Fine art on a flat roof

摘要

Part of Phil Anderson's house, near Oban in west Scotland, consists of a flat roof of about 7×4 metres covered by a butyl-rubber lining. Early in September 2018, Phil noticed that parts of it were covered by an amazing chiaroscuro of curves and spikes (see figures 1 -7). The organism responsible was never seen, but comparison with website images showed that the trails had been made by terrestrial snails grazing on algae (see also Mason, 2011 and Llewellyn-Jones, 2011). This removed the dark algal layer to reveal the original pale colour of the rubber surface. Older tracks, probably with regenerating algae, were darker and less well-defined than recent ones. Some trails were wider than others, showing that snails of a variety of sizes had fed there.