X-radiography has been found to provide a rapid, economical non-destructive method of locating very thin, fine grained tephra of Icelandic origin in Scottish peats. Tephra layers form time-parallel marker horizons, or isochrones that are of great archaeological significance because they cover large areas, offer precise chronological control for associated material and occur at times of significant cultural and environmental changes. Not all of the discrete deposits of recent volcanic fallout in Scotland are sufficiently dense to show clearly on X-radiographs, but those which do, provide particularly clear, easily located marker horizons with many applications in palaeoenvironmental studies. The tephra layer in Caithness produced by fallout from a c. 4000 by eruption of Hekla provides a particularly good example of the use of X-radiography to locate volcanic ash. The method also reveals other structures of archaeological and palaeoenvironmental significance (Butler, 1992, Journal of Archaeological Science19, 151–161).