Geologic Secrets of Stonehenge: Source of the Rocks Adds to the Mystery of the Monument

Stonehenge as long been regarded as a mystery in early human culture and technology. Although the monument has been studied extensively by archeologists, many questions still remain on who built Stonehenge and for what reason. Geologists have concluded that the rocks at Stonehenge did not come from the local area, adding questions about how the rocks were transported to their current location in the Salisbury Plain so that the monument could be built.

Archeologists have established that Stonehenge was built in stages. The current state of the Stonehenge monument is what is left from the last stage of development, which occurred about 4,500 years ago. During this final stage of configuration, the stones were arranged so that they frame the sun rising during the summer solstice and the sun setting during the winter solstice.

The Rocks of Stonehenge

Salisbury Plain itself is underlain with a soft chalk formation that was deposited during the Cretaceous period. The stones that make up the monument are of two rock types: sedimentary and igneous. The sedimentary rock is called sarsen sandstone. This sandstone was deposited over much of southern England at the beginning of the Tertiary period about 60 million years ago. These are the larger stones that make up the outer circle of the monument and rock beams across the top of the standing stones. This sandstone is not native to Salisbury Plain and petrological analysis has suggested its source as being an outcrop located at Marlborough Downs 30 kilometers away.

The smaller stones located within the monument are an igneous rock called bluestone. This rock is also not native to Salisbury Plain and must have been transported. The chemical composition of the bluestone - named so because they have a blue-gray color when wet - suggests a combination of intrusive and extrusive rock formations that make up this stone. The intrusive rock is gabbro, which is the same composition as its extrusive counterpart, basalt lava rock. The rock is also rhyolite and other types of pyroclastic rock. The bluestones are arranged in a circle inside the larger sarsen circle. Geologists have used geochmical analysis to match the bluestones to an outcrop located more than 200 km away at Preseli Hills in western Wales.

These bluestones hold another clue - although geologists have confirmed that they did not form in or near the Salisbury Plain, archeologists have come across fragments of bluestones in the Salisbury Plain that were in place long before Stonehenge was built. This evidence supports the theory that the blue stones were not transported to the Salisbury Plains by humans, but must have been transported by some other force.

There are two theories that can explain how these sedimentary and igenious rocks came to be at Salisbury Plain. One theory, favored by archeologists, is that the stones were transported there by the monument's makers. However, this theory does not explain how the bluestones came to Salisbury Plain since it has been established that they were brought there before any development of the monument started. The second theory is that glaciers transported the stones to Salisbury Plain.

In 1971, a geologist named Geoffrey Kellaway published an article in Nature suggesting that the bluestones are actually glacial deposits called "erratics" - boulders that get transported by glacial movement. Moreover, subsequent geochemical studies have established that the bluestones did not just come from one quarry, but multiple quarries throughout Preselli Hills, some as much as 13 kilometers apart.

Geologists have challenged archeologists to explain why, when already facing the task of transporting these rocks more than 200 kilometers over unforgiving terrain, the makers of Stonehenge would have gone around the country side picking up an assortment of stones instead of just sticking to the same quarry. Geologists conclude that these bluestones must have already been conveniently close to the monument's site, transported there by glaciers.

Sources

Mar 5, 2009 12:57 AM

Guest :

Are you aware that there are other sarsens lying around in the landscape of Salisbury Plain (eg the Bulford Stone, the Cuckoo Stone to name but two)?

You write: "...This sandstone is not native to Salisbury Plain and petrological analysis has confirmed its source as being an outcrop located at Marlborough Downs 30 kilometers away..."

If the Stonehenge sarsens were quarried 30km away in the Marlborough Downs and transported to the plain by human activity, are you suggesting that the other sarsen examples were too?

Mar 5, 2009 6:27 AM

Alexandra Matiella Novak :

Guest:

I'm not sure. What do you think? It's possible that these sarsen sandstones could have come from a closer source. In one of the sources listed above, "Stonehenge's Mysterious Stones", it does state, "some early geologists thought that the stones might have come from an ancient litter of sarsen stones in the immediate vicinity." Did these get here by glacier movement?

This source also states, "These geologists found no sandstone bedrock at the surface within 10 kilometers of Stonehenge."

The theory for glacier transport, I think, has been restricted to explaining how the bluestones came to Salisbury Plain. But perhaps it could explain how the sarsen sandstones got there too.

Either way, I think geologists have done a relatively good job at concluding that neither the sandstone nor the bluestones originated from the immediate vicinity. They had to have been transported at some time. Now the question: is were they transported by humans, glaciers, or maybe both?

The thing to take away from all this is it's all still part of the "mystery of Stonehenge". There's still some debate amongst geologists and archeologist regarding a transport theory, especially in the case of the bluestones.

What are your ideas? I'm just now starting to understand the geology behind Stonehenge. The first article I read was the March 2008 National Geographic article, which really accepted the human transport theory for even the bluestones. The second article provides evidence for the glacier transport theory. I recommend reading both and seeing how much this is still a debate.