Jenolan Caves

Geological Features



Beds are the original layers formed when a sedimentary rock is deposited. Bedding can be horizontal, at steep angles and sometimes almost vertical. This deformation occurs during the uplifting process. Massive limestone, in which large caves form, often shows little bedding. In many cases, bedding can be seen because fossils are aligned along bedding planes.


Breccia is a rock made of angular fragments. Breccia can be produced, where forces in the earth have sheared rock, or where collapse into a cavity has occurred.


Dykes are bodies of igneous (molten) rock, that have intruded through cracks in other rocks.


Faults are planes of weakness, along which there has been movement of rock masses. Faults can be caused by compression, tension, or sideways movement. Faults are important in controlling the movement of underground water and thus cave development.


Fossils are evidence of life in the past, and are found in most limestone caves including Jenolan. Fossils are often used to establish the age of the limestone. Four main groups of organisms are:

  • Brachiopods

Brachiopods are marine animals with two shells, which were once very abundant, but are not common in modern oceans. Although they have two shells, they are not related to Bivalves, which are Molluscs, another major group (Phylum) of animals. Large Pentamarid brachiopods are commonly seen in cross section, in the cave walls.

  • Corals

Corals are often found in Limestone, the commonest being colonial corals and solitary corals. Colonial corals are the skeletons of coral polyps that grew together in a colony. Each tube (corallite) housed an individual. Solitary corals consisted of one large polyp which had a horn-shaped skeleton.

  • Crinoids

Crinoids, sometimes called sea lilies, are marine animals that live attached to the sea floor. They consist of a body with arms, used to filter food out of the water, supported by a long stalk. The stalk has a skeleton made of disc-shaped plates call ossicles. Crinoid ossicles are commonly found, either individually or forming sections of stem.

  • Stromatoporoids

Stromatoporoids are an extinct group of marine colonial organisms, which grew as dome-shaped colonies on the sea floor. Stromatoporoids are often seen in limestones.


Joints are fracture planes in rock, where there is no observable displacement of one side relative to the other. Joints usually occur in sets, in which many joint planes are aligned in the one direction. There is usually more than one set of joints. Joints are often the result of tensional forces affecting the rock mass.  They are important pathways for groundwater through limestone. Joints control cave development not only by controlling water movement, but also by providing planes of weakness along which breakdown can occur.


Slickensides are scrape marks on the surfaces of fault planes, caused by the movement of one side relative to the other. They often develop a mineral coating.


Palaeokarst means "old karst". Palaeokarst deposits are sediments which filled very old caves, and are now seen as part of the rock in which the present caves have developed.


 Layers of 'bedding' can be clearly seen in this example.

 This example shows the angular fragments of breccia in a cave.

 In this fossil example, you can clearly see the outline of a brachiopod.

 Here is an excellent example of fossilised coral.

 The domelike shapes of stromatoporoids can be seen on this cave wall.

 Examples of joints.

 An example of slickensides.












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