DISCOVERING THE PAST|DATING WITH ACCURACY AND DIGGING THE PAST
InfoDISCOVERING THE PAST
Discovering the past is unique way to try unleashed all ancient and vanished data which disappear from our eyes.
Searching For Sites
Many archaeological sites are visible as ruins – others are lost, submerged beneath the surface of the landscape.
Modern submersible allow marine reconnaissance at great dephts.
The thick walls of their spherical chambers resist crushing water pressure.
Sophisticated techniques are now used to pinpoint buried remains and identify areas likely to yield hidden treasures.
A bird’s eye view
Sites long since invisible from the ground often become obvious when photographed from the air. Aerial photography reveals ridges and hollows, subtle variations in the colour or texture of soil, and differences in vegetation particularly in cereal crops – which may mark the outlines of buried buildings, ditches, fields, and canals.
Seen from the air, enigmatic lines etched into the arid plain near Nazca in Peru
Between 200 BC and AD 600 create huge geometric patterns, spirals, and pictures
of birds and animals. The hummingbird is 50m (165ft) long.
Scientists use radar and satellite photography to locate sites in difficult terrain such as the rain forests of Central America, where Maya canals and raised field systems have recently been revealed.
Prospecting with electricity, magnetism, and sound
If archaeologists suspect that remains lie hidden beneath land or sea, they can pinpoint them using devices that measure the changes in electro- magnetic effects. On land, changes in the soil’s resistance to the flow of electricity can be measured by passing a current through electrodes placed in the ground. The intensity of the current varies according to the resistance of the material through which the current flows.
Since stone offers more resistance than soil, high resistance may indicate the presence of walls. Another device – a magnetometer – measures distortions in the magnetic field, which varies clearly where an object made of materials such as iron or burnt clay is buried. Sonar devices are also widely used, particularly in underwater reconnaissance. The sonar uses transmitted and returned acoustic waves to locate archaeological sites such as shipwrecks through murky waters and thick sediments impervious to other detection devices.
Probes and Periscopes
In 1955 an Italian engineer, Carlo Lerici, devised a periscope that was to revolutionize Etruscan archaeology. Little was known about the Etruscans, who dominated central Italy between the 8th and 5th centuries BC. Although their artefacts
were usually buried with them in thousands of tombs, most had been looted and were empty – something usually discovered only after a tomb had been identified and opened.
Lerici’s periscope could find out whether a burial chamber was worth excavating. Once a tomb had been identified, scientists drilled a small hole into it, lowered the periscope, and studied the interior before excavation – even taking photographs of it. By the mid-1970s, scientists had used the probe to examine some 7,000 tombs, while opening only 600.
Probes are now widely used – for example, in locating and examining hundreds of pits in the burial complex that also includes the terracotta warriors at Mount Li in China, and to reveal a wooden boat buried in a trench beside the Great Pyramid at Giza in Egypt.
Traces of the past
One method of research requires no special equipment other than a sharp eye and archaeological expertise. Occasionally, construction workers in the ancient world used stones from older, dilapidated structures when building new ones. Such stones are likely to be out of style with the rest of the new building, and suggest the existence nearby of ruined structures which may be buried.
Anomalies in the present-day layout of buildings or field boundaries are sometimes evidence of earlier buildings and settlements. These are created where builders have avoided earlier structures that have since disappeared. Documents such as parish registers and old maps may also provide clues about these features.
DATING WITH ACCURACY
Since the middle of the 20th century, scientists have been able to establish the dates of archaeological discoveries with increasing accuracy. Methods range from investigating atomic particles to consulting the growth rings of the bristlecone pine.
Unleashing Ancient Energy
Ancient clay objects can be dated by measuring an effect known as thermoluminescence. After a clay pot is fired, crystals within its structure develop impurities through the process of ionization, in which electrons are stripped from the atoms of the minerals within the clay. These free electrons remain locked inside the crystals.
In thermoluminescence-dating the pot is once again subjected to fierce heat – effectively a second firing – until the trapped electrons emit photons, particles of light, which can be counted. Their number gives an idea of the age of the object – the older the pot, the more free electrons it contains. The technique also works on burnt flint.
A technique known as electron-spin resonance dating, which works on the same principle, is now being applied to other materials such as bones and tooth enamel, and has been used to date a number of early hominid teeth back to 250,000 years ago. Optical luminescence dating, another related technique, measures exposure to sunlight, and is used to date ancient sedimentary deposits containing archaeological remains.
Radioactive Remains
Carbon 14 is a radioactive isotope of carbon that occurs naturally and is absorbed by all living things. When a living thing dies (and stops absorbing carbon 14), the carbon 14 decays at a known rate – halving every 5,730 years (accurate to 30 years either way). By measuring the proportion of carbon 14 in organic remains such as bone or wood with a Geiger counter or a more modern tool – an accelerator mass spectrometer – a date of death can be calculated. The spectrometer’s accuracy diminishes with the age of the object, but it can date remains up to 50,000 years old.
Secrets in the Trees
Trees preserve a yearly record of seasonal changes in their annual growth rings. The thickness of the rings varies according to the effects of rainfall, sunlight, and temperature. Trees growing in a single area record the same conditions.
By overlapping the rings of trees of different ages – even if the trees are dead, as in wooden beams from buildings – climatic calendars can be created. These allow wooden artefacts and ancient building timber to be dated accurately.
Dendrochronology was pioneered in the south-western USA. Climatic calendars created from the bristlecone pine of the American west have been dramatically successful. These pines live for up to 3,000 years, and with them scientists have built a record stretching back almost 8,000 years. Dendrochronology has been used to double-check carbon dating. Its climatic records highlight fluctuations in atmospheric carbon 14, and provide data to correct inaccurate radiocarbon dates.
DIGGING UP THE PAST
Sites are recorded in three dimensions as successive cultures leave evidence in uneven but distinct layers, often cut through vertically by features such as walls or ditches. An analysis of the layers reveals the history of the site through the ages.
SIFTING THROUGH THE STRATA
The remains of older cultures generally lie beneath those of more recent people. By studying the strata of successive cultures, archaeologists determine their sequence.
These strata are of irregular thickness and extent, depending on what they represent-such as accumulations of domestic rubbish, the rubble from demolished building, man-made floors and roads, or soil resulting from natural erosion. The layers are usually dated by then age of the material contained within them.
Vertical features-upright structures such as walls, as well as pits and holes-will cut across the horizontal deposits into layers above and below. Such features will belong to the same era as deposits in the layer relating to them such as one containing the base of a wall or the top of a pit.
AN EXCAVATION IN PROGRESS
To reveal buried objects, workers first remove the topsoil, which is set aside and usually replaced when the excavation is complete. Next, archaeologists set up a grid of markers and string to record the positions of everything the find. The layers of the site are than carefully removed, in order.
Often only a sample portion of an extensive layers is dug, while smaller features, such as hearths, are cut in half excavated in their entirety. Fragle objects such as bones and jewelry are easily damaged-uncovering them requires infinite patience, and the use of precision tools such as dental picks and small brushes. Tiny shells, seeds, and other items too small to spot with the naked eye are recovered using wet sieving combined with fictions techniques. Meticulous plans, drawings, and written notes record the location and context of each find before its removal-information that is vital to enable an accurate reconstruction of daily life on the site.
AFTER THE DIG
Most of the material discovered on a site is sent to laboratory for careful cleaning and treatment-allowing the investigation of tiny residues of food or other material left on the surface of tools containers, which reveals what they were used for.
Conversation is also undertaken to stabilize or restore decaying of broken objects. An army specialist than sets to work analyzing the finds-from pottery, metalwork, human bones, and animal remains to microfauna and plant remains-in order to extract every scrap of information that could shed light on life at the site, and its relationship to the local landscape and the wider world.
MARINE ARCHAEOLOGY
When Jacques Cousteau and Émile Gagnan invented scuba gear in 1943, marine archaeology became a real possibility. The first to put it to use were a team under the American George Bass, who excavated a Bronze Age shipwreck off Cape Gelidonya in Turkey in 1960. Since then, many wrecks and other underwater sites across the world have yielded up their secrets.
LOOKING BENEATH THE WAVES
Researchers investigating drowned settlements, submerged harbours, and shipwrecks use scuba gear in shallow waters and submersibles for deeper work. Remotely controlled submersibles can work in deep sea locations impenetrable by humans – they were used in the investigation of the Titanic and the recovery of material from the ship.
Underwater excavation often presents problems: poor visibility; currents disturbing sediments, equipment, and finds; and the physical limitations on the time investigators can spend under the water. As in land excavations, it is extremely important to record the position of finds. Measurements can be made from grids fixed to the seabed or suspended from a ship above, though these can be difficult to keep in position.
Making detailed plans of marine excavations is far easier today than in the past. Instead of hand-drawn plans, overlapping photographs are taken. These are used to create a mosaic which gives an accurate record of excavated structures or vessels. Video cameras, operated directly or from a remotely controlled submersible, can also be used to create a precise photographic record. The pictures can be manipulated by computers to produce not only plans, but three-dimensional reconstructions.
A new method of accurately recording the position of underwater sites uses GPS – the global positioning system. GPS receivers pick up signals transmitted by satellites in orbit around the Earth. The time lapse between the transmission and receipt of a signal is used to determine an exact position, regardless of bad weather and difficult conditions at sea.
UNDERWATER EXCAVATIONS
The difficulties of recording underwater finds are often balanced by the ease of excavation. In land excavations, soil has to be laboriously dug or scraped away and then removed, but sediments in underwater sites can often be shifted simply by a gentle fanning of the excavator’s hand. The underwater equivalent of the pick and spade is the suction dredger, a machine which sucks up the overburden of mud and sand through its maneuverable nozzle without clouding the surrounding water.
Small objects are placed in a bag or box and taken to the surface by divers. Larger objects are raised with the help of cables, flotation bags, and balloons. The timbers of wrecked ships are often badly decayed, but can be recorded in minute detail by using latex rubber to make moulds of the surviving fragments. Some well-preserved wrecks have been raised to the surface in specially constructed cradles. Underwater sites often yield a wealth of organic remains, from massive timbers to tiny seeds, which will begin to decay when brought to the surface; the work needed to preserve them may involve expenses far greater than those of the excavation itself.