A geological term for a set of beds laid down in a cycle
A cyclothem is a sequence of beds deposited in a single cycle of sedimentation, e.g. in coal-bearing strata, the sequence is: sandstone, shale, fireclay, coal, shale, sandstone. Or, in numerical representation: 1,2,3,4,3,2,1.
Cyclothems are believed to represent a variety of tectonic, climatic or sedimentological factors, as exampled by alternating layers of terrestrial and marine sediments, laid down during a period where sea levels retreated and advanced.