Although each type of deposit (facies) may be continuous over a wide area, its age varies according to the position of the shoreline through time.
An example is the sandy beds near the end of the lower Carboniferous of the west of England (the Drybrook sandstone of the Forest of Dean).
[2] The detection of diachronous beds can be quite problematic since fossil assemblages tend to migrate geographically with their environment of formation.
The term may also be applied to other features that vary in age, such as erosion surfaces, areas of uplift, etc.
It is also sometimes applied to fossils which appear sporadically at different times in different places due to migration,[3] though such usage is regarded by some authors as incorrect.