CAVE GEOLOGY

Development:

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 318,000 years old.

Decorations & Formations:

The development of cave decoration 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 becomes less capable of holding the dissolved calcium carbonate and some of it precipitates out and we have the beginning of a decoration. The term used by speleologists to describe these cave decorations or formations is speleotherms and refers to any cave decoration for example: shawls, flowstone, stalactites, stalagmites, helictites, pendulums, straw, columns and pillars etc. Ngilgi Cave is renowned for the large amount and diversity of its shawls.

The caves of the South West have yielded treasure troves of material that have both archaeological and palaeontological 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.

Cave Fauna:

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 evolved longer legs and feelers as well as loss of pigmentation and sometimes sight.

Cave Flora:

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 region. Within the twilight zone fungi can be found whereas in the dark zone, there is an absence of flora, except for the trees which put down roots in search of water. The exception to this rule is a tourist cave that due to the lighting, a fungi commonly known as lampflora can be found. This is an environmental problem that has to be addressed through good management practices.

CAVE ENVIRONMENTAL ISSUES

People:

The use of caves for recreation brings with it a whole set of environmental issues. Due to the lighting being on for many hours a day and the presence of moisture creates a fungi called lampenflora.In the long term lampenflora can affect cave decorations, however, it can be managed by lowering lux levels of lighting and restricting the amount of time the lights are on. Other problems relate to the flow of people through the cave who bring in dirt on shoes, fluff from clothing, hair and dead epidermis.

Land Use:

People have only recently realised the relationship between the surface landforms and the cave environment. What is carried out on the surface will affect the cave below. The cave environment is a dynamic system, where limestone is being dissolved and deposited as speleothems. Many of the problems in the cave environment originate due to land use activities on the surface. The cave environment is affected by such things as; tree clearing, roads, pesticides, soil compaction, effluent sewerage, fires, buildings, fertilisers, erosion.