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 stillthus farthat 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
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 sufficethough 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 noteven to meroll 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, findor be foundother
phenomena to describe? Safer by far to keep them where they are … or
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 usedas noted aboveby
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
...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 graptolitethe short-ranged
and beautifully distinctive Monograptus crispusis, for instance, decidedly
diachronouson standard biostratigraphic groundsbetween 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 succumbingthough
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
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 detourwhere 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 simplifiedi.e. unifiedtime-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, 445448.
SHELDON, P. R. 1987. Parallel gradualistic evolution of Ordovician trilobites.
Nature, 330, 561563.
Department of Geology, University of Leicester