What kind of stones is stonehenge made of

Carrying out the PXRF analyses required access to the monument when it was closed to visitors and included several night shifts and one early morning analysing the lintel stones from a mobile scaffold tower. Data collection is never easy! Analysis of the PXRF data showed that the geochemistry of most of the stones at Stonehenge was highly consistent, and only two sarsens stones 26 and had a statistically different chemical signature.

This was an interesting result as it suggested we were looking for a single main source. Then came a major stroke of luck. We were able to analyse three small samples that had been taken from one of the stones in , Stone 58, part of the group of sarsens with a consistent chemistry. Using a method known as inductively coupled plasma mass spectrometry ICP-MS gave a high-resolution geochemical fingerprint for the Stonehenge sarsen. Like all good detectives, we could now compare our fingerprint with those of the potential sources.

Sarsen blocks are found widely scattered across southern Britain, broadly south of a line from Devon to Norfolk. Comparing the geochemical signature from Stone 58 against our resulting data revealed only one direct chemical match: the area known as West Woods to the south-west of Marlborough.

We could therefore conclude that most of the Stonehenge sarsens were from West Woods. Our results not only identify a specific source for most of the sarsens used to build Stonehenge, but also open up debate about many connected issues. Researchers have previously suggested several routes by which the sarsens may have been transported to Stonehenge, without actually knowing where they came from.

Now these can be revisited as we better appreciate the effort of moving boulders as long as 9m and weighing over 30 tonnes some 25km across the undulating landscape of Salisbury Plain. The so-called Altar Stone that lies within the sarsen horseshoe is a foreign sandstone — different from the sarsen sandstone. At least two other sandstone monoliths of unknown origin were also in the bluestone circle. In all, at least 20 rock types have been identified at Stonehenge. Furthermore, and perhaps most important, archaeologists have uncovered diabase fragments from a number of archaeological sites in the area that are far older than the earliest stone settings at Stonehenge — a powerful clue that the Stonehenge bluestones were already present on Salisbury Plain long before Stonehenge was erected.

The lowland plain is underlain by Cretaceous-aged soft chalk. These geologists found no sandstone bedrock at the surface within 10 kilometers of Stonehenge. The sarsen sandstone pillars that form the outer circle and the trilithon horseshoe may have come from the Marlborough Downs outcrops. On the other hand, some early geologists thought that the stones might have come from an ancient litter of sarsen stones in the immediate vicinity. These geologists also confirmed that the bluestones had no known source in southern England.

In , geologist Herbert Thomas suggested that the Stonehenge bluestones matched a suite of igneous rocks found in the vicinity of Carn Menyn, a rocky outcrop in the Preseli Hills in western Wales, more than kilometers away. Detailed petrographic studies later confirmed this match. These early geological studies were right on track. But unfortunately, in , the story took a wrong turn.

He formally announced his findings to the Society of Antiquaries in London in Geologists of the day did not seriously challenge his ideas on human transport, and in the decades that followed, this hypothesis was repeated and elaborated ad infinitum. It became accepted as fact. The only substantial differences were that later writers suggested that the stones had been transported by sea from Wales and across the Bristol Channel, and that the stones had come from a single bluestone quarry at Carn Menyn.

Even a June article in National Geographic states this as accepted fact. But why would a geologist put forward a human transport theory? You would think that a geologist would have looked for a natural explanation for the transport of these stones — and apparently Thomas did consider this option, but only superficially. Glaciers have the capacity to move gigantic rocks from one place to another. The last glacier to flow across this region was part of the Irish Sea Glacier, supplied from source areas in Scotland, northern England, Ireland and Wales probably about , years ago.

Some geologists had already shown that ice had reached the southwestern coast of England and had pressed farther east toward the edge of Salisbury Plain, but Thomas inexplicably chose to disregard this evidence.

The human transport idea became firmly fixed in the minds of generations of archaeologists. Few questioned it. Then, in , geologist Geoffrey Kellaway published a study in Nature suggesting that the Stonehenge bluestones were transported onto Salisbury Plain by glaciers. Kellaway also pointed out that, contrary to claims made by archaeologists, many sites in southwestern England have glacial deposits and other glacial remnants.

For example, the Isles of Scilly off the extreme southwestern tip of England were affected by glacier ice, and there are many glacial sites within kilometers of Stonehenge. Near Street, for example, about 60 kilometers west of Stonehenge, estuarine muds overlie old glacial deposits; at Bath, only about 40 kilometers from Stonehenge, erratics and ancient glacial deposits fill rock fissures on the downs.

In addition, the dry canyons at Cheddar Gorge and elsewhere in the Mendip Hills, about 60 kilometers from Stonehenge, were carved by torrents of meltwater that raged as the glaciers melted.

Another crucial piece of evidence is the occurrence of bluestones in a number of Early Neolithic monuments that are as much as a thousand years older than Stonehenge. Surely, Kellaway noted, that boulder must have originated as a glacial erratic. But evidence of glaciation is sketchy at best on Salisbury Plain. Farmers have been clearing stones here for more than five millennia, so today there are few large erratic boulders littering the plains.

And there are few recognizable glacial deposits either. There has never been a comprehensive survey of buildings and walls in this region to look for erratic rock types, so no one knows whether there are other sites that use glacial erratics. The standoff between the supporters of the two hypotheses continued until the s, when a group of geologists from Open University in the United Kingdom showed that the bluestones at Stonehenge had actually come from at least seven locations in the Preseli Hills, some as many as 13 kilometers apart.

When they looked at rock fragments from the pits and banks at Stonehenge, they found another eight rock types. They are made of a very pure sandstone, which is, geologically speaking, very difficult to source because there is nothing really in the stones except silica.

Theoretically, these stones could have come from anywhere. With the new study, they were able to record in considerable detail elements that are present in the sarsens in really small quantities. There are other elements still there in small quantities — in that one per cent. Recent technology developments allowed researchers to measure these trace elements and pinpoint a source site: in this case, West Woods in Wiltshire.

In the traditional narratives of Stonehenge, West Woods was never suggested as a location. Locals know about it, but archaeologists were looking for sarsens mostly out on the open downs. If we can identify this site and excavate it, we can literally prove where the Stonehenge sarsens came from. This is where it gets really exciting. If, as is quite likely, some of the dressing took place at the quarry in other words, the really rough shaping to get rid of the huge irregular lumps on these boulders.

Logically, you would think that the people who built Stonehenge would have done this before moving these huge stones many miles to their final resting point. So we can learn about how they were dressed, whereas at the moment we are completely guessing.

If we can excavate those pits, we can date the excavation of the stones. If we can do that, it would transform our entire understanding of Stonehenge itself. One of the main problems for Stonehenge archaeologists is dating the monument itself, literally dating the erection of the megaliths themselves.

If we can do that, we can learn much more about the people, landscape and societies involved in the building of Stonehenge. So this is really just the beginning of a new episode.