---
title: "Landforms"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 31
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/31-landforms
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 31 — Landforms

On the south [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of England, the white [cliffs](#def-g9-landforms-cliff) of the Seven Sisters drop straight into the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). In a normal year they lose up to a metre. In the winter of 2014, [storms](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm) took from the [cliffs](#def-g9-landforms-cliff) at Birling Gap as much as they usually lose in seven years, in two months. The chalk of these [cliffs](#def-g9-landforms-cliff) was once [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) on the floor of a [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), made of the tiny plates of microscopic algae that lived in its water. Today it stands as a wall of white [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) that the waves cut back a little further every year. Why does the land have the shapes it has, and why do some of them change so fast?

![The Seven Sisters, chalk cliffs on the coast of East Sussex (England), with the houses of Birling Gap in the foreground. The cliffs are cut back by the sea, and rock falls happen at any time. At their foot lies a beach of pebbles. Photograph Arild Vågen, 2017 (CC BY-SA 4.0, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g9-landforms/img-5ffa4a419659.jpg)

*The Seven Sisters, chalk [cliffs](#def-g9-landforms-cliff) on the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of East Sussex (England), with the houses of Birling Gap in the foreground. The [cliffs](#def-g9-landforms-cliff) are cut back by the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), and [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) falls happen at any time. At their foot lies a [beach](#def-g9-landforms-beach) of [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble). Photograph Arild Vågen, 2017 (CC BY-SA 4.0, via Wikimedia Commons).*

**You already know.**

A *[landform](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform)* is one shape of the land: a [hill](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), a [plain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), a [cliff](#def-g9-landforms-cliff) ([Chapter 2](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#ch-g1-landscapes-around-us)). The [relief](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-relief) is the shape of the land in height ([Chapter 9](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#ch-g4-world-physical-map)). [Weathering](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-weathering) breaks the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) wears it away and carries it off, and [deposition](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-deposition) lays it down elsewhere ([Chapter 10](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#ch-g5-water-shapes-the-land)). The [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) push up [mountain ranges](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-range) where they collide ([Chapter 18](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#ch-g7-plate-tectonics)); [glaciers](https://one-course.com/books/earth/1/en/chapter/27-ice-on-earth#def-g8-ice-on-earth-glacier) carve [U-shaped valleys](https://one-course.com/books/earth/1/en/chapter/27-ice-on-earth#def-g8-ice-on-earth-glacial-landforms) ([Chapter 27](https://one-course.com/books/earth/1/en/chapter/27-ice-on-earth#ch-g8-ice-on-earth)); limestone fizzes under a drop of dilute acid ([Chapter 15](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#ch-g6-rocks-and-minerals)). In chemistry, you learn that acids attack limestone and give off carbon dioxide.

## 31.1 The relief: a balance between uplift and erosion

Two kinds of forces shape the [relief](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-relief). From inside, the movements of the [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) raise the ground. From outside, water, ice, wind and the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) wear it down.

**Definition 31.1 (Uplift).**

*Uplift* is the slow rise of the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and the ground surface of a region, driven by forces inside the Earth, such as the collision of two [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate).

[Uplift](#def-g9-landforms-uplift) is slow on the scale of a life but fast on the scale of the Earth. India collided with Asia about 50 million years ago, and the Tibetan Plateau, behind the Himalaya, gained most of its height, about $4600\,\mathrm{m}$, in the last 10 million years. On average, that is $4600 \div 10^7 = 4.6 \times 10^{-4}$ m, or about $0.5\,\mathrm{mm}$ a year: less than the thickness of a fingernail, but kept up for millions of years.

As soon as the ground rises, [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) attacks it. The higher and the steeper the land, the faster [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), [glaciers](https://one-course.com/books/earth/1/en/chapter/27-ice-on-earth#def-g8-ice-on-earth-glacier) and [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) falls carry [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) away. The height of a [mountain range](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-range) at any moment is the result of a contest: what [uplift](#def-g9-landforms-uplift) brings up, minus what [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) takes away.

![The relief as a balance (a sketch, not to scale). Uplift raises the rocks; erosion carries them away to the valleys and the sea. A range keeps growing as long as uplift wins; when the plates stop pushing, erosion slowly wears it down to hills.](https://one-course.com/images/onecourse/chapters/earth-1/g9-landforms/fig-2ad4c10e3900.svg)

*The [relief](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-relief) as a balance (a sketch, not to scale). [Uplift](#def-g9-landforms-uplift) raises the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock); [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) carries them away to the [valleys](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). A range keeps growing as long as [uplift](#def-g9-landforms-uplift) wins; when the [plates](https://one-course.com/books/earth/1/en/chapter/18-plate-tectonics-the-moving-puzzle#def-g7-plate-tectonics-plate) stop pushing, [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) slowly wears it down to [hills](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform).*

[Rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) ([Chapter 10](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#ch-g5-water-shapes-the-land)) and [glaciers](https://one-course.com/books/earth/1/en/chapter/27-ice-on-earth#def-g8-ice-on-earth-glacier) ([Chapter 27](https://one-course.com/books/earth/1/en/chapter/27-ice-on-earth#ch-g8-ice-on-earth)) are the great carvers of [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). The rest of this chapter follows three other agents, which make [landforms](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) of their own: the wind, the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), and water that dissolves [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

## 31.2 Landforms made by the wind: dunes

Where plants are few, loose [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) plentiful and the wind strong, the wind can pick up [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and heap it into [hills](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform): in [deserts](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert), and behind sandy [beaches](#def-g9-landforms-beach).

**Definition 31.2 (Dune).**

A *dune* is a mound or ridge of loose [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) heaped up by the wind. Its slope facing the wind is long and gentle; its far side, the *slip face*, is short and steep.

A [dune](#def-g9-landforms-dune) moves. The wind rolls and bounces the grains up the gentle slope; at the crest they pile up until the heap becomes too steep, and a little avalanche of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) slides down the [slip face](#def-g9-landforms-dune). Grain after grain, [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) is taken from the windward side and laid on the far side, and the whole [dune](#def-g9-landforms-dune) creeps downwind without ever being lifted. [Dunes](#def-g9-landforms-dune) rarely stand alone: they cover whole fields.

Where the wind always blows from the same direction and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) is not too plentiful, [dunes](#def-g9-landforms-dune) take the shape of a crescent, a *barchan*: its round nose faces the wind and its two horns point downwind. The horns hold less [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) than the middle, so they move faster and stretch ahead of the [dune](#def-g9-landforms-dune).

![A crescent dune, or barchan (a sketch). Seen from above, its horns point downwind. Seen from the side, the grains climb the gentle windward slope and slide down the steep slip face: the dune moves downwind (dashed outline: the dune a little later).](https://one-course.com/images/onecourse/chapters/earth-1/g9-landforms/fig-0fd1d7dbeca1.svg)

*A crescent [dune](#def-g9-landforms-dune), or barchan (a sketch). Seen from above, its horns point downwind. Seen from the side, the grains climb the gentle windward slope and slide down the steep [slip face](#def-g9-landforms-dune): the [dune](#def-g9-landforms-dune) moves downwind (dashed outline: the [dune](#def-g9-landforms-dune) a little later).*

How fast? In the gypsum [dune](#def-g9-landforms-dune) field of White Sands, in New Mexico, the small [dunes](#def-g9-landforms-dune) that form at its upwind edge, a few metres high, move up to $6\,\mathrm{m}$ a year; a little further on, grown to 6 to $10\,\mathrm{m}$ high, they move about $3\,\mathrm{m}$ a year. The more [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) a [dune](#def-g9-landforms-dune) holds, the slower it goes. Barchans driven by strong winds can travel more than $100\,\mathrm{m}$ in a year, enough to bury a road or a field.

![Crescent-shaped dunes on the Atlantic coast of southern Brazil, photographed by an astronaut from the International Space Station in 2017. The dark lake, Lagoa dos Barros, is about 4.5\, km long. The sand comes from the beach, at the bottom of the photograph. NASA (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g9-landforms/img-b014a6f7b377.jpg)

*Crescent-shaped [dunes](#def-g9-landforms-dune) on the Atlantic [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of southern Brazil, photographed by an astronaut from the International Space Station in 2017. The dark [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), Lagoa dos Barros, is about $4.5\,\mathrm{km}$ long. The [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) comes from the [beach](#def-g9-landforms-beach), at the bottom of the photograph. NASA (public domain, via Wikimedia Commons).*

On a [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), a line of [dunes](#def-g9-landforms-dune) behind the [beach](#def-g9-landforms-beach) shields the land behind it from [storm](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#def-g5-natural-hazards-storm) waves. Plants that grow on the [dunes](#def-g9-landforms-dune) hold the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) with their roots: where they are trampled or cleared, the wind takes the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) again.

## 31.3 Landforms made by the sea

### 31.3.1 Cliffs

**Definition 31.3 (Cliff).**

A *cliff* is a steep or vertical face of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). On a [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), a cliff is cut by the waves, which attack its foot and make it retreat inland.

Waves hurl water, [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) at the foot of the [cliff](#def-g9-landforms-cliff). Little by little they cut a hollow at its base, a *notch*, between the levels of low and high tide. The [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) above overhangs; [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and frost weaken it further. One day the overhang falls. The waves break up the fallen blocks and carry them away, and the attack starts again on a fresh face, a little further inland. Behind it, the [cliff](#def-g9-landforms-cliff) leaves a flat shelf of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), the *wave-cut platform*, which appears at low tide.

Soft [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) retreat fastest. At Birling Gap, the chalk [cliffs](#def-g9-landforms-cliff) lose up to about a metre a year; but the retreat comes in jumps, one fall at a time, and a stormy winter can take years of retreat at once. In 1999, the old Belle Tout lighthouse, built in 1832 on top of the [cliffs](#def-g9-landforms-cliff) nearby, was moved $17\,\mathrm{m}$ back from the edge, in one piece, because the [cliff](#def-g9-landforms-cliff) was coming too close.

![How a sea cliff retreats (a sketch, not to scale). (1) The waves cut a notch at the foot of the cliff. (2) The overhanging rock falls; the waves break up the blocks and carry them away. (3) The cycle repeats: the cliff moves inland and leaves a flat rock platform in front of it.](https://one-course.com/images/onecourse/chapters/earth-1/g9-landforms/fig-89fa609a8c7b.svg)

*How a [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) [cliff](#def-g9-landforms-cliff) retreats (a sketch, not to scale). (1) The waves cut a notch at the foot of the [cliff](#def-g9-landforms-cliff). (2) The overhanging [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) falls; the waves break up the blocks and carry them away. (3) The cycle repeats: the [cliff](#def-g9-landforms-cliff) moves inland and leaves a flat [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) platform in front of it.*

**Safety.**

The edge of a [cliff](#def-g9-landforms-cliff) can give way at any moment, and [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) fall from the face without warning. [Cliffs](#def-g9-landforms-cliff) are watched from a safe distance, well back from the edge at the top and well away from the foot on the [beach](#def-g9-landforms-beach), and a [beach](#def-g9-landforms-beach) below [cliffs](#def-g9-landforms-cliff) is left before the tide comes in.

### 31.3.2 Beaches and spits

The [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) worn from [cliffs](#def-g9-landforms-cliff), and brought down by [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), do not stay where they fall: the waves sort them and move them along the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

**Definition 31.4 (Beach, longshore drift and spit).**

A *beach* is a strip of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) or [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) piled up by the waves along the shore. When the waves reach the shore at an angle, each wave pushes the grains up the beach at that angle and the water runs back straight down the slope: the grains zigzag along the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). This movement of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) along the shore is *longshore drift*. A *spit* is a long, narrow ridge of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) or [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) built by longshore drift, joined to the land at one end and running out into the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) where the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) turns.

Where the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) bends, at the mouth of a bay or of a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), the drift carries on in the same direction and drops its load in the calmer water: the [beach](#def-g9-landforms-beach) grows out into the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) as a [spit](#def-g9-landforms-beach). Its end often curls inward, bent by waves coming from other directions. Behind the [spit](#def-g9-landforms-beach), sheltered from the waves, [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settles and salt marshes grow.

![Longshore drift and a spit, seen from above (a sketch). The waves arrive from the south-west; each one pushes the sand and pebbles up the beach at an angle, and the water runs back straight down: the grains zigzag eastward. Where the coast turns into a bay, the drift carries on and builds a spit, behind which a salt marsh grows. The river keeps a channel open at the end of the spit.](https://one-course.com/images/onecourse/chapters/earth-1/g9-landforms/fig-23deaab3d9e7.svg)

*[Longshore drift](#def-g9-landforms-beach) and a [spit](#def-g9-landforms-beach), seen from above (a sketch). The waves arrive from the south-west; each one pushes the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) up the [beach](#def-g9-landforms-beach) at an angle, and the water runs back straight down: the grains zigzag eastward. Where the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) turns into a bay, the drift carries on and builds a [spit](#def-g9-landforms-beach), behind which a salt marsh grows. The [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) keeps a channel open at the end of the [spit](#def-g9-landforms-beach).*

![A sandy spit across the mouth of a bay, seen from a low aircraft (an illustration, not a real place). The waves break on its seaward beach; behind it lie calm water and salt marshes.](https://one-course.com/images/onecourse/chapters/earth-1/g9-landforms/img-f7becfa0065c.jpg)

*A sandy [spit](#def-g9-landforms-beach) across the mouth of a bay, seen from a low aircraft (an illustration, not a real place). The waves break on its seaward [beach](#def-g9-landforms-beach); behind it lie calm water and salt marshes.*

## 31.4 Landforms made by dissolution: karst

Rainwater is not pure water. Falling through the air and then soaking through the soil, it takes up carbon dioxide, which makes it slightly acidic. This weak acid does slowly what the drop of dilute acid does fast to a piece of limestone: it dissolves the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). Limestone is full of cracks, along its layers and across them. Water seeps down them and widens them, millimetre by millimetre, over thousands of years.

**Definition 31.5 (Karst, cave and sinkhole).**

A *karst* is a [landscape](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landscape) shaped by water dissolving a soluble [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), most often limestone. [Rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) there tend to vanish underground and come back out as [springs](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-spring). A *cave* is a natural underground space large enough for a person to enter, dissolved along the cracks of the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). A *sinkhole* is a hollow or a hole in the ground surface, formed where the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) below dissolves away or where the roof of a cave collapses.

![A karst landscape (a sketch, not to scale). Slightly acidic water seeps through the cracks of the limestone and widens them; underground streams hollow out caves; a cave roof gives way and opens a sinkhole; a stream vanishes into a hole and comes back out as a spring lower down.](https://one-course.com/images/onecourse/chapters/earth-1/g9-landforms/fig-3b77d2a7c463.svg)

*A [karst](#def-g9-landforms-karst) [landscape](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landscape) (a sketch, not to scale). Slightly acidic water seeps through the cracks of the limestone and widens them; underground streams hollow out [caves](#def-g9-landforms-karst); a [cave](#def-g9-landforms-karst) roof gives way and opens a [sinkhole](#def-g9-landforms-karst); a stream vanishes into a hole and comes back out as a [spring](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-spring) lower down.*

In a [cave](#def-g9-landforms-karst), dissolution can run backwards. A drop of water seeping from the roof carries dissolved limestone. When it meets the air of the [cave](#def-g9-landforms-karst), it gives off some of its carbon dioxide, and a thin film of calcite comes out of the water around the drop. Drop after drop, a hollow tube grows from the roof, then thickens into a *stalactite*, hanging like an icicle. Where the drops land, the rest of the calcite builds a *stalagmite* up from the floor. When the two meet, they make a column.

![Stalactites hanging under a large block of rock in Carlsbad Cavern, New Mexico, a cave dissolved in limestone. Each one grew drop by drop from water seeping through the rock above. National Park Service, 2018 (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g9-landforms/img-3002f27c2f55.jpg)

*Stalactites hanging under a large block of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) in Carlsbad Cavern, New Mexico, a [cave](#def-g9-landforms-karst) dissolved in limestone. Each one grew drop by drop from water seeping through the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) above. National Park Service, 2018 (public domain, via Wikimedia Commons).*

[Karst](#def-g9-landforms-karst) country holds much water underground, but it flows fast through the open cracks and is hardly filtered: whatever is spilled on the ground can reach a [spring](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-spring) quickly. And where the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) below has been dissolved, the ground can sink or collapse: where people live, a [sinkhole](#def-g9-landforms-karst) that opens suddenly can swallow roads and houses.

## 31.5 Reading a landscape

Every [landform](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) is a clue. Its shape tells which agent made it; the agent tells which process was at work, [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) or [deposition](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-deposition); and the process often tells something about the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) or the loose material underneath.

**Method 31.6 (Reading a landform).**

1. **Shape.** Describe the [landform](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) : a ridge, a hollow, a steep face, a flat shelf; its size; which way it points.
2. **Agent and process.** Decide which agent made it (a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) , ice, the wind, the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) , dissolving water) and whether it was carved ( [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) ) or built ( [deposition](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-deposition) ).
3. **Material.** Say what it is made of, or what the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) below is likely to be: loose [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) , [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) , limestone, hard or soft [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) .

| shape | agent | process | material |
| --- | --- | --- | --- |
| crescent-shaped [hills](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) | wind | [deposition](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-deposition) | loose [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) |
| a vertical [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) face above a flat [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) shelf | [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) | [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) | [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) (soft [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) retreats fast) |
| a narrow ridge of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) across a bay | [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ([longshore drift](#def-g9-landforms-beach)) | [deposition](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-deposition) | [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) or [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) |
| hollows in fields, streams that vanish | dissolving water | dissolution | limestone |
| a [U-shaped valley](https://one-course.com/books/earth/1/en/chapter/27-ice-on-earth#def-g8-ice-on-earth-glacial-landforms) | ice | [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) | any [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) |
| a fan of [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) at a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) [mouth](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-source-mouth) | [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) | [deposition](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-deposition) | [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) |

**Example 31.7 (A walk on a plateau).**

On a high grassy plateau, there is no stream at all, although it [rains](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) often. The fields are dotted with round hollows a few metres across, and in the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) below a powerful [spring](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-spring) gushes out of the foot of a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) face. *Shape*: hollows, no streams on the surface, a [spring](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-spring) lower down. *Agent and process*: water dissolving the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and flowing underground. *Material*: the plateau is most likely limestone, and the hollows are [sinkholes](#def-g9-landforms-karst). A drop of dilute acid on a loose piece of the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), put by the teacher, would fizz.

## 31.6 Exercises

**Exercise 31.1 ★.**

Which agent made each [landform](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform): the wind, the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), ice, or water dissolving the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock)? (a) A crescent-shaped [hill](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) in a [desert](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert). (b) A [cave](#def-g9-landforms-karst) full of stalactites. (c) A long ridge of [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) across the mouth of a bay. (d) A [U-shaped valley](https://one-course.com/books/earth/1/en/chapter/27-ice-on-earth#def-g8-ice-on-earth-glacial-landforms) with a [moraine](https://one-course.com/books/earth/1/en/chapter/27-ice-on-earth#def-g8-ice-on-earth-glacial-landforms) at its end. (e) A vertical chalk face with a flat [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) shelf at its foot. (f) A [delta](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-delta).

**Solution of Exercise 31.1.**

(a) The wind. (b) Water dissolving the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) (and depositing calcite in the [cave](#def-g9-landforms-karst)). (c) The [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ([longshore drift](#def-g9-landforms-beach)). (d) Ice. (e) The [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). (f) A [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

**Exercise 31.2 ★.**

Give the word for each description. (a) The slow rise of the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and the ground of a region. (b) The steep side of a [dune](#def-g9-landforms-dune), away from the wind. (c) A hole in the ground formed where the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) below has dissolved or a [cave](#def-g9-landforms-karst) roof has collapsed. (d) The movement of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) along the shore. (e) A [landscape](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landscape) shaped by water dissolving limestone.

**Solution of Exercise 31.2.**

(a) [Uplift](#def-g9-landforms-uplift). (b) The [slip face](#def-g9-landforms-dune). (c) A [sinkhole](#def-g9-landforms-karst). (d) [Longshore drift](#def-g9-landforms-beach). (e) A [karst](#def-g9-landforms-karst).

**Exercise 31.3 ★.**

True or false? Correct the false sentences. (a) The horns of a crescent [dune](#def-g9-landforms-dune) point towards the wind. (b) The slope of a [dune](#def-g9-landforms-dune) facing the wind is gentle. (c) A [dune](#def-g9-landforms-dune) moves because the wind lifts it all at once. (d) [Dunes](#def-g9-landforms-dune) form more easily where few plants grow.

**Solution of Exercise 31.3.**

(a) False: the horns point downwind; the round nose faces the wind. (b) True. (c) False: the wind moves it grain by grain, up the gentle slope and down the [slip face](#def-g9-landforms-dune). (d) True.

**Exercise 31.4 ★.**

Put these steps of the retreat of a [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) [cliff](#def-g9-landforms-cliff) in order. (a) The waves break up the fallen blocks and carry them away. (b) The waves cut a notch at the foot of the [cliff](#def-g9-landforms-cliff). (c) The [cliff](#def-g9-landforms-cliff) face now stands further inland, and the flat platform in front of it is a little wider. (d) The [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) above the notch overhangs. (e) The overhang falls.

**Solution of Exercise 31.4.**

(b), (d), (e), (a), (c).

**Exercise 31.5 ★★.**

Seen from an aircraft, the crescent [dunes](#def-g9-landforms-dune) of a [desert](https://one-course.com/books/earth/1/en/chapter/11-weather-and-climate#def-g5-weather-and-climate-desert) all have their horns pointing to the east. (a) From which direction does the wind blow? (b) On which side of each [dune](#def-g9-landforms-dune) is the [slip face](#def-g9-landforms-dune)? (c) A road runs $60\,\mathrm{m}$ east of one of the [dunes](#def-g9-landforms-dune). The [dune](#def-g9-landforms-dune) moves $6\,\mathrm{m}$ a year. In how many years will its foot reach the road? (d) A larger [dune](#def-g9-landforms-dune) moves only $3\,\mathrm{m}$ a year. Why do larger [dunes](#def-g9-landforms-dune) move more slowly?

**Solution of Exercise 31.5.**

(a) From the west: the horns point downwind. (b) On the east side, between the horns. (c) $60 \div 6 = 10$ years. (d) A larger [dune](#def-g9-landforms-dune) holds more [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), and all of it has to be carried over the crest for the [dune](#def-g9-landforms-dune) to move forward.

**Exercise 31.6 ★★.**

The chalk [cliffs](#def-g9-landforms-cliff) of a [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) retreat by up to about $1\,\mathrm{m}$ a year. In the winter of 2014, they lost as much as they usually lose in seven years, in only two months. (a) At $1\,\mathrm{m}$ a year, how much is lost in seven years? (b) Two months is what fraction of a year? (c) How many times faster than usual did the [cliffs](#def-g9-landforms-cliff) retreat during those two months? (d) Explain, using the notch and the overhang, why a [cliff](#def-g9-landforms-cliff) retreats in jumps rather than smoothly.

**Solution of Exercise 31.6.**

(a) $7 \times 1 = 7\,\mathrm{m}$. (b) $2 \div 12 = \frac{1}{6}$ of a year. (c) Seven years of retreat in $\frac{1}{6}$ of a year: $7 \times 6 = 42$ times faster. (d) The waves first cut a notch; the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) above hangs over it for a while, without moving; then the overhang falls all at once, and the edge jumps back.

**Exercise 31.7 ★★.**

A straight [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) runs west–east, with the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) to the south. The waves come from the south-west. (a) In which direction does the [longshore drift](#def-g9-landforms-beach) carry the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand)? (b) Draw the zigzag path of a [pebble](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) over three waves, labelling the stroke up the [beach](#def-g9-landforms-beach) and the stroke back down. (c) At its east end the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) turns north into a bay. What [landform](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) is likely to grow there? (d) Why does [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settle behind it?

**Solution of Exercise 31.7.**

(a) Towards the east. (b) Three strokes up the [beach](#def-g9-landforms-beach) slanting to the north-east, each followed by a stroke straight back down to the south: a zigzag moving east. (c) A [spit](#def-g9-landforms-beach), growing eastward across the mouth of the bay. (d) The [spit](#def-g9-landforms-beach) shelters the water behind it from the waves; in calm water, the fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) can settle.

**Exercise 31.8 ★★.**

(a) What makes rainwater slightly acidic? (b) Why are there [caves](#def-g9-landforms-karst) in limestone but hardly any in granite? (c) Why do the [caves](#def-g9-landforms-karst) follow the cracks and layers of the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock)? (d) Give two ways a [sinkhole](#def-g9-landforms-karst) can form.

**Solution of Exercise 31.8.**

(a) It takes up carbon dioxide from the air and from the soil. (b) The weak acid dissolves limestone slowly, but hardly attacks granite. (c) The water can only enter the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) through its cracks and along its layers; it widens these openings first. (d) The [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) below the surface is dissolved and the ground sinks slowly; or the roof of a [cave](#def-g9-landforms-karst) collapses.

**Exercise 31.9 ★★.**

(a) Explain why calcite comes out of a drop of water hanging from a [cave](#def-g9-landforms-karst) roof. (b) Why do stalagmites have no hollow tube inside, unlike young stalactites? (c) Suppose a stalactite has grown $0.1\,\mathrm{mm}$ a year (an invented rate: real rates vary a lot). How many years has it taken to grow $30\,\mathrm{cm}$? Write the answer in scientific notation.

**Solution of Exercise 31.9.**

(a) When the drop meets the air of the [cave](#def-g9-landforms-karst), it gives off some of its carbon dioxide and can no longer hold all of the dissolved limestone, which comes out as calcite. (b) A stalagmite grows from drops that splash on the floor and spread out; there is no drop hanging at its tip to build a tube. (c) $30\,\mathrm{cm} = 300\,\mathrm{mm}$; $300 \div 0.1 = 3000 = 3 \times 10^3$ years.

**Exercise 31.10 ★★.**

The Tibetan Plateau rose about $4600\,\mathrm{m}$ in about 10 million years. (a) Compute the average [uplift](#def-g9-landforms-uplift) in metres per year, in scientific notation, then in millimetres per year. (b) Suppose [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) removes $0.2\,\mathrm{mm}$ a year from the plateau (an invented figure). By how much does the plateau rise each year? (c) A chalk [cliff](#def-g9-landforms-cliff) retreats about $1\,\mathrm{m}$ a year. How many times faster is that than the [uplift](#def-g9-landforms-uplift) of (a)? (d) Why can a slow [uplift](#def-g9-landforms-uplift) still build a high plateau?

**Solution of Exercise 31.10.**

(a) $4600 \div 10^7 = 4.6 \times 10^{-4}$ m a year, or $0.46\,\mathrm{mm}$ a year. (b) $0.46 - 0.2 = 0.26\,\mathrm{mm}$ a year. (c) $1\,\mathrm{m} =
1000\,\mathrm{mm}$; $1000 \div 0.46 \approx 2200$, about two thousand times faster. (d) It is kept up for millions of years: $0.26\,\mathrm{mm} \times
10^7 = 2.6 \times 10^6$ mm, about $2.6\,\mathrm{km}$.

**Exercise 31.11 ★★★.**

Read each [landscape](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landscape) with the method of the chapter (shape, agent and process, material). (a) In a dry country, long lines of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) [hills](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), each shaped like a crescent, with a steep side facing south-east. (b) At the foot of a white [cliff](#def-g9-landforms-cliff), a flat [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) shelf covered with seaweed appears at low tide. (c) On a wide plateau of pale [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), a stream flows across a field and disappears into a hole; a [spring](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-spring) gushes out of a [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) side $2\,\mathrm{km}$ away.

**Solution of Exercise 31.11.**

(a) Shape: crescent [dunes](#def-g9-landforms-dune) with their [slip face](#def-g9-landforms-dune) on the south-east side. Agent and process: the wind, blowing from the north-west, [deposition](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-deposition). Material: loose [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand). (b) Shape: a [cliff](#def-g9-landforms-cliff) above a flat shelf uncovered at low tide. Agent and process: the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion); the shelf is a wave-cut platform left by the retreating [cliff](#def-g9-landforms-cliff). Material: white [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), probably chalk, a soft [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). (c) Shape: a stream that vanishes into a hole, a [spring](https://one-course.com/books/earth/1/en/chapter/17-water-underground#def-g6-water-underground-spring) far away. Agent and process: water dissolving the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and flowing underground. Material: limestone; the plateau is a [karst](#def-g9-landforms-karst).

**Exercise 31.12 ★★★.**

In a [dune](#def-g9-landforms-dune) field, the small [dunes](#def-g9-landforms-dune) at the upwind edge move up to $6\,\mathrm{m}$ a year; a little further on, the [dunes](#def-g9-landforms-dune), 6 to $10\,\mathrm{m}$ high, move about $3\,\mathrm{m}$ a year. A track crosses the field $45\,\mathrm{m}$ downwind of a [dune](#def-g9-landforms-dune) of each kind. (a) After how many years does each [dune](#def-g9-landforms-dune) reach the track? (b) [Dunes](#def-g9-landforms-dune) driven by strong winds can travel more than $100\,\mathrm{m}$ in a year. How long would such a [dune](#def-g9-landforms-dune) take to cover the same distance? (c) Name one way to slow the [dunes](#def-g9-landforms-dune) down, and explain how it works.

**Solution of Exercise 31.12.**

(a) The small [dune](#def-g9-landforms-dune): $45 \div 6 = 7.5$ years; the larger one: $45 \div 3 =
15$ years. (b) $45 \div 100 = 0.45$ of a year, about five or six months. (c) For example, plant grasses on the [dunes](#def-g9-landforms-dune): their roots hold the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), so the wind can no longer take it.

## 31.7 Problem: The cliff and the coast road

**Problem 31.1.**

Weekend problem — a coast road runs behind a retreating chalk cliff; the final answer is the year the cliff edge would reach the road at the measured rate

A [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) road runs along the top of chalk [cliffs](#def-g9-landforms-cliff). Every few years, a survey team measures the distance from the road to the [cliff](#def-g9-landforms-cliff) edge along the same line, from fixed markers set well back from the edge, never at the edge itself. Here are their measurements.

| year | 2004 | 2008 | 2012 | 2014 | 2016 | 2020 | 2024 |
| --- | --- | --- | --- | --- | --- | --- | --- |
| distance from the road to the edge (m) | 46.0 | 43.1 | 41.0 | 36.2 | 35.5 | 33.0 | 30.4 |

**Part I — The measurements.**

1. By how many metres did the [cliff](#def-g9-landforms-cliff) edge move closer to the road between 2004 and 2024?
2. Compute the average rate of retreat over these 20 years, in metres per year.
3. Between which two surveys did the edge retreat fastest? Compute the rate over that interval.
4. How many times faster than the average is it? Suggest what happened during that interval.

![The survey: distance from the coast road to the cliff edge (exercise data).](https://one-course.com/images/onecourse/chapters/earth-1/g9-landforms/fig-c310b99d6270.svg)

*The survey: distance from the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) road to the [cliff](#def-g9-landforms-cliff) edge (exercise data).*

**Part II — The rate.**

5. Write the average rate in centimetres per year, then in metres per century.
6. On a real chalk [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) , the [cliffs](#def-g9-landforms-cliff) retreat by up to about a metre a year. Is this [cliff](#def-g9-landforms-cliff) ’s retreat of the same size?
7. A lighthouse on such a [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) was once moved $17\,\mathrm{m}$ back from the edge. At this [cliff](#def-g9-landforms-cliff) ’s average rate, how many years of retreat does $17\,\mathrm{m}$ represent?
8. Why is the rate measured over 20 years a better guide to the future than the rate measured over two years?

**Part III — The road.**

9. The road is to be moved before the edge comes within $10\,\mathrm{m}$ of it. In which year, at the average rate, will that be?
10. If stormier winters doubled the rate, in which year would the edge reach the road?
11. Why does the survey team take its measurements from markers set well back from the edge?
12. At the average rate, in which year would the [cliff](#def-g9-landforms-cliff) edge reach the road?

**Solution of Problem 31.1.**

**1.** $46.0 - 30.4 = 15.6\,\mathrm{m}$.

**2.** $15.6 \div 20 = 0.78\,\mathrm{m}$ a year.

**3.** Between 2012 and 2014: $41.0 - 36.2 = 4.8\,\mathrm{m}$ in two years, or $4.8 \div 2 = 2.4\,\mathrm{m}$ a year.

**4.** $2.4 \div 0.78 \approx 3$ times faster. Probably a stormy winter, during which large overhangs fell.

**5.** $78\,\mathrm{cm}$ a year; $0.78 \times 100 = 78\,\mathrm{m}$ per century.

**6.** Yes: $0.78\,\mathrm{m}$ a year is a little less than a metre a year.

**7.** $17 \div 0.78 \approx 22$ years.

**8.** The [cliff](#def-g9-landforms-cliff) retreats in jumps: a two-year interval may hold a large fall, or none at all. Over 20 years, the jumps and the calm years average out.

**9.** The edge has $30.4 - 10 = 20.4\,\mathrm{m}$ to go: $20.4 \div 0.78
\approx 26$ years, so in 2050.

**10.** $2 \times 0.78 = 1.56\,\mathrm{m}$ a year; $30.4 \div 1.56
\approx 19.5$ years after 2024, so in 2043.

**11.** The edge of a [cliff](#def-g9-landforms-cliff) can give way at any moment; standing on it, or near it, is dangerous.

**12.** $30.4 \div 0.78 \approx 39$ years after 2024: in **2063**.