Geologic time scale

I thought it would be useful to have a geologic time scale on hand for reference. The Quaternary is truncated, but who cares about the Holocene? The four columns deal with different spans of time and so are not proportional to each other.

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"Ma" refers to "million years" (technically, "Ma" stands for "megaannum", "mega" for million and "annum" for years) and "Bndy Age" is just "Boundary Age" shortened. For those of you interested in such things, the colors are from the USGS, the basic design was adapted several iterations ago from a GSA timescale and has diverged in its own merry way, and dates and additional information have come from "Geologic Time Scale 2020" (Gradstein et al., editors), the International Commission on Stratigraphy, and, farther back, USGS Fact Sheet 2007-3015. (I should note that the ICS numbers are not exactly the same as the numbers in Gradstein et al. 2020.)

[Content below added 2025/10/16] 

You may also notice references to North American Land Mammal Ages in these posts and The Compact Thescelosaurus when the talk veers into the Cenozoic of North America. The land mammal ages do not line up consistently with the geologic time scale, and in fact are not necessarily firmly dated in terms of years because they're biostratigraphic. The following list of names, dates, and rough equivalents is after Barnosky et al. (2014), including the two ages they introduced which are quite useful biostratigraphically regardless of one's opinions of the Anthropocene. The dates should not be considered hard and fast.

• Saintaugustinean: AD 1540–ongoing (latest Holocene)
• Santarosean: 14,000 years before present–AD 1540 (latest Pleistocene–most of the Holocene)
• Rancholabrean: 210,000 years age–14,000 years before present (late Middle–most of the Late Pleistocene)
• Irvingtonian: 1.4 Ma–210,000 years ago (second half of the Early Pleistocene–most of the Middle Pleistocene)
• Blancan: 4.7–1.4 Ma (late Early Pliocene–middle Early Pleistocene)
• Hemphillian: 9.4–4.7 Ma (middle Late Miocene–late Early Pliocene)
• Clarendonian: 12.5–9.4 Ma (late Middle–middle Late Miocene)
• Barstovian: 16.3–12.5 Ma (late Early–late Middle Miocene)
• Hemingfordian: 18.5–16.3 Ma (middle–late Early Miocene)
• Arikareean: 29.5–18.5 Ma (late Early Oligocene–middle Early Miocene)
• Whitneyan: 31.8–29.5 Ma (middle–late Early Oligocene)
• Orellan: 33.9–31.8 Ma (early–middle Early Oligocene)
• Chadronian: 37.0–33.9 Ma (almost all Late Eocene)
• Duchesnean: 39.7–37.0 Ma (late Middle–earliest Late Eocene)
• Uintan: 46.2–39.7 Ma (early–late Middle Eocene)
• Bridgerian: 50.5–46.2 Ma (middle Early–early Middle Eocene)
• Wasatchian: 54.9–50.5 Ma (early–middle Early Eocene)
• Clarkforkian: 56.2–54.9 Ma (basically early Early Eocene with a tiny sliver of latest Paleocene)
• Tiffanian: 60.9–56.2 Ma (almost all Late Paleocene)
• Torrejonian: 63.8–60.9 Ma (middle Early–early Late Paleocene)
• Puercan: 66.0–63.8 Ma (early–middle Early Paleocene)

References

Barnosky, A. D., M. Holmes, R. Kircholtes, E. Lindsey, K. C. Maguire, A. W. Poust, M. A. Stegner, J. Sunseri, B. Swartz, J. Swift, and N. A. Villavicencio. 2014. Prelude to the Anthropocene: Two new North American land mammal ages (NALMAs). The Anthropocene Review 1(3): 225–242.