Williams, Leslie (Juni 1988): «Observations on the Flora of Wall Habitats on Yell, Shetland». Fra Ecological Studies in the Maritime Approaches to the Shetland Oil Terminal 1986-1987: Report of the Leicester Polytechnic Expeditions to Shetland, August 1986 and July 1987. Redigert av J.A. Fowler. The David Attenborough Laboratories, School of Life Sciences, Leicester Polytechnic.