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Reply: Stratigraphy over-simplified? - or a question of perspective?

Steve Walsh does indeed offer a distinct perspective on the utility of time-rock stratigraphy, and draws up a nicely-argued case for its retention. He has an excellent grasp of the relevant literature, so perforce I must match the seven-league boots of considerable scholarship with the crudely-hewn clogs of blunt enquiry. As he says, the points of contention between us are few: the field is narrowed to stratified rocks, and “upper/lower’ modifiers have been removed from the discussion. So what are the points of debate now? Let’s take them one by one.

A lack of precision in a unified nomenclature.

It seems to me still—thus far—that one can be as precise as one wants to be regarding the relation of time, rock and deduced process using either a dual or unified terminology. To take the Maastrichtian, the formulation ‘strata deposited during the Maastrichtian Age’ is of course a longer phrase than ‘the Maastrichtian Stage’ but it is just as unambiguous. In this sphere, brevity does not always equate with clarity. Furthermore, I’m not so sure that simply the use of ‘Maastrichtian strata’ would inevitably lead to dangerous confusion, as the meaning would generally be clear from the context. After all, geologists frequently use time and time-rock terms quite interchangeably in everyday communication (at least, before a sharp-eyed editor hauls them up) with little sign that the Edifice of Geological Thought is imperilled.

There is an acid test here. Can one systematically write papers on stratigraphic topics using only geological time terms? Are there any occasions where one cannot get by without time-rock terms? My limited experiments to date suggest that time terms alone will suffice—though the experiments do need to run longer.

Logical possibilities are one thing. There’s the important question of the terminology that people are used to. Using unified terminology, of course, one can’t say that ‘the base of the Quaternary’ any more than one can say ‘the base of January’; one should say rather ‘the beginning of the Quaternary’ (though one can, of course refer to the base of a Quaternary succession, just as one can refer to the base of a January snowfall). To say the ‘beginning of the Quaternary’ does not—even to me—roll so neatly off a Pavlovian tongue. Words have a history and force of their own, and evolve to mean different things to different groups of people. Thus, it would be senseless for French-speaking geologists to try to replace the universally-used ‘inférieur/supérieur’ with ‘précoce/tardif’; the former modifiers have come to have a temporal as well as a topological meaning and are universally understood.

Now, as my colleagues on the Stratigraphy Commission and I have argued, the time/time-rock distinction in the English-speaking world is decidedly not universally understood. Nevertheless, the nomenclature of both time and time-rock classification is in common, if frequently mangled, usage. The terms themselves won’t fade away quickly, whatever formal recommendations are made. How often, for instance, is the K/T boundary referred to as the K/P boundary? Still, the inextricable tangle of habit and meaning that form an ever-more-burdensome baggage around stratigraphic terms may be illuminated, if not resolved, bytrying to be clear about the phenomena they represent. Here I’m entirely at one with Steve Walsh. Which brings us up to the next point…

The incompleteness of successions.

Thus, the argument that locally a System has boundaries different from that of the equivalent Period, as in the case of the Devonian strata at Siccar Point. Well, there are probably very few places where a Period is represented by a continuously deposited succession. One can say that there are strata of Devonian age (or Devonian depositional age, if one wanted to exclude all ambiguity) at Siccar Point, without implying that the entirety of the Devonian is represented there by rock. Similarly, one can talk about a January snowfall, without implying that snow fell all through January without a break. And snowfalls generally have diachronous bases, as the snow clouds that produce them scud hither and thither. Yes, one can make a System/Period distinction in this way if one wishes, but I’m not sure that it is necessary to an effective description of what, in geological terms, happened when and where. The Devonian, as defined by GSSPs above and below, is simply an interval of time within which processes, such as the deposition of sediment, took place.

Consequent migration of the meaning of terms.

This is a novel slant, and deserves consideration. If, let’s say, the formal use of words such as ‘System’ and ‘Series’ was abandoned, would those words, now left redundant but still echoing within the crania of tens of thousands of geologists, find—or be found—other phenomena to describe? Safer by far to keep them where they are … or is it?

Words such as ‘series’ and ‘stages’ certainly have had different meanings in the past. They even have different meanings today, among different communities of geologists in different parts of the world. Changes of meaning do cause confusion, in the short to medium term. How great is the danger in this instance? I find that hard to judge. In the particular instance quoted, that of synthems, then that particular term does seem destined to fade away, lamented by few. However, the phenomenon it describes – that of unconformity-bounded units, seems mostly to be now subsumed under the general umbrella of sequence stratigraphy (which itself has an evolving terminology).

Its more likely that, whatever (if anything) is formally recommended about the time-rock terms, they will continue to be used—as noted above—by those geologists who find them useful. Such a continued use, even if informal in itself, will likely be some form of barrier to further evolution of these terms. Of course, the community of geologists who will use these terms will be… I was about to say stratigraphers, but the word stratigraphy itself has course evolved (between Hedberg and Salvador, I think) to encompass those who decipher earth history from unstratified as well as stratified rocks. We are all stratigraphers now, and about time too.

So we need a new term for stratigraphers dealing with strata, those for whom this argument is germane. Perhaps we should call this group of earth scientists ‘strata-types’, to avoid terminological confusion. Among the strata-types, there will be those who might prefer to adopt the unified terminology, perhaps because (as in my case) they might lack a little theoretical solidity. Perhaps this group of stratigraphers should be described as ‘hollow-types’. The persistence, or not, of the time-rock terms will likely, ultimately, depend on how many hollow-types continue to be preserved among the strata-types, in museums and other places of learning. Now, isn’t that a clear and unambiguous enough summary of a Darwinian struggle between competing terminologies?

I think, paddling furiously towards the safety of a sensible shoreline, that it is hard to predict how our stratigraphical language will evolve, except that it undoubtedly will. The danger of sinking deeper into a terminological quagmire shouldn’t, therefore, stop us trying to work out which direction we think we should be trying to head towards.

Blurring of rock, time and fossil units.

Steve Walsh has nicely demonstrated how the distinctions between biozones, sequence stratigraphic units and chronostratigraphic units can become blurred. The argument here is that Systems, Series and so on should be retained in order that the fossil- and sea-level-based units do not encroach too much upon the territory of geological time. I’m not sure of this: the holy trinity of rocks, fossils and time helps me, at least, maintain a grasp of the distinction. Is a holy quartet of rocks, fossils, time and time-rock quite as elegant? Here, I’ll offer the observation that it’s quite easy for time-proxy units to slip towards being thought of as time-units, especially if the proxies are pretty good. Perhaps we ask too much of them sometimes. Which brings us to…


...which might just be an example of where the lily might be being gilded to achieve a bewitching sheen, like that of fool’s gold. Now fossils offer excellent proxies for the time record, agreed. Biozones are a useful way of constructing effective quasi-temporal pigeon-holes out of a plexus of evolving lineages of any major fossil group. Biozones, though, are also imperfect, even in the Jurassic. They are always diachronous (even the base of an ignimbrite is diachronous, for a pyroclastic flow takes many billions of nanoseconds to traverse from a crater rim to the plains below; I exaggerate here, but perhaps not altogether ludicrously, as my colleagues Richard Brown and Mike Branney (2004) have resolved very finely-spaced depositional events in the accretion of a single ignimbrite).

Even on the standard rough-hewn geological timescale, biozones are often demonstrably diachronous. The (relative) range of my favourite graptolite—the short-ranged and beautifully distinctive Monograptus crispus—is, for instance, decidedly diachronous—on standard biostratigraphic grounds—between Wales and Bohemia. So fossils, while a splendid guide to time, need watching like a hawk at the high-resolution end of things.

So biozones I understand. But the role of a biochronozone… Now, as I understand it (and I stand to be corrected on this), this encompasses all the time subtended by the maximum temporal range of the defining taxon of that biozone, and represents an interval to be extended into all of the strata deposited during that time, whether fossiliferous or not. I more or less understand this idea in general, but am a little puzzled as to how it might work in practice.

Inasmuch as I understand evolution, at least that of the graptolites that I work with, they generally don’t seem to show the kind of gradual evolution that Peter Sheldon (1987) painstakingly demonstrated for the Builth trilobites. Most graptolite taxa tend to, well, just appear in any individual section, as more or less distinct entities, the spirit of punctuated equilibrium presumably hovering somewhere nearby. Evolution always, dammit, seems to have happened somewhere else. Presumably a taxon arises somewhere out there, generally as a small isolated population pace Ernst Mayr. Then, some time later, it breaks out of its geographic confines to spread around the world. It may spread quickly, like wildfire, or more slowly, or stepwise, and part of that spread will have been modulated by climatic or oceanographic conditions, and the way these changed with time. The taxon flourishes for a while, and then, outcompeted by its successors or wrongfooted by some environmental change, it dies out, but isolated relict populations may hang on for a while here and there before finally succumbing—though perhaps one or two of these may give rise to successor species which might in turn spread over a good part of the world.

As biostratigraphers, we mainly, I think, see the arrival of our key taxa into sections (sampling, at best, a few square metres of seafloor, if that, and recalling the strictures of Messrs Signor & Lipps), and we also see its local demise. These are our tie-lines, for practical purposes. We normally don’t chance upon the birthplace of a new taxon: I can recall only one half-way decent candidate of this phenomenon in the graptolitic successions I’ve looked at. And we don’t normally find the relict late populations; hence the flurry of excitement a few years ago over the Wrangel Island mammoths, which we now know were grazing that tiny portion of steppe while the Pyramids were being built. A more pronounced example of diachronous extinction was of course mooted by Sir Arthur Conan Doyle in his Lost World, an example of entirely ridiculous proportions, until, that is, one remembers the coelacanth.

Even if we had wonderful geographical control, and locate birthplaces and final resting places of taxa, we would simply encounter a different type of uncertainty: where, in a continuous and continuously recorded lineage, does one draw a boundary between two successive taxa? Peter Sheldon’s Builth trilobites, in this situation and at this scale, don’t lend themselves easily to taxonomic pigeon-holing.

So, while to seek to correlate biostratigraphic tie-lines into unfossiliferous strata is a perfectly valid exercise, the attempt to formalize this into time-units based upon ideally compartmentalized taxa takes me, at least, into unfamiliar and potentially treacherous terrain, given the amoeba-like, fuzzy-edged geometries encompassed by taxa through space and time. At least a golden spike represents a single point of certainty, even if all correlation that follows is uncertain.

So in conclusion… and after something of a detour—where should we go? Of all the reasons quoted for retention of the dual time-scale, the most telling seem to me to be the familiarity and convenience of the time-rock terms for part of the geological community working today. Tomorrow, though, a generation on, stratigraphy will have moved on (think how far it has moved in the last quarter-century), and the eternal verities may have shifted a little also. Even today, we can operate effectively without using the time-rock terms, though at times we have to use unfamiliar language, and need to consider exactly what we want to express. The last point may not be altogether a Bad Thing.

I’m conscious of the growing importance of stratigraphy to much broader-based studies of how the Earth functions, and especially of climate change. These are pressing matters, for time is short and the water around us is quite literally rising. Time is part of the lingua franca of science as a whole while time-rock is not. A simplified—i.e. unified—time-scale should not, of course, be mandatory; it might be recommended, perhaps. Nevertheless, if widely applied in practice, it might just help the sciences talk to each other more easily. They certainly need to.


BROWN, R. J. & BRANNEY, M. J. 2004. Bypassing and diachronous deposition from density currents: Evidence from a giant regressive bedform in the Poris ignimbrite, Tenerife, Canary Islands. Geology, 32, 445–448.

SHELDON, P. R. 1987. Parallel gradualistic evolution of Ordovician trilobites. Nature, 330, 561–563.

Jan Zalasiewicz
Department of Geology, University of Leicester

Created by Alan R.T. Spencer on the 2006-01-15. (Version 2.0)