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River Landscapes » Waterfalls, Gorges and Interlocking Spurs

What you'll learn this session

Study time: 30 minutes

AQA spec: 3.1.3.3

What interlocking spurs look like and how they form
How a waterfall forms and what its main features are
How a gorge forms as a waterfall retreats upstream
How to describe and explain these landforms in an exam

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Landforms of erosion in the upper course

Waterfalls form where a band of hard rock lies over softer rock

Near its source, a river has a steep gradient and cuts downwards. This is vertical erosion, and it makes a steep, V shaped valley. Three landforms of river erosion are on your specification: interlocking spurs, waterfalls and gorges. Each one is the result of the erosion processes you have already met, working on different types of rock.

Key terms:

Interlocking spurs: ridges of high land that stick out from alternate sides of a V shaped valley and fit together like the teeth of a zip.
Waterfall: a place where a river falls vertically over a step in its course.
Plunge pool: a deep hole at the bottom of a waterfall.
Gorge: a steep sided, narrow valley, often with a rocky floor.
Overhang: a layer of hard rock that sticks out above softer rock that has been worn away below.
Differential erosion: different rocks wearing away at different speeds.

Interlocking spurs

👁 What they look like

Seen from above, the river winds around ridges of land that poke out from each side of the valley. The ridges alternate, left then right, and overlap like the teeth of a zip. The river twists between them.

⚡ Where they form

In the upper course, where the valley is steep and narrow. The river is small and has little energy to erode sideways, so it does not cut straight through the ridges.

Here is how they form:

In the upper course the river is small and flows in a narrow channel. Most of its erosion is vertical, cutting down into its bed.
The river is not powerful enough to erode sideways through the higher land. So it bends around it instead.
The bits of land that the river bends around are left sticking out into the valley. These are the spurs.
Over time the river keeps cutting down. The spurs stay in place and interlock as the river winds around them.

Remember

Interlocking spurs are made by vertical erosion. The river goes around the spurs, not through them.

Waterfalls

A waterfall forms where a river flows over a band of hard rock that lies on top of a band of softer rock. The two rocks wear away at different speeds. This is differential erosion. The soft rock is eroded faster, and the hard rock is left sticking out.

Here is the formation, step by step:

The river flows over hard rock and then onto softer rock.
The softer rock is eroded more quickly than the hard rock, by hydraulic action and abrasion. A step forms in the river bed.
As the water falls over the step, it swirls around at the bottom. This erodes the soft rock even more and creates a deep plunge pool.
The plunge pool, and the water and rocks swirling in it, wear away the soft rock beneath the hard rock. This undercuts the hard rock.
The hard rock is left as an overhang with nothing to support it. Eventually the overhang becomes too heavy and collapses into the plunge pool.
The fallen rocks are swirled around the plunge pool and erode it further by abrasion. The process then starts again.

Each time the overhang collapses, the waterfall moves a little way upstream. We say that the waterfall retreats.

🌊 Main features

A vertical drop of water, a band of hard rock at the top, softer rock beneath, an overhang and a deep plunge pool at the base.

🔭 Describing one

Say how high it is, what the rock layers are, whether there is an overhang and how big the plunge pool is. A labelled sketch is worth marks.

Gorges

Gorges form as a waterfall retreats upstream over time

A gorge is a steep sided, narrow valley with a rocky floor. It is formed by the retreat of a waterfall.

A waterfall forms as above.
Each time the hard rock overhang collapses, the waterfall moves back upstream.
Over thousands of years the waterfall keeps retreating. It leaves a long, narrow, steep sided valley behind it.
This valley is the gorge. Its walls are steep because the waterfall cut straight down through the rock as it retreated.

So a gorge is found downstream of a waterfall. If you stand at the foot of the waterfall and look downstream, you are looking along the gorge that the waterfall has already carved out.

Worked example: linking the landforms

Imagine a river flowing over a layer of hard sandstone resting on soft clay. The clay is eroded more quickly and a step forms. Water falls over it and creates a plunge pool. The pool undercuts the sandstone, which collapses. The waterfall moves upstream, leaving a gorge downstream of it. One process explains both landforms.

Putting it together

⛰ Interlocking spurs

Vertical erosion. The river winds around ridges that stick out from alternate sides.

💦 Waterfall

Hard rock over soft rock. Differential erosion makes a step, a plunge pool and an overhang.

🏔 Gorge

Left behind as the waterfall retreats upstream. A steep sided, narrow valley.

Common mistakes

1. Saying the hard rock wears away first. It is the soft rock that erodes faster, which is why the hard rock is left behind.
2. Forgetting the plunge pool. It undercuts the hard rock, so it explains the overhang.
3. Saying a gorge forms before the waterfall. The gorge is created by the waterfall as it retreats.
4. Mixing up spurs with the valley. The spurs are the high ridges, and the river is in the gaps between them.

Exam-style question

(a) Describe the main features of interlocking spurs. [2 marks]

(b) Explain how a waterfall forms. [4 marks]

Model answer

(a) Ridges of land stick out from alternate sides of a V shaped valley (1). They overlap like the teeth of a zip, and the river winds around them (1).

(b) A waterfall forms where hard rock lies on top of softer rock (1). The soft rock is eroded faster, which makes a step (1). Water falls over the step and creates a plunge pool, which undercuts the hard rock (1). The overhang collapses and the waterfall retreats upstream (1).

(c) As the waterfall retreats upstream, it leaves behind a steep sided valley (1). This narrow valley downstream of the waterfall is the gorge (1).

Exam tip

For a 4 mark "explain how" question, write a clear sequence using words like "first", "then" and "eventually". Include the plunge pool and the overhang, as these are the points that score.

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