As with other limestone caves along the Leeuwin-Naturaliste Ridge, Ngilgi Cave has developed within a thick layer of aeolian calcarenites formed during the Pleistocene Epoch some one to two million years ago. This limestone ridge, which formed and consolidated over thousands and thousand of years, overlays a base of metamorphic rocks of Precambrian Time.
Geology & Geomorphology:
Ngilgi Cave is a stream cave formed progressively through the abrasive and corrosive effect of water. As passages have widened stresses were created in the overlying limestone resulting in many collapses, the collapses have then either settled or been gradually broken down by the action of water. The age of the cave is approximately 500,000 years old. The oldest piece of decoration has been dated at around 386,000 years old.
Decorations & Formations:
The development of cave decorations begins when rainwater seeps through the top-soil which is rich in carbon dioxide, due to the action of plants and rotting vegetation. The water and carbon dioxide form a weak acidic solution which percolates through the limestone dissolving the calcium carbonate. If the solution reaches a cavity, some carbon dioxide is lost to the atmosphere and it becomes less capable of holding the dissolved calcium carbonate. It deposits small particles of calcium carbonate and we have the beginning of a decoration. The term used by speleologists to describe these cave decorations or formations is speleothems and refers to any cave decoration, for example: shawls, flowstone, stalactites, stalagmites, helictites, straw, columns, pillars etc. Ngilgi Cave is renowned for the large number and diversity of its shawls.
The caves of the South West have yielded treasure troves of material that have both archaeological and paleontological significance. Digs at Ngilgi Cave have uncovered the remains of a Tasmanian Tiger or Thylacine, as well as the remains of possums, bats, kangaroos, etc. Fossils found in caves are generally well preserved due to the absence of sunlight, rain and wind which break down and dissolve many surface fossils.
There are three zones within a cave: the daylight zone which is around the entrance to the cave, the twilight zone, where there is a small amount of light filtering down and the dark zone, where there is absence of all light. Within each of these zones, different types of fauna can be found. Depending upon the type of entrance, fauna at the daylight zone includes small marsupials, insects and spiders. In the twilight zone, native cockroaches which venture into the dark zone for food can be found along with a similar range of fauna as in the daylight zone. It is the dark zone where the true cave fauna can be found, these include centipedes, spiders, millipedes, isopods, and mites. Many of these have evolved from creatures found on the surface but have made adaptations to suit their environment including longer legs and feelers, loss of pigmentation, loss of sight and slower metabolisms.
Around the entrance at Ngilgi Cave there are shelves of limestone that have been filled with soil and orchids, mosses, lichen and ferns have established themselves in this daylight zone. Within the twilight zone fungi can be found and in the dark zone, there is an absence of naturally occurring flora, just the roots from trees above the cave searching for water. In tourist caves however, a by product of the lighting is a green fungi known as lampenflora which also exists in the dark zone. At Ngilgi Cave we have recently upgrade the lighting system to low energy LED, in part to minimise the damage done by lampenflora.
Cave Environmental Issues
The use of caves for recreation brings with it a whole set of environmental issues. Consistent lighting and high moisture content creates the perfect environment for fungi called lampenflora to flourish. In the long term, lampenflora can affect cave decorations, however, it can be managed by lowering lux levels of lighting, introducing LED lighting, and restricting the amount of time the lights are on. Other problems from tourism include the introduction of foreign matter into the cave environment such as dirt or rubbish, interference with formations by visitors, soil erosion, and increased carbon dioxide levels from breathing. With the right standards and procedures all these issues can be managed and their impact minimised. Ngilgi Cave is managed in accordance to best practise guidelines.
People have only recently realised the relationship between surface landforms and the cave environment. It is a dynamic system, where limestone is continuously being dissolved and deposited as speleothems. Many of the problems in the cave environment originate from activities on the surface. Influences include, tree clearing, roads, pesticides, soil compaction, effluent sewerage, fires, buildings, fertilisers and erosion.
One of the most prominent land use issues results in a lack of water in the caves of the South West. Combined with a lower annual rainfall, land development and higher use of water bores, have lead to a 40% reduction in ground flow of water since the late 60s. Caves, being underground streams are particularly affected by this.