The Journey of a Raindrop

Doing the groundwork

Raindrops began laying the foundations for these caves in the Pleistocene era, around 1.5 million years ago. During this period, a series of mini-ice ages caused sea levels to rise and fall dramatically (as much as 200 metres), exposing large areas of seabed to the elements. And when strong westerly winds blew the sand and shell fragments inland, forming huge dunes, the rains percolated and compacted the dunes into Tamala limestone over many thousands of years.

Building cathedrals from tiny crevices

It’s believed that the caves started forming at the same time as the limestone ridge itself. To explore how, let’s zoom in on one single drop of rain falling from the sky.

As this raindrop falls, it picks up a little bit of carbon dioxide (CO₂). Once it splashes down onto the forest floor and slowly percolates through the woody debris and alkaline soil, it absorbs a bit more carbon dioxide. That CO₂ makes the raindrop slightly acidic, roughly the same PH as a cup of tea. So, when it reaches the limestone and flows down through natural cracks and crevices, it dissolves the rock

Over 1.2 million years, and many, many raindrops later, this process has created what’s called a limestone karst – a landscape riddled with cracks and fractures which became deeper and wider as the rock dissolved, eventually forming the caves.

What makes each cave so different?

That’s a really hard one to answer, because there are so many different factors that could influence their formation,” says Capes Foundation Cave Manager Rikki Andrew.

The density of the limestone and the unique features of the forest above the ground all influence cave formation. But the biggest factor is how the water entered the limestone.

“Jewel Cave is what we call a phreatic cave,” explains Rikki. “It’s formed by the water table dissolving the limestone as it moves very slowly towards the ocean. Whereas Mammoth Cave is what we call a stream passage cave, where an above-ground stream has made its way down and through the limestone ridge on its way towards the ocean. Here, the water flowed much faster than in Jewel and Lake Cave, and the rock dissolved rapidly, which is why you’ll see evidence of rock collapses at Mammoth Cave.”

Waters from the same swampy wetland area that created Mammoth Cave also carved out Lake Cave via the same process.

But by the time the water reaches Lake Cave, it’s moving a lot slower, because it’s seeping through the limestone rather than flowing in from an above-ground stream. So the formation process is slower compared to Mammoth Cave,” says Rikki.

Ngilgi Cave is different again, as cave guide Nathan Hull explains. “Ngilgi sits high up on a hill overlooking the coast and was carved out by an ancient fast-flowing river making its way to the ocean. So, there were some strong forces at play here.”

Interior design by raindrops

Rain doesn’t just carve out the cave; it builds a whole world inside it through a process called mineral deposition. By the time a single raindrop enters the cave, it’s formed a calcium carbonate solution of dissolved limestone and CO₂.

The moment the water droplet is exposed to the cave air, it releases CO₂ back into the air and begins to evaporate. No longer able to hold onto the calcium carbonate, the droplet then deposits the calcium inside the cave in the form of calcite crystal.

Where and in what shape the calcite crystal is deposited depends on how the raindrop enters the cave. There are so many variables, all with striking results.

Straws

Straws begin to form from a single raindrop hanging on the ceiling of the cave. The calcium carbonate (dissolved limestone) is deposited in a ring around the outer edge of that hanging droplet. So, drip by drip, ring by ring, they get longer and longer, forming a straw.

You’ll see some perfect examples of long straw formations in the first chamber of Jewel Cave.

Stalactites

When a water droplet flows down the outside of a straw formation, the crystal deposits are left on the outer sides of the straw, causing them to thicken into that typical carrot-shaped stalactite you see hanging from the ceilings in caves.

One of the most impressive stalactites hangs in Ngilgi Cave and is estimated to weigh around two tonnes.

Stalagmites

As a water droplet drips off the end of a straw or stalactite, it drips onto the floor of the cave and deposits what’s left of the dissolved limestone there as well, and those tiny crystal deposits build upwards, forming a stalagmite.

Stalactites and stalagmites can eventually grow so large that they meet in the middle and form a column. These can grow huge, like the Grumpy Old Man you see today in Ngilgi Cave.

Helictites

Helictites were originally called ‘cave mysteries’ because people couldn’t figure out how they were formed,” says Rikki. “These are my absolute favourites! I’m obsessed with their crazy, curly, gravity-defying shapes. In Jewel Cave, they’re everywhere, growing in every direction.

Helictites, from the Greek word meaning ‘Twist’, are strange, gravity-defying shapes that form when a raindrop enters the cave very slowly through the wall or ceiling, or a fissure in another formation. Rather than forming a droplet that’s pulled down by gravity, it can be pulled in any direction by a tiny speck of dust, a grain of sand, or the movement of air. Whatever’s happening at that time determines where the crystal is deposited and how the helictite takes shape.

In the main chamber of Ngilgi Cave, there’s a helictite that looks almost like it’s carved by hand into the shape of a little camel.

Shawls

Where water meets an angled roof, it runs down and deposits a small amount of calcite in a line along the rock surface. Over tens or hundreds of thousands of years, these lines build in layers to form a curtain of crystal known as shawls.

Shawls tell us a lot about changing weather patterns over millennia,” says Ngilgi Cave Guide Nathan Hull. “During warm and wet periods, when the trees grow tall, and the undergrowth grows thick, the rain picks up deep colours (tannins) as it seeps through the ground. That’s where you see the lovely rich oranges and browns in the shawls. Whereas, during colder periods, there’s less plant life and more water entering the caves at a faster rate, leaving thinner deposits of crystals with little or no colouration.

Two big, beautiful shawls hang close to each other in the main chamber of Ngilgi Cave, known as the Brides Vale.

Flowstones

When rain enters the cave and flows down the walls as a large sheet of water, it deposits the dissolved limestone across a wider surface area. Over many thousands of years, these deposits form a crystalised waterfall called flowstone.

Look for ‘The Frozen Waterfall’ in Jewel Cave, ‘The Snowfields’ in Mammoth Cave, and ‘The Suspended Table’ where the floor of Lake Cave has been washed away, leaving an impressive 5.3-tonne column and flowstone magically floating.

How long do these formations take to form?

That’s the ultimate question, and it’s a hard one to answer. Every formation grows at a different rate, and there are many different environmental influences. The most important one would be the drip rate.

When water is dripping really fast, it’s not forming much crystal on the ceiling above. Instead, it’s forming a lot of crystal on the surface below. But then that drip might change. It might get smaller. Or stop dripping for 200,000 years.

There is a straw formation in Lake Cave with a big black mark where a tour guide burnt it with a candle in the 1950s. Since then, the straw has grown by about one and a half inches.