Image of Stonehenge at the summer solstice from: https://www.worldatlas.com/articles/history-and-mystery-behind-the-origin-of-stonehenge.html
Stonehenge is one of Britain's most iconic prehistoric monuments and a World Heritage Site. Sitting on Salisbury Plain in Wiltshire, the ruins consists of a number of incomplete stone circles and horseshoes that are aligned to face towards the sun at the summer solstice. The building of the monument is thought to have been built over five stages between 3000-1500 BC, starting in the Neolithic and finishing with some modification in the Bronze Age. Archaeologists have long studied the when, why, how and who of the monuments history and studying the geology could be the key to answering these questions.
In terms of geology, the site is split into two sections. Firstly, the outer ring of vertical standing stones topped by horizontal lintels are known as sarsens. The inner ring is made of smaller volcanic stones which up until the 19th century where thought to be of exotic origin. These are a collection of different types of rocks and are collectively known as bluestones - a term of convenience rather than geological precision.
Plan view of Stonehenge showing the circular arrangement of the stones from: https://www.guidelondon.org.uk/blog/beyond-london/top-10-things-to-see-at-stonehenge/
The provenance of the sarsen standing stones has been relatively easy for geologists and archaeologists to figure out. They are silicified sandstone blocks, abundant in the region, thought to have been quarried about 50km from Stonehenge itself. Although these would not have been easy to transport, it is more than likely that the builders were able to either roll or drag the stones to their current positions in the monument.
Sarsen stones are made of a dense, hard rock created when sand was cemented by silica during Neogene/Quaternary weathering. Possibly when ground water rushed through buried sediment. The quartz grains making up the rock are very hard and resistant to wind and water. This durability makes them perfect for monuments - able to withstand centuries of weather without crumbling.
These rocks were first studied in the late 1950s when a core was extracted whilst operations were undergoing to restabilise parts of the monument. The core was given to Robert Phillips who was onsite and involved with the conservation efforts. Phillips took the core with him when he moved to the US in 1977 but it was returned to Britain in 2018 for scientific purposes.
The provenance of the inner bluestones however has been much more of a mystery. The term bluestone refers to all of the non-sarsen stones at Stonehenge. They are mostly dark, heavy, medium grained rocks, harder than granite and represent at least 46 different individual lithologies. The rocks are mostly low-grade metamorphosed dolerites, rhyolites and some volcaniclastics and most are now believed to have come from a place called Preseli Hills, about 250km away in western Wales. The question however, still pondered by archaeologists today, is how did they get from Wales to Wiltshire?
There are two main possibilities. The first is that they came to England via glacial transportation. This theory however comes to a dilemma as there are no other bluestone fragments that have been found in or around the area of Stonehenge as you would expect if they had been transported glacially.
The other option is that they were mined and transported by humans. In 2015, neolithic quarries were discovered in Preseli Hills at Carn Goedog and Craig Rhos-y-felin. There has been some debate in the geological community as to wether these are indeed quarries or are simply natural features carved out by glaciation. This being said, the discovery of a bluestone dolerite axe at Preseli Hills and the Boscombe Bowmen burial site near Stonehenge perhaps suggest that human quarrying and transportation could be the answer. It is very plausible that the natural volcanic pillars could have been hit at their vertical joints and the stones broken free in order to then be transported to the monument.
As there still yet to be any evidence provided for a cross land or sea transport route from the Preseli Hills to Salisbury Plain, the discussion surrounding the provenance of the bluestones is ongoing. However, the identification of rock source and knowledge of glacial transport processes brought to the discussion by geologists has and will continue to be a key part of finding the answer.
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