Exogenetic Processes Part-2Class 9 Geography Chapter 4 Notes – Maharashtra Board

Summary: Exogenetic Processes Part-2

Agents of erosion — running water, glaciers, wind, sea waves and groundwater — wear away the surface, carry the material and drop it when they slow down. In this way they keep creating new landforms.

Rivers erode most in their steep upper course, making gorges (canyons), V-shaped valleys and waterfalls. Lower down, as the slope and speed fall, they deposit sediment as alluvial fans, meanders, flood levees, flood plains and deltas.

Glaciers are slow but massive. Their erosion makes cirques, arêtes, horns, U-shaped valleys, hanging valleys and roches moutonnées; the debris they carry and drop is moraine (ground, lateral, medial, terminal), with drumlins and eskers.

Wind works best in deserts and semi-arid lands. Sand-blasting carves mushroom rocks, yardangs and deflation hollows; deposition builds sand dunes, barchans, seifs, ripple marks and loess plains.

Sea waves erode coasts into cliffs, caves, arches, stacks and wave-cut platforms, and deposit beaches, sand bars and lagoons. Groundwater dissolves limestone to make sinkholes and caves, and deposits stalactites, stalagmites and pillars (karst landforms).

Key points to remember

• Five agents: rivers, glaciers, wind, sea waves, groundwater
• Each agent erodes, transports and deposits
• River erosion: gorge/canyon, V-shaped valley, waterfall, pothole
• River deposition: alluvial fan, flood levee, flood plain, delta; meanders form in the gentle course
• Glacier erosion: cirque, arête, horn, U-shaped valley, hanging valley, roche moutonnée
• Glacier deposition: moraines (ground, lateral, medial, terminal), drumlins, eskers
• Wind erosion: mushroom rock, yardang, deflation hollow; deposition: dunes, barchans, seifs, ripple marks, loess
• Sea-wave erosion: cliff, cave, arch, stack, wave-cut platform; deposition: beach, sand bar, lagoon
• Groundwater erosion: sinkhole, limestone cave; deposition: stalactite, stalagmite, pillar
• Water table is higher in the rainy season and deeper in summer
• Jakobshavn Glacier, Greenland — one of the fastest, about 46 m a day (book figure)
• Konkan rivers form creeks (estuaries), not deltas

Exogenetic Processes Part-2 – Notes

1. Work of rivers

• Rain water flows down the slope under gravity. Many small flows (rills, gullies, streams) join to make a river.
• How much a river erodes, carries and deposits depends on the slope, type of rock, volume of water, length of flow and amount of sediment.
• Upper course: a river starts high up and flows fast, so its power to erode is greatest. Water, sand, pebbles and joining tributaries scour its bed and banks, making deep gorges or canyons, V-shaped valleys and waterfalls. Swirling pebbles drill round potholes in the bed.
• At the foot of the hills the slope suddenly drops, the river slows and dumps coarse sediment in a fan shape — an alluvial fan.
• Middle and lower course: the gentle slope makes the river slow; it swings in wide loops called meanders. A meander that is cut off becomes an ox-bow lake.
• In floods the river spills over its banks; coarse sediment piles up near the banks as flood levees, and fine silt spreads over the land as a flood plain.
• At the mouth, the very slow river drops its load and splits into many channels, forming a delta (e.g. Ganga–Brahmaputra). Konkan rivers are short and fast, flow down steep slopes and meet strong tides, so their mouths form creeks (estuaries) instead of deltas.

2. Work of glaciers

• Where temperatures stay below freezing, snow builds up layer on layer. Its weight presses the lower layers into ice, and friction and pressure melt a little at the base, so the ice mass slides slowly downhill — a glacier.
• Glaciers move from about 1 cm to 1 m a day. The Jakobshavn Glacier in Greenland is among the fastest (the book gives 46 m per day). The ice in the middle moves faster than at the sides and the bottom, which are slowed by friction.
• Erosion: because the ice is so heavy it scrapes its bed and sides hard. Landforms: cirque (armchair-shaped hollow at the head), arête (sharp ridge between two cirques), horn (pyramid peak where several cirques meet), U-shaped valley, hanging valley (a side valley left high above the main one), roche moutonnée (sheepback rock, smooth on one side and rough on the other).
• Deposition: rock debris carried by the glacier is called moraine. Ground moraine lies under the glacier, lateral moraine along its sides, medial moraine in the middle where two glaciers join, terminal moraine at its snout.
• Other deposits: drumlins (egg-shaped mounds) and eskers (long winding ridges of sand and gravel left by melt-water streams).
• In India, glacial landforms are seen in the Himalayas. In polar regions, glaciers reach right down to sea level.

3. Work of wind

• Wind is a gaseous agent. It works best in deserts and semi-arid areas where there is little vegetation and lots of loose sand produced by mechanical weathering.
• Erosion: wind hurls sand and small pebbles at rocks, wearing them by friction. The lower parts are worn more, so mushroom rocks form. Bands of hard and soft rock lying along the wind are cut into ridges called yardangs. Wind blowing away loose sand leaves deflation hollows; bare rocky desert is called hamada.
• Transport: fine particles go very far; coarser sand moves only short distances.
• Deposition: where wind slows, sand piles up as sand dunes — crescent-shaped barchans, long sword-like seifs, small ripple marks — and fine dust settles as thick loess plains (e.g. in northern China).
• Wind also builds sand dunes along sandy coasts.

4. Work of sea waves

• Winds and tides move sea water and send waves to the coast.
• Erosion: breaking waves hurl water, stones and sand at the rocks; the force of the water (hydraulic action) and dissolving (chemical action) also wear the coast. Landforms: sea cliff, sea cave, sea arch (a cave cut through a headland), sea stack (pillar left when an arch roof falls), wave-cut platform at the base of a cliff.
• Deposition: material moves to and fro with tides and becomes fine through attrition. Where wave energy is low it is dropped, forming beaches, sand bars and lagoons (salt-water lakes behind a bar).
• The Konkan coast has many examples — cliffs, beaches and sea caves can be found along the Ratnagiri and Sindhudurg coasts.

5. Work of groundwater

• Rain seeps through porous rocks and cracks and collects above a non-porous layer. This is groundwater; its upper surface is the water table.
• The water table rises in the rainy season and falls in summer; it also depends on rock porosity and rainfall.
• Erosion: groundwater dissolves soluble minerals, especially in limestone, and carries them away, forming sinkholes and limestone caves.
• Deposition: when water evaporates or holds more mineral than it can dissolve, the mineral is deposited. Stalactites hang from the cave roof, stalagmites rise from the floor, and when they join they form pillars (columns).
• Landforms in limestone are called karst, after the Karst region of Slovenia where they are well developed.

Agents and their landforms

Key terms

Exogenetic Processes Part-2 – Flashcards

Read the question, answer it in your head, then tap the card to check. 20 cards.

Exogenetic Processes Part-2 – MCQ Quiz

15 questions. Tap an option — you will see at once if it is right and why.

How to study Exogenetic Processes Part-2

⏱ About 45 minutes

1. Make a table (15 min)
   For each agent — river, glacier, wind, sea waves, groundwater — list erosional and depositional landforms.
2. Picture them (15 min)
   Sketch V-shaped valley, delta, U-shaped valley, moraines, mushroom rock, sea arch and stalactite; label each.
3. Check yourself (15 min)
   Answer the 'Try it yourself' questions, take the quiz and play 🎮 Play.