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
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,
waterways may become completely abandoned and are called fossil
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.