The six-tonne Altar Stone is of Old Red Sandstone from the Orcadian Basin, an area that extends from the Northern Isles of Orkney and Shetland to Inverness and eastwards to Banff, Turriff and Rhynie. Its geochemical composition does not match that of rocks in the Northern Isles, so it can be identified as coming from the Scottish mainland.

Its position at Stonehenge as a recumbent stone within the southwest arc of the monument, at the foot of the two tallest uprights of the Great Trilithon, recalls the plans of recumbent stone circles of north-east Scotland.

Unusually strong similarities in house floor layouts between Late Neolithic houses in Orkney and the Durrington Walls settlement near Stonehenge also provide evidence of close connections between Salisbury Plain and northern Scotland.

Such connections may be best explained through Stonehenge’s construction as a monument of island-wide unification, embodied in part through the distant and diverse origins of its stones.