Local Geology and Cave Formation

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Local Geology

~ The Ogof Ffynnon Ddu cave system lies within the Brecon Beacons National Park in South Wales, United Kingdom. Consisting predominantly of Millstone Grit and Carboniferous Limestone, this area is bordered to the north by Old Red Sandstone, and to the south by the South Wales Coalfields.
~ By far the vast majority of caves in the UK (and indeed the rest of the world) are formed in Limestone, by the chemical and physical erosion of flowing water. This particular band was laid down around 350 million years ago at a time when the area was in fact close to the equator, under shallow warm seas. Owing to continental drift, this has now turned up in our rather cooler (!) climate.

Sloping Limestone beds in Yorkshire, showing cracks and solution runnels.

What's so special about Limestone?

~ Limestone is a sedimentary rock and is generally composed of the mineral calcite, formed of the skeletons of millions of marine organisms that collect at the bottom of the sea. With time and pressure these consolidate and harden. (With more heat and pressure, marble is formed).
~ Rainfall absorbs carbon dioxide from the air, creating a mild solution of carbonic acid. It is also believed that rotting vegetation in peat bogs or soil, produces organic acids that are also capable of attacking limestone. This would explain why jungle covered karst regions in the tropics (with high rainfall), often produce such dramatic caves. eg Sarawak, Mexico.
The stream which flows into OFD first passes through a large peat bog, and so the water is very aggresive, chemically (!) before disappearing underground.

Passage Types

~ Most limestone contains small cracks or fissures, and these give the water small channels to explore. The action of this slightly acidic water is to gradually dissolve the rock, and widen the cracks and crevices. Soon any larger channels begin to dominate and to take the majority of the flow, taking in water from side branches, and swelling as a result.
~ If at this stage, the underground channels are entirely full of water, then they will grow in size over their entire surface and tend to form circular phreatic tunnels. Where there is an air surface above the water, then its action will be to cut downwards, creating vadose canyons. It is possible for a phreatic tube/tunnel to run uphill as well as downhill with the water flow, as it can behave like a pipe, as long as the final outflow is well below the inflow.

The deep Maypole Inlet Canyon, OFD II
Dip Sump in OFD I  (by Martyn Farr)

Vadose Canyon

Flooded Phreatic Tube

~ Vadose canyons always trend downhill with water flow. Often a vadose passage will have started life as phreatic, but then due to a reduction in water flow, an air space develops and the stream begins to cut downwards.

Column Hall, OFD II

Cave Formations

~ As water passes through the cave it becomes more and more saturated with bicarbonate of lime from the dissolved rock. Water dripping from the roof or trickling down the walls, evaporates leaving behind a slow-growing deposit of calcite. When they are downward-growing they are stalactites. Drips landing on the floor can form upward-growing stalagmites. If these then meet they form a column. Trickles down the walls can form curtains, whereas thin films of water can form flowstone.

Karst Geomorphology ~ J N Jennings ISBN 0 631 14032 9
Rocks, Minerals and Fossils of the World ~ Roger Phillips ISBN 0 330 29953 0
Caves and Karst of the Brecon Beacons National Park (BCRA)
~ Michael J Sims ISBN 0 900 26520 5

PCW 2004