How Do Rocks Tell Time?: Going Further 6–8

Layers in a pile

Think of a pile of old newspapers in the corner of a room. Each day’s paper goes on top. Without reading a single date, you know that the paper at the bottom is the oldest and the one on top is the newest. Unless someone has shuffled the pile, the order of the papers is the order of the days.

Rock layers form in the same way. You learned in Grade 7 that sedimentary rock is made from bits of sand, mud and shells (sediment) that settle in layers, usually under water, and are slowly pressed and cemented into rock. And in Grade 9 you will read that fossils are found in these rocks Grade 9: Evidence from fossils. A river carries mud into a lake or the sea; the mud settles on the bottom; next year more settles on top of it. Each layer of rock is called a stratum; two or more are strata.

From this, two rules follow.

a.Superposition: in layers that have not been turned over, each layer is younger than the one below it and older than the one above. Lower is older.
b.Original horizontality: sediment settles in flat, level layers, because gravity spreads it out. So if you see layers that are tilted or bent, they were moved after they were made.

Figure 21.2 Five layers, numbered from the oldest. Left: as they were laid down, flat, each on top of the one before. Right: the same layers after being squeezed into an arch. They are bent, but layer 1 is still underneath.

A story from science

The shark’s teeth that became a rule

In 1666 fishermen caught a huge shark near Livorno in Italy, and its head was sent to Nicolas Steno, a young Danish doctor working in Florence. Looking at its teeth, Steno saw that they were exactly like the “tongue stones” people dug out of rocks and sold as charms. They were not stones that had grown in the rock; they were the teeth of sharks that had died long ago.

But how could a tooth get inside solid rock? Steno reasoned that the rock had once been soft mud settling in water, which buried the teeth, and later hardened. In 1669 he wrote down the rules that follow from this: that each layer was laid down on the one below it, that layers start out flat, that a layer spreads out sideways until it thins out, and that anything cutting across a layer came after it. More than 350 years later, geologists still use all four.

These rules give a relative age: whether one rock is older or younger than another. They don’t give an absolute age, a number of years. Your pile of newspapers tells you that the bottom one is older than the top one, but not by itself whether that bottom paper is from last week or from 1990. Strata are the same: on their own, they tell the order of events, not their dates.

A cliff shows four flat layers: limestone at the bottom, then shale, then sandstone, then a thin layer of ash on top. Which layer is the oldest?

Pick an answer to see why.

A huge canyon with steep walls made of many horizontal bands of red, brown and cream rock, with green slopes between the cliffs — **Figure 21.3** The Grand Canyon, in the USA, is about 1.8 km deep. The Colorado River has cut down through flat layers like a knife through a cake, and below them into much older, crumpled rock. The layer at the rim formed about 270 million years ago; the rock at the bottom of the inner gorge is about 1.7 billion years old. Photo: Tuxyso, CC BY-SA 3.0

Clues that cut across

Lower is older is only the first rule. Rocks hold other clues to the order of events.

Think of a wall. If a crack runs through the bricks, the crack must have come after the wall was built: you can’t crack a wall that isn’t there yet. And if a poster is pasted across the crack, unbroken, the poster came after the crack. Rocks work the same way.

Cross-cutting: anything that cuts across a layer is younger than that layer. That includes a crack filled with once-molten rock (a dyke), a break along which the rock has slipped (a fault), and a valley cut by a river.
Pieces inside: if a rock holds broken pieces of another rock, those pieces are older than the rock around them. They had to exist before they could be broken off and buried.
Gaps: sometimes layers were lifted up, worn away by rain and rivers, then sunk again and buried under new layers. The worn surface between them is an unconformity, a gap in the record. The time when rock was being worn away, instead of laid down, left no layers at all.

Figure 21.4 Four more clues to the order of events, each in a cliff face seen from the side.

Looking down at a flat, pinkish-brown rock surface covered in a network of old mud cracks, with a straight band of dark gray rock running across it — **Figure 21.5** A dyke on the shore of Orkney, Scotland. The pale sandstone, with its old mud cracks, formed in a lake about 385 million years ago. The straight dark band is once-molten rock that forced its way up a crack in it more than 100 million years later. The dyke cuts the sandstone, so it must be younger. Photo: Geopersona, CC BY-SA 3.0

A desert cliff of flat, buff and brown rock layers, cut by a pale, upright band of rock that runs straight up through them like a wall — **Figure 21.6** A dyke seen from the side, in Makhtesh Ramon in Israel’s Negev desert. Once-molten rock filled an upright crack and cut straight across the flat layers. It is younger than every layer it cuts. Photo: Godot13, CC BY-SA 4.0

A story from science

A gap of 65 million years

In 1788 the Scottish geologist James Hutton sailed along the coast with two friends, the mathematician John Playfair and the geologist James Hall, looking for one particular thing. At Siccar Point they found it. At the bottom, gray layers of rock stood almost upright. Above them, cutting across their tops, lay gently sloping layers of red sandstone.

Hutton read the story. The gray layers had been laid down flat under the sea. Then they had been pushed up on end, raised above the sea, and worn down to a flat surface. Then the land had sunk again, and the red sand had been laid down over the worn edges. Each step must have taken an enormously long time. Playfair later wrote: “The mind seemed to grow giddy by looking so far into the abyss of time.” Today we know the gray layers are about 440 million years old and the red ones about 375 million: the worn surface between them stands for some 65 million years that left no rock behind.

A rocky shore by the sea: gray layers of rock stand almost upright in the front, and above them, in the middle, sloping layers of red sandstone lie across their tops — **Figure 21.7** Hutton’s unconformity at Siccar Point, Scotland: upright gray layers below, sloping red sandstone across their worn tops. Photo: Lysippos, CC BY 3.0

Slide through time and watch a cliff’s rocks being made, one step at a time. Notice where each new rock goes, and what cuts what.

Time: step 1 of 8

Sand in a shallow sea. Sand settles on the floor of a shallow sea, burying dead trilobites. In time it hardens into sandstone: layer 1.

A dark dyke cuts up through layers 1, 2 and 3 of a cliff, but layer 4, on top, lies across it unbroken. What can you say about the dyke?

Pick an answer to see why.

Now you know enough to read a cliff. Geologists don’t just state the order: for each step they give the evidence, the clue in the rock that shows it.

Worked example

Reading a cliff

A road cut shows, from the bottom: a gray shale (layer 1), a sandstone (layer 2) and a limestone (layer 3). A fault breaks all three layers. Across the top lies a layer of gravel (layer 4), unbroken. A dyke cuts through all four layers. Put the events in order, oldest first, and give the evidence for each step.

Answer

Shale, sandstone, limestone (layers 1, 2, 3), in that order. Evidence: each lies on top of the one before (superposition), and they are flat, so they haven’t been turned over.
The fault. Evidence: it breaks layers 1–3, so it is younger than them (cross-cutting). It doesn’t break layer 4, so it is older than layer 4.
The gravel (layer 4). Evidence: it lies across the top of the fault, unbroken.
The dyke. Evidence: it cuts all four layers, including the gravel, so it is the youngest.

Three cliffs, each with its rocks and features lettered. Tap the letters in order, oldest first, then check. If your order breaks a clue, you’ll be told which.

Your order, oldest first: ·······

Tap the letters in order, oldest first, then Check.

Matching layers far apart

One cliff tells only part of the story. Its layers start at some time and stop at another, with gaps where rock was worn away. To read the Earth’s whole history, geologists need to fit together the pages from many places. Matching layers from one place to another is called correlation.

Sometimes you can simply follow a layer along a valley wall. But layers in two mountains, a hundred kilometers apart, can’t be followed: valleys, rivers and plains lie between. How do you know which layer here matches which layer there?

Not by the kind of rock. Sandstone forms whenever sand settles, and that has happened at countless times in the Earth’s history. Two sandstones can be hundreds of millions of years apart. And at one moment, the same sea can lay down sand near the shore and lime mud farther out, so one time can be two different rocks.

By fossils. In the 1790s the English surveyor William Smith, laying out a canal for coal barges, noticed that each layer held its own particular fossils, and that the fossils always came in the same order, from the bottom up, wherever he went. This is the principle of faunal succession: fossils follow one another in the same order everywhere. Each kind of living thing appears, lives for a while, and dies out. Once a species has died out it never comes back, so its fossils mark one stretch of time and no other.

The best fossils for matching layers, called index fossils, come from living things that:

lived for only a short time, before changing into something different or dying out, so they pin down a short stretch of time;
were widespread, often floating or swimming in the seas, so they are found in many places;
were common and had hard parts, so they are easy to find;
are easy to recognize.

Ammonoids and graptolites (tiny floating animals that lived in colonies) are among the best. Some ammonoid species lasted less than a million years, so they can match layers to within that time. Some key layers work too: when a volcano erupts, its ash falls over a huge area within days, so a layer of ash marks the same moment everywhere it falls.

Two cliffs, far apart. Join each layer in one cliff to the layer from the same time in the other. Then check, and see what the two tell together.

Tap a layer in one cliff, then the layer from the same time in the other.

A story from science

The map that changed the world

William Smith was born in 1769 to a village blacksmith in England, and taught himself surveying. Working on the Somerset Coal Canal in the 1790s, he went down mines and walked the cuttings, and kept noting which fossils came in which layer. He found he could name a layer from its fossils alone, and so predict what lay underground: where coal might be, and where it would not.

He spent years traveling, mostly at his own cost, to map the rock layers of the whole country. In 1815 he published the result, a hand-colored map in 15 sheets, about 2.5 m tall when joined: the first geological map of a whole country. Others copied it and sold cheaper versions, and in 1819 Smith was put in prison for debt. Only in 1831 did the Geological Society of London honor him with its first Wollaston Medal, calling him the father of English geology.

An old map of England and Wales colored in many bands of pink, green, yellow, brown and gray, each color a different layer of rock — **Figure 21.8** William Smith’s map of 1815. Each color is a different layer of rock where it reaches the surface, the same layer from one end of the country to the other. Photo: William Smith (1769-1839), Public domain

Fossils can do more than match layers. Since we know when each kind of living thing was around, a rock that holds two kinds must have formed when both were alive: where their time ranges overlap.

Figure 21.9 When five groups of living things whose fossils geologists use lived, in millions of years ago (rounded). A rock with both trilobites and ammonoids must have formed in the window where their bars overlap.

Did you know?

One thin layer, all over the world

In 1980 the American father and son Luis and Walter Alvarez reported something strange about a layer of clay, a centimeter or so thick, in the rocks near Gubbio, in Italy. Below it were the fossils of tiny sea creatures of the age of dinosaurs; above it, almost none of them. And the clay held about 30 times more of the rare metal iridium than the rock around it. Iridium is rare in the Earth’s crust but much commoner in asteroids. The same iridium-rich clay has since been found in rocks across the world, always at the same place in the order of fossils: above the last dinosaurs and ammonoids, below the layers of the age of mammals that followed. It is the dust of the asteroid that hit Mexico 66 million years ago, settled all over the Earth. One thin layer marks one moment, everywhere.

A slab of dark, layered rock in a museum, crossed by a thin pale band, with a label pointing to it — **Figure 21.10** The thin pale band in this rock from Alberta, Canada, is the layer left by the asteroid that ended the age of the dinosaurs. Dinosaur bones are found below it, never above. (The label gives the older figure, 65 million years ago; it is now dated at 66.) Photo: Mike Beauregard from Nunavut, Canada, CC BY 2.0

Two cliffs 200 km apart each have a layer of red sandstone. What is the best evidence that they formed at the same time?

Pick an answer to see why.

How long did a layer take?

Strata give the order of events. Can they give the time as well? People tried, by asking how fast sediment piles up.

In some lakes fed by melting glaciers, each year leaves a pair of bands: a pale, coarse band of silt from the summer melt, and a thin, dark band of fine clay that settles slowly under the winter ice. One pair of bands, one year, is a varve. Count the varves, as you would count the rings of a tree, and you count the years.

A low bank of rock and earth with thin, wavy pale and dark stripes, and the red shaft and black tip of a walking pole lying against it — **Figure 21.11** Varves at Sandend Bay, Scotland, laid down in a lake beside the ice during the last Ice Age. Each pale-and-dark pair is one year. (The walking pole gives the scale.) Photo: Anne Burgess, CC BY-SA 2.0

Most layers are not varves. For them, geologists can only estimate how fast the sediment built up, from what is building up today in similar places.

Worked example

How long did a limestone take?

A limestone is 120 m thick. Suppose that lime mud on that sea floor built up at about 2 cm every 1,000 years (counted after being squeezed into rock). Roughly how long did the limestone take to form?

Answer

$\text{time} = \frac{\text{thickness}}{\text{rate}} = \frac{120\ \text{m}}{2\ \text{cm per 1{,}000 years}}$
Change to the same unit: 120 m = 12,000 cm.
$\text{time} = \frac{12\,000\ \text{cm}}{2\ \text{cm}} \times 1000\ \text{years} = 6000 \times 1000\ \text{years} = 6\ \text{million years}$
Roughly 6 million years.

The dates came later. From 1907, scientists learned to read the clocks inside radioactive atoms (How Old Is the Earth? in Going Further 9–12 explains them). Volcanic ash and lava contain crystals that can be dated this way, but sandstone and limestone usually can’t. So geologists date the ash beds and lava flows above and below the fossil layers: a layer lying between ash dated at 70 million years and ash dated at 68 million years formed between those two dates. The order came from the strata; the numbers were pinned onto it.

Putting the Earth’s history in order

Now you can see how the geological time scale you met in Grade 8 was built. It isn’t a list someone made up; it is a summary of the evidence in the rocks.

Geologists in many countries worked out the order of the layers in their own region, by superposition and cross-cutting.
They matched the layers between regions by their fossils, and so built one long column, the order of all the strata known, far longer than any one cliff.
They divided that column where the fossils change most sharply. Many of these breaks are mass extinctions, when many kinds of living things vanish from the rocks at once and new kinds appear above. That is why the eras are named by their life: Paleozoic (“old life”), Mesozoic (“middle life”) and Cenozoic (“new life”). The smaller divisions were often named after the place where their rocks were first studied: the Cambrian after Cambria, an old name for Wales; the Jurassic after the Jura mountains of France and Switzerland; the Permian after Perm in Russia.
Last, radioactive dating of ash and lava put years on the boundaries. The first time scale with numbers was published by the English geologist Arthur Holmes in 1913, and the dates are still being refined.

So the time scale is a way to organize the Earth’s history using the evidence of the rocks. Take any big event: geologists can say what came before it and after it, and, with the dates, when.

Worked example

How do we know the Himalaya rose after the ammonites lived?

Explain, from evidence in the rocks of Nepal, why the Himalaya must be younger than the ammonites in the shaligrams of Mustang.

Answer

Shaligrams hold ammonites, which lived in the sea. The rock around them is sedimentary rock laid down flat on a sea floor, in layers that lie on top of older sea layers.
Those same layers are now folded and raised thousands of meters high in the Himalaya (as in the Kali Gandaki valley). Layers are laid down flat, so the folding and lifting happened after the layers formed, by the rule of original horizontality: whatever bends a layer is younger than the layer.
So the rise of the Himalaya came after the ammonites lived and were buried. (Radioactive dating now puts numbers on it: these ammonites lived roughly 165 to 145 million years ago, and India began pushing into Asia about 50 million years ago.)

Activity

Layers in a jar

You need: a clear glass jar or plastic bottle with a lid, a few handfuls of soil, a handful of sand, a handful of small gravel or grit, water, and a few lentils or small buttons as “fossils”.

Put the gravel, sand and soil in the jar, fill it about three-quarters full with water, close the lid tightly and shake it hard.
Put it down where it won’t be moved and watch for a minute, then leave it until the next day.
Look at the layers from the side and draw them.
Drop a few lentils in, then gently pour in a spoonful of sand mixed with water. Let it settle for another day.
Draw the jar again. Which layer is the oldest? Where are your “fossils”, and which layers are they older and younger than?

What you should see

The gravel settles first, within seconds, at the bottom; then the sand; then the fine soil, which can take hours; tiny clay particles may keep the water cloudy until the next day. One shake makes one layer that is coarse at the bottom and fine at the top, which is one of the clues geologists use to tell which way up a layer was. The new sand you add settles on top of everything, burying the lentils: they are younger than the layers under them and older than the sand on top. You have made superposition happen in a jar.

In short

Sedimentary rock forms in layers, strata, each laid down flat on the one before. In layers not turned over, lower is older (superposition); tilted or folded layers were moved after they formed.
Anything that cuts across layers (a dyke, a fault, a valley) is younger than them; broken pieces inside a rock are older than it; a worn surface, an unconformity, is a gap in the record.
Layers far apart are matched (correlation) by their fossils, not by their kind of rock: fossils follow each other in the same order everywhere (faunal succession), and short-lived, widespread index fossils are best. Ash beds mark one moment over a wide area.
Strata give relative ages, the order of events. Rates of build-up give only rough times; radioactive dating of ash and lava pins years onto the order.
The geological time scale is the order of all the strata, divided where the fossils change most, often at mass extinctions, with dates added later.

Questions

Which of these could a geologist not learn from the strata of one cliff alone?

Pick an answer to see why.

Which would make the best index fossil?

Pick an answer to see why.

a.

Explain the difference between a relative age and an absolute age. Give one example of each for a rock.

Answer

A relative age says whether a rock or event is older or younger than another, without a number of years: “the sandstone is older than the dyke that cuts it”. An absolute age is an age in years: “the ash bed formed 72 million years ago”, measured by radioactive dating. Strata and fossils give relative ages; radioactive clocks give absolute ages.

b.

A cliff shows, from the bottom: a limestone (1), a shale (2), and a sandstone (3). A river valley is cut down into layers 3 and 2, and on the valley floor lies a layer of river gravel (4). A dyke cuts through layers 1, 2 and 3, but the valley has cut away its top. Put all the events in order, oldest first, giving the evidence for each step.

Answer

Limestone, shale, sandstone (1, 2, 3): each lies on the one before (superposition).
The dyke: it cuts layers 1–3, so it is younger than them (cross-cutting).
The valley: it cuts into layers 3 and 2 and has cut away the top of the dyke, so it is younger than the dyke.
The gravel (4): it lies on the valley floor, so it was laid down after the valley was cut. It is lower than layer 3, yet it is the youngest rock in the cliff: superposition compares layers lying on top of one another, not things side by side.

c.

Why are fossils better than the kind of rock for matching layers in two places far apart?

Answer

The same kind of rock, such as sandstone, forms whenever the same conditions occur, and that happens at many different times, so two sandstones can be hundreds of millions of years apart. And at a single time, different places get different rocks (sand near the shore, lime mud farther out). But every species lives for only one stretch of time and, once extinct, never returns. So fossils follow each other in the same order everywhere (faunal succession), and layers holding the same short-lived species must have formed at the same time, whatever the rock.

d.

A geologist finds, in one layer, fossils of ammonoids and of flowering plants (washed into the sea from the land). Use Figure 21.9 to say when the layer formed. Could the layer hold trilobites as well? Why?

Answer

Ammonoids lived from about 409 to 66 million years ago; flowering plants from about 130 million years ago until today. Both existed from 130 to 66 million years ago, so the layer formed then. It can’t hold trilobites: they died out about 252 million years ago, long before the first flowering plants.

e.

A lake bed has a core 1.5 m long, made of varves 2.5 mm thick on average. How many years does the core cover? Why can a geologist be much more confident of this answer than of an age worked out from the thickness of a limestone?

Answer

1.5 m = 1500 mm. Years = 1500 mm ÷ 2.5 mm = 600 years.
Each varve is one year’s summer and winter bands, so counting them counts years directly, like tree rings. For a limestone, there are no yearly layers: the age depends on an estimated rate of build-up, which can change greatly, and on how much the layer has been squeezed; and gaps (unconformities) may hide time that left no rock.

f.

The geological time scale divides the Earth’s history into eras such as the Paleozoic, Mesozoic and Cenozoic. Explain how geologists worked out the order of these eras, and where to put the boundaries between them, long before anyone could date a rock in years.

Answer

In each region they found the order of the strata by superposition (lower layers are older) and cross-cutting (whatever cuts a layer is younger).
They matched strata between regions by their fossils, because each species appears in the same place in the order everywhere (faunal succession). This joined the pieces from many places into one long column, the order of all the strata.
They divided the column where the fossils change most sharply, often at mass extinctions, when many kinds of living things disappear from the rocks at once. The eras are named by their life: “old life”, “middle life”, “new life”.

John Phillips published such a time scale in 1841, with no dates at all. Numbers in years were added only after 1907, from radioactive dating of ash and lava layers above and below the boundaries.

Think about it

Upside down?

In a cliff in the Himalaya, a geologist finds layers full of ammonites like those in Mustang’s shaligrams (about 150 million years old) lying under layers with trilobite fossils. Does this break the principle of faunal succession? What might have happened?

Answer

Trilobites died out about 252 million years ago, a hundred million years before these ammonites lived, so in undisturbed rock the trilobite layers would be under the ammonite layers. (Not every ammonoid is younger than every trilobite: the two overlapped from 409 to 252 million years ago, which is why the question names ammonites of a known age.) Finding them the other way round doesn’t break faunal succession: it tells the geologist that the rocks have been disturbed. Either the stack was folded so far that it is now upside down (as happens in the Himalaya), or older rock was pushed up and over younger rock along a great break. The geologist would look for way-up clues, such as mud cracks or coarse-to-fine grains, to tell which. Fossils can even reveal folds that are hard to see.

Where to go next

How Old Is the Earth? In Going Further 9–12: how the clocks inside radioactive atoms put years on the order the strata give, and date the Earth at 4.6 billion years.
How Does One Rock Become Another? Sandstone and limestone are only one kind of rock. The rock cycle, and the heat and sunlight that drive it.
Do Continents Really Move? The same fossils in rocks in India, Africa and Antarctica, matched as you matched the cliffs here, show that the continents were once joined.