Geologists and archaeologists have long known that the builders of Stonehenge made use of two main types of stone: a silcrete, known as ‘sarsen,’ was used for the large trilithons, sarsen circle and other monoliths, and a variety of ‘bluestones’ — used for the smaller standing stones — were erected in an inner ‘horseshoe’ and an outer circle. Two ancient quarries in the Preseli hills of west Wales — Carn Goedog and Craig Rhos-y-felin — have now been excavated to reveal evidence of megalith quarrying around 3000 BC — the same period as the first stage of the construction of Stonehenge.
“What’s really exciting about these discoveries is that they take us a step closer to unlocking Stonehenge’s greatest mystery — why its stones came from so far away,” said team leader Professor Mike Parker Pearson, a researcher in the Institute of Archaeology at University College London.
“Every other Neolithic monument in Europe was built of megaliths brought from no more than 16 km (10 miles) away.”
“We’re now looking to find out just what was so special about the Preseli hills 5,000 years ago, and whether there were any important stone circles here, built before the bluestones were moved to Stonehenge.”
The largest quarry was found almost 290 km (180 miles) away from Stonehenge on the outcrop of Carn Goedog, on the north slope of the Preseli hills.
“This was the dominant source of Stonehenge’s spotted dolerite, so-called because it has white spots in the igneous blue rock,” said team member Dr. Richard Bevins, from the National Museum of Wales.
“At least five of Stonehenge’s bluestones, and probably more, came from Carn Goedog.”
In the valley below Carn Goedog, another outcrop at Craig Rhos-y-felin was identified as the source of one of the types of rhyolite — another type of igneous rock — found at Stonehenge.
The bluestone outcrops are formed of natural, vertical pillars. These could be eased off the rock face by opening up the vertical joints between each pillar.
Neolithic quarry workers needed only to insert wedges into the ready-made joints between pillars, then lower each pillar to the foot of the outcrop.
Although most of their equipment is likely to have consisted of perishable ropes and wooden wedges, mallets and levers, they left behind other tools such as hammer stones and stone wedges.
“The stone wedges are made of imported mudstone, much softer than the hard dolerite pillars,” Professor Parker Pearson said.
“An engineering colleague has suggested that hammering in a hard wedge could have created stress fractures, causing the thin pillars to crack. Using a soft wedge means that, if anything were to break, it would be the wedge and not the pillar.”
Archaeological excavations at the foot of both outcrops uncovered the remains of man-made stone and earth platforms, with each platform’s outer edge terminating in a vertical drop of about one meter (3.3 feet).
“Bluestone pillars could be eased down onto this platform, which acted as a loading bay for lowering them onto wooden sledges before dragging them away,” said team member Professor Colin Richards, a researcher at the University of the Highlands and Islands.
An important aim of the scientists was to date megalith-quarrying at Carn Goedog and Craig Rhos-y-felin.
In the soft sediment of a hollowed-out track leading from the loading bay at Craig Rhos-y-felin, and on the artificial platform at Carn Goedog, they recovered pieces of charcoal dating to around 3000 BC.
They now think that Stonehenge was initially a circle of rough, unworked bluestone pillars set in pits known as the Aubrey Holes, near Stonehenge, and that the sarsens were added some 500 years later.
The new discoveries also cast doubt on a popular theory that the bluestones were transported by sea to Stonehenge.
“Some people think that the bluestones were taken southwards to Milford Haven and placed on rafts or slung between boats and then paddled up the Bristol Channel and along the Bristol Avon towards Salisbury Plain,” said team member Professor Kate Welham, from Bournemouth University.
“But these quarries are on the north side of the Preseli hills so the megaliths could have simply gone overland all the way to Salisbury Plain.”
The research was published in the journal Antiquity.
Mike Parker Pearson et al. 2019. Megalith quarries for Stonehenge’s bluestones. Antiquity 93 (367): 45-62; doi: 10.15184/aqy.2018.111
Source link: https://www.sci.news/archaeology/stonehenges-bluestones-06924.html