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CHAPTER 3: CAVES - ROCK, WATER, AND TIME

One of the special features of Jenolan Caves is its ability to inspire curiosity and spark questions in visitors. We look at nature’s handiwork and we wonder, “Why?” The history of Jenolan has always been this way. Generations of explorers and researchers have been drawn to an ongoing quest to uncover the hidden wonders of Jenolan Caves.

What forces were in play to carve out the Grand Arch that so spectacularly leads visitors into this secluded little valley? How is it that kilometres of passageways and huge caverns came to exist in solid rock? How did the famous Jenolan Caves formations come about so deep underground? Why is there such an amazing variety of intricate and massive formations? How old are they?

However, before we consider these questions, it is worth taking the time to ponder on the matter of the origins of the very rock itself.

This chapter looks at the natural history of the rock, from its beginning in a shallow sea, to its present position in a low mountain range. We follow its tortuous progress over millions of years. The story of the Jenolan rock is surprisingly complex.

Caves are carved throughout this massive limestone ridge. Grand Arch, Carlotta Arch, and Nettle Cave. Ros Gillespie

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The rocks themselves were formed on an ocean floor, hundreds of kilometres out to sea. Over the eons, volcanoes erupted nearby and completely buried the Jenolan rock under volcanic sediments. Huge tectonic forces pushed the sea floor westward, folding and cracking the entire landscape as it was heaped up and added to the ancient continent that was to become Australia. The folding created huge mountain ranges that were all the while worn down by rain and ice. Millions of years elapsed, and a vast depression formed where Sydney would eventually be built. The landscape was slowly dragged below sea level, and was again covered by thick sediments. Much later, the Jenolan rock was once more raised a kilometre above sea level.

In time, a giant rift formed and tore a huge chunk away from the continental mass, forming much of the current eastern Australian coastline. Meanwhile, around Jenolan, almost all of the overlying sediments were removed, and the rocks of the ancient sea floor were left exposed at the bottom of deep valleys.

We will examine each of these phases over the eons in an attempt to explain why Jenolan Caves looks the way it does today.

Although we already know a great deal, our understanding of the geological history of Jenolan is not yet complete. Professional researchers and amateur investigators alike continue to painstakingly piece the story together. We still have gaps in our knowledge, and if we let them, those mysteries can invoke great curiosity, and a desire to explore. The resulting knowledge not only improves our ability to manage the Caves, but we may also uncover vital facts about our environment, and in the process learn more about ourselves. The key to these great treasures is to first allow ourselves to be inspired and awed by Nature, and then to simply ask, “Why is it so?”

Visitors inspecting a tunnel-like section of the Chifley Cave.

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CAVES IN LIMESTONE

Caves have formed at Jenolan due to the special nature of the rock found there. The Jenolan Caves limestone is a hard rock, structurally strong enough to support large cavities without collapse. But this sturdy stone can also be dissolved by mildly acidic water, and it is this dissolving process that carves out the caves and caverns.

At this point it is appropriate to define what we mean by a cave. From here on, we will consider a cave to be a naturally-formed cavity in rock that is large enough for a person to enter. Smaller openings might be called fissures, crevices, or cracks. Away from direct light at the entrance, a cave goes through a gloomy half-light region until total darkness becomes permanent.

A cave is always under pressure from the weight of the overlying rock that attempts to collapse into the cavity. Weak rocks will bend or crumble, filling in any crevice, so large caves only form in very strong rock.

All of the caves at Jenolan are formed in limestone, and although caves can occur elsewhere in other rocks such as sandstone, and even volcanic rocks, limestone caves are formed by dissolving the parent rock.

To be properly called “limestone”, a rock must contain at least 50% of the compound calcium carbonate (CaCO3). Significant caves usually only occur in limestone when the rock is more than 90% calcium carbonate. The Jenolan Caves limestone is particularly pure, containing 97.6% CaCO3 along with some minor impurities of clay and magnesium carbonate . This results in a very strong rock.

Caves form in limestone when the calcium carbonate is dissolved by weakly acidic groundwater. Acidic water has two sources, above and below ground, and each produces it own distinctive type of cave.

Rain water becomes a dilute acid when it trickles down through soil that is rich carbon dioxide (CO2). As microbes devour decaying organic matter, they raise the concentration of carbon dioxide in the soil up to thirty times more than in the atmosphere2. The CO2 dissolves in the water to produce weak carbonic acid (H2CO3). Additionally, humic acids from decaying material also make the groundwater slightly more acidic.

As gravity draws the water down through the soil, it eventually reaches the underlying limestone where the weak carbonic acid slowly dissolves the calcium carbonate. This produces calcium ions (Ca+) and soluble bicarbonate ions (HCO3

-). These soluble components are then flushed away from the rock surface as the water seeps further into the limestone. Layers of freshly-exposed rock are then etched away in turn, gradually widening tiny cracks into larger and larger crevices. Soon, the groundwater becomes saturated with dissolved calcium carbonate, and it cannot dissolve any more rock. A detailed description of the limestone-acid chemistry can be found in Appendix 2.

When we talk about limestone being soluble, we should keep in mind that it is not very soluble. In fact, it is only just soluble. Each litre of saturated cave water typically contains about 0.15 grams of dissolved calcium carbonate3. An entire 50,000 litre

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backyard pool would account for only 7.5 kg of dissolved limestone, about the size of two house bricks. Prodigious amounts of carbon dioxide-rich waters are required to dissolve out even small limestone caves.

However, descending rainwater is not the only way to make cavities in limestone. Water warmed deep in the ground is known as geothermal (literally warm ground) water. This warm water often rises from great depths along fault lines associated with movement of the earth’s crust. Being under high pressure, geothermal water can be very rich in dissolved carbon dioxide, and also quite hot, making it aggressively acidic. When this hot acidic water enters limestone, it tends to carve out dome-shaped cavities.

Despite the enormous size of some of the Jenolan caverns, and the more than 20 km of passageways, the vast majority of the limestone is still solid rock. It is likely that less than 2% of the Jenolan rock mass has been dissolved away to form caves4.

DEEP TIME

The chemistry of cave formation is one thing, but caves need time to develop...lots of time.Planet Earth has been cool and solid for about the last 4,500 million years. Small

creatures evolved about 600 million years ago, although single cell organisms were around much earlier. The dinosaurs disappeared 66 million years ago. The first kangaroos appeared about 40 million years ago, and primitive humans only emerged in the last 200,000 years or so. The amount of time involved in Earth history is so immense it is often referred to as ‘deep time’.

To get an idea of how deep time really is, imagine counting out loud once a second. Each second counts one year. You might like to imagine the solar system with the Earth whizzing around the Sun once every second. Most people would reach their own age in less than a minute. It takes about half an hour to count back to the time of the Romans. After an hour you are back to the first pyramids. In a little more than two days you can meet your earliest hominid ancestors. However, to get back 66 million years to the end of the dinosaurs you will need to keep counting - nonstop day and night - for two years!

To count back to the time when the first animals left the sea you need to keep counting for another 10 years. But life has probably been evolving on Earth for the last 4,000 million years. Counting back continuously at one year per second, you will need more than 120 years to reach this far back into planetary history. Life on planet Earth really has been around for a long time.

To make some sense of the enormity of deep time, Earth scientists use the Geological Time Scale where Earth’s history is divided into Geological Periods. Each Period terminates in mass extinctions that heralded new eras in the evolution of life on a planetary scale. For example, the Cretaceous Period ended when the dinosaurs disappeared. This made way for the rise of the mammals, and of course, humans.

Just as historians are content to refer to events in the 16th Century, Earth scientists use Geological Periods such as the Jurassic or Triassic. Absolute dates can be used when

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The Geological Time Scale.

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the need arises (and the data is available). However, unlike centuries, Geological Periods are each different lengths.

The deep time history of Jenolan Caves goes back more than 400 million years to the Silurian Period.

416 MILLION YEARS AGO - ANCIENT SEAS

The limestone at Jenolan was formed in a shallow sea towards the end of the Silurian Period, around 416 million years ago.5 The rock is composed of calcium carbonate that once made up the rigid body parts of ancient sea creatures.

Certain marine animals, such as shellfish and corals, use some clever bio-chemistry to extract dissolved calcium from sea water to construct shells and other hard body parts that they then use for support and protection. The animals that make up hard corals build up entire coral reefs this way. However, the largest volume of calcium carbonate is accumulated as hard parts in the bodies of tiny free-swimming marine creatures such as plankton. Although each animal is tiny, the sheer volume of plankton is enormous.

When these animals die, they sink to the bottom where their soft tissue is quickly consumed. Tough shells and hard skeletons are left lying on the sea floor. These bits and pieces are gradually pulverised into a fine-grained mud that is rich in calcium carbonate. Over time, a considerable depth of carbonate deposits can build up. With more and more material being placed on top, the whole mass is compressed and cemented together to form solid limestone by the process known as lithification (literally: turning to stone). The term limestone comes from its use in the manufacture of lime for plaster and cement.

The geological setting of the shallow sea that produced the Jenolan limestone. After Scheibner

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By looking closely at the rock layers, and studying the fossils embedded in the Jenolan Caves limestone, we can tell that these creatures lived in an ancient shallow sea. Although there are some coral remains in the rocks, this was not an isolated coral atoll such as many Pacific Islands. Rather, it was a shallow seafloor, possibly of some extent. The small amount of impurities in the Jenolan limestone indicates that the marine environment was a fair distance offshore, and so protected from the sand and mud that is continuously being washed off the land into the sea.

The ancient ocean floor supported a thriving community of rich and varied marine life. Most of these ancient animal and plant groups are now extinct. However, by identifying each fossil species and comparing them to fossils in other parts of the world, palaeontologists are able to date the rocks. The animals of the Jenolan limestone were alive in the late Silurian geological period (443 to 416 million years ago) when the planet was generally warm, with high sea levels that formed extensive shallow seas.

At this time, the continental mass that was to become the east coast of Australia had yet to be formed, and the coastline was some 400 km west of its current location. A line of volcanic islands stood about 300 km offshore, with a shallow sea in between. The sea floor consisted of a series of troughs and rises. The Jenolan limestone was deposited on the shallow margins of one of these troughs, known as the Murruin Basin. The basins either side trapped sediments washing off the volcanoes to the east and the continent to the west. As a consequence, this protected seabed accumulated sediment derived almost entirely from the hard body parts of the marine creatures that lived there.

Close up of Crinoid stem fossils found in Jenolan limestone outcrop behind Caves House.

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Bony fish first appeared in the Silurian period while trilobites (now extinct) and hard-shelled brachiopods (classic shell shapes) lived in abundance on the sea floor along with gastropods (snails) and crinoids.

Although crinoids are marine animals, many ancient species were so plant-like that they are commonly known as sea lilies. Their waving flexible arms directed food into a central mouth that was mounted on a tall stalk. In the Silurian period, crinoids were very plentiful, and a few forms have even survived to the present day. Fragments of crinoid stalks are frequently fossilised, and can often be seen in the Jenolan limestone.

At the time the Jenolan limestone was being laid down in the Silurian Period, sea life had been fertile and diverse for quite some time, but the dry land had only just been colonised by primitive plants. A few insect-like creatures were the lone animal inhabitants of the continents.

The Jenolan sea floor community thrived in the shallow ocean, but eventually volcanic eruptions covered the calcium carbonate mud seafloor and brought limestone formation to an end.

A complete crinoid fossil. Location USA.

Sea floor dwellers of the Silurian period more than 416 million years ago. Mariya Foteva