Period· The columns of time

Quaternary

2.6 Ma → Today

The period of the ice ages, and the only one in which our own species exists: ice sheets advance and retreat dozens of times to an orbital beat.

What the evidence shows

Ice sheets grow over North America and northern Europe and melt away again, dozens of times, following slow changes in the Earth's orbit and in the tilt of its axis that alter how much sunlight the north receives in summer. The link was demonstrated in 1976, when those periods were found in the isotope record of deep-sea cores. Sea level moves with the ice through more than a hundred metres, and the coastline a map shows is only the one this interglacial happens to have. The period holds two epochs of very unequal length: the Pleistocene, which is the ice ages themselves and runs for all but the last eleven thousand years, and the Holocene, the interglacial we have not yet left.

Bounds and conventions

Its base was moved from 1.8 to 2.58 million years by a ratification in 2009, a reminder that a boundary is a decision before it is a date. Its top is the present, so there is no boundary to place, and whether the last century deserves a division of its own has been formally proposed and formally refused. The orbital pacing itself is not in doubt; what the orbit cannot explain is why the rhythm changed halfway through.

Sources

1. Cohen, K. M., Finney, S. C., Gibbard, P. L., Fan, J.-X. (2013). The ICS International Chronostratigraphic Chart. Episodes 36(3), 199–204, mise à jour continue, voir Cohen et al. (2025), Episodes.
2. Lambeck et al. (2014), PNAS · Sea level and global ice volumes from the Last Glacial Maximum to the Holocene
3. Hays et al. (1976), Science · Variations in the Earth's orbit: pacemaker of the ice ages
4. Lisiecki & Raymo (2005), Paleoceanography · A Pliocene-Pleistocene stack of 57 globally distributed benthic oxygen isotope records
5. Ford & Chalk (2020), Oceanography · The mid-Pleistocene enigma
6. Hublin et al. (2017), Nature · New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens
7. Head, M. J. (2019). Formal subdivision of the Quaternary System/Period: Present status and future directions. Quaternary International 500, 32–51.