Hooligan’s gallstones

This is the first in a series of earth-science articles by Purakanui geologist Mark Walrond. For his debut column he examines a landmark close to his Otago home: the famous Moeraki boulders.

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Arno Gasteiger

In April 2000, a small boulder that had for decades graced the garden of the old St Stephen’s Presbyterian manse in Dunedin was ceremoniously welcomed home by the people of Moeraki. The rock had begun to be seen as a lonely exile by the owners of the house, who felt the time had come for it to be returned home and reunited with its whanau. Now it resides as a taonga at the entrance to the Moeraki marae, but the curious may wonder: why did the Presbyterians take the trouble to lug this cumbersome orb to the manse in the first place?

The Moeraki boulders are an assemblage of near-perfect spheres that lie like discarded cricket balls on an east Otago beach. The largest of them span more than two metres in diameter and weigh over seven tonnes. Their setting is undeniably mysterious: lying on an otherwise unremarkable sandy beach they look as alien as igloos in India.

When faced with geometrical perfection in nature, many people suppose it to be the work of humans or gods. Perhaps we consider nature too haphazard an artisan to sculpt a perfect sphere or cube out of solid rock, or too shoddy a builder to be responsible for precise angles or straight lines in the landscape. Take the hype that surrounded the discovery of the so-called Kaimanawa Wall. In May 1996, claims were made in the media that a six­metre-long stone structure south of Lake Taupo had been built by pre-Maori inhabitants, such was the apparent orderliness of its construction. It turned out to be a natural phenomenon caused by fractures in a volcanic deposit.

The Moeraki boulders provide similarly fertile ground for exotic theorising, but no flight of the imagination is required to explain their origin. In the vernacular of the geologist, the boulders are excellent examples of concretions: hardened nodules that form within sedimentary rocks. The word “concretion”—like the more familiar “concrete”— is derived from Latin and means “grown together.”

Composed of exactly the same material as the surrounding rock, they form when a cementing mineral binds grains of sediment into a cohesive mass. Imagine taking a syringe of glue and injecting a little into a pile of sand. The resulting clump of glue-bound grains would approximate a concretion. Given enough time and enough cement, the result can be a massive boulder—a geological tumour within the body of the host rock.

In the case of the Moeraki boulders, the concretions are no longer in the place where they formed. They have been released from their soft host rock by erosion, in nuts are liberated from a peanut slab by licking off the chocolate. Freed, the boulders have simply rolled down the coastal cliffs and accumulated on the beach, where they now endure the Pacific’s restless pounding.

The cement that glues a concretion together is typically calcite (calcium carbonate), although many other minerals can also perform the role of binding agent. The cement is deposited in the first instance around a nucleus, frequently the decomposing remains of some marine organism. The reason why growth so often begins at the site of decay is not well understood, although it is thought that decomposing organic matter creates locally different chemical conditions within the sediment, and that this acts as a catalyst for the deposition of the cementing mineral. For example, rotting organisms often release ammonia, creating a halo of alkalinity—a condition favourable to calcite deposition.

The nucleus is not always a dead organism, however. Recently, while working in a Lincolnshire salt marsh, English geologists Stuart Burley and Ross Garden were very lucky not to be blown to pieces when a large concretion they were hitting with their hammers cracked open to reveal an unexploded Second World War shell. They found several more concretions in the marsh that had nucleated around bombs and bits of shrapnel. The large amount of ionic iron around these metal objects had led to high concentrations of the cementing mineral siderite (iron carbonate), causing the concretions to grow with astonishing rapidity.

Moeraki’s boulders have grown at only a fraction of the speed of the Lincolnshire ordnance-based concretions. In 1985, a study by Otago University geologist Chuck Landis and an American colleague concluded that the largest of the Moeraki concretions had taken a staggering 4 million years to grow—a period of time comparable to the entire evolutionary history of the human species. The rock within which they grew—a layer of mudstone known as the Moeraki Formation—began to form about 60 million years ago.

As the muddy sediment accumulated, seawater became trapped in the spaces between grains, and it was through this water that calcium and carbonate ions slowly migrated to their nucleation sites. These ions then combined to deposit calcium carbonate around the nucleus, setting in motion the lengthy concre­tionary growth process.

What of the roundness of concretions? Many people assume that Moeraki’s boulders must have acquired their shape after eons of being tumbled in the surf, but this is not so. If the atomic constituents of the cementing mineral are equally available in all directions—as is usually the case in a stagnant saturated mass of sediment—growth will proceed at the same rate in all directions, resulting in a near-perfect sphere.

However, not all concretions are so flawlessly shaped. South of Moeraki, on Katiki Beach, there are dozens of irregularly formed concretions, reminiscent of giant kumara. Water percolating through the sediment beneath the seafloor is thought to have caused preferential growth and elongation of the concretions in the direction of flow.

Within the world of concretions the Moeraki boulders are unusual in that some time after they had reached full size an array of veins, or septa, of golden-yellow calcite began to develop inside them, forming something of a geological honeycomb. The septa radiate outwards from the centre of each concretion, narrowing and terminating just before reaching the outside. On intact, partially worn boulders they can be seen to divide the surface into polygonal segments, an ornamentation that has, in other countries, been mistaken for fossilised turtle shells.

The septa are cracks that have filled with calcite derived from flowing ground water. The origin of these fractures has been the subject of debate among geologists for over a century. One long-held view is that the concretions existed in a moist, gel-like state while growing. The exterior then hardened to form a casing, and subsequent dehydration of the interior caused shrinkage cracks to occur. A more recent theory contends that the cracks are a result of ancient earthquakes.

What is certain is that about 15 million years ago the Moeraki Formation was lifted above sea level, where breaking waves began to wear it down. Marine mudstones are notoriously prone to slumping, and the coastal cliffs adjacent to the boulders have been particularly badly affected. Over time, the concretions have been exhumed by wave action and slumping, and have tumbled onto the beach. In the cliffs today, several boulders can be observed lying half-liberated from their enclosing sediment.

The boulders are found at just one site on the beach. One explanation of this distribution is that an ancient landslide flowed into the sea here and that the boulders are a residual deposit from that event.

Given the manner of their formation, it is not surprising that concretions are notable receptacles of fossils. Many of New Zealand’s most spectacular fossil discoveries have been found entombed within these concrete spheres. Joan Wiffen’s famous dinosaur discoveries in Mangahouanga Stream, Hawke’s Bay (see New Zealand Geographic, Issue 19), were contained within 70–75 million-year-old concretions that had been liberated from surrounding rock and concentrated in the streambed. Because of their distinctive appearance, small concretions are themselves often mistaken for fossilised eggs or nuts.

In 1983, Otago University palaeontologist Ewan Fordyce extracted an 8-metre-long plesiosaur from within a 70-million­year-old concretion at Katiki Beach. Plesiosaurs are a group of extinct marine reptiles with long necks, said to resemble a snake threaded through the body of a turtle. The Otago plesiosaur, named Kaiwhekea katiki, “squid-eater of Katiki,” is the only example of its species ever found, and the largest fossil yet recovered in New Zealand. The Moeraki boulders, however, appear to be devoid of fossils—which is just as well, because the fewer excuses for their destruction, the better.

Claims have been made that the Moeraki boulders are the only rocks of their kind in the world. The truth is that concretions are common, but are usually only a few centimetres in diameter, irregular in shape and unlikely to impress. The Moeraki boulders are deservedly world famous, both for their gigantic proportions and their pleasing rotundity. However, other magnificent examples exist, including the so-called Pumpkin Patch in a southern Californian desert.

In New Zealand, also, there are many significant concretion aggregations besides that at Moeraki. The spectacular boulders at Katiki Beach protrude from a wave-cut mudstone platform. Many have had their tops knocked off, like gigantic eggs at breakfast time. The sea has eroded their relatively soft interiors, forming a series of circular ponds, some of which merge, indicating concretions that impinged on each other as they grew.

Some Canterbury geologists maintain that examples of concretions even finer than those at Moeraki are to be found on the Waipara River. Concretions at Haumuri (or Amuri) Bluff, in Marlborough, contain the bones of marine reptiles and great concentrations of belemnites (fossilised squid), which are thought to be the remnants of mating gatherings.

At Fairfield, near Dunedin, fossil-bearing concretions preserve a snapshot of marine life as it was just prior to the Cretaceous–Tertiary extinction event that wiped out 80 per cent of life on Earth, including the dinosaurs, some 65 million years ago. The Fairfield site is significant because it contains some of the last ammonites to have lived.

At Batley, on the Kaipara Harbour, late-Cretaceous ammonite and Inoceramus shells can be found within concretions, reminding the collector of the essential transience of species.

Only at Moeraki, however, have concretions achieved icon status­ and attracted a raft of fanciful names. Some locals refer to them as Hooligan’s gallstones. Other monikers include giant gob-stoppers, aliens’ brains and—somewhat more respectfully—the Stonehenge of New Zealand.

To Moeraki’s Ngai Tahu residents, the boulders’ origin is intertwined with the arrival of kumara from distant Hawaiki. According to Te Runanga o Moeraki, the great waka Arai-Te-Uru was wrecked during a fierce storm at Matakaea (Shag Point), just south of Moeraki. The boulders are food baskets that washed ashore after being thrown overboard to lighten the load. The Katiki concretions are said to be kumara tubers and seeds from the same wreck.

All these items­ subsequently petrified, along with the many survivors of the voyage­ have become features of the landscape. No fewer than 90 place names of the North Otago landscape are said to commemorate people aboard Arai-Te-Uru.

The early Pakeha whalers who worked the Otago coast from the 1830s had their own folklore to explain the boulders. They called them The Ninepins, evoking an image of giants at play with skittles. In 1843, Edward Shortland, who bore the curious title “Sub-protector of Aborigines,” said of the concretions that some were “round enough to serve for canon balls and of all sizes from the twelve-pounder upwards.”

Frederick Tuckett, surveyor for the New Zealand Company, described them as “pudding stones . . . almost as round as if they had been cast.” They reminded him of similar stones in England that were sliced to make tabletops.

In 1848, amateur naturalist Walter Mantell viewed the boulders during an excursion in his capacity as “Government Commisioner for the final extinguishment of native claims.” Mantell’s passion for natural history ran in the family: his father, Gideon, is credited with being the first person to suggest that dinosaurs once roamed the planet. Of the boulders, Walter wrote that they “were hollowed out by the action of the waves into regular basins, which at low tide stand up from the sands full of water, and are three or four feet deep, forming excellent foot-baths for the weary pedestrian.”

The business-minded Mantell noted that the concretions were similar to ones found on the Sussex coast, which were turned into cement. He had a piece analysed, and wrote, “the results confirm the opinion that the New Zealand septaria will afford excellent cement.”

Twenty years later, in 1868, surveyor John Turnbull Thomson recommended that the government set up a cement mill at Moeraki, to be manned by long-sentence prisoners. His investigations found that the boulders would yield a product superior to Portland cement. Thankfully, his recommendation was not acted on, and the Moeraki boulders were spared the fate of being ground up for building material­ concretion into concrete.

The boulders may have escaped pulverisation, but an examination of old photographs reveals that there are far fewer on the beach at Moeraki now than there once were. Many of the smaller boulders, including the one which appealed to Dunedin’s Presbyterians, have been taken as souvenirs. At least one has made it to Australia, and one Oamaru resident reported receiving requests from North Islanders for boulders, but “refused to be involved with it.”

In 1938, concerned that the waves would soon destroy all of the boulders, William Gordon, a Dunedin businessman, offered to pay for one of the largest and most perfect specimens to be taken to the Otago Museum, so that it might there be “preserved for all time.” Block and tackle were duly erected on the beach and the boulder removed “like a huge Christmas pudding being taken from the pot.”

The great irony is that only 25 years later the boulder started to break up, while its companions on the beach remained intact. Grout was applied to its wounds, and the boulder can still be seen outside the museum on a concrete pedestal, among the shrubbery. It looks lost there—a specimen without a context. Indeed, much of the magic of the Moeraki boulders is their setting: they look stunning among the changing tide and crashing waves.

In the late 1960s, concern was expressed over names being carved into the boulders and explosives being used by rock hounds to get at the prized inner core (although why the core should be prized is not clear). Consequently, in 1971 the boulders were accorded reserve status, and the Moeraki lighthouse-keeper was appointed to ensure that none were taken from the site.

Soon after the reserve had been created, a boulder gracing the top of the Moeraki centennial monument began to break up. As no more of the originals were to be had, a replica was fashioned from concrete and chicken wire to replace it. This is said to contain a time capsule of Moeraki memorabilia, and its creator, John Kedzlie, has remarked that it should outlast the original four times over. Time will tell.

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