Geology

Cave formation at Spellbound.

The key ingredients for the formation of caves are limestone (a soluble rock), water and time.

Limestone is a sedimentary rock made from calcareous remains of marine organisms. 30 million years ago, much of NZ was under water beneath a shallow sea.

Wautoma’s limestones are composed of the remains of tiny shell fragments (mainly bryozoans), and a few spectacular deposits of giant oyster shells, giant scallop fossils also sporadically sporadically occur and are found in the limestones at Mangawihitikau.

Layers of calcareous rocks built upon layers and compacted under additional layers of mud and sand. The rock we see today was lifted upwards by techtomic forces to its present altitude. The process of uplift began around 5 million years ago and still continues today.

Much of NZ has a maritime climate, Waitomo on the Western side of the North Island receives 1800-2400mm rainfall per year. Water alone will not dissolve limestone, it needs to be acidified with the help of dissolved Carbon Dioxide gas derived from biological activity in the soil. Water does not percolate through the dense, pure Waitomo limestones. Instead, water will flow along the matrix of fractures occurring within the limestone. The slightly acidic water slowly hollows out cavities along the fractures, some develop into caves. Only large passages, partly or completely abandoned by the water are suitable for humans to visit.

Spellbound’s Mangawhitikau/ Oparure block of sedimentary rocks, 6-9 Km South of the Waitomo Caves Village, overlay Waitomo Limestones and are uniformly tilted at a 5 degree dip towards the South East. As a result ancient streamways once flowed down the dip, over the surface mudstone rocks and then cut valleys running in a SE direction. The Mangawhitikau Stream eventually cut down to the limestone layers below, then, found its way into one of the regular parallel fractures running to the Northeast and formed the glowworm cave. The result – Spellbound’s main river gorge runs SE and the cave systems run NE.

Mangawhitikau stream has been diverted several times, each time the river finds a new path along a fracture in the limestone it abandons its old path. Visitors can clearly see an abandoned dry gorge and abandoned dry cave passages (the Cave of the Spirit) during the tour.

Aerial photograph of Mangawhitikau Gorge
Aerial photograph of Mangawhitikau Gorge

Vistors to the Cave of the Spirit will notice high silt banks at the end of the cave walkway. These indicate that in times of extreme flooding the Mangawhitikau Stream has flowed along its former stream bed. The flood would deposit fresh sediments and debris inside the Cave of the Spirit, and it would also push fossil moa remains further into the cave then bury them.

Glossary;

  • Karst – A limestone landscape, originally studied by German Scientists in Croatia
  • Moa – Extinct family of Ostrich like birds that inhabited NZ
  • Bryozoa – Tiny Brachiopod type shell.

Maori Words;

  • Mangawhitikau – don’t swim across this stream Manga = stream whiti = swim across, kau = don’t
  • Oparure – village 6km S/E of Waitomo Village
  • Mairoa Ash – An air fall deposit from Taranaki Volcanism- up to 100,000 before present
  • Tomo – To enter, also used as a noun for vertical cave entrance (pot hole or chimney)