Sink holes (and cracks in karst), Garibaldi Ridge, NW Nelson (Peter Williams)
Sinkholes are bowl-shaped depressions in the ground. They are mostly formed in calcareous karst landscapes by solution weathering or downward movement of sediments. This occurs in various ways and although there is a complex classification of dolines describing these differences, here they are considered all together as sinkholes. A skeletal soil layer is usually present, but bare rock may or may not be exposed. As they represent areas of local concentration of drainage into the underground, their soils may be quite damp compared with on a flat surface nearby. This may not apply where the covering sediments are still present, and where collapse is occurring, mostly invisibly, beneath a contemporary surface.
Synonyms
Sink, snake hole, swallow hole, swallet, doline, cenote (Mexico)
Notable flora and fauna
Threatened plants include the nationally critical moonwort (Botrychium lunarii), the nationally endangered Fiordland limestone hebe (Hebe arganthera) and the naturally uncommon cream gentian (Gentianella luteoalba). Some rare moss species are Ditrichum brachycarpum and Orthothecium strictum.
Threatened invertebrates are unknown.
Threat status
Endangered (Holdaway et al. 2012)
Threats
Weed invasion is high in karst lands generally and sinkholes are no exception. Some of the largest sinkhole fields are subject to intensive agriculture, e.g. King Country and northern Otago. Direct erosion and unnatural drainage and runoff can be an issue, especially where the sinkholes are downslope from land development. Urbanisation has increased in some areas such as Golden Bay, resulting in sinkholes being used as stormwater drains and rubbish dumps, resulting in infilling and vegetation modification. Some sinkholes provide direct access into caves so recreation may pose a threat, depending on the intensity of use.
Where do they occur?
Sinkholes are dotted liberally through at least 30 major and many minor karst areas, from Northland to Southland. The main karst areas with sinkholes are in the western Waikato-King Country, NW Nelson, north-eastern Otago, and central Westland.
Further reading
Anon 1999. Karst management guidelines: policies and actions. Wellington, Department of Conservation.
Williams PW 1992. Karst in New Zealand. In: Soons JM, Selby MJ eds Landforms of New Zealand. 2nd edn. Auckland, Longman Paul.