Ordering dates when writing about geologic time?

December 2, 2010 10:09 AM

What's the proper way to order dates in geologic time when writing and why? In this example, The Iapetus Ocean existed between 600 and 400 million years ago, would it be equally correct to write 400 to 600 million years ago? Can you also direct me to a source for the rules?
posted by Kronur to Writing & Language (25 answers total)

Rules for what context? It matters, I think.
posted by empath at 10:10 AM on December 2, 2010

You may want to list the style you are using, (APA)?
posted by 6:1 at 10:14 AM on December 2, 2010

Response by poster: In this specific context, it is for interpretive signage along a nature trail. Not a scientific journal or article, but we'd still like to be correct in our usage. If there are different rules for different contexts, I would also love to know.
posted by Kronur at 10:14 AM on December 2, 2010

Response by poster: More specifically, the client keeps wanting the number spelled out least to greatest - 400 to 600 million years ago - but the source material always has the opposite usage - 600 to 400 million years ago.
posted by Kronur at 10:17 AM on December 2, 2010

I don't think saying "600 to 400 million years ago" is a common enough error for there to be a source that says not to do it. When you're giving a range, you give the smaller amount or number before the larger one. So, you list the shorter time period before the longer one. It's the same as if you said "The speech will last 30 to 40 minutes." You wouldn't say "40 to 30 minutes."

It seems like this issue is being confused with dates: "600 million B.C." is earlier than "400 million B.C." So, you would say an event happened sometime from "600 to 400 million B.C.," for the same reason you'd say an event happened sometime from "2008 to 2010" or from "November 1, 2010 to December 1, 2010" (you list the earlier date before the later one). That is not the same issue as "400 to 600 million years ago."

In other words, I agree with the client.
posted by John Cohen at 10:20 AM on December 2, 2010 [4 favorites]

I would agree with you, not the client; it makes sense to start with the longer ago date. Perhaps that's because I've taken geology classes, and as noted above, geology would state the earlier to later dates. It seems to me that other naturalist works I've read have done the same, but I don't have any references for you.
posted by ldthomps at 10:23 AM on December 2, 2010 [1 favorite]

Relevant pedantry: If John Cohen is correct, use "B.C.E." which stands from "Before the Common Era" rather than "B.C.", which stands for "Before Christ." The latter is no longer used.
posted by griphus at 10:24 AM on December 2, 2010 [1 favorite]

Use 400 to 600, in that order, since these are relative dates (relative to NOW) rather than calendar dates, and the convention is to use the smaller number first, not the earlier date. E.g. you would say "I spoke to him 45 or 50 minutes ago," or "that was two or three years ago."
posted by chesty_a_arthur at 10:27 AM on December 2, 2010 [1 favorite]

You are describing duration, not bracketing a date range, so I would vote for oldest to newest. I understand "600 to 400" as "started 600 m.y.a., lasted 200 m.y.", whereas "400 to 600 m.y.a." seems to suggest "occurred roughly 400 to 600 m.y.a.".
posted by misterbrandt at 10:31 AM on December 2, 2010 [2 favorites]

There is a difference between approximations:

The event happened 3 to 5 seconds ago (not 5 to 3 seconds ago).
The event happened 3 to 5 minutes ago (not 5 to 3 minutes ago).
The event happened 3 to 5 weeks ago (not 5 to 3 weeks ago).
The event happened 3 to 5 months ago (not 5 to 3 months ago).
The event happened 3 to 5 years ago (not 5 to 3 years ago).
The event happened 3,000 to 5,000 years ago (not 5 to 3 thousand years ago).

and ranges:

The event happened from 5 minutes ago until 3 minutes ago.
(and so on)

If you are trying to indicate a range, I think it would be less ambiguous to say something like:

The Iapetus Ocean formed 600 million years ago and lasted for 200 million years.

If your "source material" is Wikipedia, then it looks like it's an approximation. Perhaps a real geologist could weigh in.
posted by grouse at 10:32 AM on December 2, 2010 [4 favorites]

I would say that if you are describing a specific time range -- as in -- "This species inhabited the area from 1 Billion BCE to 500 million BCE", use the earlier number first. If you are using two numbers because there is some doubt as to the specific dates, then use the smaller number first -- "The species went extinct sometime between 200 and 300 million years ago."
posted by empath at 10:33 AM on December 2, 2010

Best answer: This is from the style guide for National Geographic, and may be helpful. This might have some helpful things in it as well (.pdf)

Everything I've read about geology and geologic time, whether is scientific papers in journals or material aimed at amateurs writes geologic time as [oldest period] to [youngest period], because they're generally talking about the duration of a particular thing. So, for instance, you could say that [thing] existed for 200 million years, but to indicate when that happened, you would say that it existed from 600 mya to 400 mya.

The Iapetus Ocean existed between 600 and 400 million years ago, would it be equally correct to write 400 to 600 million years ago?

In other words, no, ir would not be correct to reverse these - the Iapetus did not come into existence 400 mya and cease to exist 600 mya. It came into existence 600 mya, hung around for a couple hundred million years, and then stopped existing or became something else, starting around 400 mya.
posted by rtha at 10:36 AM on December 2, 2010 [1 favorite]

Yes, griphus is right about "B.C.E." I was speaking roughly.

I would agree with ldthomps that an "earlier date" would go before a "later date" — if that were the issue. I just don't think these are "dates" at all. The reader might mentally translate "400 million years ago" to "400 million B.C.E." (since this is such a huge time scale that we might as well be living in the year 1). But "400 million years ago" isn't really a date, just as "two days ago" isn't a date. You might mentally translate it to "November 30," but this wouldn't be any reason to say "three to two days ago" instead of "two to three days ago."
posted by John Cohen at 10:37 AM on December 2, 2010

But, John Cohen, if someone had been vising you last week, you would definitely say "he was here from Monday to Wednesday," not "from Wednesday to Monday" -- the latter would be weird and confusing. The ocean was there from 600 mya to 400 mya, the opposite seems really weird.
posted by brainmouse at 10:51 AM on December 2, 2010

But, John Cohen, if someone had been vising you last week, you would definitely say "he was here from Monday to Wednesday," not "from Wednesday to Monday"

I don't know why you preface this with "But, John Cohen..." since that's consistent with what I'm saying.
posted by John Cohen at 11:17 AM on December 2, 2010

Oh, sorry, I confused some people and some things. Never mind the But part of my sentence. I agree with you. High five?
posted by brainmouse at 11:27 AM on December 2, 2010

I'd say he was here 'from Wednesday to Monday' if he stayed over the weekend. ;)

But I agree with the distinctions made between a range or a duration. Ask the client to clarify the intent on those terms. Is there a space limitation on the signage that prevents a clearer phrasing?
posted by TDIpod at 11:33 AM on December 2, 2010

Seconding grouse - it's correct to say both-

"The event happened 3 to 5 years ago."
and
"The event happened from 5 years ago until 3 years ago."

In both of these instances, the "first encountered" chronological event is listed first. The difference between the two is the point of reference.

In the second sentence, you can swap the absolute references ("x years ago") with absolute dates, so that's why having the bigger number first makes sentence; eg, you could say, "The event happened from 2005 (5 years ago) until 2008 (3 years ago)".

That's why, when you write the dates as absolute references (indicated by "from" and "until"), it would it's incorrect to say "The event happened from 3 years ago (2008) until 5 years ago (2005)."

But you can't do that in the first sentence because "x to y years ago" is a unit, a numerical range, and countable ranges start at a domain of zero and go up.
posted by lesli212 at 11:42 AM on December 2, 2010

The intent appears to be to state the duration of the event in geologic time, therefore rtha's explanation above is correct. It's been some time since I was in academia, but I do remember this coming up in upper level writing courses.
posted by Big_B at 11:47 AM on December 2, 2010

Actually, I take back my own answer based on the comments by grouse, rtha, and Big_B. Here's the thing:

The Iapetus Ocean existed between 600 and 400 million years ago.

Without any other context, I'm not sure what that means. It's ambiguous.

I was assuming it means:

Sometime between 400 million and 600 million years ago (that is, 600 million to 400 million B.C.E.), the Iapetus Ocean was in existence.

That doesn't mean it didn't exist at other times, and that's not saying anything about when it came into or out of existence.

But on further reflection, you probably mean:

The Iapetus Ocean began to exist around 600 million B.C.E. and ceased to exist around 400 million B.C.E.

If that's what you mean, then it'd be wrong to say it existed "between 400 and 600 million years ago," since you'd be mentioning the end date before the start date. So I suppose you could say it existed "between 600 and 400 million years ago."

But since your goal here is to clearly explain some very basic information for a general audience, I wouldn't use either of those phrasings. As grouse says, it'd be better to reword the whole thing to be utterly unambiguous.
posted by John Cohen at 12:21 PM on December 2, 2010

Best answer: Real geologist here.

Correct usage is 600 to 400 Ma (not M.y.a.). "Ma" in geochronologist-speak, is exactly equivalent to "BCE" in historian/archeologist-speak. You are not specifying a duration, you are specifying a period of time between two dates. However, for a non-scientific audience, you would not be faulted for writing something like "these rocks were laid down between 400 and 600 million years ago", since the notation "Ma" is unlikely to be well-known to a non-geologic audience.

(Of course, a real purist would prefer ages to dates: "These rocks are Neoproterozoic (Edicaran) to Devonian (Gedinian) in age", but that's utterly useless even to a lot of non-chronostratigraphically savy geologists.)
posted by bumpkin at 12:26 PM on December 2, 2010 [3 favorites]

Oops, I just looked at my wall chart, and it looks like 400Ma is just within the Siegenian, not the Gedinian!! *So* embarrassed!
posted by bumpkin at 12:29 PM on December 2, 2010 [1 favorite]

Best answer: I am a geologist. A quick survey of my brain and some random papers on my computer says that "600 to 400 million years ago" is most common. (Examples: "since approximately 14-12 Ma," "...[this] resulted in large scale silicic magmatism between 50 and 20 Ma" [source: Long et al., 2009], "Rhyolite domes (1.4 - 1.2 m.a.) are related to the most recent eruptive event" [source: Portugal et al., 2000].)

If you want to talk dates, it's as John Cohen says. It's like saying "The Roman Republic existed from 509 to 27 BC." You wouldn't say it existed from 27 to 509 BC, because it implies some kind of weird reverse time scale. For the same reason that you don't say "X existed between 27 and 509 BC," you don't say that "X existed between 400 and 600 Ma."

Now, if this ocean still existed, you could say "X is 400 to 600 million years old." If you weren't sure how long it existed back in the day, you could say "it existed for 150 to 250 million years." When you're taking physical age rather than dates/chronological age, conventional numbering like your client wants is right.

I never learned any of this formally. I have never been told formal rules. Nonetheless, I would say "between 600 and 400 million years ago," and I would follow the source material. However, people are still going to understand "400 to 600 million years old." Even if you can't convince your client to change his mind, the only people who are the occasional nitpicky geologists. The point will still get across just fine to the general audience reading the sign.
posted by mandanza at 12:48 PM on December 2, 2010 [1 favorite]

Oh, I didn't see bumpkin's comment until I posted mine. Fun fact on Ma vs. M.y.a. vs. Myr: apparently you're supposed to use Myr for dates ("These rocks were emplaced at 600 Myr") and Ma for ages ("These rocks are 600 Ma old"). I'm told there was a whole session about this at GSA* this year. Not like that usage is widely practiced, judging from the wild variations I see in papers, but I guess someone decided recently that it should be that way.

Either way, I agree with bumpkin that those abbreviations shouldn't be used in a sign aimed at a general audience.

*for non-geologists: the Geological Society of America meeting, which was in Denver a month or two ago.
posted by mandanza at 12:53 PM on December 2, 2010

I am Mr. Cheminatrix, a geologist. Just to add to what my colleagues have said, you would typically order the dates oldest to youngest; e.g. the Iapetus existed from 600 - 400 Ma. The reason we do it that way is because we are commonly describing the geological and structural evolution of an area/plate/basin, etc. To understand event D it is important to remember the contributions of A,B, and C.

Bumpkin is right on the money, we prefer to use the names of ages. I think we think in terms of these names because they evoke more about the paleontological, structural, or climatic conditions of the specified time. Just a hunch.

Also, Cheminatrix is super-hot
posted by Cheminatrix at 5:22 PM on December 2, 2010 [3 favorites]