Erosion and landscapes

The earth’s exogenous forces tear down mountains, and even out the surface of the earth. One of these exogenous forces brings particles and sand from high mountains, all the way to the sea. This force is called erosion. When land erodes, water is often involved. The water in this creek runs fast along a groove in the mountain, and the water carries sand, pebbles, clay and particles.

With every particle the water takes away, the groove gets deeper. The creek digs slowly down into the landscape; the mountain erodes. After millions of years, the creek has dug a deep furrow, a ravine. At the foot of the mountain, the water runs more slowly over the flatland. The river winds its way, forming curves.

A curve like this is called a meander. In the meander, the water flows slowest here, on the inside of the curve. Sand and sludge sink to the bottom and create a layer called sediment. Here however, on the outside of the curve, the water flows a little faster and takes sand and soil away from the river bank. The bank erodes.

This makes the curves of the river wider and wider. Eventually the river breaks through the bank here. The course of the river is changed, leaving still water, a lake with a special curved shape, an oxbow lake. Oxbow lakes appear where a river runs over an area of flatland. This, for example, is what the Mississippi River looks like in the USA.

These are two examples of how erosion shapes the landscape around us. There are more. When the river has run all the way to the ocean, sand and clay sink to the bottom creating layers of sediment. If enough sediment gathers, small islands form at the mouth of the river. Sometimes this causes the river to split into branches as it joins the sea.

A pattern of islands like this at a river mouth is called a delta. Even frozen water erodes the earth and shapes the landscape. Let’s go back to a slope, where a creek has dug out a ravine. If the climate becomes colder, and the water freezes to ice, a glacier is formed in the ravine. The glacier slowly slides down the valley and digs out - erodes - the valley.

To begin with, the valley has a V shape, and is called a V-shaped valley. As the ice moves through the valley, the sharp V shape changes. Now it becomes rounder, and looks more like a U. It is a U-shaped valley. Wind can also erode the landscape and take sand and soil away.

If trees and plants grow on a piece of land, their roots form a network underground. This network ensures that the soil is not taken away by the wind so easily. The roots of the plants prevent erosion by the wind. But if humans cut down the forest, or if their cattle eat all the grass, then the plants die. And their roots disappear.

When the network of roots is gone, wind can take away the loose soil. Only rocks remain, and now it’s harder for plants to set root and start to grow again. The soil and the plants do not come back, and a rocky desert is formed. In this way, wind erosion together with human agriculture causes deserts to spread. Erosion is outer, exogenous, forces, that wear down the surface of the earth and flatten it out.

Different types of erosion form different types of landscape.