Download Geology of Tarnagulla area

GEOLOGY OF THE TARNAGULLA AREA
Foreword
The main purpose of this story is to help visitors to Tarnagulla to gain some
understanding of the geological history of the area and how that explains the presence of
volcanoes, subterranean masses of granite, and quartz reefs where all the gold was, and
maybe some still is, located.
My intention was to write as simply as possible, avoiding terminology that may not be
understood by the non-geologist. When I had to use geology terms I tried to explain
them. But, of course, everyone knows that dinosaurs were here during the Jurassic period
so you can expect me to use terminology like that. There will be readers who know some
geology so for their sake I needed, at times, to take the explanations a little deeper.
While the main idea is to describe the geology of the Tarnagulla area I have taken the
liberty at times to add information on what happened in other parts of Victoria and
Australia.
My main sources of information were
Geology of Victoria, edited by William D Birch, published by the Geological Society of
Australia (Victorian Division), Special Publication 23, 2003.
Volcanoes in Victoria, by William D Birch, published by The Royal Society of Victoria,
2009.
Tarnagulla Goldfield, Central Victoria, 1:10 000 Map Geological report, Victorian
Initiative for minerals and petroleum report 71, Department of Natural Resources and
Environment, 2001.
Encyclopaedia Britannica Ultimate Reference Suite 2013, Encyclopaedia Britannica,
Chicago.
A word of warning: I have used the above publications as my source of information,
including dating of the phenomena described. Earlier publications may give different
dates. This is because new evidence is being revealed every year and, in the light of that
new evidence, explanations and dates are revised. Also, other publications may give
contradictory information on geological matters. Hopefully, the information here is
currently the best available but that doesn’t mean that it will still be accurate in a few
years time because someone may have discovered something new. That is how science
progresses.
Leslie Dale August 2013
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The Tarnagulla area
Between Dunolly and Tarnagulla are low-lying hills most of which are covered by BoxIronbark forest. The Green Valley Ranges, which run between the railway line and the
Dunolly Road, have maximum heights around 250 to 300 metres above sea level. The
Tarnagulla township is around 200 metres above sea level, the highest point being Crystal
Hill, around 220 metres.
From the hills seasonal creeks carry water eastward to the Loddon river which in that
area is around 150 metres above sea level, and northward through plain country to
eventually join the Loddon River further to the north. The plains are from 190 to 150
metres above sea level. In general, it is flat country so any higher peaks can be seen
clearly in the distance. Those higher peaks include Mt Moliagul (525 metres), Mt
Kooyoora near Melville Caves, Mt Tarrengower (570 metres) at Maldon and Mt
Alexander (744 metres) at Harcourt.
We shall now look at the geology of the area which will explain how these, mountains,
hills and plains came to be there.
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The structure of Planet Earth
There are several alternative models of the Earth’s structure, none universally agreed
upon but all with similar characteristics. The model described here is a simplified
version.
Our planet has three main layers in its structure – the crust, the mantle, and the core. Most
of the crust is from 30 to 70 kilometres thick, depending on the thickness of accumulated
sediments, and is made up of 45 continental plates plus new crust on the sea floor (more
about that later) which is around 5 to 7 km thick. Under the crust is the mantle which
extends downwards to around 2900 km below the surface. The boundary between the
crust and the mantle is called the Moho, short for Mohorovicic Discontinuity. In Victoria,
the Moho is about 30 km below the surface. Below the mantle is the core. The centre of
the earth is around 6,400 km below the surface.
Temperatures in the inner core are comparable to those in the sun. The outer core is
cooler but it is molten metal at around 4000 to 6000 oC. In the mantle, temperatures are
around 2200 oC and at a depth of around 100 km much of the material is molten, like a
very dense, thick fluid. At a depth of around 70 km there is molten rock called magma.
Sometimes the magma makes its way up into the crustal rocks, melting and absorbing
them as it goes, forming a chamber of molten material at distances like 20 km below the
surface. From there it may come to the surface through volcanic action and flows out as
molten lava at around 1800 oC.
The heat from the core keeps the mantle viscous. The continental plates are less dense
than the material in the mantle so they float on top. Convection currents in the mantle
keep it moving and cause the floating continental plates to move. The plate movement is
very slow, of the order of 10 cm a year, but over millions of years they can move
hundreds of kilometres.
The geological time scale
Spend a few minutes looking at the representation of the geological time scale on the next
page. You might like to copy it, put it in a new file and make a printed copy. Some of the
terminology, like Jurassic, will be familiar to you. Quaternary is divided into Pleistocene,
from 2.6 million to 10,000 years ago, and Holocene from 10,000 years ago until now.
Neogene is divided into Miocene, 23 to 5.3 million years ago, and Pliocene, 5.3 to 2.6
million years ago. Paleogene is divided into Paleocene, 66 to 56 million years ago,
Eocene, 56 to 34 million years ago, and Oligocene, 34 to 23 million years ago. Some
geologists still argue about the boundary dates for each geologic period although the
figures argued about are not very different. We shall use the figures given in Figure 1.
Rocks of Pre-Cambrian age are older than 550 million years. That doesn’t mean that all
the rocks in those continents are older then 570 million years, just the oldest ones. In
most continents the oldest rock are of Pre-Cambrian age. Since then many layers of new
rocks could have been deposited as sediments on top of the oldest rocks and that is what
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normally has happened. The oldest rocks in the world that are on the surface are in
Canada and they have been dated at 3,960 million years old. Australia has some minerals
– zircons – that have been dated at 4,276 million years old. The age of Planet Earth is
thought to be in the vicinity of 4,600 million years.
Figure 1 Geological time scale (Encyclopaedia Britannica)
It was once thought that there was little life if any before the Cambrian period and the
Pre-Cambrian was called Proterozoic, meaning ‘former life’. Since then many early life
forms have been found.
Palaeozoic means ‘ancient life’ and rocks of this age are from 550 to 250 million years
old. The rocks in the Tarnagulla area are Palaeozoic. If you look at Figure 1 you will see
that the Palaeozoic Era has been subdivided into six periods, from Cambrian to Permian.
Most of the Tarnagulla rocks, indeed most of the rocks in Victoria, are OrdovicianSilurian so are from 420 to 500 million years old.
Mesozoic means ‘middle life’ and dates from 250 to 60 million years ago. Like the
Palaeozoic it has been subdivided into smaller time sequences. This diagram is an
American one and differs to some extent from the nomenclature used in Australia for the
Mesozoic period.
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Gondwanaland and continental drift
Our story begins around 600 million years ago when close to half of the world’s
continents were clustered together around the South Pole forming one massive continent
known as Gondwanaland, sometimes abbreviated to Gondwana. When we think about
that we need to keep in mind that by then the Earth was 4000 million years old.
This could be a good time to talk about the huge numbers associated with geological
changes. Not only are the continents moving, however slowly, but equally slowly their
surface is wearing down as the result of rainfall, erosion, frost, floods, landslides and
earthquakes. Over more than 3000 millions years the surface of Gondwanaland could
have been lowered by several kilometers. The sediments formed are carried down the
slopes to lower levels where they fill valleys and form plains. Much sediment is carried
downwards by rivers to the sea where, in some cases, huge deltas are formed such as the
delta of the Ganges river on which Bangladesh is located. The Ganges delta has a width
of over 350 km and an area of over 60,000 square kilometres. Much sediment is carried
further out to sea forming huge underwater deltas with layers of sediment that can be
kilometres thick and hundreds of kilometers long. If you want to understand geological
changes you will need to get used to huge numbers.
Figure 2 Gondwanaland around 600 million years ago (from Geology of Victoria)
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In 4000 million years sediments had accumulated on the continents to an average depth
of 40 km thick. The sediments were originally in horizontal layers but enormous
pressures had distorted them over time (more about that later). In many places,
interspersed with the sediments were volcanic lava flows. As you can imagine, much
more sedimentation and geological change had taken place by the time our story begins
than has occurred ever since. At times the continental surface was below the sea and new
sediments accumulated underwater, on top of what was there already. At times the land
was pushed up above sea level, or the sea level fell, so erosion took place once more,
gradually wearing down the surface and depositing more sediments in the sea. Some of
that erosion was enormous, removing a kilometre or more of the land surface.
In that way, Gondwanaland was formed as a single continent. But then it began to break
apart and over many millions of years Gondwanaland broke up into the shapes of the
continents as we now know them. The new continents gradually drifted northwards,
leaving Antarctica in or close to its original position over or near the South Pole. This
may seem strange to you, that they all drifted northwards, until you realize that since they
were all at the South Pole there is really only one direction they could go – north.
The process of continental drift is now called plate tectonics. The sections in Figure 2
called shields are the continental plates which drifted northwards due to underlying
movements in the mantle, a kind of convection current of semi-molten rock-like material.
The mantle is more dense than the continental plates so the continents float on top of it.
The drift is very slow, something like 10 centimetres (4 inches) a year which is around
the rate at which our fingernails grow.
So, you say, how could Australia have possibly moved something like 3,500 kilometres
to the north when it was traveling so slow? Try doing some calculations for yourself – 10
cm a year means 1 metre in 100 years, 1 km in 100,000 years and 10 km in a million
years. So how far in 100 million years?
Figure 3 Mid-oceanic ridges
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But the continental plates floating on the semi-fluid mantle are not limited to what was
there 500 and more million years ago. New continental plate is being created all the time,
at what are known as mid-oceanic ridges. These ridges are lines of weakness in the
Earth’s crust where magma oozes to the underwater surface and turns into molten basalt
which soon cools. As new basalt is created it may flow out on top of what is already there
or it may push the cooled basalt out of the way and in turn cools and solidifies. This
gradually accumulating basalt forms a new continental plate which over the years can
grow to from 5 to 7 km thick and a massive size, and the forward edge of which is
moving at about the same rate as continental drift. As you can see from Figure 3, the new
sea floor spreads out from the oceanic ridge and the further it goes, the older it is. But the
oldest sea floor is only 206 million years old, that is, of Jurassic age. Anything older has
been buried under continental plates.
Where a new continental plate is pushing against a continent it does so with enormous
force, so forceful that it can crumple up the edge of the continent, folding the existing
rocks and sometimes forming mountains. Most times the new plate slides down under the
older continent it is pushing against. This process is called subduction and because of the
massive forces involved it can have drastic consequences including volcanic action,
earthquakes, folding and mountain formation. Where subduction takes place, a deep
undersea trench is often formed, such as the Marianas Trench which is 11 kilometres
deep. There are many similar trenches including one, the Kermadec Trench, which runs
north from New Zealand and past Tonga.
What happened in Gondwanaland
A new geological term in Figure 2 is ‘orogen’. An orogen is a massive geological
upheaval that has significantly affected the sedimentary rocks in the area causing folding
and faulting, volcanic action, earthquakes and the building of mountains. You can see
from Figure 2 that the mountains of New Guinea and the Himalayas in northern India
were formed during the Mesozoic-Tertiary period, around 245 to 20 million years ago.
The Andes in South America and the mountains of New Zealand are older, having been
formed around 300 to 100 million years ago. There is still volcanic action in both of those
locations.
The orogen of interest to us is part of the Tasman Orogen which is better called the
Tasman orogen system. Figure 4 shows the area affected by the Tasman orogen system
and also shows the southern component parts which have been given different names but
are still part of the Tasman system. They are shown in more detail simply because they
have been investigated in greater detail. The Thompson orogen is also a sub-section of
the Tasman orogen system.
The Tasman orogen system was massive, not limited to Australia but extending some
20,000 km from the Andes in South America, through the Pacific margin of Antarctica to
eastern Australia. It is shown in Figure 2 in pink, where it is called Palaeozoic orogen.
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Figure 5 gives more detail of the southern part of that system. The part we are interested
in is the Lachlan orogen. From now on I shall try to avoid using the term ‘orogen’ as, for
the purposes of this story, it is better to think of the area affected as a fold belt, an area
where there has been intensive folding and faulting of the sediments. Sometimes it is
called mountain building.
In the figures the thin lines show the directions of the folds. The thick lines are major
fault lines. The term ‘craton’ means underlying base rocks which in this case are PreCambrian. The Delamerian orogen is older than the Lachlan orogen, having taken place
during the Cambrian period, around 500 million years ago,
Figure 4 The Tasman orogen system, east of the Tasman line (from Geology of Victoria)
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Figure 5 The Lachlan orogen, shown in the centre in grey (from Geology of Victoria)
How did all this happen?
At around 600 million years ago, what is now the east coast of Australia was facing the
sea. Over many millions of years the surface of the continent of Gondwanaland had been
continually eroded and the resulting stones, gravel, sand and mud was washed down onto
lower slopes and into rivers. The rivers carried much of the sand and smaller-grained
material down into the sea and a huge deposit of alluvial material was deposited as
sediment on the ocean floor. This accumulated material was in layers, each new layer
forming when floods brought down a large quantity of sediment. The deposit was
massive, stretching over almost the whole of eastern Australia, Antarctica and western
South America and up to 700 kilometers out to sea (look again at Figure 2). These
sediments were of Ordovician-Silurian age and a small part of them were to become the
sedimentary rocks of the Tarnagulla area.
There were three main layers of rock. The bottom one was about 2 km thick, of volcanic
origin (basalt) and of late Cambrian age. This was the base of the new continental plate.
The next layer, up to 900 metres thick, was mainly shales of Ordovician age. The top
layer, something like 4 km thick, was of mudstones, shales and sandstones of OrdovianSilurian age.
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At possibly around 500 million years ago the sea floor, which was part of a new
continental plate, was affected by massive forces pushing from the east towards the
continent of Gondwanaland. Slowly, over thousands of years, the new continental plate
slipped down under the Gondwanaland continent. As it did so, the softer layers of
sediment which were around 12 to 15 km thick and on top of hard layers of basalt, were
jammed up against the hard Gondwanaland continent and started to crumple. Think of
them as being like layers of wet paper on top of a hard layer of cardboard carrying them.
They crumpled into folds, not just one big fold but hundreds of small folds. The ridges of
the folds ran north and south because the forces causing them came from the east. The
sediments first affected were those now in eastern Victoria as they were the furthest out
to sea. The folding of the rocks now in the Tarnagulla area took place around 400 million
years ago.
The area affected by those forces was the Tasman fold belt which you can see in Figures
2 to 5. As well as folding occurring there was also a great deal of faulting, the formation
of almost vertical cracks through the rocks, varying in size from relatively small to huge,
some many kilometers long and kilometers deep. The main faults in the Lachlan fold belt
are shown in Figure 5, As well as the folding and faulting there was much volcanic action
and outpourings of lava, but more about that later.
The subduction continued for millions of years, from something like 500 to 400 million
years ago, and then stopped, by which time the sediments were in almost vertical folds
with the crests running north and south and the sediments had been reduced to about 30%
of their original west to east length. When on the ocean floor these sediments would have
extended around 370 km out to sea. They are now folded up and only 110 wide. You can
get some idea of what a fold looks like from Figure 6, photos of folded rocks in a railway
cutting at Tarnagulla. By around 390 million years ago the eastern part of the Australian
continent was complete.
While the pressures causing the folding and faulting were continuous, their effects were
jerky, although the jerks could have been many years apart. Pressure would build up
until the sediments, which were gradually solidifying into rocks, would suddenly give
way as either a fold decreased in width or slippage occurred along a fault line. Each time
that happened there would have been a massive earthquake. Fortunately there was no one
there to suffer as the aborigines didn’t come to Australia until something like 70,000
years ago.
As the folding took place the once level sediments were folded into deep folds that could
have been several kilometres from top to bottom. As a result, the tops of the folds were
raised above sea level and eventually became dry land. In effect, the continent of
Australia had a large area of rock added to it along the eastern seaboard and the sea
retreated to around where it is today.
This is a simplified version of what happened. In fact, geologists are still arguing about
when the folding occurred and what caused it.
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Figure 6 Folded rocks in Tarnagulla railway cutting (from geological report)
The journey north
I now want to talk about the journey that Australia took northwards to where it is now,
some 3,500 km from Antarctica. That journey has taken over 300 million years and the
Australian continental plate is still drifting northwards. It has started to collide with
Indonesia and Papua New Guinea but that is another story.
But before Australia could start to move northwards the continent of Gondwanaland had
to start breaking up. Deep cracks had to develop in the land mass, so deep that magma
could well up and start pushing the continents apart causing them to diverge (separate).
That kind of crack is called a rift and rifts are common throughout the world. The most
famous rift is the Great Rift Valley which runs for 6,400 km from Mozambique in Africa
to Jordan in the Middle East. The two sides of the valley average 50-60 km apart and they
are slowly moving further apart. The valley is from around 600 to 900 metres deep and
has been developing for around 30,000 years. Mid-oceanic ridges are formed over rifts
that are thousands of kilometers long.
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Figure 7 An artist’s interpretation of the formation of oceanic rifts and ocean trenches
Figure 7 shows the formation of a mid-ocean rift and an ocean trench and consequent
volcanic activity. Look at it carefully. Volcanoes form at subduction zones, where the
continents converge and an oceanic plate slides beneath a continental plate, enabling the
rise of magma to the surface. At mid-ocean rift zones, shield volcanoes form as two
oceanic plates pull slowly apart and magma oozes upward through the gap. Another type
of volcano, a hot spot volcano, forms where a plume of magma rises from deep within the
mantle to the surface, far from any plate margins. The Hawaiian islands and Yellowstone
National Park are over magma hot spots.
So that is what happened to Gondwanaland. Deep cracks developed, followed by rifts
that gradually widened, so separating the continents. That would have been a time of
violent upheaval, massive earthquakes and much volcanic action. The continents would
all have separated but at different times, very slowly and then began their voyage
northwards at the rate of around 10 cm a year, leaving Antarctica still over or near the
South Pole. It is thought that this separation began during the Late Jurassic period, around
160 million years ago and continued until the late Cretaceous period, some 95 million
years ago. Seldom does anything happen quickly in geology – it took around 65 million
years for the continents to separate from Antarctica. India eventually collided with
Eurasia some 50 million years ago, forming the Himalayan mountains, while the
northward-moving Australian plate has just begun its collision with Indonesia, a collision
that is still under way today, causing massive earthquakes in southern Indonesia.
Another momentous event at this time took place in eastern Australia when the shallow
sea that had covered nearly half of Australia during the Early Cretaceous (around 140
million years ago) retreated and rifting in what is now the Tasman Sea carried New
Zealand away from Australia. Rifting started in Bass Strait but didn’t last long.
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Fossils
Figure 2 shows the life associated with each geological period. We know what life forms
were present because of fossils, remnants of living things that have been found in
sedimentary rocks. The dates when those rocks and fossils were formed have been
discovered by dating minerals in the rocks, using radioactive decay dating of minerals.
The main problem with fossil-hunting is that when most living things die their remains
just decay and disappear. In order for fossils to form, the conditions have to be just right
and most times that just doesn’t happen. If the organism has bones or a shell then they are
the parts most likely to be preserved as a fossil but most times even those crumble and
disappear. In all the rocks of Ordovician-Silurian age in Victoria, including those in the
Tarnagulla area, there are very few fossils and those that have been found are not very
exciting. The only fossils found in the Tarnagulla area are graptolites, the remains of
small floating colonies of animals. These are preserved in mudstones and shales as black,
flattened carbon impressions. Each individual animal lived within a cuplike structure,
many of which were spaced along one or more branches. The entire colony sometimes
was connected, by a thread to a central float.
There are only two reports of graptolites having been found in the Tarnagulla area, one in
1912 and the other in 1988. If you want to look for them, select rocks that look as though
they are made of mud (mudstones) and are still relatively soft, or try harder shales. Both
split open reasonably easily. Look on the split faces for signs of graptolites. They will
look like smudges of black carbon. If you find one, send it to the Museum of Victoria and
your name may go down in geological history.
There’s no point in looking for more recent fossils as the youngest sedimentary rocks in
the Tarnagulla area are of Silurian age.
Figure 8 A graptolite - Monograptus priodon (Encyclopaedia Britannica)
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Volcanoes
During the Cambrian period most of Victoria was under the sea but there was much
volcanic activity on the ocean floor. Lava poured out and solidified on the ocean floor
making extensive deposits of basalt. Over many millions of years that basalt changed in
mineral composition and took on a greenish tinge which is why those deposits are now
known as greenstones. Outcrops of greenstones occur in several places in Victoria, just
south of the Grampians, east of Lancefield and west of Heathcote, and in several small
locations further east. There are no greenstones in the Tarnagulla area.
Calderas
Around 100 million years later, during the Devonian period when most of Victoria was
dry land, there was another outburst of volcanic action. This time the volcanic action was
much more explosive, sometimes forming calderas which were huge craters 8 km and
more across. At least six giant calderas were in action. It is thought that there were no
lava flows from these volcanoes but much lava was blasted high into the sky and fell
back as molten particles or volcanic ash which flowed down slopes, filling valleys and
hollows. Some of those accumulations were over a kilometre thick. That was so long ago
that considerable erosion has since taken place so that all we can see now are
mountainous areas like the Grampians or a plateau like the high plains in the east. The
softer sedimentary rocks and volcanic accumulations have been eroded away and what is
left is the hard basaltic rocks that once filled a caldera. The caldera that formed the ranges
from Lake Mountain to Eildon was about 27 km in diameter. That would not have been a
good time to live in Victoria. All of the plants and any animals living there would have
been completely wiped out.
Volcanic rocks of Silurian and Devonian age that formed in calderas include the
Grampians, Mt Macedon, the Dandenongs, Arthurs Seat, the mountains between
Healesville, Warburton and Eildon, and the northern part of the Strathbogies.
Let’s think about the conditions in the area around a new caldera and imagine what
would happen if a new caldera formed in the Tarnagulla area today with its centre just
east of Llanelly. The first indication of trouble would be a volcanic eruption and the
formation of a volcanic cone in a farmer’s paddock, something like 1000 or more years
ago. Then there might be several hundred years of relative quietness followed, without
any further warning, by an enormous explosion, throwing rock fragments high into the
atmosphere. The only recent explosion of that size was when the top blew off Mt
Krakatoa in Indonesia in 1883. There had been volcanic action at Krakatoa for over 1000
years but nothing of consequence since 1680. The explosion occurred at 10 am on 27
August 1883 and was heard 3,500 km away in Australia. It threw into the air around 20
cubic km of rock fragments, lava and ash. Large quantities of ash fell over an area of
some 800,000 square km. to a depth of up to 60 metres. The surrounding region was
plunged into darkness for two and a half days because of ash in the air. The fine dust
drifted several times around the Earth, causing spectacular red and orange sunsets
throughout the following year.
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Figure 9 Crater Lake, Oregon is in a caldera. There is a small more recent volcanic cone within the lake.
Scenics of America/PhotoLink/Getty Images Encyclopaedia Britannica
After the Llanelly explosion there would be conditions similar to those at Krakatoa.
There would be a huge hole in the ground that would fill quickly with bubbling lava.
Four townships would have disappeared – Llanelly, Tarnagulla, Newbridge and Arnold –
and the whole area for kilometres around would have been completely devastated. Not a
pleasant thought but back in Silurian and Devonian times that happened at many places
throughout Victoria
Let us now think about erosion and its effects on the landscape. Erosion takes place at
about the same rate as continental drift and in a million years the average ground level
could be lowered a much as a kilometer. Think about a mountain like Mt Macedon.
When the molten rock in the caldera that formed that mountain cooled to make basalt, the
top of that basalt would have been little higher than the surrounding area. A caldera is
often like a big hole in the ground, not like a volcanic cone. Now the top of Mt Macedon
is 1013 metres above sea level, a kilometer above the surrounding countryside. It didn’t
start off as a mountain and it wasn’t lifted up to that height. What happened is that the
softer rocks around it and all over the countryside were eroded away leaving the hard
basaltic rocks exposed as a mountain.
Lava flows
The next noticeable volcanic action in Victoria after the end of the Devonian period
started 200 million years later during the Jurassic period when rifting began to separate
Australia from Antarctica. It continued through the period until around 80 million years
ago, when further rifting opened up the Tasman Sea. New volcanoes kept forming and
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this didn’t stop until around 20,000 years ago. There were numerous volcanoes and much
lava flowed out, covering 15,000 square kilometers in southern South Australia and
Victoria, to an average thickness of about 60 metres. The lava fields include what is now
the Werribee and Keilor plains, the flows from the north down to Melbourne, Philip
Island and the Mornington peninsula and in the central highlands around Ballarat and
Maryborough. The basalt between Newbridge and Marong is of that age.
In Victoria, over 400 volcanoes of that age and younger have been distinguished and
many of there are obvious in the Western District, around the Ballarat and Maryborough
districts and north of Melbourne around Creswick and Gisborne. The oldest obviously
visible volcanoes, which are 6 to 7 million years old, are Camels Hump which is on top
of Mt Macedon, and Hanging Rock. The Western District volcanoes are between 2 and
4.5 million years old. The youngest are around 20,000 years old. Volcanoes would erupt
in one area for a few million years then go quiet, only to start up again somewhere else.
Many volcanoes do not have obvious craters, being just low hills from the sides of which
very runny lava streamed away quietly. Some have steeper slopes because their lava
wasn’t as runny. The stickiest lava came from prominent steep-sided cones that formed
Camels Hump and Hanging Rock. An example of the runny lava type is Bald Hill near
Carisbrook, the lava from which covered many hectares of land east of Carisbrook.
Individual volcanoes may have erupted intermittently for millions of years, erupting
perhaps once every 10,000-25,000 years. A recent study of volcanoes in southern
Victoria suggests that the average time between eruptions was more like 5000 years.
Geologists now believe that many of Victoria’s volcanoes are not extinct, as we may
have thought them to be, but merely resting between eruptions. We have to wonder when
and where the next eruption will be - perhaps near Llanelly?
Granitic intrusions
Not all mountains are of volcanic origin. Some, such as Mt Alexander near Bendigo and
Mt Tarrengower near Maldon, are made of granitic rocks. Sometimes pockets of magma
formed in the crust and did not break out on to the surface as volcanoes. Instead, over the
years they gradually cooled forming granitic rocks kilometres below the surface.
Geologists use the term ‘igneous rocks’ to describe all rocks that have been formed by
cooling of molten magma. Volcanic igneous rocks are those that formed from lava and
have the general name of basalts, although there are many different kinds of basalt.
Plutonic igneous rocks formed beneath the surface and are generally known as granitic
rocks. A big magma chamber that cooled to make granitic rocks is called a pluton.
Rocks that form from molten magma contain much the same minerals but the size of the
mineral crystals varies according to how fast the magma cooled. The faster it cooled, the
smaller the crystals. Granites have large crystals from around 2 to 5 cm in size because
they have cooled slowly. Gabbros have even larger crystals that can be 100 mm or more
across. Granodiorites have smaller crystals than granites. Diorites also have small
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crystals. The different kinds of granitic rocks also differ in mineral composition. Granitic
rocks at Mt Alexander, Mt Tarrengower and near Tarnagulla are granodiorites.
Granitic intrusions took place at much the same time as folding, faulting and volcanic
action. The intrusion that formed the Tarnagulla pluton has been dated at about 390
million years ago, during the Devonian period, although it occurred in several phases.
The pluton is west of but very close to the Tarnagulla township and extends west as far as
Moliagul. You can see its location in Appendix 1, and also the location of granitic
boulders in Appendix 3.
Figure 10 shows the locations of the main intrusions in the Tarnagulla area. The
Tarnagulla intrusion extends to Rheola and the Melville caves, Mt Moliagul is part of it.
The Maldon intrusion extends from Baringhup to Mt Alexander. The intrusions are of
Devonian age and are 5 to 7 km deep.
Figure 10 Granitic intrusions in the Tarnagulla area (Geology of Victoria)
The high temperature of the intruding magma and pressures it imposed on nearby
sedimentary rocks caused changes to take place in those rocks up to 5 km away. The heat
caused re-crystallisation to take place and the pressure sometimes produced pressure
lines. Slate is a metamorphic rock and so is marble. The most obvious indication that a
rock has been metamorphosed is the presence of more than usual quantities of shiny
flakes of mica. If the mica appears to be in layers between other minerals, the rock is
called a schist. If the mineral grains are larger and in more obvious layers the rock is
called a gneiss. If a sandstone is re-crystallised it makes a quartzite.
Slates, schists and quartzites can be found near Tarnagulla, close to the granodiorite.
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Quartz reefs
Figure 11 Granite is an igneous rock. Its main minerals are feldspar, quartz, and one or more kinds of
mica. Encyclopædia Britannica, Inc.
Quartz is a component of all granitic rocks. It is silicon dioxide, commonly known as
silica. Feldspar is a silicate so also contains silica. Silica and silicates make up around
95% of all the chemicals in all of the rocks on Planet Earth. When granitic rocks erode
and decay the feldspar forms clays but the quartz is resistant to chemical change and
weathering so stays as quartz but becomes grains of sand. Most sands are mainly grains
of quartz.
At about the same time as the granitic intrusion, large volumes of very hot, liquid started
to penetrate gaps and cracks in the folded sediments. The layers of rock at the bottoms of
folds were the sections most broken up by the folding and the liquids were able to
penetrate there, making their way up on the underside of the folds, sometimes for a
kilometre or more. They also found their way up along fault lines, most of which were
parallel to the folds but some of which cut across them at angles.
That liquid was like milky molten glass but it also contained many minerals. At first it
was molten and very hot, around 1800 oC or hotter, but it eventually cooled to make
quartz. This quartz now appears as veins in the sedimentary rocks around Tarnagulla. It is
very conspicuous because it is so white and looks like big lumps of translucent white
glass. In fact, quartz is possibly the most common and easily recognised mineral in the
Earth’s crust. The bigger of those quartz veins are Tarnagulla’s quartz reefs. They
are often associated with black slates.
The quartz reefs run north and south, in the same direction as the folds in the sedimentary
rocks. They vary greatly in size, from a few millimeters to a metre or more in thickness, a
kilometer or more in length and a kilometer or more in depth. Some of the reefs outcrop
on the surface, others start some distance underground. The quartz reefs are the source
of all the gold in the Tarnagulla area.
Appendix 1 show the location of the Poverty reef, the richest source of gold in the area.
Just a few words in explanation of some of the terms in Appendix 1. Cordierite is a
mineral formed by heat metamorphosis of clay-rich sediments. It marks the boundary of
the metamorphosis due to the adjacent granitic intrusion. An anticline (or anticlinorium)
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is the peak of a fold where the sedimentary rocks slope away downwards in both
directions. The railway anticline is the one exposed in the railway cutting.
The only way to get the gold out of the reefs was to put down mine shafts, bring the goldbearing quartz to the surface, crush it and extract the gold. Most of this happened
between 1852, when the Prince of Wales shaft was put down to mine the gold in the
Poverty reef, and 1994 when the Nick O Time shaft was put down to mine the newly
discovered Nick O Time shoot, close to the Poverty reef. In all, 48 shafts were put down
over the many reefs in the Tarnagulla area. The depth of most of the workings was
between 20 and 100 metres but ten shafts went deeper. The deepest was the Yorkshire
shaft at 353 metres, mining the Watts reef. Three shafts were put down to mine the
Poverty reef, one in 1852 and one in the 1860s, to 314 and 201 metres. This mine closed
in 1879. The new Poverty shaft in 1994 went down to 277 metres Appendix 2 will give
you some idea of the complexity of the workings.
Most the shaft mining had stopped by the early 1900s but a new geological survey in the
early 1990s revealed a rich source of gold in the Nick O Time shoot, an offshoot of the
Poverty reef. A shaft was put down there (you can see the workings behind the Methodist
church) by reef Mining N.L. in 1994 and mining continued there until 1999.
An enormous amount of gold was recovered from the quartz reefs, the total of recorded
production being 561,120 ounces. The most gold recovered, 360,000 ounces, was from
the Prince of Wales shaft, extracted from 122,000 tonnes of rock and gold-bearing quartz
brought to the surface. The Nick O Time shaft brought 57,400 tonnes of rock to the
surface and obtained 53,000 ounces of gold.
Alluvial mining
Tarnagulla first attracted attention in 1852 when gold was found and the first gold rush to
the area began. At the time, Tarnagulla was known as Sandy Creek and was re-named
Tarnagulla in 1961.
The gold found then was alluvial gold, found on the surface and in shallow shafts. Many
nuggets were found.
Let us think about how that gold got there. The only source of gold in that area was a
quartz reef so over millions of years there must have been gold-bearing reefs in the area
which eroded away and were no longer visible on the surface. That erosion would have
removed many metres of rock and soil from the surface of the land. As the reef eroded,
any gold and the quartz would have fallen on to the new surface and that process would
have continued for millions of years. Big pieces of gold, perhaps weighing 10 ounces or
more were too heavy to be washed down to lower levels by running water so they stayed
more or less where they fell. Smaller pieces would have been carried down by any
streams of water, even if those streams were seasonal, and would have accumulated in the
beds of such streams. Those streams would have been more or less in the same vicinity as
present day creeks and gullies but not necessarily following the same paths. For example,
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the old stream bed in Ironbark Gully is a few metres south of the present gully bed. Over
millions of years of erosion the level of lower-lying land would have been built up from
sediments washed down from higher ground. That resulted in the old stream beds being
buried as new stream beds formed at higher levels. In Ironbark Gully the old stream bed
is something like 5 metres below the level of the surface. An old stream bed like that,
carrying gold, was known as a lead. Miners would try to find a lead, put down a shaft,
locate what they thought was the bed of the old stream (they called that the bottom),
bring material from immediately above that old stream bed to the surface and wash it to
extract any gold present.
The gutters of the various leads were rich but along Ironbark and Nuggety Gullies the
findings were described as occasional nuggets or nothing. The bottom wash, down
something like 5 metres from the surface, was the richest.
Appendix 3 shows the main alluvial fields adjacent to the township of Tarnagulla. This
map was taken from a book on the Tarnagulla goldfields published by John Tully of
Dunolly. Prospectors using metal detectors use maps like this as guide to where they
should search for gold. Every year, many small pieces of gold are found in this way in the
Tarnagulla area.
Glaciers and climate changes
To talk about glaciers in the Tarnagulla area may seem rather strange but in fact they
were there at one time and the evidence for them remains.
As you know, a glacier is a river of ice that moves slowly down a valley. As it does so it
is like a giant bulldozer, scraping rocks from the bottom and the sides. A U-shaped valley
is a common result and valleys like that, once carved by glaciers, can be seen in New
Zealand. Rocks become embedded in the bottom layer of the ice and they add to the
scraping effect as they are dragged down over the underlying rocks. Over time, the effect
on the rock material carried by the glacier is much the same as what happens to rocks in a
river bed, they become worn, breaking up into rounded pebbles and sand.
Some of this scraped-out rock material is often pushed to the side and left there. It
becomes what is called a lateral moraine. Some is pushed along in front of the glacier
and when the glacier finally melts that material forms a terminal moraine, a hill of
rounded pebbles. If glaciers have been in the area for millions of years, when they finally
melt away and all the ice disappears the whole surface of the land can be left covered
with worn rocky material which is known as glacial till. The till can vary in thickness
from a few metres to a hundred or more metres.
That is what happened in the Tarnagulla area. A hill of glacial till, possibly a terminal
moraine, can be seen at Hard Hills, near where Hangman’s Lane meets the TarnagullaRheola road. It is used as a gravel pit and the stones, although not white, are close to it as
they are mainly of very hard quartz. The gravel is often used to make concrete. If you go
there and look at the stones you will see how worn they are.
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As you drive around the area you will often see dam banks that are white, unlike the
normally reddish-brown colour that dam banks usually are. They are of glacial till. In
some places you will see where mines have put down to see if gold could be found under
the till. The mullock heaps (the heaps of rocky material brought to the surface and
dumped near the mine shaft) are often quite high, indicating that the shafts are deep so
here must be a very deep layer of till in that area. Some gold has been found under till,
especially at the bottom of what once were deep valleys, but not a lot. From south of
Carisbrook to Newbridge there is an almost continuous line of glacial till of Permian age,
possibly all one long glacial valley.
The glaciers were there in Gondwanaland late in the Ordovician period which is believed
to have been one of the coldest in the history of Planet Earth. Much ice accumulated on
Gondwanaland around that time and glaciers formed. So much ice accumulated that it
removed an enormous quantity of water from the sea and sea levels fell as much as 170
metres. Much of that water was returned to the sea during the Silurian period when the
on-land ice melted. During the Devonian period Gondwanaland was free of glaciers and
there were no ice caps at either the North or Poles. By the Cretaceous period all of the
continents had warmed, probably the warmest they have ever been, so there was still no
ice in Gondwanaland.
At the start of the Permian there was widespread glaciation with ice sheets across
southern Australia. At that time, Australia was still attached to Antarctica so there was
continuous ice over 1 km thick across both Antarctica and southern Australia. The
glaciers moved from south to north carrying sub-glacial tillite pebbles tens of kilometers.
When the ice retreated late in the Permian, tillites covered most of Victoria.
Towards the end of the Permian much of the interior of southern Australia was covered
by broad basins. Rich seams of black coal in Queensland and New South Wales were
deposited during the final 10 million years of the Permian period. Other economic
resources that formed around that time were the reef gold in Victoria, lead and zinc in
New South Wales, and natural gas in South Australia.
The climate had warmed up towards the end of the Permian and continued through the
Triassic and Jurassic. In the late Jurassic the climate in Australia was temperate to subtropical and this continued in the Cretaceous so still no ice. The sea level fluctuated
greatly during that period and parts of many countries, including Australia, were flooded
creating shallow swamps.
By 230 million years ago, in the late Triassic, a second epoch of black-coal formation
opened in south eastern Queensland and Tasmania and in South Australia (Leigh Creek).
A new set of basins, including the Great Artesian Basin, subsided over the east-central
part of Australia. Thick sand was deposited over the area during the Late Jurassic (about
160 to 145 million years ago) and in the west. Subsequent burial of the sand by sediment
of late Mesozoic and Cenozoic age (about 65 million years old or younger) generated the
giant natural gas field at Rankin on the North West Shelf.
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During all that time, Australia was still part of Gondwanaland. Rifting commenced
around 160 million years ago during the Jurassic and Australia began to separate from
Antarctica. The separation was not complete until around 95 million years ago. During
and following the separation, erosion of land surfaces added sediments to the ocean floor
south of what was then the Victorian coastline. Sedimentation continued until the
Pleistocene period around 2 million years ago and by that time sediments were up to 6
km thick. They were later uplifted, forming the Otway and Strzelecki ranges. Associated
with these sediments are the brown coal deposits in the Latrobe valley. River sands
deposited in the Paleocene and Eocene epochs (65 to 34 million years ago) at the foot of
the Eastern Highlands were later shaped into broad folds to become the reservoirs of the
giant oil and gas fields in the offshore Gippsland Basin.
At about 60 million years ago the ocean flooded into the low-lying Murray basin forming
an inland sea that extended as far north as Broken Hill and as far south as almost to
Bendigo. It covered the Wimmera and Mallee and also Wycheproof, Shepparton and
Wangaratta. Over many millions of years, land erosion added sediments to the sea floor.
The sediments filled all low-lying areas to the extent that when the sea finally receded in
the late Miocene period, around 6 million years ago, the whole area was one vast plain,
overlying all earlier land surfaces. That plain is now the wheat farming area in the
Wimmera, Mallee and northern Victoria and the fertile plain of the Riverina. The layer of
sediments, in general, varies from around 50 to 250 metres in depth although at Mildura
it is 600 metres deep.
By the time Australia had fully separated from Antarctica, during the Cretaceous period
around 95 million years ago, the former Gondwanaland continents had cooled and by 40
million years ago ice had returned to Antarctica. Extensive ice sheets developed both on
land and the ocean, glaciers developed forming rivers of ice moving in general from
south to north. Slowly the surface of the whole planet became cooler and huge ice sheets
developed in the northern hemisphere. The Ice Age had begun in the northern hemisphere
although it had little effect on Australia which remained relatively free of ice, although
there were ice caps in the Alps and Tasmania.
At around 20,000 years ago, the height of the Ice Age, 30% of all land on the planet was
under ice. For comparison, only about 10% is under ice now, mainly in the northern
hemisphere. Ice covered the northern half of both Europe and North America.
During the peak of the Ice Age 18,000 years ago, so much water was taken out of the
ocean to form the ice on the land that the global sea level was some 90 metres lower than
it is today, and New Guinea and Tasmania were joined by dry land to the mainland. The
arid zone in Australia was even wider than it is at present, summers were dry, hot, and
windy, sand was moved about in dunes and sheets and dust was blown out to sea. Giant
ancestors of the Holocene animals became extinct, but humans survived as they had for
the previous 20,000 years.
At around 9000 years ago most of the ice melted, the planet warmed and conditions as we
know them today began. We are now in the Holocene period and the climate has varied
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little since the start of the period around 10,000 years ago, although there is some
evidence of increasing aridity in Australia.
Climate change has always been with us, from the time Planet Earth formed some 4,600
million years ago until the present day. It will always be with us as the planet ages, the
continents continue to drift, ice sheets and glaciers form and melt, and ocean current
change in direction and temperature. Climate is mainly a local phenomenon. The climate
at Cairns is very different from the climate at Uluru, in Hobart and at Tarnagulla. The
climate at Tarnagulla is much drier than the climate in the Latrobe Valley. The factors
that determine the climate of any one area are very complex, so complex that while
forecasting of the weather is now much more accurate than it was 50 years ago,
forecasting climate change is almost impossible. Geologists have had great difficulty
trying to determine what the climate might have been at any one time in the distant past.
They use such evidence as the presence of water and of ice, the growth rate of trees as
shown by the distance apart of tree rings, the presence of pollen of different kinds that
show what plants were present, and the presence of animals of different kinds as
indicated by fossil remains. That kind of evidence is often very elusive so there are huge
gaps in our knowledge of what the climate might have been in a given area thousands or
millions of years ago.
The Tarnagulla area today
So there we have it.
500 million years ago the Tarnagulla area was just part of a layer of mud and sediment on
the bottom of the ocean, close to the South Pole.
Once we know how it happened it is relatively easy for us to understand how a huge
thickness of sediment accumulated and was then folded into thick almost vertical folds as
the result of massive pressure from the east.
We can understand how the rift opened and Australia was slowly but steadily pushed
northward until it reached where it is today.
We know enough about volcanoes to understand how violently explosive they can be and
the huge quantities of lava that can spread from them, filling hollows and valleys and
covering very large areas of land. Even so, it is difficult to imagine the violence of an
explosion that makes a caldera and the associated destruction for many kilometres
around.
We can believe that big masses of magma could have cooled deep under the surface, at
least one or two kilometres below ground level, because we have seen granite and
granodiorite and other rocks formed in that way. We can understand that very hot liquids
from that magma could have penetrated the folded rocks and made quartz reefs like those
in the Tarnagulla area. Many of us already knew that the gold found in this area came
from those quartz reefs.
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We know that even the hardest rocks slowly crumble away as the result of weathering
and the fine-grained material produced is carried by water and wind from higher to lower
ground levels, filling hollows and valleys and forming plains. We know that this process
has been going on for many millions of years and so must have had a big effect on the
land surface, gradually reducing the height of mountains and hills. We realise that some
very hard rocks like granites and basalt are very resistant to weathering so are likely to be
left exposed on the tops of hills and mountains.
What I, and possibly some of you, find hard to believe is all that weathering and erosion
has removed something like two kilometres in depth of the original land surface in the
Tarnagulla area. Huge granite boulders in the Maldon area are now sitting up high on
hills when once they were at least a kilometer underground.
This means that thousands and maybe millions of tons of fine-grained material has been
carried down the slopes by water and deposited at lower levels. No wonder we have so
much level plain country to the north of Tarnagulla. In a few more million years maybe
the Tarnagulla hills will have been worn down too and will be part of those plains, with
the exception of course of any lava flows and the granitic mountains like Moliagul,
Tarrengower and Alexander. And, I hasten to add, provided that no caldera suddenly
appears at Llanelly.
Timeline for the Tarnagulla area
In the story of the geology of the Tarnagulla area, so many geological periods and times
in terms of millions of years ago have been mentioned that much of that information is
impossible to remember. You may find the timeline in Appendix 4 a useful reminder of
what happened when.
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Appendix 1 Tarnagulla reefs (from geological report)
25
Appendix 2 The Poverty mine shafts (from geological report)
26
Appendix 3 Alluvial fields in the Tarnagulla area (from Tarnagulla Maps by John Tully)
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Appendix 4
Timeline for the Tarnagulla area
million geological
years ago period
4,600
what happened
Formation of Planet Earth
600
Proterozoic
Tarnagulla area was deep underwater off the east coast of
Gondwanaland. Sediments accumulating.
500
OrdovicianSilurian
Huge accumulated sediments were beginning to fold
underwater due to continental pressures from the east.
450
Silurian
Violent volcanic action across Victoria. Calderas formed.
400
-390
Devonian
Lava flows on sea floor produced greenstones. Folding and
faulting of rocks in Tarnagulla area. Land level began to rise and
sea began to retreat eastwards.
390
Devonian
Granitic intrusion between Tarnagulla and Moliagul and elsewhere
in Victoria including from Maldon to Harcourt.
350
Devonian
Sediments in the Tarnagulla area now rocks on dry land.
300
Carboniferous Folding and faulting in Victoria stopped.
270
Permian
Australia still part of Gondwanaland. Glaciers created glacial till
all over Victoria including in Tarnagulla area.
160
Jurassic
Rifting started the separation of the continents.
140
-80
Cretaceous
Rifting separating Australia from Antarctica.
Extensive volcanic action in Victoria created large lava fields
including near Marong and Maryborough.
95
Cretaceous
Australia now separated from Antarctica and drifting northwards.
Global cooling and start of the Ice Age in northern hemisphere.
80
Cretaceous
Rifting created Tasman Sea
70
Cretaceous
All the continents were now separated from Antarctica.
60
Paleocene
The sea flooded the Murray basin. Sediments began to deposit on
the basin floor and also in the sea south of Victorian coastline.
28
6-7
Miocene
The sea retreated from the Murray basin leaving extensive plains
over northern and western Victoria. Volcanic action in Victoria
formed Camels Hump and Hanging Rock.
4.5-2 Pliocene
Western District volcanoes formed ash cones and lava fields.
2
Uplift and folding of southern accumulated sediments formed the
Otway and Strezlecki ranges. Extensive brown coal deposits in
Latrobe Valley. Oil and gas fields offshore.
Pleistocene
years ago
70,000 Pleistocene
Aborigines came to Australia
20,000 Pleistocene
The most recent volcanoes in Victoria.
9000
Most of the Ice Age ice had melted. Start of a period of stable and
temperate climate in Victoria and the Tarnagulla area.
161 years ago
(1852)
Gold found at Sandy Creek. Many people came to the Tarnagulla
area to make their fortunes.
An additional note about fossils
Dinosaur fossils have been found in rocks from Devonian to Pleistocene age across
southern Victoria but none in the Tarnagulla area because there are no sedimentary rocks
of that age there. Australian dinosaurs were not affected by the extinction of dinosaurs in
the northern hemisphere at the end of the Cretaceous period.
Some remains of giant mammals - a rhinoceros-sized Diprotodon, giant wombats, giant
kangaroos, a marsupial lion and a giant flightless bird - have been found in southern
Victoria, dated as of Pleistocene age. All became extinct at the end of the Pleistocene but
why that happened is not known.
Many other kinds of fossils – vertebrates, shells, fish, corals, trilobites, graptolites,
echinoderms, sponges, foraminifera, whales, dolphins, plants, pollen and many others –
have been found at Victorian locations but, with the exception of graptolites, none in the
Tarnagulla area.
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About the author
Leslie Dale was a Tarnagulla boy. He attended Tarnagulla Primary School followed by
Maryborough High School.
He studied science, including geology, at the University of Melbourne and became a
secondary school science and mathematics teacher. Years later, as a member of staff of
the Australian Council for Educational Research he led the team of writers in the
Australian Science Education Project. That led to recognition for his work in science
education and, during the 1970s and 1980s, a number of short term consultancies with
UNESCO, mainly in south-east Asian countries.
Since retirement he has maintained his interest in science education, has taught courses at
Manningham U3A and has put five courses online with U3AOnline at
www.u3aonline.org.au. He can be contacted at [email protected].
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