From: Geoscience Society of New Zealand 12: 24-30.
PROTECTING NEW ZEALAND’S HERITAGE OF SPHERICAL CONCRETIONS
Bruce W. Hayward
Natural occurrences of spherical boulders and cobbles have attracted attention since
earliest human colonisation of New Zealand and today are tourist attractions in several parts of
the country. Spherical boulders lying loose on a beach, or occasionally on a hillside, have
triggered a number of colourful stories to explain their origins – from the Maori traditional
stories of the origins of Moeraki Boulders to the more recent explanations such as the
unhatched eggs of dinosaurs or their placement by extra-terrestrial aliens. As geologists we
know them as concretions that have grown within thick beds of sandstone or mudstone and
subsequently have been eroded out by removal of the softer enclosing host rock. Indeed
geologists treasure exposures of the concretions within the host rock even more than those that
are now lying free on the surface. An interesting variety of these boulders are septarian
concretions (Pearson and Nelson, 2005), often called “turtle-back” stones by the public,
evoking even more fantasmagorical explanations of their origins.
The best known New Zealand example of spherical concretions is Moeraki Boulders,
North Otago (Fig. 1), where there is a privately-run visitor centre with souvenir shop, café, and
overlook just above the beach where they occur. This stretch of Koikohe Beach was set aside
as Moeraki Boulders Scientific Reserve in 1971 to try to stop the pilfering and vandalism that
was occurring. Today the boulders’ popularity, the presence of a nearby visitor centre and the
large size of those remaining, now provides as much protection as their legally protected status.
Prior to 1971 however, it is clear that many concretions were removed from this stretch of
beach and now lie half-forgotten in gardens, as door stops and one or two in museums. In
December last year, the advertising of a smaller (17 cm diameter) Moeraki concretion on trade-
me attracted considerable public criticism and even featured on TV news
outcry/tabid/423/articleID/325964/Default.aspx). At the time of writing there had been no bids
with a $995 reserve.
Moeraki Boulders are of Paleocene age but nearby on the north side of Shag Point there
are similar, sometimes larger, late Cretaceous concretions protruding from their host rocks (Fig.
2). Some of these grew around marine reptile bones and one was excavated by Ewan Fordyce’s
team at Otago University a few years ago to produce the magnificent Shag Pt plesiosaur fossil
(Kaiwhekea katiki) now on show in Otago Museum.
In Northland there is a 2-3 km section of coastline at Koutu on the south shore of
Hokianga Harbour that is a rapidly growing tourist attraction for its giant spherical concretions
(Fig. 3), some up to 5 m across. Public accessway tracks have recently been installed and it is
advertised in tourist brochures (http://hokiangatourism.org.nz/activities/attractions/koutu-
boulders). The walk is best done within 2 hrs either side of low tide. A shorter and more remote
section of coastline on the northern side of Hokianga Harbour has similar boulders. The two
Hokianga sites have no legal protection and both are probably subject to pilfering of the smaller
concretions. GSNZ has recently made submissions to Northland councils seeking their addition
to their schedules of outstanding natural features but this will not prevent their slow attrition
The Hokianga shorelines are perhaps the richest examples in Northland of these late
Cretaceous spherical concretions that weather out of the Punakitere Sandstone (Northland
Allochthon) but other small pockets occur scattered around the north as far south as Silverdale.
In 1971, four 1 m-diameter examples were shifted from excavations for the Whangaparaoa
interchange at Silverdale to become feature stones in a roadside reserve in Stanley St, below
Auckland University (http://www.geocaching.com/geocache/GC29DEW_four-more-
Several others were left as features on the side of the interchange and periodically were
decorated with spray painted faces. Just recently the interchange was modified and these large
concretions moved but local outcry ensured that they were reinstated when works finished. One
of the most vociferous groups was that claiming that these concretions were moved to the
Silverdale hilltop locality by pre-Maori Celtics for their “solar-related function”
Smaller cobble-sized spherical concretions are sometimes known as “cannon-ball
concretions” (Figs. 5, 6). Good examples of these occur in a number of places around the
country, with two sites listed so far in the NZ Geopreservation Inventory (see below). Many
other concretions of non-spherical shapes are common around New Zealand – their shapes
often determined by the thickness and character of the bedding, by the original shapes of the
object or trace fossil around which they have formed, or by the shape of the subsurface
plumbing of cold seeps (e.g. Nyman et al., 2010). This note is restricted to those of spherical
In addition to the scientific and aesthetic values of spherical concretions, some are
valued more highly for the fossils that they preserve inside them. These fossils may be of any
fossil group, but particularly well-known from spherical concretions are Mesozoic marine
reptile bones from Mangahouanga Stream, Waipara River (Fig. 6) and Shag Pt. Many of New
Zealand’s best preserved ammonites of Jurassic and Cretaceous age have come from inside
spherical to slightly oblate concretions, such as those that used to carpet the foreshore of
Whangaroa, Hokianga, Kaipara (Fig. 7) and Kawhia harbours and in the Catlins. The best
preserved specimens of New Zealand’s fossil crabs also come from inside spherical
concretions, particularly from Glenafric (Fig. 8) and Motunau in North Canterbury.
Sites in NZ Geopreservation Inventory primarily for their spherical concretions
GSNZ’s Geopreservation Inventory’s goal is to list all the scientifically and
educationally geoscience sites of international, national or regional importance. The inventory
is now used as the first port of call by most local authorities in compiling their schedules of
protected outstanding natural features which is used when assessing development and
earthwork permit applications. Thus listing spherical concretion sites in the inventory is the
first step in seeking their protection. The descriptions of many of the sedimentary sites already
in the NZ Geopreservation Inventory (http://www.geomarine.org.nz/NZGI/) include
concretions, but only the following 16 sites appear to be primarily included in the inventory for
the scientific, educational or aesthetic values of their spherical concretions. Assessed
significance: A = international; B = national; C = regional.
Koutu giant concretions, Northland (Fig. 3); B
Rangi Point giant concretions, Hokianga, Northland; C
Musick Point cannon-ball concretions, Auckland (Fig. 5); C
Kawhia Harbour, Ohaua Point Jurassic fossils, Waikato (mostly for fossil molluscs); B
(Fleming and Kear, 1960)
Cray Bay boulders, Hawkes Bay; Protected in Hawkes’s Bay Regional Coastal Environment
Mangahouanga Stream Cretaceous vertebrate fossils, Hawkes Bay (mostly for fossils); QEII
covenant; A (Keyes, 1984)
Chancet Rocks Cretaceous/Tertiary boundary, Marlborough (mostly for paramoudra);
Scientific Reserve; A (Lewis and Strong, 1984).
Glenafric Miocene crab fossils, Canterbury (mostly for fossil crabs) (Fig. 7); Hurunui District
Scheme schedule; B (Glaessner, 1960)
Motunau Beach Pliocene fossils, Canterbury; Hurunui District Scheme; A.
Limestone Glens cannon-ball concretions, Canterbury; Hurunui District Scheme schedule; C
Waipara River Cretaceous-Paleocene sequence, Canterbury; Hurunui District Scheme
Waipara River Cretaceous "Saurian Beds", Canterbury (mostly for reptile fossils) (Fig. 6);
Hurunui District Scheme schedule; A (Gregg, 1971)
Fairfield quarry Cretaceous-Paleocene sequence, Otago (mostly for fossil ammonites); B
Moeraki Boulders, Otago (Fig. 1); Scientific Reserve; B (Boles et al., 1985; Forsyth and
Shag Pt plesiosaur concretions, Otago (Fig. 2); B (Cruickshank and Fordyce, 2002)
Do you think this list includes all of NZ’s more iconic spherical concretion sites and
the best, more easily accessible representative examples around the regions? If not I would be
pleased to hear of any additions or alternatives, preferably with photos (send your suggestions
to b,firstname.lastname@example.org). Inclusion of a site in a District Scheme schedule may only
protect it from major developments – it is not protected from fossickers, vandals, and over-
zealous rock-hounds and geologists armed with sledge hammers. Only education and
responsible, judicious sampling can help maintain these treasures for future generations to
observe in the field and speculate upon their origins.
Boles, J.R., Landis, C.A., Dale, P. 1985.The Moeraki Boulders: anatomy of some septarian
concretions. J of Sedimentary Research 55: 398-406.
Cruickshank, A.R.I., Fordyce, R.E. 2002. A new marine reptile (Sauropterygia) from New
Zealand: further evidence for a Late Cretaceous austral radiation of cryptoclidid plesiosaurs.
Palaeontology 45: 557-575.
Fleming, C.A., Kear, D. 1960. The Jurassic sequence at Kawhia Harbour, New Zealand. NZ
Geological Survey Bull 67.
Forsyth, J., Coates, G. 1992. The Moeraki Boulders. Inst. of Geological and Nuclear Sciences
Geological Brochure 1.
Glaessner, M.F. 1960. The fossil Decapod Crustacea of NZ and the evolution of the order
Decapoda. NZ Geological Survey Paleontology Bull 31.
Gregg, D.R. 1971. Late Cretaceous marine reptiles of New Zealand. Records of Canterbury
Museum 9: 1-111.
Keyes, I.W. 1984. Joan Wiffen and the Mangahouanga reptiles: profile. GSNZ Newsletter 63:
Lewis, D.W., Strong, C.P. 1984. Chancet Rocks Scientific Reserve. Geol Soc NZ Newsletter
Nyman, S.L., Nelson, C.S., Campbell, K.A. 2010. Miocene tubular concretions in East Coast
Basin, New Zealand: Analogue for the subsurface plumbing of cold seeps. Marine Geology
Pearson, M.J., Nelson, C.S. 2005. Organic geochemistry and stable isotope composition of
New Zealand carbonate concretions and calcite fracture fills. NZ Journal of Geology and
Geophysics 48: 395-414.