More thoughts on illustrating geological time

What is the best way to represent geological time? When I was working on my little timescale project, I had one very specific aim in mind: I wanted a scale that clearly showed the names and ages of all the different geological periods, without becoming ridiculously big. As it turns out, this isn’t very easy. From a temporal perspective the divisions of the timescale are somewhat irregular, ranging from tens of thousands of years in length (the Holocene) to a couple of billion (the Proterozoic). In fact, if you plot the entire 4600 million years of Earth history linearly, even the periods which last tens of millions of years, which is most of them, are pretty narrow.


You can see from the comparison that my timescale significantly expands the last 600 million years at the expense of the preceding 4000 million, which certainly hides the full extent to which the entire period of macroscopic life (the Phanerozoic) is a pretty new development as far as the Earth is concerned. However, in return for this sacrifice it is now possible to see exactly when a geologist is talking about when he talks about the Silurian.
As a linear scale clearly doesn’t fit our purpose, the observation that the further back we go, the longer the absolute length of the different periods seems to get immediately brings to mind a logarithmic scale. As it turns out, this doesn’t really work either; the last 20 million years of geological history manages to crowd out pretty much everything else, which may be great for the hominid-centric among us but is hardly very useful for the rest of us (I note with some amusement that a young earth creationist chronology is compressed into nothingness off the left of this scale – how apt).


Clearly, the ideal scaling lies between those two extremes, which is what eventually led me to my multiply-rescaling linear axis. It occurs to me that, in a way, the need for such fudging tells us something quite important about the way the geological timescale has come to be. It’s based on something other than a striving for mathematical neatness; indeed, given that divisions such as the Cambrian and Devonian were first created almost a century before we were able to assign them absolute ages with any degree of certainly, that’s impossible. Instead, as geologists have tried to write the Earth’s biography, they’ve divided up the timescale in a phenomenological manner: each period represents a time in earth history with a distinctive tectonic, climatic, geochemical and biological ‘flavour’. Thus there is no maximum or minimum limit on how long these different chapters can last; also, as we go further back in time, the accuracy of our information is diminished, meaning that we can’t go into as much detail as we can for more recent events, and the divisions become wider.
The idea that geologists are writing Earth’s biography is quite a useful one for understanding the structure of the timescale they have created. When reading someone’s biography, it’s noticable that the different phases of somebody’s life can cover markedly different lengths of time, with hugely important and influential events getting their own chapter even though they may only actually last a few weeks or months. Likewise, the childhood of a famous person is described in a much less detailed manner than his later life, not because this time was necessarily any less eventful or important, but simply due to limited sources; instead of multiple eyewitness accounts, newspaper articles, and TV and radio recordings, all biographers may have to go on is the rememberances of a geriatric former schoolteacher, who may or may not be actually recalling the antics of the specific young lad or lass that you’re interested in.
As it is for people, so it is for the Earth; and so, also, it is for my timescale. I don’t deny that grasping the true depths of Deep Time is important (indeed, it’s essential if we’re ever going to create a sustainable civilisation), but I think getting to grips with the different chapters of geological history, where different events fall in the narrative, and their sequence, is also an important part of understanding the planet we live on. That’s what I’m trying to aid with my rather warped little plot; whether it helps, or hinders, remains to be seen. Either way, I appreciate the encouragement and/or feedback you’ve all been giving me, so please keep doing so.

Categories: basics, deep time, geology

Comments (16)

  1. John McKay says:

    Geology is an historical science and is a perfect illustration of a problem faced by all fields of history. That is, we don’t actually study “what happened” (as one of the founders of the modern history profession, Leopold von Ranke, had hoped), we study those things that we know happened, which is a subset–sometimes very small–of what actually happened. This is probably too obvious to even need stating out loud for most people who practice historical disciplines, but it’s at all not obvious to outsiders.
    As a historian, one of my personal pet peeves is the common impression is that people without written history live in an unchanging state where every generation is exactly the same as the one before. That’s as wrong in history as it is in climatology, geology, or astronomy. Just because we don’t know what happened doesn’t mean nothing happened. The expansion of time periods as we go further back is not a reflection of fewer things happening, it’s a reflection of less data surviving.
    Thanks for taking a shedding a little light into that particular dark corner.

  2. SimonG says:

    How about making use of modern technology? It may not be practical for your purposes but I can imagine a PDF showing the various periods to scale. One could then zoom in on the bits of interest, to see more detail.
    It might be helpful to make use of colour, too: particularly since the fine detail in the relatively recent periods would be practically invisible when viewing the whole page. Perhaps a rainbow scale along one edge, red at the dawn of life up to violet now, (or vice versa if you want to confuse people). But not arranged evenly, so the red bits would be much bigger than the blue bits. You could even attach some extra meaning to the colours, perhaps showing the sorts of lifeforms coming into play. (Cellular; multi-cellular; chordate; whatever.)
    Maybe have multiple layers, so that the viewer could choose between looking at particular types of life.
    Of course, that’d be an awful lot more work than your black and white diagram, (which incidentally looks quite good).

  3. Silver Fox says:

    I think this version is great for showing both the geologic periods and the fact that almost all of geologic time is Pre-Cambrian. I think it would give people a sense of deep time, and why it can feel like the Laramide orogeny was just yesterday!

  4. Silver Fox says:

    Btw, I was referring to the upper diagram.

  5. chezjake says:

    Of these two, I prefer the upper one — looks very useful.
    Perhaps you might persuade some Sciblings in paleontology and anthropology (Greg Laden, Afarensis, and any other interested parties) to come up with variants for their fields — the time scales are a bit shorter, but getting the various eras in the right sequence is equally challenging.

  6. Ian says:

    “What is the best way to represent geological time?”
    Left to right….
    Seriously, why plot the whole thing every time? Why not just say what the period is in the text and link that to an entire separate page which shows in detail, top (=recent) to bottom (=beginning) – or whichever way you think is best.
    This is not a textbook, Chris, it’s a web page. There are no contraints here. You can bend it to whatever needs you have to meet and use whatever links/means are necessary. It’s not a grant proposal or a peer-reviewed paper, it’s entirely for you and us!
    You could have more than one page, the first showing the overall, with clickable sections leading down to greater detail (and even more pages which are more detailed if you need to).
    That’s what sci-blogs needs – a good glossary that everyone can use.

  7. BAllanJ says:

    I’ve got another idea…. how about you just say how many years ago something happened? These geology names may mean something to geologists, but they’re not very descriptive are they. Lose the historical names and use the age (in years, or mega years, whatever). If you have to use names for descriptive purpose, use ones that mean something, like name an age after the supercontinent that existed then, or by the dominant life form. This whole geological naming convention is just a barrier to outsiders who haven’t earned their secret decoder ring. Whenever I read these articles (and I do find geo articles interesting) I spend most of my time off in trying to fit the info into a reasonable synthesis based on how long ago it happened.

  8. BAllanJ says:

    …although, I must say, this is a very nice “secret decoder ring”. I like….and will be printing out a copy….thankyou.
    I just think it may be time for geologists to drop the old nomenclature. I know it’ll be hard, and may make it hard to read old papers, but us physics types left behind the esu and computer geeks left behind octal (i think), so it can be done.

  9. PaulG says:

    FAO BAllanJ: The thing is that knowing the order of periods lumps things nicely together in a verbal rather than numerical package, so that when someone who knows loads of stuff tells me that something is Devonian, I can nod my head sagely, because now at least I know that it’s older than the Carboniferous limestones I look at. That makes me feel like I’m not completely useless.
    You’ve probably come across a few mnemonics already, but the one I used (ok, ok, use) is:
    Camels Often Sit Down Carefully, Perhaps Their Joints Creak.
    This bit starts at Cambrian and works upwards to Cretaceous. The next bit just deals with the Cenozoic:
    Possibly Early Oiling Might Prevent Premature Rusting.
    I know you’ve got three Ps in there, but if you remember that palaeo is old, then you’ve only got two to deal with and both premature and pleistocene are longer than their partners. It’s been working for me since my old prof. came out with it on a field trip to Co. Antrim in 1992.

  10. Lab Lemming says:

    Did you know that the ICS has a “timescale creator”?
    They also have plenty of handy charts for DL.

  11. Lab Lemming says:

    “I’ve got another idea…. how about you just say how many years ago something happened?”
    That isn’t always appropriate. The stratigraphic timescale (all the names) and the numerical time scale (from radioactive decay) are independent. A lot of people do a lot of work trying to cross correlate them, but those corellations change as newer and better data and methods become available.
    An example of how the numerical age for the cambrian (and other paleozoic periods) have changed over the past 70 years is here:
    In order to avoid confusion or revision, it is important to use the timescale appropriate to the method you use to determine the age. So if you get the age of a granite from the fossils in the sediments that were deposited before and after it crystallized, then you say it is late Jurassic to early Cretaceous in age. If you get the age my measuring the radioactive decay of potassium, then you say it is 122 Ma.

  12. Lab Lemming says:

    What software did you use to make all the cool wavy scale zooms?

  13. CherryBomb says:

    I always thought that Campbell’s Onion Soup Doesn’t Make People Popular, but then in America I had to accomodate Mississippian and Pennsylvanian instead of Carboniferous. (In either case, they could have chosen names easier to spell.)
    Anyway, at the risk of piling on, BAllanJ, I suggest you bite the bullet and learn the names. It’s a bit like suggesting that reporters stop writing “A cyclone struck Myanmar yesterday” and just give the latitude and longitude so we don’t have to learn all those country names. “Myanmar” is more informative because it not only locates the event, it brings up associations in ones mind (rice, rubies, military junta). It’s the same with geologic periods. If someone says something happened 480 million years ago, I have to look it up to see “when” that was in geologic time for it to make sense to me. I am fuzzy bout dates in the lower Paleozoic, but if you say “Lower Ordovician” I know what you are talking about.

  14. Radge Havers says:

    You’re to be highly commended for trying to create a good illustration.
    A slightly different approach might be rounder. For instance, you could have the major eras in a sort of pie chart at the center and successively refined periods blown up (and out) in surrounding arcs. In that scenario, perhaps the ultimate origins could be treated with open or blurred lines or with symbology indicating indeterminance.
    Lately I’ve been going back and looking at how lattices and symmetry are illustrated in mineralogy texts. It really makes you wonder how firm a grasp the authors actually have on their subject or if they’re just too lazy to even proofread their designer’s work. “Educators” indeed. Unbelievable.

  15. BrianR says:

    Chris … I love it. I think the top diagram is fantastic. I think this is incredibly useful. I will use it very soon. I think simply saying the period in the text (w/ approximate age paranthetically) is important… and then having this illustration in addition to that. Then, all bases are covered. Those that are familiar with the time periods can skip the figure, those that want to learn can do just that, and those that want to see the time in context of Earth history can appreciate that. Trying to satisfy everybody’s experience and tastes is difficult, I also think you should be commended.
    As for if recent should be on right or left … there is NOT one way, there is no convention. I’ve seen it both ways so many times (in geology and paleoclimatology). I think people will have to suck it up if it’s opposite of what “feels” right for them. Getting everybody to go the same direction is a pipe dream and a giant waste of time in my opinion.
    Great job!!