Stonehenge, one of the most examined prehistoric structures globally, is located on Salisbury Plain in southern England. The long-standing debate has continued for decades regarding whether glaciers transported some of the stones to Salisbury Plain or whether Neolithic people transported them over long distances.
A new geological study led by researchers from Curtin University in Australia employs a novel methodology in order to resolve the controversy by utilizing tiny mineral particles, or grains, from sediment deposited in contemporary rivers rather than the stones of Stonehenge.
Researchers from the Curtin University School of Earth and Planetary Sciences collaborated with Curtin University’s John de Laeter Centre in this study to try to establish whether glaciers existed in the region of Salisbury Plain. If they did, then the very presence or absence of a trail of minerals would indicate how the out-of-area stones were transported to Stonehenge.
Dr. Anthony Clarke of Curtin University and his coworkers have analyzed millions of microscopic mineral grains obtained from rivers flowing in and around Stonehenge. Mineral grains serve as geological passports for their formation location and travel routes.
The Curtin University team selected two types of minerals for the geological passport test: zircon and apatite. Zircon is a highly chemically and physically durable mineral that survives repeated cycles of weathering and transportation and preserves its age using uranium-lead isotopes. Although apatite minerals are chemically less likely to endure through prolonged weathering, they retain molecular records of changes concerning the surrounding rock formation. Consequently, the combination of these two minerals creates a unique process to verify whether the stones originated from glacial movement.
According to Clarke, “If glaciers carried rocks from Scotland or Wales to Stonehenge, there would have been a clear mineral signature on the Plain of Salisbury from the rocks.” As time passed, those rocks would have eroded and left tiny grains behind that could be dated to determine where they originated from.
The results of the study stated that the old sarsen stones were probably mined from the West Woods approximately 25 km from the Stone Circle. The smaller bluestones to the south-east of Stonehenge were similar to the Old Red Sandstone from Mynydd Preseli, which is approximately 230 km away and originated in the area of Stormont.
The Altar Stone, which weighs about six tons, is thought to come from the area of northeast Scotland, which is over 700 km away from Stonehenge.
“Salisbury Plain is principally composed of chalk deposited during the Late Cretaceous, while the only mineral present in chalk is zircon.” Therefore, it is an extremely appropriate sample to conduct this test. If glaciers transported zircon from northern British-derived areas or Mynydd Preseli to the Plain of Salisbury, researchers would expect to still find zircon in today’s river sand.
All CID team members used advanced instrumentation to date over 550 zircon grains in four rivers near Stonehenge. Of those 550 grains, 401 were able to accurately provide their respective ages. In addition, those zircon grains encompass a vast time frame that ranges from over 3 billion years to approximately 285 million years.
A large proportion of the zircon grain clusters indicate ages of long-extinct continental crust believed to originate from the northern portion of the United Kingdom and surrounding regions.
Only a small percentage, eight grains, of the range of grain ages is younger than 541 million years. There is only one zircon grain, believed to have originated from the River Wylye, that provides a definitive link to the 464 million-year-old Welsh bluestones.
Researchers indicate that a single zircon grain in a collection of hundreds cannot be viewed as sufficient evidence to support the glacial delivery theory. Zircons are also able to survive numerous cycles of sediment movement, and similar zircon ages can be found in recycled sediment deposits throughout the southern portion of England as well. If large amounts of Welsh material were being carried by glaciers, repetition of the age signal would be expected.
“There was enough material in the sands of the rivers flowing near Stonehenge to indicate the presence of glaciers,” states Clarke. “But we could not identify any such material, suggesting the possibility that humans transported the stones is more plausible.”
The team examined samples of apatite, encompassing approximately 250 apatite grains. For the majority of apatite grains, the ages were found to be approximately 60 to 65 million years, which is younger than the chalk bedrock formed between 77 and 70 million years ago.
The authors conclude that the apatite samples come from the phosphatic zones of the chalk bedrock and that subsequent chemical alteration altered the age of those specific apatite grains. Microscopic imaging revealed features of individual mineral grains consistent with local sedimentary sources as opposed to glacial transport. Zircon and apatite data also suggest that the landscape was created through sediment recycling and not from glacial deposits.
Evidence suggests that large tracts of ancient Paleogene deposits were once located over Salisbury Plain, particularly the Thanet Formation. Deposited approximately 59 to 56 million years ago, these sand deposits contain zircon populations that are strikingly similar to those found presently in streams in the area. As erosion removed these layers, resilient zircons were released and subsequently recycled.
This process accounts for the documented occurrence of northern-type zircons with no corresponding evidence of glacial movement. It also accounts for the presence of local chalk geochemistry in apatites and not those of more distal localities.
“These results provide supporting evidence for 150 years of debate related to ancient sedimentology. Our approach has allowed us to evaluate hypotheses that have persisted for over a century by examining minerals smaller than a grain of sand,” co-author Professor Chris Kirkland shared with The Brighter Side of News.
The findings of this study will alter current views regarding Stonehenge and prehistoric engineering. By removing the glacial transport option, this new evidence fortifies the argument supporting intentional transport by land and water in the Neolithic era of large stones to their final resting locations throughout Britain. Therefore, there was a high degree of planning, joint action, and technical knowledge within ancient cultures.
The findings of this study also illustrate how mineral signatures can answer questions that archaeology alone cannot. This method can be applied to other ancient sites where the mechanism of movement of materials is uncertain.
Furthermore, the results highlight how the landscape provides evidence of its historical nature through mechanisms that last long after ice, rivers, and humans have reshaped the Earth’s surface.
Research findings are available online in the journal Communications Earth & Environment.
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