To understand the past, we divide history into different pieces, some big, some small. Human history has been divided into Ages, (e.g. Stone Age, Bronze Age) then smaller periods like Dynasties, then by the reigns of single rulers. Geologists deal with much longer periods of time, but they divide the history of the Earth in a similar way. A trained archaeologist can find a piece of pottery and know that it was made during a particular period of time – the Ming Dynasty say. Geologists use fossils – the remains of ancient animals – in the same way. To find out how, let’s learn how about the people who first discovered how to do this.
Nineteenth Century Britain was a time and place where rapid development of industry and advances in science went hand in hand, each helping the other. William Smith worked as an engineer involved with canals and coal mines and so saw a lot of sections cut through rocks. He realised that he saw the same layers in different places, always in the same order and with the same types of fossil in each layer. He proposed that this was a universal scientific “law of faunal succession”. In any series of layers of sedimentary rock, as you look at each layer going up, fossils will appear in a specific reliable order. There are fossils in the coal-bearing rocks that are not seen in the younger limestones higher up, instead different ones are seen, sticking out of the walls of buildings in Oxford and Cambridge Universities. The rocks on top – the youngest – like the Clay under London contain a different set of fossils again. William Smith was the first person to create a geological map of Britain, that was published in 1815. He used his understanding of the fossils and his law of faunal succession to help him create the map. He produced ‘cross-sections’, pictures of a vertical slice down through the earth. These are of practical use. Dig down into rocks that you know sit above rocks that contain coal and you can make a mine. Dig down into older rocks and you are just wasting your time.
Modern scientists would use different words, but William Smith’s ideas are now known to be correct and useful in understanding geology across the world. Stratigraphy is the modern name for the study of layers of rock. Nicholas Steno, a European scientist working a century before William Smith first defined the laws of stratigraphy. These ideas are simple. Think about an old house that has been decorated many times. The walls are covered in many layers of paint, each on top of the other. If you think about it, it’s obvious that the layers closest to the wall were painted on first, and the ones above later. Think of a nail stuck in the wall. It will cut the layers of paint that were there when it was banged in. If it’s covered by different layers, then they were laid down after the nail was put in. Apply these ideas of layers of rock laid down on the surface of the earth and you get the laws of stratigraphy. Using fossils to understand these layers is known as biostratigraphy.
European scientists in the generation after William Smith studying rocks in other countries found the same fossils, but in different types of rock. What might be a limestone in one country could be a mudstone in another, but with the same types of fossils. Realising that these rocks were of the same age they started naming periods of time defined by the fossils. These are the geological periods that you may be familiar with. The oldest rocks that contain obvious fossils was named the Cambrian period after the name for Wales (part of Britain) in Latin (an ancient European language). The Ordovician and Silurian were named after the Latin names for ancient peoples from Wales. The Devonian was named after a part of Britain called Devon, the Carboniferous after the element Carbon as these rocks are rich in coal. The Permian is named after a Russian city, the Triassic in Europe contains a series of three different types of rock and ‘tri’ means three in Latin. The Jurassic was named after a mountain range in France, the Cretaceous after the latin for chalk, a rock common at this time.
The geological periods were being named when Charles Darwin was a young man, studying geology and other sciences. For example he studied with Adam Sedgwick who soon after named the Cambrian period. At the time, people were realising that the earth must be very old. They could see that layers of rock were kilometres thick. The rocks themselves were like like layers of sand seen in the modern sea or rivers. Knowing that layers in the modern world form slowly they realised that kilometres of rock would require millions of years to form.
A world that was millions of years old. Fossils that changed gradually over time. These are the ideas that were in Charles Darwin’s head as he sailed across the world studying modern plants and animals (and Geology) on a sailing ship called HMS Beagle. All these experiences led him to produce his theory of evolution by natural selection, the foundation of modern biology.
The theory of evolution also explains why biostratigraphy works. As animals and plants slowly change and evolve into new species, the fossils found in rock layers also change. Once an animal becomes extinct it is never seen again.
We now know that the way animals and plants change over time isn’t always a calm gradual process. Mass extinctions are events where many types of creature die out, due to meteorite impacts, or massive volcanic eruptions or other reasons. Many of these sit on the boundaries between different geological periods. The extinction of the dinosaurs happened at the Cretaceous-Cenozoic boundary. The biggest extinction ever – sometimes called “The Great Dying” – happened at the transition between the Permian and the Triassic. At this time 96% of marine species became extinct as massive volcanic eruptions poisoned the air and the seas.
Often geologists talk about how many millions of years old something is. We were only able to measure the age of rocks in the Twentieth century, once we understood radioactivity in rocks better. Using fossils to divide time doesn’t require you to know exactly how old they are, just that this rock is older than that, or that these are the same age. Geological periods are the most familiar divisions of geological time, but there are others, some bigger, some smaller. In the nineteenth century, all rocks older than Cambrian period were lumped into the ‘PreCambrian’ and were thought to have no fossils at all. Now we are able to find out the ages of rocks without fossils, using radiometric dating. PreCambrian rocks make over 85% of the history of the earth, so geological periods called Eons are used to divide up this vast time. All of the Cambrian and later are known as the Phanerozoic eon, the word means ‘visible life’ in the Greek language. The eon older than this, from 2500 to 541 million years ago, is the Proterozoic, meaning ‘earlier life’. Even older rocks are from the Archean eon, meaning ‘beginning’. Rocks on earth older than 4 billion years old (they are very rare) come from the Hadean eon. The earth at this time was extremely hot, covered in molten rock and hit by frequent meteorite impacts, conditions seen as hellish. Hadean is named after Hades the Greek god of Hell.
Coming down a step, between the vast Eons and the more familiar Periods, we have Eras. The Proterozoic is divided into three, early, middle and late, or Palaeoproterozoic, Mesoproterozoic and Neoproterozoic. The Phanerozoic (Cambrian and later) is also divided into old, middle and new. These eras are the Palaeozoic, during which life first left the seas onto land, Mesozoic, when the dinosaurs roamed the earth and the Cenozoic when mammals became dominant.
The better known periods of the Phanerozoic are divided up still further into Epochs. Typically a period is divided into 2 or 3 epochs, often early, middle and late. Our next divisions are called Ages. Let’s get into some examples. The Cambrian period was named after Wales in Britain. Its youngest Epoch is the Furongian, meaning Lotus, another name for Hunan province in China. The Furongian Epoch is split into 3 Ages, the first two named Paibian (named after a village in Hunan Province) and Jiangshanian (named after a village in Zhejiang Province). The third Age of the Furongian Epoch is not yet named. All of these divisions of time are known as ‘chrons’. The term can be used to refer to any slice of time that can be well defined, even those shorter than geological Ages. Why are some Epochs and Ages named after villages in China? It’s because that when dividing finer and finer periods of time, it’s important to have a well-defined definition that can be used in rocks across the world.
Wales has lots of rocks of Cambrian age, but Hunan province in China has some of the best sequences of rocks from the Furongian Epoch. What geologists are looking for are continuous sequences of rocks rich in marine fossils that change rapidly over time. It’s not uncommon for piles of sedimentary rock to have periods of time when no sediment was deposited, when the record is broken. The Furongian Epoch, starts with the Paiban Age. It’s defined officially defined as the first appearance of a fossil trilobite species, called Glyptagnostus reticulatus (no, I don’t know how to pronounce it either). This animal was a little like a woodlouse that lived in the seas, widely across the planet. It’s found today in six different continents and so a perfect way to divide up time. The official reference point that defines the start of the Paiban is a sequence of rocks near the village of Paibi. This place, called a GSSP (for Global boundary Stratotype Section and Points) was chosen by a global group of geologists called the International Commission on Stratigraphy (ICS). Their mission is to define the boundaries between all geological Ages in terms of specific fossils and a place that best shows the boundary. More poetically (but not accurately) GSSPs may be called ‘golden spikes’, a place where humans have nailed down the flow of time to particular place and event. The work of the ICS is not complete, they have more ‘golden spikes’ to define. Maybe they’ll put one near where you live? Maybe they already have.
Not all golden spikes are defined by the appearance of disappearance of fossils. The boundary between the Cretaceous Period and Palaeogene Period is defined by a layer enriched in Iridium a rare element on earth but more common in space. The layer was formed by a massive meteorite impact (the crater is in the Mexican Gulf in North America). This is also when the dinosaurs became extinct which is probably no coincidence).
The basic principles of stratigraphy – layers above other layers are younger, for example – are universal. What if you had a planet with no fossils, that you’ve never visited but where you had a good set of photos sent by a robot, could you define geological Periods? Of course! We’ve done it for Mars after all. Martian geological periods, the Pre-Noachian, Noachian, Hesperian and Amazonian cover the same period of time as earth ones, but are entirely different. They are far less well defined too (no Epochs or Ages) but they answer the same human questions – How old is that? What’s its history? What stories can we tell from it?
If intelligent beings in the future were to look at the earth, they’d be able to use stratigraphy to understand the geological history of our times. Probably they’d use the same ideas and look for extinctions of animals, or unusual layers of rock to divide up time. What would the layers being laid down right now look like? They’d be interesting, I think.
First publication by Xiaoduo Media in Front Vision. Front Vision is a Chinese online science magazine for children. Reproduced with permission.
