Loess landforms
What is loess?
Loess is a pale-yellow soil made of very fine grains of dust (silt). The wind carried it from the dry lands of the north and north-west, such as deserts, and dropped it over the plateau. The layer is very thick, tens to hundreds of metres in places.
Why it is easy to carve
- The grains stick together only lightly.
- It stands in steep walls when dry.
- It breaks and flows when it gets wet.
The three main shapes
- Yuan (tableland): a wide flat top with steep sides. Good for farms and villages.
- Liang (ridge): a long narrow hill top, like a back.
- Mao (hill): a small round hill, often a broken piece of a ridge.
Between them run gullies: deep, narrow channels cut by running water.
Soil erosion: why and how much
Soil erosion is when water or wind removes the top soil. The Loess Plateau is one of the most eroded places on Earth.
Causes
- Natural: loose soil, steep slopes, and heavy rain that comes in a few summer storms.
- Human: cutting trees and grass, over-grazing, and ploughing steep slopes.
Effects
- Fields lose good soil and grow less.
- Gullies grow and cut villages and roads.
- Mud fills the Yellow River, raises its bed and makes floods more likely downstream.
In the past the river carried over a billion tonnes of soil each year to the sea.
Ecological restoration
Restoration means helping the land heal. Plants are the key: roots hold soil and leaves soften raindrops.
- Plant grass, shrubs and trees on steep slopes. Farmers get help to stop farming there (often called "returning farmland to forest and grass").
- Terraces: steps cut into a slope to make flat fields that hold water and soil.
- Check dams: small earth dams in gullies that trap mud and make flat farmland behind them.
- Controlled grazing: keep animals in pens so grass can grow back.
Results: in the last twenty years the green cover has risen a lot and the Yellow River carries much less mud. The challenge now is water: trees need water and the region is dry, so planting must match the rain.
Try it: build a slope
- Pile damp sand in a tray to make a slope. Pour water from a watering can on top. Watch gullies form.
- Press stones or sticks in steps (terraces) and put a few pieces of grass or moss on top. Pour again. Which one loses less sand?
- In the 3D, first set plants and terraces to 0, then change one at a time.
Key formulas and definitions
- Loess: very fine, yellow, wind-blown dust (silt) laid down in thick layers
- Yuan: flat-topped tableland; Liang: long ridge; Mao: round hill
- Gully: a deep channel cut by running water in loose soil
- Soil erosion: removal of top soil by water or wind
- Terrace: a flat step cut into a slope for farming
- Check dam: a small dam in a gully that traps mud
Worked examples
1. Why does the Yellow River look yellow?
It flows through the Loess Plateau. Rain washes loose yellow soil into it, so it carries a lot of silt.
2. A slope is bare and steep. Suggest two ways to cut erosion.
Plant grass or trees so roots hold the soil, and cut terraces to make steps that slow water. Check dams in the gullies below also help.
3. A river carried 1,600 million tonnes of soil a year. After restoration it carries 300 million. How much less?
1,600 − 300 = 1,300 million tonnes less each year, about 81% less.
Common mistakes
- Thinking the soil is yellow because of the river. The river is yellow because of the soil.
- Thinking loess comes from the plateau's own rocks. It was carried in by wind from drier lands.
- Believing planting any tree anywhere is always good. In dry land, trees may use too much water. Match plants to rain.
- Thinking terraces make rain stop. They slow the water and hold the soil.