Cave Formation

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Phreatic Tubes
Keyhole Passages
Breakdown Chambers
The Courtesan, Agen Allwedd

A Phreatic Tube

Phreatic cave passages (or phreatic tubes) are formed below the water table by water flowing under pressure.  They can vary in size from a few centimetres to several metres in diameter.

Later as water levels drop a stream flows in the bottom of the phreatric tube and downward cutting begins.

A stream flowing along the bottom of a phreatic tube

The stream gradually begins to carve a trough in the bottom of the tube.  This is called the vadose stage of development.

A Keyhole Passage

Eventually a "key-hole" shaped passage may be formed with the stream flowing at the bottom of a deep rift.  Progress along these passages may be made by walking along the bottom of the rift.  However, the rift may be so narrow that cavers will need to traverse along the tube at the top.


Avens are vertical tubes found in the roof of cave passages.  They are not formed by water finding its way down a vertical joint from the top, but from the bottom by water, under extreme pressure swirling around.  This means that there is often no way on at the top of an aven.

A Breakdown Chamber

Eventually the water may form enormous caverns and a period of breakdown, where large boulders crash down from the cave roof, begins.

Ancient waterways may become completely abandoned and are called fossil passages.

Boulder Chokes

A cave passage can become totally blocked by fallen boulders forming a boulder choke.  A route may be found by squeezing through the boulder pile to find virgin cave passage on the other side.

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