataka plateaus). Geologically, it is one of the **oldest and most stable landmasses** of India, composed primarily of ancient crystalline igneous and metamorphic rocks.

The general slope of the Peninsular plateau is from **west to east**. This is evident from the flow pattern of most of its major rivers (e.g., Mahanadi, Godavari, Krishna, Kaveri), which originate in the Western Ghats and flow eastward into the Bay of Bengal. However, there are exceptions, such as the Narmada and Tapi rivers, which flow westward into the Arabian Sea through rift valleys.

Important physiographic features found within the Peninsular plateau include:

• **Tors:** Isolated masses of weathered rock.
• **Block Mountains:** Mountains formed by faulting, where large blocks of the crust are uplifted or tilted.
• **Rift Valleys:** Linear depressions formed by faulting, where blocks of the crust have subsided (e.g., Narmada, Tapi).
• **Scarps and Spurs:** Steep slopes and ridges.
• Bare rocky structures and series of hummocky hills.
• Wall-like **quartzite dykes**, which are resistant igneous intrusions that often stand out as ridges and can naturally impound water, creating sites for reservoirs.

The western and northwestern parts of the plateau, particularly the Deccan Trap region, are characterized by the widespread presence of **black soil**, derived from the weathering of basaltic lava flows.

The Peninsular plateau has experienced repeated episodes of uplift and subsidence throughout its geological history, accompanied by crustal faulting and fracturing. Some faults, like the Bhima fault, are associated with recurrent seismic activity, indicating that even this 'stable' block is subject to internal stresses. These geological variations contribute to the diversity in the plateau's relief, including areas with complex eroded terrain, such as the ravines and gorges in the northwestern part (e.g., the ravines of Chambal, Bhind, and Morena).

Based on prominent relief features, the Peninsular plateau is divided into three broad geographical groups:

1. The Deccan Plateau
2. The Central Highlands
3. The Northeastern Plateau

The Deccan Plateau

This large triangular plateau region is bordered by the **Western Ghats** in the west, the **Eastern Ghats** in the east, and the Satpura, Maikal range, and Mahadeo hills in the north. The Western Ghats are a continuous chain of mountains running parallel to the west coast. They are known by different local names, such as Sahyadri in Maharashtra, Nilgiri hills in Karnataka and Tamil Nadu, and Anaimalai hills and Cardamom hills in Kerala. The Western Ghats are significantly higher in elevation (average $\sim 1,500$ m) and more continuous than the Eastern Ghats, with height generally increasing from north to south.

The highest peak of the entire Peninsular plateau, **Anaimudi** (2,695 m), is located in the Anaimalai hills (part of the Western Ghats). The second highest peak is **Dodabetta** (2,637 m), located in the Nilgiri hills. Most of the major east-flowing Peninsular rivers (Godavari, Krishna, Kaveri) originate in the Western Ghats. The Eastern Ghats are a discontinuous series of lower hills that have been heavily eroded and dissected by the major rivers flowing towards the Bay of Bengal. Notable ranges in the Eastern Ghats include the Javadi, Palconda range, Nallamala, and Mahendragiri hills. The Western and Eastern Ghats converge and meet at the **Nilgiri hills**, forming a junction between the two ranges.

The Central Highlands

The **Central Highlands** lie to the north of the Narmada river and are bounded by the **Aravalli range** to the west. The Satpura range, characterized by a series of scarped plateaus (typically 600-900 m elevation), forms the southern boundary of the Central Highlands and the northernmost boundary of the Deccan Plateau. The Aravalli range is a classic example of **relict mountains** – very old, highly eroded, forming discontinuous ranges. The Central Highlands are generally tilted and slope towards the north and northeast, as indicated by the drainage of rivers in this region.

The northern tributaries of the river Yamuna, such as the Chambal, Sind, Betwa, and Ken, originate in the Vindhyan and Kaimur ranges (part of the Central Highlands). The Banas river, a significant tributary of the Chambal, originates from the Aravalli range in the west. An eastern extension of the Central Highlands includes the Rajmahal hills. To the south of these hills lies the Chotanagpur plateau, a region renowned for its large reserves of mineral resources.

The extension of the Peninsular plateau is also traceable westward as far as Jaisalmer in Rajasthan, where it is covered by desert features like longitudinal sand ridges and barchans (crescent-shaped dunes). This western part of the Central Highlands has undergone significant metamorphic processes, evidenced by the presence of metamorphic rocks like marble, slate, and gneiss. The general elevation of the Central Highlands is between 700 and 1,000 m above mean sea level.

The Northeastern Plateau

This region, including the Meghalaya and Karbi Anglong plateaus, is geologically an **extension of the main Peninsular plateau**. It is theorized that during the powerful northeastward movement and collision of the Indian Plate with the Eurasian Plate, a large fault or down-warped zone (the Malda fault) was created between the Rajmahal hills and the Meghalaya plateau. This depression was subsequently filled by sediment deposited by numerous rivers, detaching the Northeastern plateau from the main Peninsular Block.

The Meghalaya plateau is further divided into three prominent hill ranges, named after the tribal communities inhabiting them: the **Garo Hills**, the **Khasi Hills**, and the **Jaintia Hills**. The Karbi Anglong hills in Assam are considered an extension of this plateau. Like the Chotanagpur plateau, the Meghalaya plateau is rich in various mineral resources, including coal, iron ore, sillimanite, limestone, and uranium.

This region receives extremely high rainfall, particularly from the southwest monsoon, due to the funneling effect of the Khasi Hills and its location relative to the Bay of Bengal moisture flow. This intense rainfall has led to a highly eroded surface on the Meghalaya plateau. Places like Cherrapunji and Mawsynram, located on the windward side of the Khasi hills, are among the wettest places on Earth, and their surfaces often show bare rocky areas due to intense erosion.

The Indian Desert

To the northwest of the Aravalli hills lies the **Great Indian Desert**, also known as the **Thar Desert** or **Marusthali**. This region is characterized by an undulating topography featuring various types of sand dunes, including longitudinal dunes (aligned parallel to the wind direction) and barchans (crescent-shaped dunes). The climate is arid, with very low annual rainfall (below 150 mm), resulting in sparse vegetation cover.

Geological evidence suggests that during the Mesozoic era (around 180 million years ago, based on wood fossils found at Aakal Wood Fossil Park near Jaisalmer, and marine deposits near Brahmsar), this region was submerged under the sea. Although the underlying rock structure is an extension of the stable Peninsular plateau, the extreme arid conditions have meant that the surface features are predominantly shaped by **physical weathering** and **wind action**. Characteristic desert landforms found here include mushroom rocks (formed by differential wind abrasion), shifting sand dunes, and oases (fertile areas with water, often found in the southern part).

Based on its orientation, the desert slopes generally towards Sindh in the north and towards the Rann of Kachchh in the south. Most rivers in the Indian Desert are **ephemeral**, meaning they are temporary and flow only during or immediately after rainfall. The **Luni river**, flowing in the southern part, is the most significant perennial-ish river, though it often disappears into the sands before reaching the sea. Some streams exhibit **inland drainage**, terminating in lakes or playas (temporary salt lakes) instead of reaching the ocean. The water in these lakes and playas is often brackish or saline, providing a source for salt production.

The picture likely shows sand dunes. If they are crescent-shaped with horns pointing away from the wind, they are barchans. If they are long ridges parallel to the wind, they are longitudinal dunes. If they are ridges perpendicular to the wind, they are transverse dunes. (Assuming Figure 2.6 shows barchans). (This refers to Figure 2.6).

Based on the likely depiction of crescent-shaped dunes, the type of sand dunes shown in Figure 2.6 are likely **Barchans**.

The Coastal Plains

India has an extensive coastline, stretching for over 7,500 km including its islands. The coastal areas can be broadly divided into two major plains based on their location and the dominant geomorphological processes shaping them:

1. The Western Coastal Plains
2. The Eastern Coastal Plains

(Figure 2.7 provides a map showing the coastal plains). (This refers to Figure 2.7).

The Western Coastal Plains

The **Western Coastal Plains** are generally considered an example of a **submerged coastal plain**. Geological evidence, such as the submerged ruins of the ancient city of Dwaraka off the Gujarat coast, supports the idea that parts of this coast have sunk relative to sea level. This submergence has created a relatively narrow coastal belt, particularly in the middle section. However, this narrowness and the presence of submerged features are advantageous for the development of natural **ports and harbors** because deeper water is often found closer to the shore. Important natural ports along the west coast include Kandla, Mazagaon, Jawaharlal Nehru Port (Nhava Sheva), Marmagao, Mangalore, and Cochin.

Extending from the Gujarat coast in the north to the Kerala coast in the south, the Western Coast is further subdivided into regional sections:

• The Kachchh and Kathiawar coast in Gujarat.
• The Konkan coast in Maharashtra.
• The Goan coast.
• The Malabar coast in Karnataka and Kerala.

The western coastal plains are generally wider in the northern and southern parts but are quite narrow in the middle (Konkan-Goa coast). The rivers flowing westward across this narrow plain are short and fast-flowing due to the steeper gradient from the Western Ghats. Consequently, these rivers **do not form significant deltas** at their mouths; instead, they often form estuaries.

The Malabar coast in Kerala has unique features called **'Kayals'** (backwaters). These are shallow lagoons or systems of interconnected lakes and canals running parallel to the coast, separated from the sea by barrier islands or spits. Kayals are used for fishing, inland navigation, and are a major tourist attraction. The famous Nehru Trophy boat race is held annually in the Punnamada Kayal.

The Eastern Coastal Plains

The **Eastern Coastal Plains**, stretching from the delta of the Mahanadi river in the north to Kanniyakumari in the south, are generally **broader** than the western plains and are considered an example of an **emergent coast**. This suggests that this coast has risen relative to sea level or sea level has fallen, resulting in a wider, flatter plain. The gently sloping topography and the long courses of east-flowing rivers allow for the formation of large, **well-developed deltas**. Important deltas on the east coast are formed by the Mahanadi, Godavari, Krishna, and Kaveri rivers.

Due to its emergent nature and the presence of extensive deltas and a broad continental shelf that extends up to 500 km into the sea, the Eastern Coastal Plain has **fewer natural ports and harbors** compared to the west coast. The shallow waters and accumulating sediments in deltaic regions make it challenging to develop deep, natural harbors, although some significant artificial ports exist. Examples of ports on the eastern coast include Chennai, Visakhapatnam, Paradip, and Kolkata (on the Hooghly river delta).

The Islands

India includes two major groups of islands, located in the seas bordering the mainland.

(Figure 2.8 shows a representative island). (This refers to Figure 2.8).

The Bay Of Bengal Island Groups

This group consists of the **Andaman and Nicobar Islands**, located in the Bay of Bengal. The archipelago comprises approximately 572 islands and islets, situated roughly between $6^\circ$N and $14^\circ$N latitudes and $92^\circ$E and $94^\circ$E longitudes. The two main clusters are Ritchie’s archipelago and the Labyrinth island group. The entire chain is conventionally divided into the Andaman group in the north and the Nicobar group in the south.

The Andaman and Nicobar groups are separated by a significant channel of water known as the **Ten Degree Channel** (at $10^\circ$ North latitude). It is believed that these islands are primarily the elevated tops of **submarine mountains** that are part of a submerged mountain range extending southward from the Rakhine Yoma mountains of Myanmar. However, some smaller islands within the group are of **volcanic origin**. **Barren Island**, located in the Nicobar Islands, is the only active volcano in India.

Important mountain peaks in the Andaman and Nicobar Islands include Saddle Peak (738 m) in North Andaman, Mount Diavolo (515 m) in Middle Andaman, Mount Koyob (460 m) in South Andaman, and Mount Thuiller (642 m) in Great Nicobar. The coastlines of these islands feature beautiful beaches and some coral deposits. Due to their location in the tropics and the influence of the surrounding warm ocean, they receive **convectional rainfall** and support lush **equatorial vegetation**, characterized by dense evergreen forests.

On 26 December 2004, the Andaman and Nicobar Islands were severely impacted by a devastating natural calamity, the **Indian Ocean Tsunami**, triggered by a massive underwater earthquake off the coast of Sumatra. This tsunami caused widespread destruction, loss of life, and significant environmental changes in many coastal regions around the Indian Ocean, including parts of India, Sri Lanka, Indonesia, and Thailand. A major consequence for the Andaman and Nicobar Islands was significant inundation, damage to infrastructure, and ecological disruption. Geographically, the southernmost point of India, **Indira Point** in the Nicobar Islands, was submerged following this tsunami.

The Islands Of The Arabian Sea

This group includes the **Lakshadweep Islands** and **Minicoy**, located in the Arabian Sea off the coast of Kerala. These islands are scattered between $8^\circ$N and $12^\circ$N latitudes and $71^\circ$E and $74^\circ$E longitudes, lying at a distance of 220 km to 440 km from the Kerala coast. Unlike the Andaman and Nicobar Islands, which are mainly tectonic/volcanic, the islands of this archipelago are primarily built up from **coral deposits**. They are examples of atolls, formed on submerged volcanic platforms.

The group comprises approximately 36 islands, of which only 11 are inhabited. **Minicoy** is the largest island in terms of area (4.53 sq km). The entire island group is broadly divided by the **Nine Degree Channel** (at $9^\circ$ North latitude), with Amini Island situated to the north and Cannanore Island to the south of this channel. The eastern side of the islands features 'storm beaches' composed of unconsolidated pebbles, shingles, cobbles, and boulders, indicating the influence of wave action, particularly during storms.

Exercises

Multiple Choice Questions

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Answer The Following Questions In About 30 Words

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Answer The Following Questions In Not More Than 125 Words

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Project/Activity

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