This story is at https://www.livescience.com/stonehenge-pillars-mesozoic.html … and https://www.sciencealert.com/rediscovered-chunk-of-stone-henge-gives-a-g… … Back in 1958, during restoration work at Stonehenge, one of the uprights was found to be cracked. Three cylindrical cores were drilled out of the stone. One of them was kept as a souvenir by a chap who later emigrated to the US, taking the core with him. Later, the stone was returned, when he was in his 90s, some 60 years later. It has a tale to tell. In order to avoid any further damage and the crack expanding, possibly leading to a break up of the upright, three cores were drilled out by a diamond cutting company from Basingstoke. Metal rods were inserted into the cores to strengthen the stone and prevent a split at some point in the future, They were capped by 3 small sarsen plugs. Since it was returned to the UK the core has been CT scanned, zapped with x-rays, and analysed under microscopes. The chemistry and sedimentology of the sarsen have proved interesting – as much to the mainstream, as it could, perhaps, to the odd catastrophist. See also https://archaeology.co.uk/articles/news/getting-to-the-core-of-stoneheng…
It has long been recognised by geologists that the sarsens, a type of sandstone, were formed during the Paleogene period – at an indeterminate point in time. The Paleogene is dated immediately after the catastrophic asteroid strike at the K/Pg boundary. The Paleogene, we are told, is dated from 66 to 23 million years ago – a very long period of time. The Paleogene and the Neogene divide the Tertiary [hence the former designation, K/T boundary event]. The Paleogene itself is divided into 3 periods – the Paleocene, Eocene, and the Oligocene. The Neogene is divided by the Miocene and the Pliocene, 23 to 2 million years ago. This, on the face of it, opens the date of sarsen formation to a very wide period of time. The sarsens are provisionally dated around 20 million years after the K/Pg boundary event. Even though no geology exists between the chalk and the sarsens [a silcrete, or cemented sandstone]. Therefore it is possible the sand came first but was later crystallised at some point within the Paleogene, although later layers of geology such as the clays on top of the sandstone and chalk have to be accounted for. Peter Toghill in 'The Geology of Britain' is worth looking at as a rough guide. It is also worth pointing out there is a bit of a controversy over when the sarsens actually formed but Toghill does say the sequence is clearer in northern France. Toghill makes the point that a considerable amount of erosion occurred on the top of the chalk formation prior to the laying down of the sarsen layer. As a silicate or silcrete, it is something special to break it up into naturally occurring blocks, some very large but many quite small. That something, it is alleged, was freeze and thaw during the Pleistocene Ice Ages, and the tundra conditions that existed on occasion on the chalk plateau. So, even though the sarsen was laid down a long time ago it was still subject to earth processes in the last couple of million years [assuming the prognosis is right]. However, we may note the fact that no geology was laid down for 20 million years is something of a mystery – between the chalk and the sandstone. Either the date of the formation is wrong or the sediments are over dated by uniformitarian methodology. The amount of evolution that occurred during the Paleogene would suggest a long period of time had elapsed. That is not a problem for catastrophists who view most of the geology as a relic of catastrophes in the past – and a period without any geology laid down merely means no catastrophes occurred within the 20 million years. Uniformitarians, on the other hand, assume geology is being formed slowly all the time, a continuous process – yet they have a gaping hole. Even if the post K/Pg geology had been washed away one would think it would have survived in patches in hollows and crannies.
When looked at under a microscope the sarsen core was found to be 99.7% quartz. A quartz cement held firm to medium quartz grains to form an 'interlocking mosaic fo crystals'. This made the stone durable – or very hard. At https://howtofindrocks.com/how-is-quartz-crystal-formed/ … we are told quartz is a hard and crystalline material that consists of two oxygen and one silicone atoms. It is thought to take years to form the mineral under extrmee pressure – but there is still the possibility it could have formed quickly as it is an assumption. However, true or not, quartz is the second most abundant mineral on earth and is thought to be the crystalline form of silica dioxide.
Quartz crystals typically grow in silica rich 'molten' rock – within watery solutions of silica. As well as pegnatites. The crystals form as a result of hydrothermal processes. Hydro = water and thermal = heat. This means quartz crystals are formed due to high pressure in the presence of charged water solutions. The temperatures are thought to range from 100 to 450 degrees celsius. The pressure was intense, and also, high. The sarsens therefore formed under watery solutions of silica [sand]. Boiling water it might be said. Igneous processes appear to be involved – tectonic or volcanic eruptions. On the other hand, some of the sand grains date back to the Mesozoic era, 252 to 66 million years ago. They were contemporary with the dinosaurs – existing prior to the K/Pg boundary event. Some of the sand grains were even older – going back a billion years ago. A mystery of geology it would seem. However, the sarsen sandstone sits on top of the Cretacious chalk formation, formed apparently, in a succession of algal blooms in the ocean. So, was the sand left on top of the chalk and later transformed into a silcrete by igneous processes [or a catastrophic event].