Altar Stone    fossil patterning 

Only the upper surface of the Altar Stone can be seen at Stonehenge, yet it gives us a distinctive Lacustrine patterning that is found only on the Rousay Flagstones. The Rousay Flagstone formation extends to Papa Westray with a yellow softer Flagstone lying on top of a hard purple/green coloured micaceous sandstone.  Both kinds of stone was used in the  construction in the Neolithic houses and tombs on Papa Westray.  On Westray is the Pierowall spiral carved stone the most dramatic of all prehistoric art found in Orkney. This is carved into the softer upper Rousay Flagstone ubiqiuitous throughout Westray.  This yellow coloured stone was used as a wall divider in the Neolithic houses, the oldest known being on Papa Westray.