One of the most important
parts of the archeological
and fossil records is
understanding how old
things are.
When it comes to
understanding the real ages,
the dates of geological layers
of paleontological sites
and of archeological sites, we
use very specialized techniques,
and geoarcheologists and
geologists are real specialist
in this kind of dating.
Andy Herries who's an Australian
Research Fellow, who's the head
of the Archaeomagnetism
Laboratory
at La Trobe University in
Victoria, Australia is an expert
in dating the sites
of South Africa.
Sites in caves like
Drimolin where I found him,
and those cave sites are
among the hardest to date.
When I talked to
him at Drimolin,
we talked about some
different dating methods.
We talked about potassium
argon dating
and its use in East Africa.
We also talked about uranium
series dating and other methods
that are used in the
South African cave sites.
I tried to get him to give us an
appreciation of the complexity
of dating sites in the
archaeological record,
and the way that our
understanding of the dates
of African sites
has really improved
within the last ten years.
How do we know how old
anything is that we find?
Okay. In South Africa
working at how old most
of our fossil sites are
has been a real problem.
So in East Africa, where a lot
of hominin fossils come from,
they have a lot of
volcanic material
that you can date
using a method known
as potassium argon
or argon argon.
And that very precisely dates
volcanic layers to sit above
and below the fossils.
In South Africa we don't
have any recent volcanics.
So traditionally,
what's happened is
that we actually use the fossils
themselves and we basically say,
well we have these fossils
here in South Africa.
We have similar looking
fossils in East Africa
that we know the age of,
and we can compare the two.
So we're talking about fossils,
you mean like, all of the kinds
of fossils of the site.
You know animals and Yeah.
So, mostly traditionally,
it would be pigs
in particular are
extremely good.
Carnivores are quite
often quite good.
Various types of antelope
are particularly good.
And sometimes even micro mammals
like things like rats, voles,
and those sorts of things.
Fundamentally, the real
problem with that, obviously,
is that the sites in East
Africa are, you know, thousands
and thousands of
kilometers away.
Sure. And we don't really know
that the same species are
wandering around the landscape
in South Africa as in East
Africa at the same time.
Or really, that they are
exactly the same species.
Often they have some
similarities,
but they're not always
the same thing.
Okay. So, your work involves the
geochemistry, the geology of it.
How does it, how do you
proceed to try to do better?
Yeah. So, about 15 years
ago I started doing my PhD.
So I do a method known
as paleomagnetism.
So, the Earth has
as magnetic field.
And basically, when any sediment
is washed into a pool of water,
as all the little grains fall
out to suspension of the water,
the little magnetic minerals
inside them align themselves
altogether with the earth's
magnetic fields at that time.
That they then settle
on the floor.
It becomes compacted and
that fossil direction becomes
actually set into that sediment.
And what not lots
of people know is
that the earth's magnetic field
actually completely reverses
itself every now and then.
Right. So, 780,000 years ago,
the earth's magnetic field
was going the other way.
Yeah. So your compass
would point the other way.
What is South at the moment
would essentially be North.
So, we can actually then go
to the sequences in the caves.
And we can measure which
direction the sediment is going.
Is it towards North
or towards South?
We basically work out
where those points are.
And we know the age
of those reversals
because as the sea
floor is being formed,
we get this perfect record
of it because it's set
in as it cools on the sea floor.
So we know for the
last 5 million years,
quite well what the
record is like.
Or at least we know
the major changes.
The unique thing about South
African sites -- the caves --
is that you can get very
high resolution sequences.
To manage to get these really
really small short events
that only last a
few thousand years.
So we are really working on this
sort of finer grain silk stones.
And also on, yes, the
speleothems, or the flow stones.
And the flow stones are perfect
recorders once the grains has
set in, they've set in,
fossilize and they don't move.
And they also record longer time
period because they take longer
to form in the sediments, which
actually wash it a lot quicker.
And the sediments could always
have problems, you know,
mud cracking, so you get slight
changes but not huge amounts.
The big trick is that we
know that there's a reversal
in the site, for example.
We need to know which
one there is.
There could be more
than one possibility.
So we can use the form
that sometimes gets
in a particular place.
Sometimes we're lucky and
that tells us everything.
But what we generally
tend to do now is
that I do the paleomagnetic work
and then we will
date the flow stones.
So we do that using the
method known as uranium lead.
So, there is another method
known as uranium thorium,
it's part of the same
decay chain of minerals.
And that only unfortunately
goes back to 500,000 years.
Using uranium lead is a
much more recent method.
We couldn't do it until
recently on younger sites
because essentially not a
long enough time had elapsed
for one mineral to
decay to the other.
Sure. One isotope to the other.
So, but now we have a situation
where we can date the
flowstones in the caves.
So that basically
gives us a marker,
we know that that is that age.
Of course, the fossils
aren't in the flowstones.
Right. That's where combining
methods is the real trick.
So you date that layer there.
And you go, okay, so this
is sort of let's say,
1.89 million years or
something like that.
And then you can basically
then say, oh well I know
that there's it's some
reversals around there.
And we used the paleomagnetism
to track down to them.
And then we get that layer and
we say, right, that's that age.
That's that age.
And then we can date
the fossil layers.
So, and again, if you've got
a complex geomorphic history,
which again, brings
us back to geology.
And it has to ultimately also
come back to statrigraphy.
Yeah. Caves, how they form,
the stratigraphy that's there.
And I think traditionally, most
of the sites they've studied,
they've studied, you
know, the later phase,
which is where all the
collapse was happening.
All the fossils are there.
But what we really need to do
is understand the cave's life
history from beginning to end.
How did the cave form?
What was its speleogenesis?
How did all the passages form?
How was it then cut
open to the surfaces,
that one entrance,
two entrances?
And only then, can we sort
of put it all back together.
Because I've been caving
since I was 17 years old.
And I've been in a lot of
caves and a lot of places.
And they're complex things.
And, you know, not
many geologists
or paleomagnetists
really work in caves.
They have their own
unique set of rules.
You can get weird situations
where traditionally,
you think of, you
know, the sediments
at the top are the
youngest and the ones
at the bottom are the oldest.
But there's a, I mean,
there's a fossil site
in Australia called
the Wellington Caves.
And then artifacts there,
the oldest stuff that's
like 2.6 million years old,
is above the stuff that's
about a million years old.
So it's completely inverted.
And unless you understand
those complexities,
you can end up getting
yourself in a real muddle.
People probably imagine,
you know,
from folks that are specialists
that are working on dating
that sort of thing, sort of
parachuting into the site
and taking samples
and then they're gone.
They go back to the lab.
You're out here working
excavating.
I am. And that's the
difference ultimately I think,
because I'm an archaeologist.
Yeah. I'm trained
as an archaeologist.
I studied archeaology science.
I then did geoarchaeology and
it wasn't until later I moved
into geophysics and
geomagnetism.
I do this because I'm interested
in hominins, ultimately.
I'm interested in the big
questions of how, you know,
you can look at how all these
species relate by looking
at their difference in anatomy.
You can also do that by putting
them into a temporal line
and saying this one is this
age, that one's that age,
this could've evolved, and
that could've evolved to that.
And ultimately that's
what I'm interested
in to a certain degree, the
paleomag and the geology.
I love caves as well.
That's my other obsession.
So I've got both.
But, you know, I
do those methods
to really understand
that question.
And there are more people
now who have come that route
from archeaology into
those disciplines.
Yeah. But still, a lot of
people that work on site geology
and dating are geologists
or geophysicists.
And they're interested
in other questions.
And they do this work
and give you the dates.
But they're not as
integrated in the same way.
They don't understand the
archaeological questions
as much, maybe.
And so I think that's
one of the real benefits.
And I think, again, that's
sort of what I'm trying to do.
Where I teach in Australia is
to bring more people
into the discipline.
Sure. Doing that geological
thing geoarchaeology etc.
So if a student is interested
in getting into this field,
what would you suggest they do?
My personal view I mean,
for me I had a very
wide-ranging background.
You know? I always did science.
Science in archaeologies is
more scientific these days.
And I think, if you want to
be a field archaeologist,
you want to go out
there and dig,
one of the big challenges
is that you have
to understand a bit
about everything.
Yeah. You have to realize
that you don't know
everything about everything.
But you have to sort
of at least be able
to understand a little bit
about all there is people
that come into your site do.
People doing dating,
paleoenvironments, geology.
So I think it's very
good to have that sort
of very broad background.
Sure. And the other
thing what I always say
to my students is also think
about what all the other
people are doing in the world.
There's 600 people
doing stone tools.
You know? Find something
that's a bit different,
a niche that you can get into.
Yeah, sure.
So, I mean, I'm very lucky I get
invited to go to lots of sites.
See lots of things.
Do lots of dating.
Travel the world.
And it's great fun, so.
Well, brilliant.
Thank you so much.
Thank you.
