We are at Waihi beach which is on the South Taranaki coastline
not too far from the town of Hawera.
The geology exposed along this particular part of the Taranaki coastline
is a fantastic example of the really fascinating interplay
between tectonism and sea-level change
right throughout the last five million years of New Zealand's geological history.
A really obvious feature of the cliff-face in this particular locality
is the very very clear change from the blue-gray fine sandstones below
to a thick succession of more brown-colored sands and muds and shoal layers
So there's a very obvious change of rock type in the cliff face here,
very clearly indicating that something quite significant
has happened geologically at this locality
As we pass up the cliff face
we progressively get into more and more muddy sediments.
There's also layers of what we interpret to be peat and plant material.
And right at the very top of the cliff
we have layers of gravels.
To see if we can understand this really prominent change
from the grey mudstone to the shells and sands above,
let's walk down to the face and have a closer look at it.
When sedimentary geologist describe sections such as this,
we start by describing the rocks at the bottom
and then we work our way up towards the top.
This is because the rocks towards the bottom
are generally the oldest and were laid down first,
and the section will get progressively younger
as we move up through the layers.
One thing that stands out in this particular outcrop here
is these lovely shoal layers through here.
The sediment that the shells themselves
are preserved in is reasonably soft.
We can pick it out with our fingers here,
and it's a fairly fine grained sandstone.
There's a huge variety of animals
preserved within this rock.
Here we have a fantastic example of large oysters.
Various types of scollops.
And we have an outline here of a large bivalve.
So collectively, when we look at all these different
bivalves preserved in the rock here,
and we think about where they lived
out on the ancient sea floor,
we come to the conclusion that the sediments here
were probably deposited about 20 to 50 meters below sea level
on a fairly sandy sea floor.
As we move up through the section,
we're getting into sediments that are progressively younger
because sedimentary rocks are laid down in layers.
We hit this very prominent surface.
We can see it's a very sharp horizon,
and there's a really abrupt change
between this grey coloured rock
with the scattered and slightly layered shells,
into a much coarser, much more friable
- see the sand dropping away -
But a much more shell rich horizon here
so clearly environmentally, there's been quite a significant change
between this level of rock and this level of rock here.
There's some really intriguing clues
as to what might potentially have happened across this transition here
just simply from the shells that are preserved within it.
There's a lovely example here
of a large shell of the genus Paphies.
this includes things like pipis, totaroa and tuatua.
This here is a cockle.
The same kind of cockle, Austrovenus,
that we currently find in harbours and estuaries
all around New Zealand.
We also have here a lovely shell of a pipi.
Another type of Paphies, but a pipi.
Again, really widespread, all around New Zealand in the present day
in estuaries and harbours all around the country.
We've got an environment here
that was perhaps 20 to 50 meters under sea water
on sandy sea floor
and here we've got much coarser much more friable sediment
and the shells are telling us that were probably
in an estuary or very sandy, shelly water beach somewhere on a coastline.
One of the coolest things about this particular outcrop here,
Are these shells, preserved vertically within this rock.
These are examples of a species called Barnea.
Barnea is pretty remarkable little beastie
in that it can actually burrow itself into rock and sediment.
We think they've actually burrowed their way
from the surface above down into these rocks.
So they actually relate to the much younger horizon.
These sorts of shells generally only live in intertidal environments.
There are other parts of the cliff around this area here
where right on top of the surface
we have large chunks of rock that are volcanic rock,
the same sort of composition
as what we see in modern Mount Taranaki,
so we interpret that they've actually come down
from the mountain and been deposited on top of this surface.
This then leads to the big question
'what does the surface actually represent?'
The fossils and the types of rock laid on top of it,
the presence of intertidal specific animals
really indicates quite strongly
that this is a fossilised beach.
and based on the fossil material itself,
on correlation regionally,
on dating volcanic ash deposits,
we estimate that this was a beach 125,000 years ago,
during the peak of the last interglacial.
A cool thing about being a geologist
is that we can actually play time traveler.
and here I am, above sea level at the moment,
able to put my fingers on a beach that existed 125,000 years ago,
and still have a fantastic window
into the types of life that lived
in this environment all that time ago.
By contrast, the rocks themselves down here
we estimate to be about 3 - 3.5 million years old.
So that's quite an age gap.
3.5 million years old here, 125,000 years here.
That also leads to some really intriguing questions
of what's gone on across this surface.
We have a lot of geological time that's missing here with no record of it.
That leaves us on quests inland to try and find the rocks
that are eroded here, but are still preserved
in other parts of Taranaki and Whanganui
So if this is a beach,
what's it doing all this way up above the sea?
This is where the tectonic side of the story comes in.
This part of the Taranaki and Whanganui coastline
is ever so slowly rising and rising higher.
So by finding fossilised beaches like this up in the landscape,
we can actually start to estimate the rates
at which the landforms here are rising,
imperceptibly it seems to us,
but actually quite rapidly through geological time