Discovery Education Science P4: Term 2, Unit 4, pp. 135–148
GoalExplain how a delta forms by deposition and how a glacier carves land, and compare these with canyons and dunes.
New words
delta · a fan-shaped landform built where a river drops sediment as it meets the sea or a lakesilt · very fine sand, clay and mud carried by waterglacier · a very large mass of ice that flows slowly downhill
Explanation
Not every landform comes from erosion. A delta is built by deposition. A fast river carries silt, which is very fine sand and mud. Where the river enters the sea or a lake, the water slows down and loses its energy, so it drops the silt. The silt piles up in a fan shape. The Nile Delta in Egypt is the most famous: its soil is rich, and farmers grow many crops there. Plants in the wetlands slow the water and their roots trap more sediment. A glacier is the opposite kind of traveller. It is a huge, slow river of ice that forms where snow does not melt, in polar lands and on high mountains. Gravity makes it flow downhill. The glacier scrapes the land and carves valleys, and when it melts it leaves heaps of rocks. There are no glaciers in Egypt.
Landform
Main process
What makes it
canyon
erosion
river water
valley
erosion
river or glacier
delta
deposition
river meeting the sea
dune
erosion and deposition
wind
Worked examples
fast river carries silt→enters the sea and slows→silt drops→a fan-shaped delta
Delta steps. A fast river carries silt. It reaches the sea and slows. The silt drops. After a very long time the dropped silt makes a fan of new land.
Farm fields in the Nile Delta. Mariam’s uncle grows rice and vegetables there. The soil is rich in fine silt left by the river over a very long time. Deposition gave him good farmland.
Glacier speed. A glacier in a cold land moves 2 m every day. In a week (7 days) it moves 2 × 7 = 14 m. Many glaciers move much more slowly than this.
Class activity
Make a delta (outdoors, a grown-up near). Tilt a baking tray with a pile of sand and soil at the high end. Put a shallow dish of water at the low end as the “sea”. Pour a cup of water down from the top and watch where the sand and soil settle when the water slows at the “sea”. Draw the fan shape you see. Write what this shows about how the Nile Delta formed.
Practice
1
Is a delta formed by erosion or by deposition?
By deposition.
2
Why does a river drop its silt when it reaches the sea?
The water slows down and loses energy, so it cannot carry the silt.
3
A glacier moves 3 m a day. How far in 5 days?
3 × 5 = 15 m.
4
Are there glaciers in Egypt? Where are glaciers found?
No. They are found in polar lands and on high, cold mountains.
