The geology of Stonehenge.As this post goes up, the sun is rising over the Heel stone at Stonehenge,

The geology of Stonehenge.As this post goes up, the sun is rising over the Heel stone at Stonehenge, in a man-made alignment at least 5500 years old. Today is the summer solstice, one of the only two nights of the year when people are allowed amongst the stones (the second being the somewhat chillier winter solstice in 6 months time). A mixed mob of hippies, druids, wizards and witches, tourists, local farmers, their families and astronomy enthusiasts are all greeting the coming dawn with a wide variety of noises, ranging from shouting and chanting to sounding traditional style curved Celtic horns. Drums have been beating all night, bottles and joints passed around, and a strong feeling of shared camaraderie has grown between these very diverse individuals. People are standing atop the trilithons, gazing awestruck, as our distant ancestors must have done far back in the mists of time. The vigil is over, the moment all have awaited is come. There is a powerful shimmer of excitement hanging in the air that you can almost taste as the dawn approaches, and every hair on your body is prickled up in goose-bumps. With luck, it won’t be raining, and the sight of the sun’s first peep over the Hele stone between the pillars of the central trilithon will be clear and filled with glowing rosy light. In this climactic minute, the whole crowd becomes one entity.Stonehenge was built on Salisbury plain, a plateau landscape of rolling grassy hills underlain by Cretaceous chalk, sitting below the prehistoric Ridgeway that served as a road from the Thames Valley to the West coast. Had you visited it at solstice time in 2600BCE, when the large trilithons (one stone atop two pillars) were being installed, you might have seen a similar scene. Analysis of cattle bones excavated in the nearby population centre of Durrington Walls (the largest stone age site in north-western Europe) shows that they came from all over the isles, and were mainly slaughtered at midsummer and midwinter, when the ceremonial gatherings (and vast communal feasts) presumably took place. Scientists estimate as much as 10% of the population of our isles travelled there every year, and speculate that the building of the henge brought the nation together for the only time in its early history. The first use of the site dates from about 3100BCE, marked by a chalk bank and ditch. Over the next few centuries, a series of cremation burials were marked out by the inner ring of stones. Wooden structures were also part of the site, and the first astronomical alignments were marked out by posts. Then, around 2600BCE the ‘main’ circle of posts and lintels and the horseshoe of uprights and cross stone trilithons was erected, while the inner ring stones were moved inside it. The two rings are made of different rocks sourced in distinct localities, and both show deliberate astronomical alignments. Stonehenge is in fact a complex structure, a palimpsest that was built, modified and rearranged over the 1600 years that it was used. Those in the older ring are smaller (at a mere 4 tonnes each), and were lugged from 240Km away in Wales, though some have suggested that they were locally available erratics, transported by ice age glaciers. These Bluestones (named after their bluey-grey shade when wet, a common event if you’re a British rock) are a mixture of spotted dolerite (a coarse grained basaltic intrusive rock containing big blobby crystals of plagioclase feldspar), silica rich Rhyolitic volcanics, tuffs (compressed volcanic ash) and sandstone. Twenty different rock types have been identified.The Ordovician dolerites have long been known to come from a distinctive outcrop in the Preseli hills of Wales, and are part of an arc volcanic sequence dating from the closure of the Iapetus ocean (the first version of the Atlantic) as it subducted beneath England, joining Scotland’s fate to the rest of the isles. The source of the other bluestones remained mysterious until 2011, when the quarry site for the felsic volcanics was finally pinpointed to a 65 metre outcrop (within the same hills) using detailed petrographic and zircon analyses. Such bimodal volcanic (basalt and rhyolite) rocks are common in arc sequences and continental rifts. Some claim that the arrangement in the stones in the henge reflect that of the outcrop, implying that those hills were a very special site, but with the rearrangements that have happened (only 43 out of an estimated 80 bluestones survive) during the monument’s history, stating such a thing with any certainty is impossible. These quarries are associated with healing springs to this day, and were also a source for handaxes found all over Britain.The huge outer ring stones (including the Heele stone), weighing 25- 50 tonnes, were shaped from a Paleocene silicified sandstone (known as Sarsen stone in old mining speak) and were transported from 'nearby’ outcrops in the Marlborough Downs 40 Km away. They are the remains of a Tertiary silcrete (sand cemented by silica bearing fluids) that once covered most of southern England, a relic of a more arid era, now mostly eroded away by glaciers. Held together by mortise and tenon joints, similar to those used in carpentry, they have been dressed to millimetric precision, even tapering slightly so they look the same from the ground all up their length, negating the effects of perspective. No one know how the rocks were transported, nor the mix of sea and land travel. The glacial erratics theory is mostly discredited, but their feats of transport would challenge modern equipment, and the logistics deployed imply that the bluestones were held to be uniquely special. The Grand Menhir in Brittany, a short hop across the English channel (or The Sleeve as the French call it), weighs an estimated 340 tonnes and travelled a minimum of several miles. No one knows the actual purpose of the complex ritual landscape of the Salisbury Plain, which includes Woodhenge by the Avon river, an avenue linking it to the stone one, many barrows, other stone circles such as Avebury and the largest man made hill in Europe (Silbury Hill). Whether you subscribe to the idea that the site was an astronomical observatory, healing centre, link between the lands of the living and dead or other, the wonder inspired by the presence of these ancient stones is hard to express in words. Knowing the geological story behind them adds another dimension to this electric sensation. I leave you with a poem written by Layamon in 1215CE, based on the 10th century account of Stonehenge (stating that Merlin brought the stones from Ireland) by Geoffrey of Monmouth, who shares with Gildas and the venerable Bede the distinction of being our first historian: The stones are great; And magic power they have; Men that are sick; Fare to that stone; And they wash that stone; And with that water bathe away their sickness. Normal tours take you around the stones, but not amongst them, and are rather expensive and filled with coachloads of tourists. If you ever get the chance to go for the solstice night vigil (I personally recommend summer, though the winter gatherings are more intimate)…Go! Its the only time you can actually touch these mysterious pieces of history and let the awe at being a living human in this special place fill your soul.LozNote: English Heritage rents nearby fields as car parks for solstice night, and they are clearly signposted. The stones open roughly from dusk until 8am the next day (9 in practise), and the car park has to be clear by noon. Food/drink/first aid tent and portable toilet facilities are provided, so logistics is no obstacle to enjoying the vigil. Image credit: Max Alexander, via APOD.http://www.english-heritage.org.uk/daysout/properties/stonehenge/summer-solstice/access-times/The geology of Wiltshire (includes map): http://www.wiltshiregeologygroup.org.uk/geology/http://phys.org/news/2013-03-stonehenge-ancient-rave-theory.htmlhttp://www.guardian.co.uk/culture/2013/mar/16/stonehenge-conflicting-theories-spa-cemeteryhttp://www.guardian.co.uk/science/2013/mar/09/archaeology-stonehenge-bones-burial-groundhttp://www.smithsonianmag.com/history-archaeology/light-on-stonehenge.htmlhttp://ngm.nationalgeographic.com/2008/06/stonehenge/alexander-text/1http://www.earthmagazine.org/article/stonehenges-mysterious-stoneshttp://www.wired.co.uk/news/archive/2011-12/19/stonehenge-rocksPaywall access: http://www.sciencedirect.com/science/article/pii/S0305440310003699Please google search solstice sunrise at stonehenge images to get a feel for the event and enjoy some of the colourful characters who go there. You might even spot me and some buddies (to whom this piece is dedicated) in a few of them. -- source link

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