A biogenic sedimentary structure formed by the behavioural activity of an animal on or within a given substrate. The study of trace fossils is called ‘ichnology’. Traces are most frequent at the interface between different lithologies (e.g. sandstone and shale), and are classified on various criteria including morphology and preservation. Of these two, the second is preferred as a toponomic classification (i.e. classification by place of occurrence) and, apart from the processes of preservation, considers the position of the trace within the depositional unit concerned. In 1970, A. Martinsson divided traces into four groups dependent on their relationship to the casting medium: epichnia are surface ridges or grooves; endichnia are tubes or burrows formed within the casting medium; hypichnia are grooves or ridges preserved on the lower surface of the main body of the casting medium; and exichnia are formed by bioturbation outside the main body of the casting medium. See fossilization.