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I wrote "3.5–1.75 billion years ago" in an academic manuscript recently (referring to a period that began 3.5 billion years ago and ended 1.75 billion years ago), and a professor suggested I change it to "1.75–3.5 billion years ago" because he considered the first version "strangely ordered". Neither of us is an earth scientist, but the context is earth science–related, so I thought it appropriate to ask for advice here.

Is there some rule or standard way of writing these ranges? Should the oldest part (i.e. the largest number) come first or second?

It seems like the oldest part comes first in these sources at least:

https://iwaponline.com/hr/article/53/7/958/89350/Hydrogeology-and-groundwater-quality-in-the-Nordic

[Note: the authors define m.y. as million years ago.]

In this area, the various rock types in the bedrock were formed during the Archean (3,500–2,500 m.y.) and Svecokarelien (2,500–1,950 m.y.) periods. The Precambrian bedrock in the southern parts of Finland and Northeastern parts of Sweden was formed during the Svecofennian orogeny (1,950–1,750 m.y.).

https://www.britannica.com/science/geologic-time

Formal geologic time begins at the start of the Archean Eon (4.0 billion to 2.5 billion years ago) and continues to the present day.

Modern geologic time scales additionally often include the Hadean Eon, which is an informal interval that extends from about 4.6 billion years ago (corresponding to Earth's initial formation) to 4.0 billion years ago.

https://en.wikipedia.org/wiki/Hadean

4567.3 ± 0.16 – 4031 ± 3 Ma

https://en.wikipedia.org/wiki/Archean

4031 ± 3 – 2500 Ma

https://en.wikipedia.org/wiki/Proterozoic

2500538.8 ± 0.2 Ma

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    $\begingroup$ Is this an estimate of a particular time in geological history, or is it the beginning and ending dates of a period of time? $\endgroup$
    – Spencer
    Commented May 29 at 16:15
  • $\begingroup$ The latter. I've updated the question. $\endgroup$
    – KQUB
    Commented May 29 at 18:50

3 Answers 3

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If this is an estimate of the time a given event happened, then it's OK to use the youngest date first. But it's also OK to say it the other way.

"The KQUB Formation consists of heavily metamorphosed amphibolite, and is hard to date, but its placement can be bracketed to between 1.75 Ga- 3.5 Ga."

But if you're describing the beginning and end of a period of time, definitely use the oldest date first.

"The KQUB period lasted from 3.5-1.75 Ga."

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    $\begingroup$ I was referring to the latter situation. Do you have any reasoning or authoritative resource explaining why one should "definitely" put the oldest date first? Is it standard practice? If so, where? $\endgroup$
    – KQUB
    Commented May 29 at 18:48
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I do not have a specific resource for this, but in general geologists like to narrate from old to young. This is also in agreement with the exerpts you posted!

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    $\begingroup$ I Think this is more an comment than an answer :) $\endgroup$
    – Weiss
    Commented May 31 at 12:48
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Abbreviations and words can have slightly different rules around their usage and this looks like one of those cases. A similar thing happens with BC dates, if you're using the shorthand notation then it's BC 800-400, but if you write the full form it's 800-400 years Before Christ. Similarly "1.75-3.5 billion years ago" and "1.75 to 3.5 billion year ago" are different sentences with slight different rules because English is strange. 3.5-1.75Ga would be a correct notation for defining the time period, 1.75 to 3.5 Billion Years ago would also be correct though because again English is weird. There's probably also an issue here about generational shifts in notation protocols meaning that how your Prof. is used to writing dates and what is now broadly accepted don't necessarily match.

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