^Ritchie, Graham; Anna (1981). Scotland: Archaeology and Early History. London: Thames and Hudson. p. 9. ISBN 0-500-27365-0.
^Royal Commission on the Ancient and Historical Monuments in Wales (1976). An Inventory of the Ancient Monuments in Glamorgan. Royal Commission on the Ancient and Historical Monuments in Wales. p. 17. ISBN 978-0-11-700588-4.