Reading history in rocks
All these rocks hold clues to the history of the Earth and, through many years of hard work by hammer-wielding geologists, this information has been painstakingly unlocked. In a saga of endeavour generally traced back to the work of William Smith in early-nineteenth-century England, the geological tale of the Earth has been assembled bed by bed. The geological column you see here conveys only the barest bones of their achievement. Geologists have divided up the record into thousands of units and sub-units, characterized mostly by the distinctive fossils they contain, noted the folding and faulting that distorted them, and marked both the ways they change across space and the way surprisingly similar systems can be found thousands of kilometres apart.
Although this activity is generally called geological surveying, it is very far from the flat compilation of a catalogue. Instead, it yields true Earth stories. An example of vast economic importance is the formation of coal. The rocks called coal measures are not just thick layers of coal. Instead the coal is found in seams, some many metres thick and some too pitiful to be worth mining. Between them are layers of coarse and fine sandstones, muddy rocks and even limestones. This means that the coal – laid down originally in a kind of peaty swamp – was later buried by sediments laid down in first shallow and then deep water, as finer sediments are associated with a more peaceful, deep-water, marine environment. Eventually the basin fills up and the cycle starts again with more peaty deposits.
Most geological effort has been put into describing the past 570 million years of the Earth's history, known as the Phanerozoic eon. This starts with the Cambrian, which lasted 60 million years and was marked by the “Cambrian Explosion” of species, a unique episode in Earth history in which life suddenly became far more common and varied. All of a sudden (geologically speaking) life was abundant and diverse, at least in the sea, to which it was confined at this era. This makes the story simpler to unpack because the arrivals and extinctions of species allow geologists to date the rocks in which their fossils are found. In any case, most of the rocks we see come from the Phanerozoic, even though it makes up only the last 11 percent of the Earths history. By comparison, the period before this was for long known simply as the Precambrian. Even now, as the chart shows, its subdivisions are few and simple, mainly because there are too few species to allow more detail to be drawn up.
The Cambrians most recognizable denizen is the trilobite (left), although, as with many of the fossils mentioned below, they lived well beyond the period which they characterized.
Most species found in the fossil record are marine ones and many are shells, often looking at first sight much like the seashells you might find today. But as ever in the fossil record, this tells you nothing about what things were like at the time. Having a hard shell increases an organisms chance of being seen in a museum in a few hundred million years. But we infer from todays ecology that there were also plenty of soft-bodied creatures around in the past. In the case of the Cambrian, we have detailed knowledge of
some of them from rock formations such as the Burgess Shale in Canada, where freak conditions preserved a weird and wonderful array of them. It formed from material shed down a continental slope.
At this time, too, things were shifting on the tectonic front. Most of the present land mass of the southern hemisphere was joined together in one supercontinent, while most of the northern hemispheres present-day land mass was flooded, save for a few smaller continents more or less corresponding to Siberia, northern Canada and Scandinavia today.
Although they look definite on the charts, and are often marked by substantial changes in rock type, these geological periods are human artefacts and not inherent in nature. The Ordovician, which ran from
510 to 439 million years ago, is a case in point. It was dreamt up to end a long-running argument about where the Cambrian ended and the next period, the Silurian, began. Its interpolation ended a lengthy period of scientific warfare among the mountains of Wales, of which the Silures and the Ordovices were inhabitants before and during the Roman era.
The most distinctive Ordovician fossils are those of graptolites (right), now-extinct marine organisms that floated about ancient seas in colonies up to a metre or more in length, and which appear in the rocks as sawtooth-like fossils. This period also saw the first primitive fish. Most of the land was in the southern hemisphere at this time and traces of the glaciers that formed in what is now north Africa can still be seen.
This extended glaciation of areas now far from the poles continued into the Silurian (439-409 million years ago), despite the drift of land, including present-day Siberia and Australia, across the Equator. Graptolites were common in this period too, and they have been used to mark out the major layers into which the Silurian is divided, which have names mainly derived from Wales and the areas of England adjacent to it such as Llandovery and Much Wenlock. The big news story in the Silurian was the arrival of plants on land, and there were also many shallow-water life forms such as corals, and freshwater animals that may have made the odd trip onshore. Comedians have often viewed this move as life's biggest error.
From Wales, the action now moves to south-west England, as the next period, the Devonian (409 to 363 million years ago), takes its name from the county of Devon. The massive sandstone deposits of that era in Devon are known as the Old Red Sandstone. They were laid down by erosion from the Alpine-size mountain range which dominated Wrales at that time but which has now been worn away to nearly nothing. During the Devonian, Europe and North America were equatorial territory. Although life expanded, with many more species of fish and the growth of forests and insects on land, the end of the Devonian was marked by a mass extinction of many marine species. Its cause is still a matter of speculation.
Most of these geological periods are classified in the same way the world over but the Carboniferous, which lasted from 363 to 290 million years ago, is an exception. Although North Americans do use the term, they prefer to think of these rocks in two halves, the Mississippian and the Pennsylvanian. But the word “Carboniferous” is an eloquent one. Although there are some important younger coal deposits, the Carboniferous ones are the biggest and the best. There are no older ones because before this time, there were too few land plants to rot down and make coal.
The North American distinction between the Mississippian and Pennsylvanian is more or less reflected in Europe by a division of the Carboniferous into the “Lower” and “Upper”. The Lower Carboniferous rocks tend to have accumulated in more or less shallow seas as the continents converged, while the Upper Carboniferous, as we have seen, produced the coal measures through the steady, cyclical drying-out and inundation of coastal areas. The lush forests whose plants were later to turn into coal were home to the biggest animals yet seen, with large amphibians and reptiles on the land and in the sea.
However, these creatures' descendants did not enjoy what happened next, in the Permian, which ran from 290 to 248 million years ago. It was at this time that Wegener's Pangea was formed, with essentially all the land we see today gathered in a ring around a sea called Tethys, while a single mighty ocean, Panthalassa, took up most of the Earths surface. The Permian was marked by many mass extinctions that removed almost all land and marine species. The lower Permian has many signs of life such as limestones, rocks which are essentially remains of shells and corals, and the coal accumulation of the Carboniferous continued. But the upper Permian is characterized by rocks which indicate drying out and cooling. The cause may have been a reverse greenhouse effect generated by a lack of carbon dioxide in the atmosphere after so much had been swallowed up into coal measures.
The end of the Permian is also the end of the Palaeozoic era, which began with the Cambrian. These longer pieces of geological time are not vital to understanding but they mark big changes to life on Earth through time.
The next of these eras, the Mesozoic, starts with the Trias (or the Triassic), which is so continuous with the Permian that they are sometimes lumped together as the Permo-Trias. It lasted from 248 to 208 million years ago, during which time the map of the world did not change much. Volcanic lavas, and sedimentary rocks of both land and marine origin, formed in and around Pangea. Many of the land-based ones are associated with dry or even desert conditions, such as salts from the drying out of lakes, and desert sands. The ancestors of both dinosaurs and warm-blooded present-day mammals gained ground at this time.
The Jurassic rocks of Europe are probably the easiest place on Earth to find fossils. Mostly they are shells and corals formed in warm water, but the origins of big-league fossil collecting are in these rocks too, along the Jurassic coast of England. These rocks yielded the fossils that led to the coining of the term dinosaur. The first bird, archaeopteryx, also dates from this era. Most of these fossils are found in rocks that developed in shallow water. But the Jurassic saw the final break-up of Pangea as a result of sea-level rise and renewed plate tectonics. Much previous land was flooded, and North and South America were separated by a seaway. India became an island, as did Antarctica, one of the few land masses to be more or less in its present-day position in this era, but at that time attached to Australia.
The final period of the Mesozoic is also the point at which the map of the world starts to look more recognizable, with the Atlantic starting to open and Australia heading north. Called the Cretaceous (creta is the Latin for chalk), it lasted from 146 to 65 million years ago. The high sea levels of this period left vast chalk deposits across large areas of the world, with the white cliffs of Dover the most sung-about. The chalk is formed from coccoliths, the shells of tiny animals. At this time there was also extreme tectonic action, with huge volcanic outpourings forming thick deposits of basalt called the Deccan Traps, in India.
The end of the Cretaceous is probably the most discussed event in Earth history), and will remain a talking point. However, the asteroid impact idea would now take some dislodging from both science and the popular imagination.
The disappearance of the dinosaurs at that time meant that our mammal ancestors dominate the rest of history – or at least, they make up most of the big land animals. Although whales are the largest animals, fish still dominate the sea while the only notable flying mammals are bats.
The rest of the geological column is the Cenozoic era, and this is the one we are living in. Its rocks are much-studied and their divisions and subdivisions have been rejigged with some enthusiasm over time. Nowadays its lower half, the Tertiary, is itself divided into the Palaeogene and the Neogene. The Palaeogene lasted from 65 to 23 million years ago and was marked by further opening of the Atlantic and the beginning of the formation of the Himalayas as India pushed north into Asia and the crust between the two became ever more compressed. This process continues today. Major evolution on land included the development of ever larger mammals.
But quality counts as well as quantity. The Neogene, from 23 to about 1.8 million years ago, is notable for one big item in evolution, the separation about 5 million years ago of human ancestors from their last ape relations. It is divided into the older, and still more familiar, Miocene and Pliocene. The boundary between these is best left to the professionals. It is defined by the percentage of still-existing molluscs in fossils at different times.
The final phase of the story, the Quaternary, lasts just 1.8 million years. In the geological nomenclature it has been renamed the Pleistogene and has equal status with, say, the Jurassic, which lasted 70 million years. But it is only reasonable that we care more about our own times, especially as there is more detailed information available on them.
All of this period except the last 10,000 years is called the Pleistocene. This is too short a time for much to have happened on the plate tectonics front. Instead it was marked by the formation of most of the landscapes of the northern hemisphere through a number of episodes of more or less severe glaciation. Go for a country walk almost anywhere in the northern hemisphere and you will see the traces, described on pp.219-23.
All this cold did not prevent Homo sapiens spreading out from Africa across the world. Indeed, the lower sea levels caused by water that is today in the oceans being turned into ice made the task simpler, for example providing a land bridge between what are now Siberia and Alaska.
The time period we are living in is called the Holocene. This has been persistently warmer than an ice age and is referred to as an interglacial because the pessimists in charge of the terminology think that the warm period is merely a prelude to the next ice age. As well as the gross effects, it is apparent that the growth and reduction in ice volumes in polar and high mountain regions has other more subtle consequences, affecting climate and temperature far from the ice itself.
However, it is also fair to say that the increasing numbers and impact of humans on the world have been among the most striking changes of the Holocene era.