||One of the "advantages" of living to be a
more "senior" person is the hindsight gained in looking
back over your lifetime. In the half-century (plus a little) that
I have been interested in geology, the geological time scale has been
refined time and time again. For example, when I started university,
back in 1961, the base of the Cambrian period was 600 million years,
then placed at 530 million (1982), back again to 590 million (also
in 1982), then to 570 (1990) and now, perhaps finally, placed at 542
± 0.2 million years (Ma). Remember that the Cambrian and younger
rock sequences only represent a small part of the geological time
scale that has been established over 200 years of geologic observations.
But perhaps I am getting a little beyond myself.
Certainly at the age of about 10 or 11 I was looking at rock sequences
in my hometown of Barry in South Wales. I would like to share a
few of these observations since they throw some light on the perplexing
problem of sorting out the age of the world. I was really fortunate
in living in a part of Britain where the geological structures were
only moderately affected by earth movements in the past and so it
was very easy to observe the juxtaposition of beds. This brings
me to the first aspect of dating termed:
1: Horizontal basal Jurrasic strata, Old Harbour, Barry,
Figure 1 shows a rock sequence exposed in the sea cliffs about
one kilometre from my childhood home. The beds here are horizontal
and they display three of the fundamental laws that cover stratigraphy.
The Principle of Superposition first expounded (or
at least documented) by Nicolaus Steno over 300 years ago states
that the oldest (first deposited) bed is always at the bottom of
a given sequence and the youngest (last deposited) bed is always
at the top. (The inherent assumption is that the sequence has not
been totally inverted). The Principle of Horizontality
is another assumption that the sediments that form the bed were
laid down horizontally. In a sequence that is as well defined and
undisturbed it is easy to see the fundamentals of a third principle.
This is the Principle of Lateral Continuity; where
one can follow a bed for hundreds of metres (or even kilometres)
until the bed finally either grades into another lithological unit,
thins out and vanishes, or is cut off by some other factor. It might
drop below beach level, be cut by topsoil at the ground surface
or terminate in a rock break such as a fault or a sharp contact
with another rock type.
Figure 2: Angular unconformity at Friar's Point, Barry
Island, South Wales.
Figure 2 illustrates another outcrop on a nearby headland. Again
the geology is relatively simple but it shows a fourth important
principle. This is the Principle of Cross-cutting Relationships.
The Principle of Cross-cutting Relationships states that in a given
rock sequence if "something" (and the something might
be an igneous intrusion, a mineral vein, a fault or younger strata)
cuts through a sequence of beds then the "something" must
be younger than the beds through which it cuts. In Figure 2 it is
fairly easy to see that the lower beds (originally deposited horizontally)
have been subsequently tilted and then cut by a second (horizontal)
sequence. Even as a younger person it was not difficult to assume
that the tilted sequence was older than the horizontal strata above
the dipping beds. More importantly this illustrates the concept
of relative ages. Let's put this in order of events, so we start
at the bottom (the oldest part).
Stage 8: Formation of topsoil and growth of grass.
Stage 7: Erosion of the upper, horizontal strata (note
that the erosion surface is more irregular)
Stage 6: Accumulation of the upper beds as horizontal
strata on the flat erosion surface
Stage 5: Erosion of the lower unit (note that the erosion
surface is horizontal)
Stage 4: Uplift of the lower unit
Stage 3: Tilting of the lower unit.
Stage 2: Compaction and lithification of the sediments.
Stage 1: Deposition of material as horizontal sediment.
(Start here and work upward).
By working your way from one local outcrop to the next "up"
or "down" through each sequence, it is possible to find
additional regionally younger and older units and to determine how
beds might grade laterally from one lithology into another. This
provides a "relative" way of assessing the oldest and
youngest beds in the area being studied. Perhaps one of the most
famous examples in the world is "Hutton's unconformity"
at Siccar Point in Berwickshire, Scotland (Figure 3). It was at
this locality that James Hutton recognized the true immensity of
geological time and helped the natural historians of the day to
break away from the religious dogma that supposedly (according to
the Irish Bishop Ussher) dictated that the world was created in
4004 BC. If you are dealing with metamorphic or igneous terranes
that have no fossils then you can probably at least determine which
are the oldest and youngest rocks and the age progression of each
by using cross-cutting relationships. An example is illustrated
in Figure 4.
Figure 3 (ABOVE): "Hutton's Unconformity"
In sedimentary units your observations will not only include the
lithology of the beds that you are mapping (which allows a determination
of the type of environment that existed when the sediments were
deposited), but also the fossil content of the beds concerned. Fossils
may be blatantly obvious, or they may be totally absent. Most sedimentary
rocks do contain macroscopic or microscopic fossils and a careful
determination of the families, genera and species might also allow
the "age" of the beds to be determined. Certain fossil
groups are "generalists" existing for huge blocks of geologic
time with little or no apparent change. The small intertidal brachiopod
Lingula, looks the same today is it did in rocks as old as 500 million
years. On the other hand, more rapidly-evolving organisms such as
certain graptolites or ammonites, changed morphologically over "short"
periods of geologic time (perhaps 500,000 years). We use the fossilised
remains of such rapidly-evolving animals or plants as zonal fossils.
Figure 4 (BELOW): Igneous rock near Kingston,
ON. The light angular unconformity (grey overlying beds) contains
clasts from the lower sequence, and must be younger.
Zonal fossils have certain requirements. In a rather simplistic
way, these are: rapid evolution (they exist for short time
frames); easily recognised (usually a function of hard
parts that are easily preserved); wide geographic distribution
(found in many different parts of the world), and presence in
rocks of different lithologies (many depositional environments).
Many excellent zonal fossils were "drifters" in marine
sequences of the past. Because zonal fossils existed for short widows
of geologic time one can use their presence to correlate one outcrop
with another. This is true in regional contexts but might also be
true for world-wide correlation. Lingula, for example, would make
a most unsuitable zonal fossil because finding specimens in a rock
sequence would not provide much information about age (only that
the rock sequence falls somewhere in the last 542 million years
of geologic time)! On the other hand finding Psiloceras planorbis
(an ammonite) would be extremely useful over much of western Europe
since it only exists in the lowest zone of the Jurassic (199.6 +/-
0.6 Ma). Putting a geological age such as this on a fossil takes
us out of the realm of relative dating and into:
"Absolute dating" implies "absolute dates" and
this is not quite correct. To a geologist it illustrates that a
certain radiometric decay series has been used (sometimes more than
one) to determine the rock age. It does not imply precise calendar
years, but it does provide a framework which, with limitations,
provides an approximation of the ages of rocks. For example the
age determinations on the junction between Cretaceous and Paleogene
(Tertiary) rocks are 65.5 +/- 0.3 Ma. Multiple determinations using
different dating techniques on this boundary from many different
areas of our planet average at this age. Rocks older than this (Cretaceous)
contain dinosaur remains. Rocks younger than this (Paleogene in
age) do not contain dinosaur remains, unless odd bones have been
weathered out and are incorporated in younger sediments. If you
wander the badlands of Alberta you will find dinosaur bones (sometimes
lots of them; - see What On Earth, Fall issue 2003), but this is
not an indication that dinosaurs are alive today. Dinosaur bones
can therefore be incorporated into sediments that are being laid
down today in the Red Deer River valley, but to a future (insectoid?)
geologist it should not be an indication that dinosaurs were alive
in the Quaternary Period (the time frame that we live in today).
On the other hand, bones of humans cannot be found in sediments
of Cretaceous age because humans simply were not alive on Earth
at that time.
Table 1: Main radiometric dating techniques
|Elements and isotopes involved
||Age range provided
||Dates obtained from
|Uranium - Lead Series
|238U - 206Pb
||4.6 Billion years
||Age of Earth
|235U - 207Pb
||713 Million years
||Age of Earth
|Potassium - Argon
|40K - 40Ar
||1.3 Billion years
||Age of Earth
||Micas (volcanic rocks)
|C14 - N14
||-150 to ~50,000
Painstaking analyses of different rock sequences all over the world
by generations of geologists have provided observations that allow
us to state, with a fair degree of confidence, what types of fossils
should occur when and where. Of course Mother Nature throws us an
occasional odd curve ball, such as the discovery of living coelacanths
in the Indian Ocean, when the last fossil specimens found in rock
sequences date back to about 70 Ma!
I have implied above that we use different types of dating methods
to provide ages on rock sequences. A discussion of each of them
is beyond the scope of this article, but I would like to mention
the main techniques and the ages most suited to that determination.
These are summarised in Table 1.
A detailed discussion of each of these methods is inappropriate.
Suffice to say that the parent product will decay to a daughter
product (often through a whole series of subsidiary decay series).
The measurement of the remaining "parent" isotope to the
abundance of "daughter" isotope provides a measurable
difference that allows a calculation of the "age" of the
rock under examination. If you are really interested in following
this topic I refer you to the webpage of the International Stratigraphic
I have provided a simplified example using radiocarbon-14 as an
example. This is also known as Carbon-14 or radiocarbon dating.
Radiocarbon dating is extremely useful to Quaternary geologists
looking at the younger events of the last ice age and to archeologists,
studying the past 40,000 to 50,000 years since practically all organic
materials - shells, bones, teeth, skin and hair, peat, wood, (and
structures made from wood), or charcoal - can be dated. There are
some problems with the technique. Human use of fossil fuels (coal
and oil particularly) has added vast quantities of "infinitely
old" carbon to our atmosphere over the past 200 years. Furthermore,
above-ground testing of large nuclear bombs, especially in the 1960's,
added radioactive contaminants that complicate analyses of natural
decay rates. For these reasons dates prior to about 1800 are considered
reliable and dates following 1800 are not usually attempted. At
the more distant end of the methodology dates beyond about 50,000
are also considered unreliable.
How does this technique work? Cosmic rays from our Sun, enter the
upper atmosphere forcing the conversion of Nitrogen14 to Carbon14.
Most Carbon14 is oxidized to form Carbon Dioxide (CO2). The CO2
is then absorbed by plants, which are then eaten by other organisms.
However, some of the original Carbon14 is not altered and the ratio
of original Carbon14 (the unstable form) to CO2 (the stable form)
is about 1 atom to one trillion atoms. When the plant or animal
dies it stops taking in CO2 and the decay of Carbon14 begins. Since
the dead plant or animal cannot take in any more CO2 or Carbon14
the "radiometric clock" within the organism begins to
count down. We know the precise decay rate of Carbon14 since it
has been measured against annual rings taken from long-living organisms,
sections of giant Sequoia trees, for example that are >2,000
years old or Bristlecone Pine (>4,000 years). The Carbon14 in
the sample decays to Nitrogen14 in a constant manner, with 50% changing
(or "one-half life" changing) every 5,730 years. Put in
another way, 5,730 years after the organism dies only 50% of the
Carbon14 contained within the organism remains. After another 5,730
years (or at 11,460 years after the organism died) only 25% remains.
Unfortunately by 7 half-lives (age 40,110 years) less than 1% of
daughter isotope is left. This explains why dates greater than 50,000
years before present are considered unreliable, since the amount
of measurable activity is exceedingly small. Incidentally, "before
present", is by convention before 1950. Conversion scales for
"carbon-14 years" to real "calendar years" are
also available in the very youngest part of time.
Radiometric dating, therefore, depends on when the "parent"
material was formed and the rate of decay of that parent material
to the daughter product. In the case of igneous rocks it depends
on the time of crystallization of the parent mineral in the rock
that is being dated. Let's take the case of a granite, an igneous
intrusion that is, in reality, two billion years old. The atomic
clock starts counting down when the magma cools. Unfortunately the
rock sequence is involved in a later reheating (perhaps another
mountain-building episode) at one billion years. The atomic clock
within the granite is "reset" by this later reheating
and partial melting episode, and acquires the "new" later
age. Geologists must be aware that they are dating this later episode
of mountain building and not the original age of the granite. Careful
mapping and dating elsewhere in the region can usually resolve such
When the granite disintegrates a geologist might date grains of
feldspars (using Potassium40 dating) that are trapped in sedimentary
rocks. The sedimentary rocks are, perhaps, 600 million years old.
However, the geologist would have to be careful to realize that
they might be dating grains that come from the original granite
that cooled 2 billion years ago, or from the "reset" feldspar
ages that were derived from the altered granites one billion years
ago. Neither of these ages would reflect the "true" age
of the sedimentary sequence that is 600 million years old.
The net result is that with reliable dating methods that have been
developed largely over the past three or four decades, coupled with
meticulous geological mapping over the past 200 years, the geoscience
community has been able to unlock the history of our world. We know
how old it is, when life forms began to inhabit the Earth; when
major catastrophes and calamities have overwhelmed our planet; when
new species have appeared or evolved, in many cases, provide the
rates of evolution. Finally we have been able to say when certain
mineral deposits (including all the energy minerals, so necessary
for our survival) were created and also how our world has physically
evolved. Much remains for future generations of geologists! The
current geological timescale is featured in the centrefold of this
Some comments on the Timescale and Teminologies use
The timescale illustrated in the centrefold is the latest version
of many. This is the currently accepted scale with the latest refinements
using absolute time. The meeting of the 32nd International Congress,
to be held in August of 2004 in Florence, Italy, will ratify a number
of the latest amendments.
Please note that I have not used the correct colours for the various
divisions of geological time (there are indeed designated colours)
and I have introduced a few of my own views. For example, in the
left side of the diagram in the Proterozoic I have kept the "Aphebian"
"Hadrinian" and "Helikian", rather than just
absorbing the newer terms (Siderian to Ediacaran); although the
latter are included. The Edicaran (latest period of the latest Precambrian)
is now known to have many fossils in various parts of the world.
My suspicion is that sometime in the future this might be absorbed
into the Phanerozoic Eon, although I am unaware of any suggestions
for this to take place. At the very bottom of the timescale I have
included the oldest radiometrically dated rocks (extra-terrestrial
and on Earth). Note that some terrestrial minerals (zircons) are
dated to between 4.3 and 4.4 billion years.
In the Paleozoic (Carboniferous Period) I have indicated the current
time break between the "lower" (Mississippian) and "upper"
(Pennsylvanian) subdivisions used extensively in North America.
In the Cenozoic, one of the most important changes will be to delete
the long-familiar "Tertiary Period" and replace it with
the Paleogene and the Neogene, divided at 23.03 million years. There
is also a proposal to delete the Quaternary and replace it with
the "Pleistogene". I have not included this in the chart.
to see the full Geological Time Scale