The impossible journey of an ancient stone

Near the centre of the 5000-year-old stone circle known as Stonehenge is a six-tonne stone known as the “altar stone”. It was believed to have been transported from Wales to its current resting place on Salisbury Plain in southern England.

Stonehenge features two types of rocks, the larger sarsens and the smaller bluestones. The sarsens are sandstone slabs found naturally in southern England and weigh 20 tonnes on average. The bluestones are smaller and made of sandstone mixed with volcanic material.  They are called bluestones because they look blue-grey when wet or freshly broken.

The largest of the bluestones is the altar stone and it weighs six tonnes and is 5m long. It is thought that the altar stone was installed in the second phase of the construction of Stonehenge, sometime between 2620 BCE and 2480 BCE. At the winter solstice, the setting sun would have shone between the largest of the trilithons (two upright stones capped by a horizontal lintel) and onto the altar stone. It was a significant part of the Stonehenge set up and for years it was thought to have come from Wales. This alone is an impressive enough feat of Stone Age engineering and transport, but new research has thrown new light on the altar stone.

Analysis of mineral grains in two microscopic fragments of the altar stone found that the stone’s source was the Orcadian Basin in northeast Scotland in the area of modern-day Inverness, the
Orkneys and Shetland. This means that to reach Salisbury, the altar stone would have needed to travel at least 745km by land or 1000km by sea.

Transporting the altar stone by land meant taking the six-tonne rock across rivers, around mountains and through dense forests. The sea route would mean putting the stone on a Stone Age boat and navigating the often-treacherous coastal waters.

We know that the altar stone made this heroic journey, but how and why remains a mystery.