Laser scans probe Stonehenge secrets

Caren B. Les, caren.les@photonics.com

“In ancient times,” goes the Spinal Tap song, “hundreds of years before the dawn of history, lived a strange race of people: the Druids. No one knows who they were or what they were doing, but their legacy remains, hewn into the living rock ... of Stonehenge.”

The song might not be factually accurate – carbon dating proves that the Druids came along too late to take credit – but it does capture the mystery that has surrounded Stonehenge for ages. And now a comprehensive 3-D laser scan of the ancient monument has given researchers new evidence of how it was built and what its purpose was.

Data from this first full laser survey on Stonehenge, commissioned by the London-based English Heritage organization, reveals significant differences in the ways the stones were shaped and worked.

A full 3-D laser scan of Stonehenge offers evidence that the northeast view of the prehistoric monument was especially important to its creators.

Archaeological analysis of the scans shows that the stones in the outer sarsen circle, visible from the northeast approach, were completely pick-dressed – that is, their surface crust was removed to expose a bright gray-white surface. In contrast, the outer faces of the surviving upright stones in the southwestern part of the circle did not receive this technical attention. The data also shows that the sides of the stones bordering the solstice axis were carefully worked to form very straight and narrow rectangular slots.

The scan analysis strongly suggests that special effort was made to dress the stones that flank the northeast/southwest axis to allow a more dramatic and obvious passage of sunlight through the stone circle on midsummer and midwinter solstices, English Heritage said.

The variations show not only that the monument aligns with the solstices, but also that its view from the Avenue – the ancient processional way to the northeast – was especially important to the early architects.

To approach and view the stone circle from this direction means that the midwinter sunset had special meaning to prehistoric people; English Heritage said that the builders of Stonehenge made deliberate efforts to create a dramatic spectacle for those approaching the monument from the northeast.

“We didn’t expect the results to be so revealing about the architecture of Stonehenge. It has given further scientific basis to the theory of the solstitial alignment and the importance of the approach to the monument from the Avenue in midwinter,” said Susan Greaney, senior properties historian at English Heritage.

The data also recorded the monument’s visible graffiti, damage, weathering and restoration; identified known prehistoric carvings as well as new carvings; and provided information on the incompletion of the sarsen circle. “The new presentation of Stonehenge will enable visitors to appreciate the importance of the solstitial alignment far better,” said Loraine Knowles, Stonehenge director at English Heritage. “[That’s] why we are closing the A344 [road] which severs the alignment, to enable the stone circle to be reunited with the Avenue.”