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• Cover Photo from the collection of Don Weston , University of Otago
INTRODUCTION
The famous Moeraki Boulders lie scattered along the shore between Moeraki and Hampden
on the east coast of New Zealand's South Island . They are remarkable for their size and
roundness and there is nothing quite like them anywhere else in New Zealand.
Since the first human settlement the boulders have been a source of fascination. They were
the inspiration of Maori legend, and in the early days of European colonisation smaller
specimens were taken as "souvenirs". Today, however, the boulders are reserved for all to
enjoy. Every year thousands of visitors ask the same questions. Why are the boulders so
round? Where did they come from? How did they form? We hope this pictorial memento will
provide you with some answers to these questions and enhance your appreciation of this
natural phenomenon.
THE MAORI EXPLANATION
The Ngai Tahu people of the area relate the Moeraki Boulders to the wreck of the great
canoe Arai Te Uru . As it was travelling south, the canoe foundered in a storm near Matakaea
(Shag Point). Its cargo was washed up on the nearby beaches - the round food baskets
and wa ter gourds are Te Kai Hinaki (the Moeraki Boulders), while the seed kumara (sweet
potatoes) are the irregularly shaped boulders further south .
The hull of Arai Te Uru forms the reef which extends into the sea at Shag Point, and the
large rock nearby is Hipo, the navigator. Several of the hills between Moeraki and
Palmerston carry the names of crew members: Pukehiwitahi (Pakihiwitahi), Puketapu, Nga
Tamariki a Hekura (Hekura's children) and Poutaiki. The hills can also be taken to represent
the great waves which overwhelmed the canoe.
PHOTO 1 Model of a Polynesian ocean-going canoe , and gourds used by the Maori to carry water on
long sea voyages . Photos.' courtesy of the Otago Museum
THE SCIENTISTS' EXPLANATION
The boulders may look as though they were washed up onto the shore, but a geologist
would direct your attention in the opposite direction . The Moeraki Boulders were once
buried in the mudstone cliffs at the back of the beach (e.g. Photos 2 & 3). For millions of
years the sea has been eroding these cliffs , washing away the soft mUdstone which
surrounds the resistant boulders. In the process
PHOTO 2 Once hidden from view within the
the cliffs have con stantly slumped seaward and
mudstone cliff thiS concretion has been
the boulders have been left lying on the
exposed by erosion. Photo. J Forsyth
shoreline. There are countless more still
embedded in the mudstone waiting to be
uncovered .
To geologists the Moeraki Boulders are
septarian concretions. Their roundness has
nothing to do with being washed by the surf ­
they were round when they formed in the
mudstone. The process by which they formed
began some 60 million years ago (in the
Paleocene period). At that time much of North
Otago was covered by the ocean. Muddy
sediment, with fragments of plants and shells,
accumulated on the sea floor. As great
thicknesses of mud piled up over millions of
years, chemical changes occurred in the wet
sediment and the concretions gradually grew
within the mudstone.
About 15 million years ago, during the Miocene
period , the mudstone and other rocks of the
region were lifted up above sea level and the
agents of erosion began to work away at the
new landmass. Eventually the modern coastline
was sculptured, exposing the mudstone beds
containing the concretions.
WHAT ARE CONCRETIONS?
Concretions are hard masses which form in sedimentary rocks (rocks which are laid down in
oCean s, lakes or rivers). They range in size from small pellets to huge spherical bodies three
metres across. Some of the Moeraki boulders are giants of their kind at over two metres in
diameter.
"Concretion", like "concrete", means a hard, cemented material. In a concretion the cement
is usually the mineral calcite (calcium carbonate), but may also be silica (silic on oxide),
pyrite (iron sulphide) or an iron oxide.
HOW DO CONCRETIONS FORM?
Small amounts o f th e cementing minerals are dissolved in sea water. During deposition of
sediment on the ocean fl o or, sea water becomes trapped between the sedimentary particles.
Decaying organic matter in the sediment, such as shell, bone or plant fragments, acts as a
nucleus aro und which the con cretionary minerals gradually crystallise. The chemical
components in the wet sediment migrate toward the nucleus and continue to collect around
it until con ditions change or the supply runs out. This takes place slowly over thousands or
millions o f years.
If the crystall isation of mineral s proceeds equally in all directions from the nucleus , the
concretion grows in a spherical shape; otherwise a flat or irregular shape develops.
SO HOW DID THE MOERAKI BOULDERS FORM?
Geological studies of the Moeraki Boulders have identified a number of stages in the
formation of the concretions we see today . The following illustrations give a schematic
history of boulder formation spanning 65 million years. Illustrated by Geoffrey Cox.
Abou t 60 milli on years ago muddy sediment slow ly accumulated on the sea lIoor.
Shell fragments and plant
debns were incorporated Into
the layers of sedi ment during
depoSItion.
2. As the mudstone at Moerak l
accumula ted on the anCient sea
Iloor, It was continuous ly
mixed by burrowing animals '
fiSh, marine worms, molluscs
and sea urchins. Th is
destroyed any original layering
In the sediment.
3. Within the wet sedimenl the
cement in g mineral calcite
gradually crystallised around
organ ic nuclei to form
spherical nodules Geologists
ca lculate that this took about
120 000 years lor a small
concretion (less than 0.5
metres across) and about lour
million years lor a large one
(over two metres across) .
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4 The outside surface of the
developing concretions then
became hard and brrllle. and
the inside material began to
dehydrate by chemical
reactions. caUSing sh rink age
cracks to propagate outwards
Irom the core to the rim .
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5. Subsequently the cracks
became fi lled with ca lcite
crystals which grew in Iwo
stages. Tiny crystafs of brown
ca lcite grew first. followed by
larger crystals 01 ye ll ow ca lCite
(see Photo 6) .
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6. By the time the yellow calcite
began to form , Ihe region had
been uplifted on the edge of
the new New Zealand
landmass. This period of
mountain building IS known as
the Kalkoura Orogeny. It
began In the MIOcene period
and IS conlinuing today.
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7. Finally, in the last few million
years, erosion of this landmass
exposed the mudstone beds
containing the Moeraki
Boulders as seen in Photo 3.
The weathering effects of rain ,
surf and sun have worn away
the outside of many of the
bou Iders reveahng the network
of calcite-filled veins (Photos
4, 5 & 6) . Fragments of the
boulders litter the beach
PHOTO 3 Progressive
erosion of the coastal cliffs
IS slowly brtnging more
Moerak i Boulders down
onto the beach . Photo.' 0
Weston
PHOTO 4 The Moeraki Boulders display well developed "veins" which radiate out from the cent re These
veins were once Internal cracks which became infilled with the secondary mineral calci te. They often
form within concretions and are known as septa (meaning dividing walls or partitions) , hence the name
septarian concretion Photo ,' J. Forsyth
PHOTO 5 This photograph shows a Moerakl Boulder
with the distin ctive surface pattern which forms when
erosion exposes Ihe internal network of veins This
pattern Is tYPical of septanan concretions and has given
rise to the names 'turtle backs" or "turtle stones" PIIOtO.
J Forsyth
PHOTO 6 ThiS close-up of the smoothly eroded surface of a Moerakl Boulder shows the lWO kinds of
calcite wh ich crysta llised In cracks during Ihe formation of the boulders. The finely crystalline. early­
stage calcite (A) formed as the cracKs first developed The coarse ly crystal line ye llow ca lCite (8) (ormed
milch later. SClenlists have found the Is%pic signature of rainwater wllhln the yell ow calcite, Indicating
that this minerai formed after the region had been uplifted to form a new landmass Photo. J Forsyth
THE SHAG POINT CONCRETIONS AND THE PLESIOSAURS
South of Moeraki lie the long sweep of Katiki Beach and the headland of Shag Point. Here
there is another set of concretions, less spectacular than the Moeraki Boulders, but more
exciting in that some are formed around the bones of two kinds of extinct marine reptiles,
plesiosaurs and mosasaurs.
A few years ago a near-complete skeleton of a 7-metre plesiosaur was discovered near Shag
Point (Photos 7 & 8) It was excavated with some 15 tonnes of the enclosing rock, and is
being prepared for display in the Geology Museum at Otago University, Dunedin. This
plesiosaur belonged to the long-necked elasmosaurfamily, and had a large body, a small
head, four streamlined flippers and a short stumpy tail. Elasmosaurs lived from 205 to 65
million years ago (Jurassic to Cretaceous time) and died out during the worldwide extinction
of the dinosaurs and many other organisms at the end of the Cretaceous period.
Bones of a small mosasaur have also been found near Shag Point. Mosasaurs were large
swimming reptiles closely related to living lizards, but they too became extinct at the end of
the Cretaceous. The Shag Point concretions date back about 65 million years and are
somewhat older than the Moeraki Boulders.
PHOTO 7 A large volume of
rock conta ining th e remains
of a 7 metre-long plesiosaur
IS seen here being excavated
from a site near Shag Point In
1984 Photo. courtesy of
REFordyce
PHOTO 8 The ske leton of
the plesiosaur found at Shag
POint is being recovered
pa instaking ly from the
enclosing mudstone In the
Geology Department at the
UniverSity of Otago. Photo.
o Weston, courtesy of
R E Fordyce