Caves of the Leeuwin–Naturaliste Ridge

Many of us, especially as children, have experienced the thrill of exploring a cave we’ve discovered. Whether it’s an overhang in a river bank or a whole system of caverns and tunnels, creeping into the dank darkness of a cave appeals to our innate curiosity about the unknown.

There are more than 150 limestone caves along the stretch of coastline between the lighthouses at Cape Naturaliste and Cape Leeuwin in the southwest of Western Australia. It’s how Caves Road got its name and is part of the reason visitors come to the region.

This story is about how caves form, the secrets hidden inside them and why these particular caves are here. Then we’ll have a peek into some of the caves in this region that you can actually visit, as only a handful of them are open to tourists. We will also discover how caves can provide a unique record of past climates that can be accurately dated back several thousand years.

Acknowledgement of Country

We respectfully acknowledge the Wardandi and Bibulman Peoples of Noongar Country (which stretches from Kalbarri to Esperance in Western Australia) as the Traditional Custodians of the land on which this story takes place. We acknowledge their enduring connection to the lands, waterways and communities and pay our respects to Elders past and present.

Any natural opening into the Earth large enough for human exploration may be called a cave. A large expansive, deep cave is often regarded as a cavern.

Caves exist worldwide and come in many shapes and sizes. Some are filled with air, some are flooded with water, and not all have an entrance at the surface.

Speleology is the science of exploration and study of all aspects of caves and cave environments.

The majority of caves, including those along the southwest coast of Western Australia are formed in limestone rock by dissolution. However, caves can form in almost any rock type and by other means – the lava tubes of volcanic landscapes such as on Hawai’i are one example, and the surreal Blue Ice Caves within the Vatnajökull glacier of Iceland are another.

Limestone is a sedimentary rock composed of, or cemented with, calcium carbonate (CaCO ₃ ) typically in the form of the mineral calcite.

When rainwater combines with carbon dioxide in the atmosphere it becomes slightly acidic, forming a very weak acid called carbonic acid (H ₂ CO ₃ ).

🌏 Explore the rock cycle 🌏

This fluid then filters down through the soil where it absorbs more carbon dioxide released by tree and plant roots, and decaying plants and animals. This makes it a little more acidic.

When this weakly acidic water (carbonic acid) encounters and penetrates limestone it can start to dissolve the rock.

Limestone often contains fractures and cracks which the weakly acidic groundwater seeps into, gradually dissolving the rock and widening the cracks.

Over time, and with large enough quantities of this weakly acidic groundwater flowing through, dissolution of the rocks leads to the formation of caves. 🌏 Explore chemical weathering 🌏

The type of landscape primarily resulting from this process, is called karst. Characteristic features of karst are caves, gorges, limestone pavements, sink holes and dolines (enclosed depressions or sink holes). 🌏 Explore weathering and erosion 🌏

The Pinnacles in Nambung National Park near Cervantes are an example of karst and they formed in the same rock unit (the Tamala Limestone) as the Leeuwin–Naturaliste Ridge caves.

Over time, groundwater flowing through limestone gains more and more dissolved calcite. When this carbonate-saturated solution reaches a larger cave that is well ventilated (i.e. has lower levels of carbon dioxide), carbon dioxide gas is released from the acidic water. This discharge of carbon dioxide reduces the water’s ability to hold onto minerals in solution and excess bi-carbonate ions are precipitated as calcite.

This is the reverse of the reaction that dissolves limestone to form a cave.

Over time, the accumulation of these precipitates can lead to the formation of impressive cave deposits called speleothems.

Speleothems are all the different cave formations created from the dissolved chemicals in the water flowing through a cave. They adorn cave walls, ceilings and floors with all sorts of strange and wonderful forms. Speleothems usually take many thousands of years to form and they vary greatly in size and weight, from helictites that can be only millimetres long, to massive pillars and flowstones that weigh several tonnes.

Although mostly composed of white to cream coloured calcium carbonate (the mineral calcite), small amounts of impurities can give speleothems a variety of colours. Iron or organic matter can result in red, brown, yellow and orange colours; manganese, copper and organic material can produce unusual colours such as black, blue or pink.

Perhaps surprisingly there is a great diversity of life that inhabits caves. Cave organisms are classified as troglobites (organisms that are specially adapted to live their entire life in the dark) or trogloxenes (creatures that spend varying amounts of their life in caves). Trogloxenes include bats that leave to find food, and birds that just roost or nest in caves.

Caves have some advantages over other places to live. They are sheltered from the elements such as wind and rain. They have a relatively constant temperature, being cooler than the outside temperature in summer and warmer in winter. This more constant temperature also allows the humidity to remain high.

Organisms that do leave the cave often provide important food resources for those that don’t. They can drop scraps of food as they pass through the cave – bat guano (excrement) is also a vital source of food for many species of insects like cockroaches and mites. These insects then form the base of the food chain for predators such as spiders and pseudoscorpions, who are then eaten in turn by centipedes and snakes.

Troglobites can’t use sight like typical animals do to find food, detect predators or navigate their environment. In fact, many have no eyes at all instead relying on other heightened senses such as touch, hearing, smell and taste.

Some have longer antennae, legs and pincers to help feel their surroundings; bats use echo-location so they can avoid crashing into walls and each other; and some crustaceans have chemosensory hairs to detect prey. Troglobites are adapted to a very narrow range of environmental conditions such as temperature and humidity which means they are particularly sensitive to change.

There are also ‘accidentals’ which are creatures that fell or were washed into a cave and became trapped and subsequently died. This can include frogs, kangaroos, possums, lizards and even goats. They account for some of the fossils that have been discovered in caves.

Caves are also excellent locations for finding fossils of the animals living at the time the limestone was forming. Some have provided great insight into past climates, indicating environmental conditions that were vastly different from today, such as ancient coral reefs.

🌏 Explore fossils and evidence of change 🌏

🌏 Fossil clues to the past 🌏

🌏 Explore types of fossilisation 🌏

Several fossils have been found in Mammoth Cave:

• embedded in the cave wall a 50 000-year-old fossil jaw bone of Zygomaturus trilobus, a long extinct marsupial species that was roughly the size and shape of a small hippopotamus
• fossils of megafauna – giant animals that weighed more than 45 kg and became extinct about 46 000 years ago. These include a giant 2 m tall kangaroo, giant emus and a tree-climbing marsupial sometimes called a marsupial lion.

Footprints and bones of the now extinct Tasmanian tiger (thylacine) have been found inside Jewel Cave. Tasmanian tigers were striped like today’s tigers but were smaller, more like the size of a kelpie. They were marsupials with both sexes having pouches. It is likely the tiger fell into the cave and later died.

Caves along the north–south oriented Leeuwin–Naturaliste Ridge are formed in Tamala Limestone. This is a calcium carbonate-cemented coastal dune sand that was deposited on a platform of much older, hard, weather-resistant, metamorphic rocks – the Leeuwin Inlier – that form the spine of the ridge.

You can move and zoom in on the map using your mouse or the buttons, and elements of the map can be selected to find out more information about the rocks and features.

The Leeuwin Inlier is around 20 km wide and is composed of strongly metamorphosed igneous rocks that have undergone several episodes of complex folding and partial melting. They are dated to between 1100 and 520 million years ago. Outcrops of these metamorphic rocks are generally limited to coastal headlands (see pink coloured rocks on the map) and are predominantly granitic gneiss with lesser amounts of mafic gneiss.

In contrast, the Tamala Limestone has a maximum age of ~1.5 million years, and a youngest estimated age of about 10 000 years. Thus, wherever we see Tamala Limestone lying directly on the basement rocks, the contact between them represents a time gap of at least 520 million years!

Here you can see the unconformity between the Tamala Limestone (at the top) and the Leeuwin Inlier granitic gneiss (at the bottom). The white dashed line shows the contact between them.

The Tamala Limestone is a coastal dune limestone that formed during the Pleistocene Epoch when sea level was up to 130 m lower than it is today. It has been recognized along most of the Western Australian coast as far north as Cape Range and Exmouth Gulf. It can reach thicknesses of up to 330 m in places but is often only about 10–20 m thick along the coastline due to weathering and erosion.

The Tamala Limestone was derived from windblown, carbonate-rich sand, fragmented shells, calcareous algae and other material blown up from the beaches. The material was broken up by wave action, carried ashore and then blown into dunes and finally cemented by calcium carbonate. This makes the rock a softer, very porous type of limestone. This is what has allowed it to dissolve easily and what has, over a few thousand to a few hundred thousand years, created the hundreds of cave systems along this section of the coast.

Other Cenozoic sedimentary rocks found in the area include surficial deposits such as eolian (windblown) sand, unconsolidated coastal and floodplain deposits, and laterite.

The caves that are open to the public differ in terms of the level of skill, or mobility, required to access them. A few businesses offer guided tours of some caves. Others may be visited without guides, although a torch and a reasonable level of agility may be needed to explore safely. Some of the most popular or interesting caves are described here:

Indigenous Australians have used caves for tens of thousands of years as places to live, cook, seek shelter and bury their dead. They have also been places to record their history with rock art and as workshops for making stone tools.

Devil’s Lair is a very special cave in this region since it provides the earliest known evidence of human occupation in Western Australia, and is one of the most reliably dated sites showing early occupation in Australia as a whole. The cave is a single chamber cave with a floor area of about 200 square metres. It is not open to the public though, due to its immense significance and risk of damage.

In the late 1960s, a small number of stone tools and a human tooth were identified in some of the paleontological samples that had been collected nearly a decade earlier from Devil’s Lair cave. As a result, the Western Australian Museum began archeological evacuations here in 1970. They uncovered numerous stone and bone artefacts, intact campfire ash-beds, and enormous quantities of mammal and other bones. Radiocarbon dating of these finds show that Aboriginal groups first occupied Devil’s Lair cave more than 47 000 years ago.

The information from Devil’s Lair and other cave excavations in this area show the use of caves appears to have been occasional. People most likely sheltered here during colder or wetter times. More artefacts have been recovered dating from the last glacial period up until the Last Glacial Maximum (around 21 000 years ago), but as the climate started to warm up and become drier, the use of caves decreased. Indigenous Australians continued to use the caves closest to the coast within the last 800 years as overnight camps and places to gather for meals.

Managing caves can be tricky. There are often many conflicting interests, and changes in the surrounding area far from the caves themselves can have a major effect. Human activity including farming, factory or industrial waste, mining and quarrying, and changing the natural vegetation can cause changes to the water flow and chemistry, or affect sediment levels and volume. These may then have huge impacts on the cave ecosystems.

Opening caves to the public can create its own set of problems. Cave tourism clearly has economic and recreational benefit, and promoting an interest in cave systems is to be encouraged. However, challenges arise in ensuring visitors don’t upset or damage the fragile ecosystems they have come to see.

Many visitors are oblivious to some of the issues their presence can cause, and there are sometimes simple measures that can be used to overcome them:

Measures to protect caves have come too late in some cases, and there have been instances of irreparable/irreversible damage, as seen here with the straw formations at Calgardup Cave:

Caves can provide us with a wealth of information about past climates; from temperature, to rainfall, and even when wild fires took place. We can accurately date the calcite in speleothems back several thousand years using U–Th ratios which enables us to determine the timing of the changes.

The calcite (CaCO ₃ ) comprising speleothems contains oxygen (O), and deviations between the ratio of the two isotopes of oxygen ( ¹⁶ O and  ¹⁸ O), called the  ¹⁸ O/ ¹⁶ O ratio or δ ¹⁸ O, can provide vital information about variations in precipitation, temperature, and atmospheric circulation over time. When water and atmospheric temperatures are colder, such as during an ice age, there is a higher abundance of  ¹⁸ O in the atmosphere because  ¹⁶ O is preferentially locked up in ice. As the abundance of δ ¹⁸ O varies in the atmosphere, it also varies in precipitated minerals, such as the calcite of speleothems.

Measurements of δ ¹⁸ O from stalagmites in Golgotha Cave have been taken with the aim of providing a record of climate change in the southwest of Western Australia over the past several thousand years. Research and analysis of the results is currently underway.

Speleothems also record changes in the chemical compositions of the water over thousands or even millions of years. For example, the presence of a specific chemical compound only produced when a wild fire occurs above the cave can be used to reconstruct how severe wild fires were in the past, and when they took place. This is another reason why protecting speleothems is of such scientific importance.