Forming the Rock

The Jenolan Caves are formed within limestone. As scientists learn more of the geology of New South Wales, they revise their estimates of the rock as well as how it came to be formed. Current theories suggest that the limestone was formed about 400 million years ago in late Silurian/early Devonian period. Although it is possible to find some fossilised corals in the rock, most of the rock is comprised of what is known as a calcareous ooze. In a warm shallow sea there is a great deal of life to be found. These were the conditions under which the rock formed. As various organisms grew they exuded calcium carbonate from their skeletons into the surrounding sea water. This settled to the sea bed forming what is best visualised as a grey mud. Mixed in with this were corals and sea shells. Over millions of years a combination of pressure, heat and chemical recombination served to convert this ooze into a solid rock. Surrounding the limestone rock are beds of cherts and shales.

Approximately 350 million years ago the rocks were uplifted to form a high mountain range. Around 335 miilion years ago it is believed that the limestone was first exposed allowing caves to start forming. This was in the early Carboniferous period. These are not the caves that we see today though. The sea level rose and covered the limestone belt filling in the caves with a coarse limestone. This process of uplift, cave formation and then subsidence is believed to have happened several times, up until about 120 million years ago. At that point the rock was uplifted a final time. Erosion of the landscape commenced in the late Cretaceous period with a great deal of development suspected of happening up until about 2 million years ago. The level of the creek involved in forming the caves is believed to have been at the base of the Carlotta Arch around 10 to 15 million years ago.

Note that when the rock was lifted up from the seabed it was tilted up at an angle as well as twisting the rock somewhat as well. Generally this had the effect of standing the bed of limestone on its edge at an angle of approximately 60 degrees. Depending on where you stand in the Jenolan valley though you will find that this angle varies. This is signficant as it effects the way caves.

Formation of the Landscape.

Jenolan Caves are an example of a karst landscape. A karst landscape is one where the terrain has been predominantly shaped by the action of water dissolving the base rock. The rock is easily dissolved relative to other rock types. For this reason, when a creek flows over limestone, the rock beneath the creek is dissolved away leading to the creek bed dropping. this process keeps happening leading to very steep sided valleys or even canyons forming. In the process of being uplifted, the limestone is fractured, allowing water to seep into it. The water will start dissolving the rock from around the edges of these fractures, leading to caves being formed. For more information on the chemical process formed, click here.

When the rock is uplifted the fractures tend to form parallel with the sea bed, that is it is as though there are lots of layers of rock stacked one upon another like a stack of pancakes. If the rock did not tilt when it was uplifted, water would seep in and start developing caves that are predominantly horizontal. If the rock is tilted these fractures will also be tilted. If the rock were to be stood up one its edge then the water would seep straight down and cause very deep caves to form. Since the limestone at Jenolan is tilted at a variety of angles there are some caves that are horizontal but there are also many such as the River Cave, that have a large vertical component to them.