cyclothems

thumb|Originally proposed by Harold Wanless of the University of Illinois, to describe a Pennsylvanian-age rock succession in western Illinois

In geology, cyclothems are alternating stratigraphic sequences of marine and non-marine sediments, sometimes interbedded with coal seams. The cyclothems consist of repeated sequences, each typically several meters thick, of sandstone resting upon an erosion surface, passing upwards to pelites (finer-grained than sandstone) and topped by coal.