Curious concretions

Press, 18 December 1986, Page 17

Curious concretions

Recently I visited a local museum which had a nice display of Maori" artefacts, among them some fishing sinkers. All of these except one were of the traditional kind, a suitably sized stone cobble, around which a groove had been sawn for the attachment of a line. The odd man out caught my eye.

It consisted of two spheres of hard mudstone, joined with a narrow neck — rather like a squat dumbell. While it is possible that it could have been used as a sinker, I doubted it. It was a wholly natural object, owing nothing to the handiwork of man, and unusual only be-

cause of its shape. It was in fact, a concretion.

Concretions are not uncommon. They are found in sedimentary rocks such as mudstones and are nearly always round or oval in shape. They are thought to form when minerals accumulate in the pore spaces between the grains of material which make up the sediment and they usually form around some object, often a fossil or a fossil fragment.

Most concretions are very dense and hard and are therefore much more durable and resistant than the sediments in which they occur. When that

sediment starts to’ weather away, or be eroded by a river or the action of the sea, the concretions are left exposed and it is then that they are frequently collected and the smaller ones find their way into museums or become garden decorations. And of course they are always a target for fossil hunters, although some are so large and solid that they require heavy machinery to move them and are extremely difficult to break open. Some very important fossiliferous concretions have been found in North Canterbury and Marlborough. In the Waipara River, large concretions containing the bones of extinct marine

reptiles were discovered as early as 1859, and by 1869, similar finds had been made at Haumuri (Amuri) Bluff south of Kaikoura. Smaller specimens, mainly from the Motunau area, have yielded some beautifully preserved fossils, especially the remains of crabs, but also of early penguins and a unique and exciting extinct “bony-toothed” bird similar in form to a pelican. Some of these can be seen displayed at Canterbury Museum. Probably the best known concretions in New Zealand are the Moeraki Boulders, -south of Oamaru. Here, the large spherical shapes are eroding by wave action out of a soft, grey mudstone sea-cliff, falling down on to the beach and lying awash in the inshore waves. Some weigh several tonnes and are two to three metres in diameter.

Of course the Maori people had a much more romantic idea of the origin of the Moeraki Boulders, and one certainly more imaginative than our mundane geochemical explanation. According to legend, a canoe travelling from the Maoris’ traditional homeland capsized near Moeraki spilling out on to the beach its cargo of food baskets and kumaras, both at Moeraki and further south at Katiki. Here they were eventually transformed into the boulders we see today. It doesn’t really matter whether that odd-shaped piece of

rock you have collected is manmade or not. The "fishing sinker” in the local museum was merely incorrectly labelled; it was, however, an interesting object in its own right. Products of nature such as concretions are often just as curious and collectable as those items made by man.

By

BEVERLEY McCULLOCH