Caves are an integral part of limestone country. Sometimes they are too narrow for humans to enter, or access is blocked by soil. In the few million years that New Zealand carbonate rocks have been subjected to karstification, there has been ample time to develop large caves.
New Zealand’s longest and deepest
Many kilometres of cave passages have formed in the marble country of north-west Nelson. The total explored length of passages in the Ellis Basin system on Mt Arthur is close to 29 kilometres. The total in Bulmer Cavern beneath neighbouring Mt Owen is almost 51 kilometres. Some of these passages originally formed below the water table, but are now high and dry. They are more than 700,000 years old.
Nettlebed Cave on Mt Arthur has a total depth of 889 metres, and Bulmer Cavern is 749 metres deep.
In limestones in other areas, caves are shorter and shallower. The longest in the Waitomo region is Gardner’s Gut Cave at 12.2 kilometres. Other long caves are Honeycomb Hill Cave (13.7 kilometres) and Megamania (14.8 kilometres), both north of Karamea on the West Coast.
Cave formations known as speleothems develop where percolating water deposits calcite (calcium carbonate) over thousands of years. These include the well-known icicle-like stalagmites (which grow from the ground up) and stalactites (from the roof down), as well as fragile straws and odd-shaped helictites (twisting, curling straws).
Flowstone floors develop where water seeps over the rock, leaving calcite deposits that look like flowing water. Rimstone dams form where water has deposited calcite on the edge of a pool or stream.
Rivers and streams that disappear underground and then re-emerge in another spot are common in karst landscapes. The place where a river re-emerges is called a resurgence.
On Mt Arthur the water table lies about 900 metres beneath the upper slopes. Its level is controlled by the main outflow spring, the resurgence of the Pearse River. Divers in the Pearse Resurgence have found that flooded passages reach downwards well beyond the current limit of exploration (177 metres below the water surface). The marble continues to an unknown depth.
The level of the water table changes with the season and with storms. In some karsts, such as at Cave Creek near Punakaiki on the West Coast, the level can change by up to 75 metres. This presents serious safety hazards for explorers in Xanadu Cave, part of the Cave Creek groundwater system.
People and limestone landscapes
Māori used limestone caves for shelter and burial, and drew pictographs on the walls. Modern cavers have explored many subterranean passages, especially around Waitomo, Mt Owen and Mt Arthur. New discoveries continue to be made. Black-water rafting (rafting through cave streams) was first developed in Ruakurī Cave near Waitomo in 1987. Some limestone outcrops, such as those at Castle Hill in Canterbury, are also important rock-climbing locales.
The marble landforms around Tākaka have inspired some impressive landscape works by New Zealand artists, including Colin McCahon’s 1948 painting ‘Takaka: night and day’. Painter Leo Bensemann also took the karst landscapes around Tākaka, his birthplace, as a theme.
Karst landscapes also attract revellers. Starting in 1997, and for a number of years after, a huge New Year’s Eve dance party known as The Gathering was held at Canaan Downs on top of Tākaka Hill. The saucer-shaped depressions (dolines) were used as amphitheatres.