Cyclothems and the ice-driven sea
Sea level swings hundreds of times as ice sheets grow and melt, stacking rock in repeating cycles.
- When
- Known to
- ± 15 million years
- Era
- Carboniferous, Paleozoic
Gondwanan ice sheets advancing and retreating drive repeated global sea-level changes of tens of metres. Each cycle deposits the same sequence in coastal basins — marine limestone, then shale, then coal, then sandstone — and these cyclothems are stacked dozens deep across North America and Europe. They are among the clearest records of orbital climate forcing in the entire geological column.