Geo News

Ventifacts—sandblast from the past

In 1869 a strange faceted stone, shaped a little like the segment of a small mandarin, was found near an old Maori kitchen-mound at Lyall Bay, Wellington. It was duly exam­ined by a trio of renaissance men, Captain William Travers, Dr James Hector and Mr Walter Mantell, all of whom agreed it must have been fashioned by humans. They specu­lated it might once have been used as an arrowhead or perhaps a tool to cut and polish the grooves in hei tiki. Maori were consulted but claimed never to have seen an implement like it, and scoffed at the hei tiki hypothesis. With an eagerness to unravel the mystery, the men headed for the discovery site, whereupon it soon became clear that the alleged artefact was, on the contrary, a product of nature. The site was lit­tered with many similar stones that together formed part of a boulder bank, separated from the sea by sand dunes. Sand was constantly being pushed over the boulder bank by the prevailing winds and surprisingly, it was the cutting-action of this mobile sand that had fashioned the stones into their remarkable shapes. Stones that have been sculpted and polished by long-term exposure to abrasive wind-driven sands are called ventifacts—from the Latin “make by wind”. Even these days, ventifacts are commonly mistaken for the artefacts of stone-age people, such as those recently misidentified as tools of Indian origin at Cape Cod, Massachusetts. Extraterrestrial ori­gins have also been suggested ahead of the wind. The semi-circular Ma­son Bay, on the west coast of Stewart Island was, in the 1980s, seriously suspected of being an asteroid im­pact crater, some 20 km in diameter. Evidence for this theory included some unusual fan-shaped grooves that had been carved into granite outcrops. The grooves seemed aligned to the centre of the bay and were suspected of being shatter cones, a phenomenon associated with massive impacts. However, in 1991 geologists Graham Bishop and Tony Reay dismissed the cosmic link, saying that the grooves had instead originated from sandblasting by the exceptionally strong westerly winds that slam into Stewart Island. Mason Bay is classic ventifact country. The main requirements for ventifaction to proceed are strong winds that blow persistently in the same direction and a plentiful supply of sand that can be mobilised by those winds. In such environments, ordinary pebbles and boulders suffer severe abrasion and slowly begin to take on seemingly unnatural shapes. Their surfaces become flattened and smoothed on the windward side and given time, a variety of alluring shapes can result. The classic forms are those with multiple facets that intersect along distinct ridgelines and terminate in pyramidal peaks. Highly prized among collectors are the so-called dreikanter or ‘three-ridged’ types that look somewhat like minimalist sculptures of Mt Aspiring. How such ventifacts form has been the subject of some confusion. Some two- or three-sided forms might in­dicate that strong winds have blown from more than one direction, as is true at Lyall Bay. New facets can also be added if the ventifact is moved during its period of sandblasting, which probably explains those rare ventifacts with up to twenty facets. One way that they can move is when the ground upon which they rest is scoured out on the lee side by wind eddies. Unsupported, the ven­tifacts then roll over, thus exposing their underbellies to the blast. The erosive power of the wind is relatively insignificant when com­pared to that of moving water and ice. However, in deserts and those regions frequented by tumultu­ous gales, the wind becomes an important destructive agent. Wind abrasion occurs in two ways. Very fine particles can be held aloft by strong winds before striking and abrading obstacles in their path. Most ventifaction, though, results from the impact of sand grains that are too large to be held in suspen­sion. Instead these grains roll along, generate some uplift and then jump a short distance before settling again. When the grains encounter a rock in their path they strike its surface with enough force to produce a minute star-shaped fracture, much like those seen on windscreens. Microscopic shards of rock are spalled off and after myriad impacts a polished or etched appearance results. Surpris­ingly, winds armed only with ice particles can also abrade rock, as shown by the smoothly worn, west-facing rock surfaces at Browning Pass, in the Southern Alps. When ice is near its melting point it is softer than a fingernail, but its hardness increases progressively with cooling until, at minus 40° C, it is as hard as quartz. The rate at which a ventifact forms depends on many factors, such as wind strength and duration, sand availability and the hardness of the original rock. In the tempests of Nova Scotia, ventifacts are known to have formed in only ten years, although in most climes millennia are probably required. The surfaces of ventifacts can be decorated with deep pits, elongated grooves and scallop-shaped flutes. Pitting tends to occur in soft sedi­mentary rocks where sand grains can become trapped in depressions by the wind. The grains then oscil­late within the holes, wearing and deepening them, until the rock looks like Swiss cheese. Ventifacts are not particularly useful, although these surface decorations can record the wind direction at the time of forma­tion, which is of interest to scientists who decipher past climates. Most ventifacts, though, are mere­ly of use as trinkets. I know of ven­tifacts employed as paperweights, worry stones and mantelpiece curios to break the ice at parties. It seems that many people harbour a feverish desire to own ventifacts, an affliction that got out of hand near the mouth of the Waitotara River in the 1970s. Ventifacts were being plundered there by the trailer-load and one county official even saw a truckload taken. When it comes to conservation, native birds and plants are now thankfully in steady receipt of our preservation efforts, yet rocks are rarely, if ever, considered. Many outstanding geological features have been destroyed by development and plundering. Some Auckland beaches, long seen as free-for-all landscape supply centres, are now almost de­void of sediment. At Waitotara River, many ventifacts had been senselessly smashed to pieces with hammers. Another threat to these ventifacts were plans by local authorities to stabilise drifting sands with lupin and marram grass, a move that would have obscured them for good. Upset by this, the Geological Society of New Zealand applied to have some ventifacts formally protected with the result that several scientific re­serves now exist along the Taranaki-Wanganui coastline. Trailers are most unwelcome in these areas now and thieves risk hefty fines. In Antarctica also, we have progressed from plunder to preser­vation. Unquestionably, the oldest landscapes on earth are to be found in the severe polar deserts known as the Dry Valleys, within the New Zea­land-administered Ross Dependency. The artefacts here include ventifacts of incomparable beauty and many were taken by early visitors, although such acquisitive tendencies are now strictly forbidden. It is impractical to return the gathered ventifacts to Antarctica, so a collection has been assembled for the public to appreci­ate in the foyer of the Dunedin office of the Institute of Geological and Nuclear Sciences. One resembles a Bishop’s mitre both in size and shape. Others are perfectly pyrami­dal and one is faceted like a well-cut gemstone. It took a couple of decades of research to glean the true signifi­cance of the Dry Valleys and it is now known that their rocks have lain undisturbed for many millions of years. Rain never falls but dust-laden winds blow every day. The valley sides control the winds and therefore only two directions are possible: the katabatic westerly that flows down the valleys from the massive Antarctic ice cap, and a less severe summer sea breeze that blows in from the Ross Sea. Salt also corrodes the exposed rocks of the Dry Valleys. The winds trim their tops, while salt crystals attack from beneath, prising them apart crystal by crystal. Bizarre arch-like forms result and ultimately this destructive collaboration reduces the rocks to mere shells of their former selves. In places the thinned rocks emit haunting edge tones during strong winds, and have thus been dubbed the “singing rocks”. The shapes are best described as “out of this world”, and indeed NASA used the Dry Valleys as a Martian analogue when planning and testing for the Viking missions of the early 1970s. It is no surprise then that Mars too has ventifacts. NASA’s Spirit Rover, which landed on the red planet in January of this year, has sent back spectacular photographs of orange-hued ventifacts, all of which now bear Christian names such as Adirondack, Cake and Blanco. It is thought that these Martian ventifacts are relics from a time when great floods deposited sands that were then blown about on the planet’s surface. Photographs of sand tails on the leeside of pebbles indicate that the direction of the Martian wind is from the NE. Interestingly, two ventifacts named Soufflé and Stimpy reveal this has not always been so because grooves on their facets at­test to a dominant SE wind that once blew on Mars. Life is hard where ventifacts are formed. Recently, while walking on the mercilessly exposed south coast of the South Island, I saw a pine tree cowering like a reprimanded dog. Despite its 10 m long trunk, the tree never extended more than two metres above the ground – a botani­cal ventifact. Lichen grew on nearby rocks but never on their windward sides. The Waitotara ventifacts are perhaps the finest examples in New Zealand. They have been said to resemble Brazil nuts or Chinese hats and are between one and twenty centimetres in length. They are sculpted from a variety of rock types but one Waitotara ventifact holds a special intrigue for geologist Vince Neall because it is formed from a rock type not known from the North Island. This “exotic” specimen is sculpted from garnet-mica schist and was found by a Mr T. Smith of Ara­moho, however, it is now missing. To Vince Neall the specimen suggests that rocks from the South Island might have been transported to the North Island during past glacial pe­riods, the last of which was 20,000 years ago. During these times of lower sea level, the North and South Islands were joined between Nel­son and Taranaki by a land bridge and gravel from the South Island is thought to have drifted northward up that now severed coastline. Sea lions provide another way in which “exotics” can be transported. The animals are known to swallow stones to aid their digestion and witnesses have reported seeing sea lions at rest on land regurgitating piles of stones weighing several pounds. Such stones are known as gastroliths and sea lions, in their role as geological agents, can transport pieces of rock hundreds of kilometres from their original source. At the mouth of the Waikato Riv­er, accumulations of once rounded pumice boulders have been planed down by the prevailing sou’wester to the extent that their upper portions are entirely removed. They look like a neat artificial pavement made of inlaid cobblestones. Although for the most part venti­facts in New Zealand are a coastal phenomenon, they can be found in Central Otago. These are sculpted from hard silica boulders that lie strewn upon much of Central’s schist surface. In places these have been ventifacted by the nor’west föhn winds. Strange looking ventifacts also occur around the Clarence River in Marlborough. Here the medium is white limestone and the upper surfaces of the ventifacts have taken on a texture comparable to the hide of an elephant. Tall security fences now encircle the site of the Lyall Bay ventifact discovery of 1869 but not for reasons of conservation. Wellington Airport occupies the area today. Whiteknuckled passengers aboard gyrating aircraft will readily confirm that the intensity of these winds has not abated.


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