Cave Sediments

Sediments can be brought into caves in a number of ways. Soil and dust can be washed or blown into cave entrances. Sediment can be transported into caves by streams. Animals can deposit mounds of guano, and minerals can be deposited from solution.

Cave Entrance Deposits

Surface material can easily collect in cave entrances. Soil is washed in by rain. Rubble can roll downhill. Air-borne dust can be caught and preserved, where elsewhere it would rapidly wash away. Cave entrance deposits often contain bones of animals that have either fallen into the cave or been brought in by carnivores. Important paleontological discoveries have been made in caves all over the world, including Jenolan.

Fluvial Sediments

Fluvial sediments are deposited by the action of running water. Streams draining off the insoluble rocks surrounding caves carry with them a load of sediment. When they sink underground, sediment is carried with them. Very coarse sediment remains at the surface and is carried down the dry valleys during floods. Where caves are large enough to carry the flood flow of streams, coarse sediment is able to enter them.

When sediments enter caves, they may pass through or they may be deposited. This depends on a number of factors, mainly the velocity of the water, the grain size of the sediment and the geometry of the cave. In complex cave systems, sediment may take a long time to pass through, and may move from one part of the cave to another.

Coarse fluvial sediment, gravel, is found in a lot of caves and in places that contain large rounded boulders. The gravel fills large volumes of cave, and many of the speleothems are developed on gravel, rather than bedrock. In places, caves are completely developed in cemented gravel.

Still Water Deposits

When caves contain bodies of still water, fine sediment may be deposited, brought in by flood pulses or falling down from higher levels in the cave. Fine-grained sediment, such as clay, frequently has very thin layers or beds, representing each period of deposition. In places, the layers may be graded, with coarser grains at the bottom and fine at the top. Fine layered sediment is often deposited where caves become blocked.

Guano

'Guano' is the most common organic deposit in caves. It consists of bat droppings. In caves where bats are roosting, very large piles of guano can be produced. Guano is the most important source of phosphate in cave sediments.

Reaction Rims

Sediments deposited in caves often react chemically with the cave's limestone walls. This produces a reaction rim, between the sediment and the limestone. Reaction rims are found in many of the caves, at the boundary between sediments and limestone. The rims consists of a pure white layer and a black layer. Reaction rims are mostly produced when phosphatic solutions in the cave sediment (derived from guano) react with the limestone.

Here is an example of very coarse sediment, including smooth river rocks, now forming part of a cave roof.

Here is an example of very coarse sediment, now part of the cave wall. The old light bulb gives a clear idea of the size of the rocks in the sediment.

Here is an example of fine clay particle sediment, which would have settled in still water.

The dark area is the reaction rim.