A vertical sequence of several different kinds of distinctive sedimentary rock units that is repeated upward through the stratigraphic succession. Originally defined in the rock succession of Pennsylvanian age in the Illinois Basin in the 1930s, the rock types include coal, limestone, sandstone, and several types of shale and mudstone. Cyclothems were soon recognized elsewhere in rocks of this age in the central and eastern United States. Those in the Midcontinent (Kansas and states to the northeast) are dominated by several types of limestone and shale, with less coal and sandstone. Those in the Appalachian region are dominated by coal, mudstone, shale, and sandstone, with less limestone. Some workers generalized the term, and simple cyclothems became recognized in coal-bearing successions of other ages, but the term never achieved wide usage outside of rocks of Pennsylvanian age.