Geological analysis explains durability of Stonehenge megaliths

Researchers on Wednesday described a battery of examinations that provided a glimpse inside one of Stonehenge's 52 sandstone megaliths, known as sarsens, gaining insight into its geology and chemistry.

They studied a core sample extracted from one of the sarsens, called Stone 58, during 1950s conservation work. It was kept in the United States for decades before being returned to Britain for research in 2018.

The sarsens are made of stone called silcrete that formed gradually within a few yards (meters) of the ground surface as a result of groundwater washing through buried sediment.

The examination clarified Stone 58's internal structure. It showed that the silcrete is comprised of mainly sand-sized quartz grains cemented tightly together by an interlocking mosaic of quartz crystals. Quartz is extremely durable and does not easily crumble or erode even when exposed to eons of wind and weather.

"This explains the stone's resistance to weathering and why it made an ideal material for monument-building," said University of Brighton geomorphologist David Nash, who led the study published in the journal PLoS ONE.

In a remarkable engineering achievement by late Neolithic people, the sarsens were erected at the site in Wiltshire, England around 2500 BC. Stone 58, one of the giant upright sarsens at Stonehenge's center, stands about 7 meters (23 feet) tall, with another 2 meters (7 feet) underground, and an estimated above-ground weight of 24 tons.

The core sample is a rod of stone, about an inch (2.5 cm) in diameter and roughly a yard (meter) long. Its cream color is brighter than the pale-gray exterior of the megaliths, which have been exposed to the elements for millennia.

It was given as a souvenir to a man named Robert Phillips who worked for a company involved in the conservation work and was on-site during drilling. Phillips took it with him with permission when he emigrated to the United States in 1977. Phillips decided to return it to Britain for research in 2018. He died in 2020.

"Getting access to the core drilled from Stone 58 was very much the Holy Grail for our research," Nash said. "All the previous work on sarsens at Stonehenge involved samples either excavated from the site or knocked off from random stones."

The researchers used CT-scanning, X-rays, microscopic analyses and various geochemical techniques to study fragments and wafer-thin slices of the core sample - such testing being off limits for megaliths at the site.

"This small sample is now probably the most analyzed piece of stone other than moon rock," Nash said.

It remains unclear precisely when the rock formed, though the researchers found that some embedded sand grains dated to as long ago as the Mesoproterozoic Era, 1 billion to 1.6 billion years ago.

Nash led research published last year https://www.reuters.com/article/us-science-stonehenge/scientists-solve-mystery-of-the-origin-of-stonehenge-megaliths-idUSKCN24U2VG involving the same core sample that showed that 50 of the 52 sarsens share a common origin about 15 miles (25 km) from Stonehenge at a site called West Woods. Stonehenge's builders may have either dragged or moved the huge stones on rollers.