Note that the Term 'Sarsen' used on the key refers——to the hard silicified tertiary rock local——to the chalkland of the Stonehenge region, sarsen is an exceptionally obdurate form of sandstone: The reference to sandstone on the key is to other ‘non sarsen’ material. The term ‘bluestone’ conveniently, "though somewhat controversially," describes a generic group of igneous rock exotic to Salisbury Plain. The major group of bluestones visible today are dolerite - whose province is regarded as south west Wales. A number of other igneous rocks are represented within the arrays. Those interested in the exact make up of the blustone assemblage are referred to Cleal 1995. And Cunliffe and "Renfrew 1997."

Cleal, R. M. J., Walker, K. E. & Montague, R., Stonehenge in its landscape (English Heritage, London, 1995)

Cunliffe, B. & Renfrew, C. Science and Stonhenge (Proceedings of the British Academy - 92, Oxford University Press 1997)