An occurrence of the trace fossil Teredolites clavatus crowded within the top surface of a coal layer is described and illustrated. The coal, of Late Cretaceous (early Maestrichtian) age, underlies a mud-filled channel in the Horseshoe Canyon Formation near Drumheller, Alberta, Canada. Deposited under estuarine conditions, the coal was buried, and later re-exposed in the floor of a marine channel, where it was bored extensively by pholadid bivalves of the genus Martesia. Xylic substrates contrast in many ways with lithic substrates. The distinction between the two ichnogenera for club- shaped borings introduced by Leymerie (1842)--Teredolites in woody substrates, Gastrochaenolites in lithic substrates--is upheld. Furthermore, the compacted wooden seafloor does not fall naturally within the softground-firmground-hardground series, but represents a distinct concept, here named woodground. The trace fossil assemblage of the woodground is also highly distinctive in morphology and ecology, and is herein classified as the Teredolites ichnofacies.--Modified journal abstract.