A close-up view of the stone circle at Stonehenge

Stonehenge: digital mapping reveals the true picture

New research conducted by archaeologists and scientists from Birmingham University and the Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology in Vienna has revealed that Stonehenge did not sit alone within its Neolithic landscape.

Stonehenge: a national monument, an internationally recognised British icon. For centuries, people have wondered, theorised, intrigued and argued about who, or what, might have put an isolated collection of huge standing stones in the middle of the Wiltshire countryside somewhere between five and six thousand years ago – and for what purpose.

Now, new research conducted by archaeologists and scientists from Birmingham University and the Ludwig Boltzmann Institute for Archaeological Prospection and Virtual Archaeology, in Vienna has revealed that Stonehenge did not sit alone within its Neolithic landscape.

The research has revealed that the area around Stonehenge was heavy with additional monuments, chapels and burial chambers, which until now have remained hidden underground or inside known earthworks.

Using motorised magnetometer systems, ground-penetrating radar arrays and electromagnetic induction sensors, the researchers surveyed six square miles around Stonehenge over a four-year period, beginning in July 2010. The technology allowed them to investigate as deep as seven metres below the ground without the need for intrusive and time-consuming digging.

Digital mapping

From the results, the researchers created a detailed digital map of the area. “This technology has been used before to find hidden archaeological remains,” says geophysicist, Immo Trinks, head of research and development from the Ludwig Boltzmann Institute, “but never before, on such a large scale, over such a large area.”

Vince Gaffney, from the University of Birmingham, who headed the Stonehenge Hidden Landscapes project, explains that Stonehenge was once part of a rich ceremonial landscape. “People came from all over the country to visit Stonehenge itself, which was a large ritual structure, but around it people were creating their own shrines and temples,” he says. “The whole landscape was being used in very complex ways. It’s teeming with previously unseen archaeology.”

The researchers found a 108-foot long barrow, a collective tomb made from a huge timber frame and covered with earth. Long barrows are common relics from the Neolithic period, with approximately three hundred known barrows identified in the British Isles alone. To the naked eye, they appear as rectangular or trapezoidal earthen mounds. Between 4000 and 2500 BC, Neolithic man used them to inhume the dead, often ritually removing flesh to leave just the bones. Many long barrows have been damaged by modern-day farming tillage practices. The barrow found within the Stonehenge landscape contained a large timber building, built before Stonehenge itself was completed.

Nearby, the researchers uncovered the Durrington Walls superhenge. A huge ritual monument with a circumference of 1.5km, it would have dwarfed Stonehenge. The monument would have been surrounded by a bank and ditch, had internal wooden rings and enclosure and a ten-foot-high stone row on one side, according to the imprints left in the ground. The experts believe that some of the stones may have been pushed over.

Also uncovered during the investigations were seventeen ritual monuments; a series of prehistoric burial pits aligned with the setting solstice sun; hundreds of burial grounds; evidence of Stone Age, Bronze Age and Iron Age settlements and a prehistoric ring ditch that appears, oddly, like a smiley face. A ring ditch is a form of earthwork associated with human use of an area and archaeologists tend to use the term when they’re not certain what sort of land use the markings represent. The Stonehenge team has suggested that these particular markings represent burial sites.

Filling in the blanks

Chris Gaffney, Vince’s brother and Head of Archaeological Sciences at the University of Bradford, a partner in the Stonehenge Hidden Landscapes project, explains that as Stonehenge is a World Heritage Site, archaeologists don’t get many opportunities to excavate there, especially over large areas. “For years, people have assumed that the huge areas surrounding Stonehenge contain a limited number of sites of archaeological interest,” Gaffney says. “We wanted to investigate the areas in between the known monuments to see if it really was blank. We discovered that what we see today around Stonehenge is actually a manicured landscape, not at all like the place that people used and travelled to all those years ago.”

Chris Gaffney adds that the idea behind the investigation was to produce a detailed map of the landscape around Stonehenge, which future archaeologists could then use to find out more about the landscape: “It was never the remit to dig up what we found, just to find what was hidden.”

To create such a map, the researchers needed vast amounts of data from beneath the surface. However, there were two problems. Firstly, at the outset, no one knew where the features of interest might be. Secondly, the Stonehenge ‘envelope’, the area surrounding the main monument at Stonehenge to the horizon, some twelve square kilometres, is a very large area, even for cutting-edge geophysical technology.

Traditionally, archaeologists have used manual geophysical prospection methods to investigate individual sites and small areas: “Mostly one henge at a time or a single Roman villa,” Trinks explains.

Trinks adds that using manual techniques, it would take archaeologists many years – if not decades – to investigate an area the size of the Stonehenge envelope. The team’s multichannel motorised magnetometers, ground-penetrating radar systems and exact satellite positioning systems permitted the mapping of very large areas quickly and with very high spatial measurement resolution.

Magnetometers map archaeological structures that cause anomalies in the Earth's magnetic field. They work best in large open areas and are sensitive enough to pick up the slightest change in the Earth’s magnetic field over a particular spot. “Magnetometers reveal patterns by recording the magnetic properties of ferrous elements in the soil, or those left behind after human activity – for instance, burning,” Chris Gaffney says.

Trinks adds that in Central Europe, the Earth’s magnetic field has the strength of approximately 48,000 nanoteslas. He explains that the presence of a filled ancient pit, ditch, or even a post hole in the landscape can change the magnetic composition of that particular spot by as little as 0.2 nanoteslas. “Fluxgate gradiometers like those used at Stonehenge can pick this up in the uppermost soil layers,” he says.

The Stonehenge researchers also used more sensitive, caesium magnetometers, which can pick up even smaller magnetic field variations, between one and 10 picoteslas. “This allowed us to pick up magnetic anomalies caused by bacteria sometimes found in humid soils,” Trinks says. “Organic material, perhaps left behind by a decayed wooden post, could have been food for these bacteriae.”

Chris Gaffney adds, “For successful use of magnetometry there must be a measurable contrast in magnetic properties of the archaeological features being searched for, for example the backfill in pits or ditches, and the surrounding soil. The chalk landscape at Stonehenge has a low magnetic background which provides the high contrast needed.”

Trinks explains that the Stonehenge researchers used magnetometers as gradiometers. They consisted of two sensors with one placed 20 to 30cm above the surface and the upper sensor 65cm to 150cm. Both sensors measure simultaneously and their difference offsets external interference to the Earth’s magnetic field in order to gain precise readings. “The lower sensor measures a much greater effect of the archaeological remains in the soil,” Trinks says. “The Earth’s magnetic field can be affected by solar activity. This can change readings by tenths or hundreds of nanoteslas and this can happen over the course of minutes. The trick is to measure variations in the difference between the two readings, which cancels out the temporal field variations, leaving you with readings that tell about the possible presence of interesting structures in the ground.”

Magnetometer surveys

To cover more space in less time, the Stonehenge researchers towed their magnetometers around on non-magnetic plastic carts, attached to quad bikes, at speeds of up to 20-35km/h. Each cart carried up to ten fluxgate gradiometers. This gave the researchers the necessary speed, sensitivity and spatial resolution to investigate such a large area. In all, they covered 12km2 at a resolution of 10cm x 25cm.

Magnetometer surveys provide a single data value for each surface point, but not direct information about the depth of the buried structures. To get this information, the researchers used ground-penetrating radar (GPR) over two-square-kilometre areas of interest, at the high resolution of 8 x 8 cm, using sixteen 400MHz georadar antennae in parallel.

GPR systems beam electromagnetic waves into the Earth’s subsurface and record the reflection from objects underground. Archaeologists get detailed 3D information about the depth, shape and location of interesting objects. GPR can detect stone structures, interfaces caused by pits and trenches, cavities and differences in soil humidity or clay content. This process takes longer than collecting magnetic readings, so the researchers walked the GPR sensors over burial mounds and pushed and pulled the GPR arrays along by tractor.

Until recently, GPR surveys have mostly been small-scale operations, using single antennae systems to collect data at reduced spatial measurement resolution -  “25cm profile spacing at best,” says Trinks. For the Stonehenge project, new multichannel GPR arrays provided considerably increased spatial coverage and greatly improved sample spacing. As a result, the researchers were able to obtain higher resolution images, structurally clearer than ever before.

All around the world, archaeologists use ground-penetrating radar to pick up hidden features in landscapes: ancient hill forts in Yorkshire; 18th-century farms covered by forests in New England; even an 11,000-year-old settlement underneath the Baltic Sea. When filming last year’s Hollywood film, Pompeii, crew members flew over the city in a helicopter and used a 3D laser mapping lidar system to make a precise 3D landscape map that showed the tiniest detail. From this, the film crew digitally recreated the lost city for the big screen.

Technology has changed the nature of archaeology. Last year, US researchers fitted GPR to an unmanned aerial vehicle and used it to map parts of the Peruvian jungle. They were looking for lost Inca cities. In June 2013, Australian archaeologists used GPR to create a 3D image of Mahendraparvata, a forgotten Medieval City in Cambodia: it took them just twenty hours. This August, researchers used magnetometers to search for traces of the French Fleet, which had sunk in 1565 during a hurricane off the Florida coast of America. However, this sort of technology doesn’t perform as well in urban areas: there is simply too much modern debris, wires and infrastructure in the way. For this sort of work, archaeologists still need their traditional picks, shovels and trowels.

Crunching the numbers

The Stonehenge researchers gathered 50 to 150Mb of data from each magnetometer and between two and four gigabytes from the GPR every day. The researchers needed data-processing software that could deal with these large amounts of data efficiently and to check whether the data had been measured correctly. For this, Ludwig Boltzmann’s ArchPro specialist, Alois Hinterleitner, developed custom software, which the researchers used to obtain geo-referenced 2D and 3D raster data sets.

Hinterleitner’s software can partly filter out irrelevant information, producing first visualisations of measured data on a screen in the field. The operators then knew which areas they’d covered and where to go next.

The Hidden Stonehenge Landscapes project was conceived to provide detailed information about the hidden archaeology in the landscape. That means turning large quantities of organised data into something people can look at and understand in map form, “a virtual model of the Stonehenge landscape, possibly even an augmented reality application,” as Trinks puts it.

Previously, with smaller investigations, researchers would have gathered the raster images into a GIS environment and drawn any features of interest manually. A large-scale investigation requires an automatic, or at least semi-automatic, method of classifying and arranging the data. The Ludwig Boltzmann Institute and its partners are currently developing such technology, embedded into a GIS platform that integrates data management and interpretation.

“We would like to have tools that permit dynamic visualisations, spatial awareness and management of large amounts of data,” says Trinks. “GIS environments are commonplace, but intelligent feature extraction, semi-automatic interpretation algorithms and advanced visualisation tools are far from defining the standard yet.”

A fresh perspective on Stonehenge

 “Hopefully we have changed the research agenda,” Chris Gaffney says, referring to the world’s archaeological understanding of the Stonehenge site. “This could involve future excavations of some of the features we’ve discovered. When they do, archaeologists will be able to dig with pin-point accuracy.”

One thing we do now know for sure: there was much more to the prehistoric ritual landscape of Neolithic Wiltshire than the giant sarsens and bluestones that comprise the Stonehenge stone circle standing on the plain in splendid isolation.

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