After the Second World War the ‘Black Belt’ therefore formed the heartlands of the civil rights movement. Rosa Parks refused to give up her bus seat to a white traveller in December 1955 in the city of Montgomery, Alabama, smack in the middle of this curving strip of 75-million-year-old Cretaceous rocks. Even today, virtually all the counties in the US with the highest proportion of African-Americans lie along this same arc within the south-east.60 Persisting after many African-Americans had migrated north and west, these populations are almost like an erosional remnant staying in place after the economic tide has swept millions elsewhere.
Without major development from industry or tourism, this formerly economically productive region has long suffered socio-economic problems of high unemployment and poverty, low levels of education and poor health care. Thus the electorate here has traditionally tended to vote for the policies and promises of the Democrat Party, producing the distinct curving band of blue in the presidential election maps. There is a clear causal chain taking us from the politics and socio-economic conditions of today, to their roots in historical agricultural systems, and then further back to the geological tapestry of the ground beneath our feet. The exposed band of ancient seafloor mud is still imprinted on our political map.
Chapter 5
What We Build With
Who built the pyramids?
Your immediate answer may well be the pharaohs of ancient Egypt. And you would be right, of course. It was the all-powerful god-kings of the fertile Nile Valley who, over 4,500 years ago, were able to muster and orchestrate the manpower needed to quarry, transport and assemble giant stone blocks into the colossal pyramids towering over the Giza Plateau. The largest of these is the Great Pyramid, built during the reign of the pharaoh Khufu – or Cheops, as he is also known – and completed around 2560 BC. Until the completion of Cologne Cathedral in 1880 it was the tallest human-made structure in the world.1
The main bulk of the Great Pyramid is made up of about 2.5 million limestone blocks, each weighing 2.5 tonnes on average, laid atop each other in 210 layers. These were quarried from a limestone deposit nearby, dragged on sledges to the construction site and then hauled to the top of the growing pyramid up earthen ramps. This pointed construction was then covered in outer casing stones, using a much higher-quality limestone quarried further away on the other side of the River Nile, which were fitted together tightly and then beautifully polished. The Great Pyramid would originally have gleamed spectacularly in the sun, but most of these casing stones have since been removed. The large granite blocks, some weighing up to 80 tonnes, used for lining the interior chambers were quarried much further away in Aswan, about 400 miles upriver.
It is thought that the construction of the Great Pyramid took several decades and a workforce of tens of thousands of skilled labourers, who were paid in bread and beer. They worked without iron tools, pulleys or wheels, instead using copper chisels, drills and saws. But although the scale of the Great Pyramid is absolutely staggering, and the human effort involved in its construction was truly colossal, what is perhaps equally amazing is the nature of the building materials. It turns out that they were created by some of the simplest organisms on the planet.2
BIOLOGICAL ROCK
If you can get up close to the massive building blocks making up the core of the Great Pyramid – now exposed with the removal of the outer casing stones – and peer intently at their surface, you’ll notice a very curious texture. The limestone blocks are made up of scores of coin-like disks. Search for some that have cracked open and you might be lucky to see their internal structure: an impressively intricate spiral, subdivided into small chambers. You are looking at fossils of sea creatures called foraminifera, or forams. And most impressive of all, given that each shell can be up to a few centimetres across, is the fact that the organism that created them is single-celled. The largest human cell, a woman’s egg cell, is about a tenth of a millimetre across, and only just visible to the unaided eye. The sea creatures that make up the limestone of the pyramids are absolutely colossal in comparison. They belong to a kind of giant foram called Nummulites (meaning ‘little coins’ in Latin).
Deposits of nummilitic limestone are found not just around the Nile, where they offered construction material for the ancient pyramid-builders, but across a huge area ranging from Northern Europe to North Africa, from the Middle East to South East Asia. This expansive region of nummulitic limestone was laid down in the warm, shallow margins of the Tethys Ocean, 40–50 million years ago. During this period of the early Eocene, global temperatures were elevated for longer than the PETM extreme temperature spike that we explored in Chapter 3, although they weren’t as high. The high sea levels caused the Tethys Ocean to flood in great arms stretching to Northern Europe and across North Africa. Huge numbers of forams lived in the warm waters, and when they died great drifts of their coin-shaped shells made of calcium carbonate sank down and carpeted the seafloor. Over time, they became cemented together to form the nummulitic limestone.
These particular limestone formations have become exposed in many different places. Where the distinctive coin-like fossils have eroded out of the bedrock in North Africa to be scattered among the desert sands they are known to the Bedouins as ‘desert dollars’.3 And in the Crimean Peninsula, craggy outcrops of this nummulitic limestone form the very jaws of the ‘Valley of Death’ that witnessed the disastrous Charge of the Light Brigade during the battle of Balaclava in 1854, as memorialised in Alfred, Lord Tennyson’s poem.4
Thus the enormous blocks of rock that form the Great Pyramid at Giza were quarried from what is essentially a huge slab of limestone stretching across Eurasia and Africa. Composed of countless foram shells, this nummulitic limestone is a biological rock. So while the Egyptian pharaohs may have ordered their construction from enormous blocks of limestone, it was another life form that built the pyramids. The tombs of the pharaohs are made of innumerable drifts of the skeletal remains of a giant single-celled sea creature.5
The pyramids are one of the most enduring symbols of human civilisation, revealing what we can build when we put our minds, and coordinated efforts, to it. Throughout history many of the grandest edifices have been constructed out of devotion to the divine: the step pyramids of Mesoamerica, the temple complexes of Sanchi Stupa and Angkor Wat, or the medieval cathedrals across Europe. But the materials with which these monuments were made are the same as those used for buildings constructed for more pragmatic purposes – dwellings, civic buildings, bridges, harbours, fortifications. At the root of all this fervent construction is a fundamental human requirement: to find shelter from the elements. And throughout history we’ve turned to the natural materials we found around us.
WOOD AND CLAY
Many cultures around the world, especially nomadic peoples, have constructed temporary structures like the wigwam, tipi and yurt out of branches, bark, reeds or animal hides. Timber, of course, is one of the oldest construction materials. A variety of different trees can be worked into supporting beams, poles and planks, as well as cladding slats or roofing tiles. And before metal was widely available, timber was also used for mechanical components.fn1 The cross-grained fibres of elm make it resistant to splitting and so perfect for the hubs of cartwheels. Hickory is particularly hard and so was used for the gear teeth in the drive systems of waterwheels and windmills. And pine and fir trees grow exceptionally tall and straight and so are well suited for ships’ masts.
The simplest material suitable for fashioning into solid walls is clay. The early city-dwellers of Mesopotamia, the land between the rivers, lived in a world of mud. Although a perfect environment for productive agriculture, the area is woefully lacking in natural resources like timber, stone and metals, all of which had to be imported. A succession of ancient Mesopotamian civilisations – Sumerians, Akkadians, Assyrians and Babylonians – survived by trading their food surplus for cedarwood from Lebanon, marble and granite from Persia and Anatolia, and metals from Sinai and Oman.6
Nonetheless, most of their structures were constructed from what was locally available. Houses and palaces, city walls and forts alike were all built with sun-dried adobe bricks. Even the cores of their great ziggurats – the tiered, flat-topped pyramids that served as temples – were made of sun-baked bricks. More durable kiln-fired bricks were only used for the facing of the palaces and ziggurats and they were decorated with colourful glazing. Mud even became a writing material, when the Sumerians invented writing by pressing a stylus into tablets of soft clay.7
In fact, long before providing the ancient Mesopotamians with earthen bricks and soft tablets for the earliest forms of writing, clay had proved transformative to human existence. The innovation of firing clay into earthenware pots gave us brand new capabilities. Pottery provided vessels in which food could be cooked by boiling or frying. Not only does cooking deactivate certain plant poisons that are present in the potato and the cassava, for example, and so make more foodstuffs available to us; it also breaks down complex molecules to release more nutrients that our bodies can absorb. In short, pottery enabled greater processing of food to make it more digestible to us. Lidded containers fashioned out of clay also enable food stores to be protected from pests and vermin, as well as making them far more portable for travelling and trade. Pottery can be made more watertight and more attractive by glazing it – coating it in a solution of certain powdered minerals before kiln-firing – and this may well have been what made humanity stumble across the process of smelting metals like lead or copper.
Fired clay proved critical to our development through history not only because it is hard and watertight, but also because it can be extremely heat-resistant. Firebricks are ideal for lining kilns and furnaces: they insulate the heat inside without being affected themselves, and so allow very high temperatures to be achieved. Thus ceramics enabled humanity to truly master fire, not just for keeping the night-time cold at bay or for cooking, but for taking raw materials gathered from the environment and transforming them into some of the most useful substances of history: smelting metal out of their ores, calcining lime to create mortar, or producing glass.
The Mesopotamians built with dried mud for want of harder, more durable materials. But elsewhere around the world we have used the geology beneath our feet. We not only build our cities within the landscape – near the coastline, in a fertile river valley or close to hills with mineral resources – but we also make them of the landscape. In this chapter we’ll see not just how the Earth made us, but how the planet provided the solid materials that we have used for construction. The story of civilisation is the story of humanity digging up the fabric of the planet beneath our feet and piling it up to build our cities.
There are three fundamental rock types on Earth, and throughout history we’ve built using all three kinds. Sedimentary rocks are formed by the deposition and then cementation together of material which either eroded from older rocks or was produced biologically – sandstone, limestone and chalk are all examples. Igneous rocks such as granite, on the other hand, solidify from volcanic lava or magma still deep underground. And when sedimentary or igneous rocks are subjected to high temperatures and pressures – caught in the crunch of continental collisions or when magma intrudes up into them – they are transformed physically and chemically, becoming a metamorphic rock like marble or slate.
The ancient Egyptians were the first civilisation to extensively quarry and build with natural stone, and they made use of a variety of different rocks. Nubian sandstone was available from the cliffs flanking the Nile in Upper Egypt. The Great Temple of Rameses II at Abu Simbel and the Luxor Temple in Thebes, for example, were carved of this yellow-brown stone. Further north, the Nile cuts through the nummulitic limestone that overlies the older Nubian sandstone which, as we saw earlier, was quarried to construct the pyramids at Giza. In the Eastern Desert, the rifting open of the Red Sea exposed the ancient basement rocks that form the very foundations of the African continental crust. The granite and gneiss (formed by granite’s metamorphosis) here are over half a billion years old. Hard and durable, they were prized by the Egyptians for carving statues and obelisks, and taken by barge down the Nile to be exported around the Mediterranean world.8
Let’s take a look now at some of the most important rocks we’ve used through history, and how our planet created them.
LIMESTONE AND MARBLE
As we have seen, the nummulitic stones used to construct the pyramids are a form of limestone. But it is just one kind of this very widespread rock type. Calcium carbonate rock is also created at the mouth of volcanic hot springs where, as the water cools, the minerals precipitate out of solution and rapidly form layers of limestone on the ground. This form of limestone is known as travertine. The main pillars and external walls of the Colosseum in Rome, for example, were made of travertine quarried from Tibur (today’s Tivoli, a town about 30 kilometres north-east of Rome), and hot-spring limestone from the same place was used for the Getty Center in Los Angeles.9
However, most limestone is not formed in volcanic hotspots on land like the Tivoli mineral springs, but on the seafloor as biological rock. Much of the limestone found across Europe and the rest of the world was formed during the Jurassic Period, as warm, shallow seas flooded the land. Marine reptiles like pliosaurs and ichthyosaurs swam these tropical seas,10 whilst on the sea floor calcium carbonate from the shells of sea creatures – such as forams – precipitated as a limy mud. As particles of sand or shell fragments rolled back and forth around the seabed with tidal currents they became coated with concentric layers of calcite mineral to form tiny balls called ooliths (from the Greek for ‘egg-stone’). These little spherules then became cemented together with more calcite to form oolitic limestone.
In Britain, oolitic limestone formed during the Jurassic has resurfaced as a great sash across the country, stretching from East Yorkshire, through the Cotswolds, and down to the Dorset coast (see map here). Oxford lies in the middle of this band and many of its university colleges were built of this glorious, golden stone.11 At the very south-western tip of this diagonal stripe of Jurassic limestone lies the Isle of Portland, a promontory jutting out into the English Channel, its hard rock resisting the pounding of the waves. The limestone exposed here dates to the very end of the Jurassic Period, 150 million years ago.
Portland stone is a fabulous building material, and not only for its delightful creamy tint. The ooliths that made it were cemented together in just the right amount: it is durable enough to resist weathering and crumbling, but not impractically hard for masons to cut and carve. Portland stone is known as a freestone: its fine-grained texture can be cut smartly in any direction and it has been used as a building material since Roman times. Portland stone became the rock of choice for many of Britain’s monumental or civic buildings. Its pure hue appears in the Tower of London, Exeter Cathedral, the British Museum, the Bank of England, and the facing of the east side of Buckingham Palace (including the famous balcony). Sir Christopher Wren selected it for the rebuilding of St Paul’s Cathedral, as well as many other London churches, after the Great Fire of London in 1666. Portland stone has also been used around the world, for example in the United Nations building in New York.
The United States has its own sources of limestone. Some of the highest quality is quarried in southern Indiana where it was laid down, much earlier than Portland stone, during the early Carboniferous Period around 340 million years ago. Indiana limestone has been used in the facing of the Empire State Building, the Yankee Stadium in New York, the National Cathedral in Washington, DC, and the Pentagon. This stone was also used extensively in the reconstruction of Chicago after its Great Fire of 1871, emulating the rebuilding of London’s monumental landmarks after its own conflagration two centuries earlier.
Much of the northern Mediterranean coastline that we explored in the last chapter is also made up of limestone rocks originally laid down on the bed of the Tethys Sea. Now thrust up above the waves, it has been dissolved aw
ay by rainwater trickling underground, a process which has created an extensive networks of caves. Perhaps unsurprisingly, many of these have become associated with the Underworld of classical mythology. At the tip of the Mani Peninsula, the southernmost point of Greece, for example, is the entrance to a cave where the legendary Orpheus is said to have descended into the Underworld to find his deceased wife Eurydice. The beauty of Orpheus’ lyre playing won over the god Hades, who allowed him to take Eurydice back to the land of the living on one condition: that he should not look back. But as soon as Orpheus had reached the upper world he anxiously turned round to check that she was following, and so Eurydice disappeared for ever.12
Where this Tethyean limestone has been baked underground in the convergent plate boundaries around the Mediterranean – by magma rising up and intruding into it, or by its being caught in the tectonic vice crunching up mountain ranges like the Alps – it is metamorphosed into marble. This is the signature stone of Classical Greek and Roman sculpture, monuments and grand public buildings. Some of the most highly prized marble in the world is still quarried around the city of Carrara in northern Tuscany. Here the Apuan Alps contain mountains of the pure-white stone which has served as a building material since the days of ancient Rome, when it was used for the Pantheon and Trajan’s Column, for example. Carrara marble was also a favourite of Renaissance sculptors: it provided the material for what is probably the most famous statue in the world – Michelangelo’s David. It has also been exported around the world to construct some of the most iconic global landmarks: Marble Arch in London, the Peace Monument in Washington, DC, Manila Cathedral, Sheikh Zayed Mosque in Abu Dhabi and Akshardham in Delhi.
Origins Page 12