Limestone forms fascinating shapes

Press, 18 March 1988, Page 21

Limestone forms fascinating shapes

Part one of a three-part series

Limestone is perhaps the most interesting, attractive, and useful of all Sedimentary Jocks. There are a number of different! varieties of! limestones, but the most comnjon are those which ( were, laid down beneath the sea and are composed primarily of the skeletal remains of marine creatures. |often mixed with a certain amount of sand or other rock particles. Such marine limestones lifted up and exposed by great earth movements, can |be seen in numerous places throughout the South Island, and ate quite plentiful in Canterbury! and the surrounding regions. I

Because much of the material originally laid down under the sea has been removed by erosion since it was exposed, this limestone often occurs in discrete areas. Some of them, for example those at Castle Hill and Broken River on! the way to Arthur’s Pass, now remain in inland isolation. In this, the first ■ article of a series of three "looking at limestone" in Canterbury and adjacent regions, the fascination of limestone in its natural state, both above and! below the ground, is discussed. Subsequent articles will look at some of the uses that man has; made of this

Where limestone outcrops j above the ground it (may weather | into intriguing, and often quite j beautiful, shapes. In! a number of : areas spectacular i outcroppings j can be seen from I the road, in I which travellers with imagina- ! tion can visualise ! all sorts of i forms and figures! Such areas are often like natural studios, full of sculptures that might well be • the envy of many ah art gallery. ! Some are very ! well known, ! and the-limestone outcrops have, | in some instances; been given I individual names, or a particular ! formation may give! a name to an | area. Most New Zealanders and I

thousands of overseas tourists are familiar with the Pancake Rocks at Punakaiki, on the West Coast, where the limestone has been laid down in layers or beds. Weathering has produced the effect of stacks of pancakes piled one upon the other. Interesting outcrops have given rise to other evocative names in the Castle Hill limestone area closer to home: Parapet Rock, where uptilted beds of limestone jut out like the defensive wall of an ancient fortress, or Flock Hill, where grey-white limestone boulders are scattered over a slope for all the world

like a flock of woolly sheep camped on a hill. Castle Hill itself rises from the rocky limestone "castles" of the surrounding countryside. Still nearer to Christchurch is the Weka Pass area of North Canterbury. The road there winds between massive shapes of grey limestone. Most familiar of these is Frog Rock which crouches over the road on the eastern side. It is best viewed when travelling south when its frog-like appearance is quite unmistakable.

Just north of there on the same side of the road is Seal Rock (sometimes called the Lizard), also very easily recognised. Further to the north again, this time on the western side, three humped forms have been known to generations of children as the Elephants Backsides. Complete with legs and tails they stand patiently, backs to the world, timeless and enduring. But in looking at these curiosities of the Weka Pass limestone, travellers should not overlook its inherent beauty such as the magnificent scarp face which runs eastwards up towards the slopes of Mount Donald, its cream-and-grey weathered surface dotted with interesting plants, like the leafless clematis and prostrate kowhai, and relics of the eastern forests which once covered the area.

Those who appreciate beauty, however, will not find anything to surpass a limestone rock overlooking the Pyramid Valley moa swamp, one of several such outcrops which give the property its! name, “The Pyramids.” This rock (shown in the accompanying illustration)’ of fossil-rich, gold, sandy limestone, silhouetted against a clear blue North Canterbury sky, equals for sheer magnificence anything that was ever sculpted by man. But it is not just the naturally occurring limestone that can be seen above ground that holds a fascination for the i human species. Most of the great underground . cave systems, especially those of the West Coast-Buller-Nelson areas have formed in limestone.

| This highly calcareous carbon- ’■ ate rock is particularly subject to dissolution by water, particularly rainwater charged with carbon dioxide. The water works its way

underground through cracks or joints in the limestone until eventually a through-drainage system is established. By this channel, dissolved material is carried away along a routeway that is continually enlarged until a whole system of underground passages and caves may be formed, often opening to the; surface by funnel-shaped holes which channel the water down. These sinkholes (often called potholes or swallow-holes) are a favourite route by which the speleologists, who explore these caves, gain access. They also form natural traps for the unwary.

Quite large of the bones of ground-dwelling animals are frequently found at the bottom of sinkholes and of particular importance to New Zealand scientists have been the bones of moas and other extinct birds which have been (recovered in quantity from such sites. Within the caves is'generally quite a lot of water seeping through cracks in rooLand walls. This water is rich in; lime dissolved from the surrounding rock. Calcium carbonate from that solution may be! deposited from drops of water hanging from a roof, or on! the floor below when the drop falls. Thus formed are the long icicle-like pendants, called stalactites which descend from above, and the

thicker stalagmite columns which grow; upward from the floor. (How does one remember the difference? Well, the stalactites have to hang on “tight,” or they’ll fall off the roof.) The accompanying photograph show’s the natural beauty of some of these formations, lit by the photographer’s flash in a world of perpetual darkness. This cave is part of a complex system at Oparara in the Buller, appropriately named Honeycombe Hill. More than five kilometres of tunnels, with numerous openings to the surface, were discovered there as late as 1980 and have still not been fully explored. Some of the deepest limestone

caves in the Southern Hemisphere are found in the’South Island. While they are not easily accessible to the general public as ! the above-ground outcrops,' they are nevertheless a source of great attraction | to the caversj whb continue tq find new and more exciting ways into their depths. . Ilike ’many of the outcrops, they too have been given names: Harwood’s Hole, or Xanadu, or my! favourite. Profanity. Enough said. i ( The natural world of limestone provides continual interest and pleasure for many people. But limestone is more than just interesting; it is: very useful to man. In both its natural and in a modified state it has been used by humans in New Zealand for at least a thousand years. In two concluding articles, its use by both Polynesians and Europeans will be examined.

By

BEVERLEY McCULLOCH