Weathering, erosion and the agents of gradation

Mountains are pushed up from inside. But on the outside, the weather, water and wind are always wearing them down. Two processes do most of this work: weathering and erosion. Over a very long time they wear down mountains, carve valleys, build plains and create caves, cliffs and deltas.

Weathering (apakshay) means rocks breaking into smaller pieces where they are. Nothing is carried away. Three types:

• Physical weathering: temperature changes (hot days, cold nights), frost, or wind crack the rock.
• Chemical weathering: water, air or acids react with the minerals and turn them into new substances.
• Biological weathering: living things do it: plants, animals or micro-organisms. For example, a plant's roots grow into a crack and split the rock.

Weathering also helps make soil.

Erosion (aparadan) means soil and rock are worn away and carried somewhere else, by water, wind, ice or waves. Types:

• water erosion: rivers, rain or ocean waves;
• wind erosion: common in dry, sandy areas;
• glacial erosion: moving ice scrapes and carries rock;
• coastal erosion: sea waves wear away the shore.

Watch out for the one difference that decides most answers: weathering only breaks; erosion breaks and carries. If the broken bits stay put, it is weathering. If they move, it is erosion.

"Why should a farmer care?" Erosion touches people's work and homes:

• Farmers lose the fertile topsoil their crops need, so harvests fall.
• People living near rivers and coasts can lose land, houses and roads.
• In building and mining, eroded land becomes unsafe.
• Tourism and fishing suffer when beaches, rivers and fertile land are destroyed.

India has saved its soil and water for thousands of years. The Sindhu-Sarasvatī civilisation used contouring, bunding, terracing, dams and canals. Texts such as the Vedas, the Kṛiṣhiparāśhara, Kauṭilya's Arthaśhāstra and the Vṛikṣhāyurveda describe these methods; the Arthaśhāstra even sorts land by how fertile it is.

Three methods, in plain words. Picture a hillside where rain rushes down and takes the soil with it:

• Contour trenches: ditches dug across the slope, along its contour lines, to catch rainwater so it soaks in.
• Bunding: low earth walls built along the contour lines to slow the water.
• Terracing: turning the slope into a staircase of flat steps, so water cannot race down.

Two more from the book: the Zabo system of Nagaland uses earthen bunds on hill slopes to save soil and water as part of its farming. And check dams across small streams slow the water down, stop erosion, let mud settle, and help rainwater soak into the ground.

Finally, the agents of gradation: the natural forces that wear down, carry and lay down material, slowly levelling the land. There are five:

• running water (rivers) makes valleys and plains;
• glaciers carve U-shaped valleys;
• wind shapes deserts;
• waves shape coasts into cliffs, beaches and bays;
• groundwater dissolves rock like limestone into caves and sinkholes.

Together they lower the high places and fill the low ones.