---
title: "Sediments and Fossils"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 21
exercises: 12
source: https://one-course.com/books/earth/1/en/chapter/21-sediments-and-fossils
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 21 — Sediments and Fossils

The Grand Canyon, in Arizona (United States), is on average $1.6\,\mathrm{km}$ deep. Its walls are made of layers of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), one on top of the other, red, brown, grey and cream. In the topmost layer, the Kaibab Limestone, the rangers point out shells: the shells of brachiopods, animals that lived 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). That [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) formed at the bottom of an [ocean](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), and it now forms the rim of the canyon, high above the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

![The Grand Canyon from Hermits Rest, on its southern rim: layer after layer of rock, from the rim down to the bottom. NPS photograph by Kristen M. Caldon (public domain, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-sediments-and-fossils/img-44731965e087.jpg)

*The Grand Canyon from Hermits Rest, on its southern rim: layer after layer of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), from the rim down to the bottom. NPS photograph by Kristen M. Caldon (public domain, via Wikimedia Commons).*

How does a pile of loose [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) become solid [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock)? How can a shell last for so long that it ends up inside a cliff? And what do layers and [fossils](#def-g7-sediments-and-fossils-fossil) tell us about places as they were long ago?

**You already know.**

[Rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) are slowly broken into pieces at the surface (*[weathering](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-weathering)*); water, wind and ice carry the pieces away (*[erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion)*) and lay them down where they slow down and stop (*[deposition](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-deposition)*): the heaviest first, the fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) only where the water is almost still ([Chapter 10](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#ch-g5-water-shapes-the-land)). A *[sedimentary rock](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-rock-families)* forms at the surface from grains of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), or from shells, laid down in layers and hardened: sandstone, shale, limestone. The [rock cycle](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-rock-cycle) turns the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) of each family, slowly, into the others ([Chapter 15](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#ch-g6-rocks-and-minerals)).

## 21.1 Sediments settle in layers

Wherever water, wind or ice drops what it was carrying, a loose deposit builds up: [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) on a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) bed, [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) on the bottom of a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) on a beach or in a dune, shells on the floor of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

**Definition 21.1 (Sediment).**

A *sediment* is loose material laid down by water, wind or ice: [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), or the shells and skeletons of living things. It collects at the bottom of [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) and [seas](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), and on land where the wind or the ice drops it.

[Sediments](#def-g7-sediments-and-fossils-sediment) settle in a particular order. When muddy water slows down, the largest and heaviest grains sink first and the smallest and lightest last. Each time the water changes — a [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) that brings [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), then calm weeks when only fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) reaches the bottom — the grains that settle change too, and a new layer starts. Little by little, the bottom of a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or of a [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) is covered with a pile of layers, the oldest at the bottom of the pile.

![Sediment settling in a lake (a drawing, not to scale). After each flood a layer of sand settles, then fine mud in the calm weeks that follow: layer after layer builds up on the bottom.](https://one-course.com/images/onecourse/chapters/earth-1/g7-sediments-and-fossils/fig-f97056b42977.svg)

*[Sediment](#def-g7-sediments-and-fossils-sediment) settling in a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) (a drawing, not to scale). After each [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) a layer of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settles, then fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) in the calm weeks that follow: layer after layer builds up on the bottom.*

**Definition 21.2 (Sedimentary layer).**

A *sedimentary layer*, or *stratum* (plural *strata*), is a sheet of [sediment](#def-g7-sediments-and-fossils-sediment), or of the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) it became, laid down at one time. It has a base and a top, and it differs from the layers above and below it in the size of its grains or in what it is made of.

[Sediments](#def-g7-sediments-and-fossils-sediment) settle from water or air, so they are laid down flat, like the pages of a book lying on a table. When layers are found tilted or folded, something moved them after they formed. Layering is the clearest mark of a [sedimentary rock](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-rock-families): you saw it in the walls of the Grand Canyon.

**In the lab — Layers in a jar (in class, with the teacher).**

Put a handful of [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), two handfuls of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and a few spoonfuls of soil in a tall glass jar; fill it with water, close it tightly and shake it. Set it down and watch for a few minutes: the [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) reaches the bottom at once, the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) a little later, and the fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) keeps the water cloudy for hours. The next day, add a little more soil and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), shake gently, and let it settle again. How many layers can you count? Which grains are at the bottom of each one?

## 21.2 From sediment to rock

A layer does not stay on top for long. Other layers settle over it, then others, and it is buried deeper and deeper. Under the weight of the layers above, the grains are pressed closer together and the water between them is squeezed out. Water seeping through the pile leaves [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) between the grains, a natural cement that sticks them together. Over a long time, with more layers piling up and slow chemical changes, the loose [sediment](#def-g7-sediments-and-fossils-sediment) at the bottom of the pile becomes [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

Each kind of [sediment](#def-g7-sediments-and-fossils-sediment) gives its own [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

| **[sediment](#def-g7-sediments-and-fossils-sediment)** | **the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) it becomes** |
| --- | --- |
| [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) and [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) | conglomerate |
| [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) | sandstone |
| [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) | mudstone, or shale when it splits into thin sheets |
| shells and skeletons | limestone |
| plant remains, buried in a swamp | coal |

![From sand to sandstone (a drawing of the grains, much enlarged). Left: loose grains with water between them. Middle: buried under new layers, the grains are pressed together. Right: minerals left by the water stick the grains together.](https://one-course.com/images/onecourse/chapters/earth-1/g7-sediments-and-fossils/fig-df381ab51cca.svg)

*From [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) to sandstone (a drawing of the grains, much enlarged). Left: loose grains with water between them. Middle: buried under new layers, the grains are pressed together. Right: [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) left by the water stick the grains together.*

Most of the [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) you can see at the surface of the Earth are sedimentary. The pile of layers they form is the history book of the place: each layer is a page, and the oldest pages are at the bottom.

## 21.3 How a fossil forms

**Definition 21.3 (Fossil).**

A *fossil* is the remains, or the trace, of a living thing of long ago, kept in [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). Some fossils are the hard parts of the body — a shell, a bone, a tooth, wood — turned to stone. Others keep only a shape: a *mould* is the hollow imprint left in the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) where a shell or a bone has dissolved; a *cast* is the copy that forms when a mould is filled with new [mineral](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) matter or [sediment](#def-g7-sediments-and-fossils-sediment). A *trace fossil* is a sign of what an animal did — a footprint, a burrow, a trail — and not a part of its body.

A [fossil](#def-g7-sediments-and-fossils-fossil) starts its life as a dead animal or plant on the bottom of a [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) or a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). If it stays uncovered, bacteria break it down and the water slowly dissolves what is left. It has a chance only if it is buried quickly, under [sediment](#def-g7-sediments-and-fossils-sediment) that settles over it and shuts it off. Then, as the [sediment](#def-g7-sediments-and-fossils-sediment) turns to [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), water seeping through the layers brings [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) that fill every tiny hole of the shell or the bone: it turns to stone. Long afterwards, the layers above may be worn away, and the [fossil](#def-g7-sediments-and-fossils-fossil) appears at the surface.

![How a fossil forms (a drawing, not to scale): a shell is buried quickly under mud; buried deeper under new layers, it turns to stone; long afterwards, erosion wears away the layers above and the fossil appears at the surface.](https://one-course.com/images/onecourse/chapters/earth-1/g7-sediments-and-fossils/fig-2d533286a421.svg)

*How a [fossil](#def-g7-sediments-and-fossils-fossil) forms (a drawing, not to scale): a shell is buried quickly under [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand); buried deeper under new layers, it turns to stone; long afterwards, [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) wears away the layers above and the [fossil](#def-g7-sediments-and-fossils-fossil) appears at the surface.*

Most living things leave no [fossil](#def-g7-sediments-and-fossils-fossil) at all. Soft bodies rot before they can be buried; even shells and bones break or dissolve if they lie uncovered for long. The best place to become a [fossil](#def-g7-sediments-and-fossils-fossil) is the bottom of a quiet [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) or [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), where fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settles over the remains and protects them. That is why the [fossils](#def-g7-sediments-and-fossils-fossil) found in [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) are mostly shells, and the tiny shells and remains of microscopic plants and animals; dinosaurs, the most famous [fossils](#def-g7-sediments-and-fossils-fossil), are only a very small part of them.

![Two fossils of animals with hard shells. Photographs: James St. John (CC BY 2.0, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-sediments-and-fossils/img-e84bd86eb23b.jpg)

![Two fossils of animals with hard shells. Photographs: James St. John (CC BY 2.0, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-sediments-and-fossils/img-b6c9623f7756.jpg)

*An ammonite, an extinct animal with a coiled shell, a relative of the squids and octopuses, $5.7\,\mathrm{cm}$ across, from the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of Yorkshire (England).*

*A trilobite, an extinct [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) animal whose body has three lobes, about $2.6\,\mathrm{cm}$ across, from Utah (United States).*

*Two [fossils](#def-g7-sediments-and-fossils-fossil) of animals with hard shells. Photographs: James St. John (CC BY 2.0, via Wikimedia Commons).*

Sometimes the shell itself does not last. Buried in [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), it is slowly dissolved by the water seeping through the grains, and only a hollow remains, with the shape of the shell printed on its walls: a [mould](#def-g7-sediments-and-fossils-fossil). If [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) later settle in the hollow and fill it, they make a copy of the shell: a [cast](#def-g7-sediments-and-fossils-fossil).

![A mould and a cast (a drawing, cut through the rock). The mould is the hollow, with the ribs of the shell printed on its wall; the cast is a stone copy of the shell, made of the minerals that filled it.](https://one-course.com/images/onecourse/chapters/earth-1/g7-sediments-and-fossils/fig-d96e459871d4.svg)

*A [mould](#def-g7-sediments-and-fossils-fossil) and a [cast](#def-g7-sediments-and-fossils-fossil) (a drawing, cut through the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock)). The [mould](#def-g7-sediments-and-fossils-fossil) is the hollow, with the ribs of the shell printed on its wall; the [cast](#def-g7-sediments-and-fossils-fossil) is a stone copy of the shell, made of the [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) that filled it.*

Animals also leave [fossils](#def-g7-sediments-and-fossils-fossil) of what they did. An animal walking on the soft, wet [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) of a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) bank or a shore leaves footprints. Most are washed away by the next wave or the next [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood). But if [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) covers them before they are erased, the footprint is filled, the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) later turns to sandstone, and when the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) splits along the layer, it shows the footprint and, on the other side, its [cast](#def-g7-sediments-and-fossils-fossil).

![Dinosaur footprints at Dinosaur Ridge, near Morrison, Colorado (United States): dozens of tracks left in soft ground, now a tilted surface of rock. Most were made by plant-eating dinosaurs, a few by meat-eaters with slender toes. Photograph: Dexter Perkins (CC0, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-sediments-and-fossils/img-3719cd1727e0.jpg)

*Dinosaur footprints at Dinosaur Ridge, near Morrison, Colorado (United States): dozens of tracks left in soft ground, now a tilted surface of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). Most were made by plant-eating dinosaurs, a few by meat-eaters with slender toes. Photograph: Dexter Perkins (CC0, via Wikimedia Commons).*

Biology uses [fossils](#def-g7-sediments-and-fossils-fossil) to follow the long history of living things. In this book, [fossils](#def-g7-sediments-and-fossils-fossil) are clues of another kind: they tell us what the place was like when the animal lived there.

## 21.4 Layers and fossils, a record of the past

Each layer keeps the marks of the place where it was laid down. Reefs of coral grow today in warm, clear, shallow [seas](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean); ripple marks form on [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) in shallow water, at the edge of a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or a [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean); coal formed from the plants of ancient forests and marshes; brachiopods, crinoids and trilobites lived only in the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). When the same clues are found in an old layer, the place was probably like that when the layer formed.

| **clue in the layer** | **the place when the layer formed** |
| --- | --- |
| corals | a warm, shallow, clear [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) |
| shells of [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) animals (brachiopods, crinoids, trilobites) | a [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) |
| ripple marks on the surface of a layer | shallow water, at the edge of a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or a [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) |
| footprints | soft, wet [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) or [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), often on a shore or a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) bank |
| coal | a swamp forest or a marsh |
| leaves and dragonfly wings | land, with plants growing |

**Method 21.4 (Reading a layer).**

1. Look at the grains: coarse grains settle where the water moves fast, fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) only where it is calm.
2. Look for [fossils](#def-g7-sediments-and-fossils-fossil) and marks (corals, shells, footprints, ripple marks, plant remains, coal) and use the table above.
3. Put the clues together into one picture of the place: land or [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) , shallow or deep, calm or moving water.

![Ripple marks on a slab of sandstone in Capitol Reef National Park, Utah (United States): the sand was once rippled by moving water, like the sand of a beach or a stream bed today. Photograph: James St. John (CC BY 2.0, via Wikimedia Commons).](https://one-course.com/images/onecourse/chapters/earth-1/g7-sediments-and-fossils/img-e65fdfc45031.jpg)

*Ripple marks on a slab of sandstone in Capitol Reef National Park, Utah (United States): the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) was once rippled by moving water, like the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) of a beach or a stream bed today. Photograph: James St. John (CC BY 2.0, via Wikimedia Commons).*

**Example 21.5 (The top of the Grand Canyon).**

The Kaibab Limestone, the top layer of the Grand Canyon, is full of brachiopod shells and the stem pieces of crinoids, both [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) animals. It formed 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). Today the same layer lies up to about $2700\,\mathrm{m}$ ($9000$ feet) above [sea level](https://one-course.com/books/earth/1/en/chapter/9-the-physical-map-of-the-world#def-g4-world-physical-map-altitude): since it formed, the whole region has been raised.

A pile of layers can be drawn as a *column*: the layers stacked in the order they formed, with their thickness to scale and their clues noted at the side. The column of the Varden Hills, the invented region of the exercises of this series, is below.

![The column of the Varden Hills, the invented region of this series: the layers in the order they formed, the first at the bottom, with their thicknesses to scale. The fossils and marks noted at the side are exercise data.](https://one-course.com/images/onecourse/chapters/earth-1/g7-sediments-and-fossils/fig-1e69d5b1306b.svg)

*The column of the Varden Hills, the invented region of this series: the layers in the order they formed, the first at the bottom, with their thicknesses to scale. The [fossils](#def-g7-sediments-and-fossils-fossil) and marks noted at the side are exercise data.*

![A warm, shallow sea floor of long ago, as it might have looked while shells and coral pieces were settling into a future limestone (AI-generated illustration of a generic scene).](https://one-course.com/images/onecourse/chapters/earth-1/g7-sediments-and-fossils/img-74d3b69e4736.jpg)

*A warm, shallow [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor of long ago, as it might have looked while shells and coral pieces were settling into a future limestone (AI-generated illustration of a generic scene).*

**Remark 21.6 (Look, photograph, leave it).**

In many protected places, taking a [fossil](#def-g7-sediments-and-fossils-fossil) away is forbidden: in the Grand Canyon, for example, it is against the law to dig up, move or take any [fossil](#def-g7-sediments-and-fossils-fossil). A visitor who finds one takes a photograph or makes a drawing, leaves it where it is for others to see and for scientists to study, and tells a ranger.

**In the field — Sediments being laid down today.**

On a beach, a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) shore or the bank of a stream, with an adult, look at the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) under shallow water: the moving water shapes it into ripples, like the ones kept in the old sandstone. Leave a footprint in the wet [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) near the water’s edge and watch what the next waves do to it. After a rainy day, look at a puddle that has dried out: a thin skin of fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) often covers the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) at its bottom — the finest grains settled last, when the water was still. Draw what you see, and note where and when.

## 21.5 Exercises

**Exercise 21.1 ★.**

Match each word with its meaning: [sediment](#def-g7-sediments-and-fossils-sediment); [stratum](#def-g7-sediments-and-fossils-layer); [fossil](#def-g7-sediments-and-fossils-fossil); [mould](#def-g7-sediments-and-fossils-fossil); [cast](#def-g7-sediments-and-fossils-fossil); [trace fossil](#def-g7-sediments-and-fossils-fossil). (a) A hollow left in a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) where a shell has dissolved. (b) A footprint or a burrow kept in a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). (c) Loose material laid down by water, wind or ice. (d) A stone copy made when a hollow fills with [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral). (e) A sheet of [sediment](#def-g7-sediments-and-fossils-sediment) laid down at one time. (f) The remains or trace of a living thing of long ago, kept in [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock).

**Solution of Exercise 21.1.**

(a) [mould](#def-g7-sediments-and-fossils-fossil); (b) [trace fossil](#def-g7-sediments-and-fossils-fossil); (c) [sediment](#def-g7-sediments-and-fossils-sediment); (d) [cast](#def-g7-sediments-and-fossils-fossil); (e) [stratum](#def-g7-sediments-and-fossils-layer); (f) [fossil](#def-g7-sediments-and-fossils-fossil).

**Exercise 21.2 ★.**

Put these steps in order, to tell how a shell becomes a [fossil](#def-g7-sediments-and-fossils-fossil) at the surface: (a) [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) wears away the layers above it; (b) the animal dies on the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor; (c) new layers pile up and the shell turns to stone; (d) [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) covers the shell quickly.

**Solution of Exercise 21.2.**

(b) the animal dies; (d) [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) covers the shell quickly; (c) new layers pile up and the shell turns to stone; (a) [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion) wears away the layers above it.

**Exercise 21.3 ★.**

Use the table of clues. Where did each layer form? (a) A limestone full of corals. (b) A sandstone whose surface shows ripple marks. (c) A dark layer of coal. (d) A shale with leaf prints and the wing of a dragonfly.

**Solution of Exercise 21.3.**

(a) A warm, shallow, clear [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). (b) Shallow water, at the edge of a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or a [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). (c) A swamp forest or a marsh. (d) Land, with plants growing (and dragonflies flying), near water where the [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settled.

**Exercise 21.4 ★.**

True or false? Correct the false ones. (a) Every animal that dies becomes a [fossil](#def-g7-sediments-and-fossils-fossil) one day. (b) A footprint can be a [fossil](#def-g7-sediments-and-fossils-fossil). (c) When muddy water slows down, the fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settles first. (d) [Sediments](#def-g7-sediments-and-fossils-sediment) are laid down flat.

**Solution of Exercise 21.4.**

(a) False: most living things leave no [fossil](#def-g7-sediments-and-fossils-fossil); they rot or dissolve before they are buried. (b) True: it is a [trace fossil](#def-g7-sediments-and-fossils-fossil). (c) False: the largest and heaviest grains settle first, the fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) last. (d) True.

**Exercise 21.5 ★★.**

Use the column of the Varden Hills. (a) Give the thickness of the sandstone, the limestone and the shale. (b) What is their total thickness? (c) What percentage of this total is limestone? Round to the nearest whole number. (d) Why does the column give no thickness for the chalk?

**Solution of Exercise 21.5.**

(a) Sandstone $120\,\mathrm{m}$, limestone $180\,\mathrm{m}$, shale $100\,\mathrm{m}$. (b) $120 + 180 + 100 = 400\,\mathrm{m}$. (c) $180 \div 400 = 0.45$: **45 %**. (d) Its top has been worn away by [erosion](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-erosion): what is left is not the whole layer, and how much is left changes from place to place.

**Exercise 21.6 ★★.**

A pupil shakes a jar of water with [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), and lets it settle. (a) Draw the jar the next day, and label the layers. (b) Why is the water still cloudy after one hour? (c) She adds [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) again and lets it settle. What does she see at the bottom now? (d) Which layer of the jar formed first?

**Solution of Exercise 21.6.**

(a) From the bottom up: [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), then [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), then [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) on top, under the clear water. (b) The tiny [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) grains sink very slowly: they are still falling after an hour. (c) A second set of layers on top of the first: [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), then [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) again. (d) The [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) at the very bottom.

**Exercise 21.7 ★★.**

Which of these has the best chance of becoming a [fossil](#def-g7-sediments-and-fossils-fossil), and which the least? Explain. (a) A jellyfish washed onto a rocky shore. (b) A snail shell buried in the [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) of a calm [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). (c) A bone lying for years on a dry [plain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). (d) A clam shell covered by [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) in a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood).

**Solution of Exercise 21.7.**

Best: (b), a hard shell, buried quickly under fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) in calm water. Then (d): the shell is hard and buried, but a [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) can also move and break it. Least: (a), a soft body with no hard parts, on [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) where no [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settles to cover it. (c) is not much better: the bone stays uncovered and is broken down and worn away.

**Exercise 21.8 ★★.**

A shell is buried in [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) that becomes sandstone. 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) dissolves the shell completely. (a) What is left in the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock)? (b) Later, [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral) fill this hollow. What forms? (c) Is the shell itself still there? (d) Why are [moulds](#def-g7-sediments-and-fossils-fossil) and [casts](#def-g7-sediments-and-fossils-fossil) found more often in sandstone than in shale? (Think about how easily water passes between the grains.)

**Solution of Exercise 21.8.**

(a) A hollow with the shape of the shell printed on its walls: a [mould](#def-g7-sediments-and-fossils-fossil). (b) A [cast](#def-g7-sediments-and-fossils-fossil), a stone copy of the shell. (c) No: the shell has dissolved; the [cast](#def-g7-sediments-and-fossils-fossil) is made of new [minerals](https://one-course.com/books/earth/1/en/chapter/15-rocks-and-minerals#def-g6-rocks-and-minerals-mineral). (d) Water passes easily between the large grains of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), so it can reach and dissolve the shell; in fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) it passes much less easily.

**Exercise 21.9 ★★.**

The Grand Canyon is on average $1.6\,\mathrm{km}$ deep. (a) Write this depth in metres. (b) Suppose the canyon walls were made of layers $80\,\mathrm{m}$ thick each: how many layers would fit from the rim to the bottom? (c) A layer of $160\,\mathrm{m}$ is what percentage of the depth?

**Solution of Exercise 21.9.**

(a) $1.6 \times 1000 = 1600\,\mathrm{m}$. (b) $1600 \div 80 = 20$ layers. (c) $160 \div 1600 = 0.1$: **10 %**.

**Exercise 21.10 ★★.**

Dinosaur footprints are found in a sandstone. (a) Were the dinosaurs walking on hard [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock)? Explain. (b) Why do most footprints left on a muddy shore disappear? (c) What had to happen soon after the dinosaurs passed for the footprints to be kept? (d) Is a footprint a body [fossil](#def-g7-sediments-and-fossils-fossil) or a [trace fossil](#def-g7-sediments-and-fossils-fossil)?

**Solution of Exercise 21.10.**

(a) No: a foot leaves a print only in soft ground, wet [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) or [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand); the ground became [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) later. (b) The next wave or [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) washes them away. (c) New [sediment](#def-g7-sediments-and-fossils-sediment) had to cover them and fill them quickly, before they were erased. (d) A [trace fossil](#def-g7-sediments-and-fossils-fossil): it shows what the animal did, not a part of its body.

**Exercise 21.11 ★★★.**

In a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), every [flood](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) leaves a layer of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) about $2\,\mathrm{cm}$ thick, and the calm weeks after it a layer of [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) about $5\,\mathrm{mm}$ thick (exercise data). (a) How thick, in millimetres, is one pair of layers? (b) A [core](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) taken from the [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) bottom is $1\,\mathrm{m}$ long and made only of such pairs. How many [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood) does it record? (c) Near the [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) the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) layers are thicker, far out they are thinner. Explain why. (d) Why is there more [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) than [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) far from the [mouth](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-source-mouth)?

**Solution of Exercise 21.11.**

(a) $20 + 5 = 25\,\mathrm{mm}$. (b) $1000 \div 25 = 40$ pairs: the [core](https://one-course.com/books/earth/1/en/chapter/16-inside-the-earth#def-g6-inside-the-earth-layers) records **40 [floods](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-flood)**. (c) The [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) is heavy: it settles soon after the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) enters the [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), near the [mouth](https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land#def-g5-water-shapes-the-land-source-mouth); only a little of it travels far. (d) Far out, only the fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) is still in the water: it settles there slowly, when the water is calm.

**Exercise 21.12 ★★★.**

The [fossils](#def-g7-sediments-and-fossils-fossil) found in [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) are mostly shells and tiny remains; [fossils](#def-g7-sediments-and-fossils-fossil) of soft animals are rare, and dinosaurs are only a very small part of all [fossils](#def-g7-sediments-and-fossils-fossil). (a) Explain why shells are so common. (b) Explain why soft animals are rare. (c) A scientist wants to know all the animals that lived in an ancient [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). Can the [fossils](#def-g7-sediments-and-fossils-fossil) give the full list? Explain.

**Solution of Exercise 21.12.**

(a) Shells are hard: they last long enough to be buried, and many animals with shells lived on the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) floor, where [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settles over them. (b) Soft bodies rot before they can be buried. (c) No: soft animals almost never leave [fossils](#def-g7-sediments-and-fossils-fossil), so the list from [fossils](#def-g7-sediments-and-fossils-fossil) is incomplete; it shows mostly the animals with shells or bones.

## 21.6 Problem: The column of the Varden Hills

**Problem 21.1.**

Weekend problem — the layers of the Varden Hills are read one by one, their fossils tell where they formed, and the time needed to pile up the limestone is worked out

Use the column of the Varden Hills in this chapter. Its sandstone shows ripple marks and footprints; its limestone holds corals and shells; its shale is made of very fine grains, with ammonites; the chalk at the top has been partly worn away.

**Part I — Reading the layers.**

1. List the four layers, from the first one laid down to the last.
2. Give the thickness of the sandstone, the limestone and the shale, and their total.
3. What percentage of this total is limestone?
4. Why can nobody say how thick the chalk was when it formed?

**Part II — The fossils in them.**

5. Where did the sandstone form? Give two clues.
6. Where did the limestone form? Give the clue.
7. The shale is made of very fine grains. Was the water calm or moving when it was laid down? Explain.
8. Tell the story of the place in three sentences, from the sandstone to the shale.

**Part III — How long it took.**

9. Write the thickness of the limestone in centimetres.
10. Suppose the sandstone piled up by $1\,\mathrm{cm}$ every 20 years. How many years did its $120\,\mathrm{m}$ take?
11. Suppose the limestone piled up by $1\,\mathrm{cm}$ every 100 years. How many years did $1\,\mathrm{m}$ of limestone take?
12. How many years did the $180\,\mathrm{m}$ of limestone take? How many times longer is that than the sandstone?

**Solution of Problem 21.1.**

**1.** Sandstone, limestone, shale, chalk (the first at the bottom of the column).

**2.** $120\,\mathrm{m}$, $180\,\mathrm{m}$ and $100\,\mathrm{m}$; total $400\,\mathrm{m}$.

**3.** $180 \div 400 = 0.45$: **45 %**.

**4.** Its top has been worn away: the upper part of the layer is gone, and nobody can measure what is no longer there.

**5.** In shallow water at the edge of a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or a [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean): ripple marks form on [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) in shallow water, and footprints are left in soft, wet [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) or [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), often on a shore.

**6.** In a warm, shallow, clear [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean): the corals.

**7.** Calm: fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settles only where the water is almost still.

**8.** First the place was a shore, with shallow water and soft [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) where animals walked. Then the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) covered it: a warm, shallow [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) where corals grew and shells piled up. Later the water became calmer, perhaps deeper, and only fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settled, with the shells of ammonites, animals related to squids and octopuses.

**9.** $180 \times 100 = 18\,000$ cm.

**10.** $120~\text{m} = 12\,000$ cm, and $12\,000 \times 20 =
240\,000$ years.

**11.** $1~\text{m} = 100$ cm, and $100 \times 100 = 10\,000$ years.

**12.** $180 \times 10\,000 = 1\,800\,000$: about **1 800 000 years**, almost two million years. That is $1\,800\,000 \div 240\,000 = 7.5$ times longer than the sandstone.