In geology, cyclothems are alternating stratigraphic sequences of marine and non-marine sediments, interbedded with coal seams. Unique to the Carboniferous period, they apparently formed as a result of marine transgressions and regressions related to decay and growth of ice sheets, respectively, as the Carboniferous was a time of widespread glaciation. Cyclothems were possible because of the extremely low topography of the interior lowlands the seas covered and uncovered, which is why they have been absent in the current ice age.