Caves

Although caves are not confined to limestone areas (they can occur in volcanic lava flows, for example, and any kind of hard rock on the coast), those developed almost entirely by solution are characteristic of a karst region.

What are caves?

Calf Holes, Ingleborough

Caves are natural underground tunnels that may widen in places to form caverns.

An exact definition is not available: to a scientist a cave is any void greater than a centimetre in diameter, whereas to a potholer it has to be large enough to be accessible and explored.

Most caves in Yorkshire are less than about five metres wide, although some are considerably larger, and many cave systems and passages extend for many kilometres.

The strength of the limestone and the joints, faults and fractures control just how large a cave can become.

Caves take a considerable period of time to develop. The evolution of a cave system is closely related to what is going on at the surface, especially the climate and most especially rainfall, because caves cannot form without water.

All limestone caves are, or have been in the past, channels that have carried rivers.

Movement of water through limestone regions

Hunt Pot

Massive limestones, like the Carboniferous limestone, have low porosity — the water cannot soak into pores as it can in chalk, for example. Water is forced to flow over the limestone along joints, fissures, bedding planes and faults.

Water may enter a cave system as rainfall seeping through the joints or as a river flowing down a swallow hole, and follows the cave system to the resurgence. Water attempts to take the shortest route to the resurgence, but this is not always possible underground where the geology may prevent it.

Underground, water picks out lines of weakness and solution is concentrated at these points.

Bedding planes are particularly vulnerable to attack because they are geographically widespread zones of weakness.

Joints are more localised and when the river is forced to change level, wider or weaker joints are picked out.

Waterfalls tumble down over joints and in some cases hydrostatic pressure forces the water upwards through water filled joints and passages (sumps).

In some respects the cave system is similar to a surface river system:

  • water is concentrated into one place
  • the passages are equivalent to valleys
  • abandoned passages (fossil caves) can be compared to dry valleys

However, cave systems differ because the flow of water is controlled by the geology and can flow 'uphill' when there is sufficient hydrostatic pressure and where the structure of the limestone permits it.

Also, underground passages in limestone areas are made almost entirely by solution rather than erosion.

Cave formation

Why does the water pick out a specific location to create a cave? This is controlled by local factors:

  1. A bedding plane at the top of a particularly hard bed in the limestone (or perhaps an impermeable bed of shale within the limestone) could be more resistant to solution and erosion, so that rain water percolating down from the surface preferentially dissolves the overlying limestone.
  2. Water is able to seep along the bedding plane too, so that the sites where solution is happening are joined up by tiny channels.
  3. Solution continues and the channels widen from a few millimetres to a few centimetres.
  4. One channel may start to take more water than the others and enlarges to become a small phreatic cave.
  5. Water flows more easily through the cave, solution is more rapid and the cave increases in size.
  6. The increase in water flow also increases solution (and erosion) of the more permeable limestone below, and eventually the cave grows downwards as well as upwards.

Eventually, a cave forms. If it is entirely below the water table (phreatic) solution takes place on the floor, roof and walls and the cave takes on a more circular or tube-like shape. If much of the water drains from the cave and it becomes vadose, solution takes place towards the floor of the cave and the cave becomes keyhole shaped.

As the cave increases in size, parts of the roof or walls may become unstable and collapse, so that the caves become all sorts of shapes and sizes.

Thor's Cave, Derbyshire
Dow Cave

The shape of cave systems

The shape of the cave system also determines how water flows through it and is controlled by the local geology such as a weakness at a fault, jointing or a particular bedding plane.

On Simon Fell at Ingleborough for example, the cave entrance (a swallow hole) of Alum Pot follows a fault. In nearby Pen-y-ghent Pot a zone of jointing forms a long passage.

Horizontal beds

If the beds of limestone are more or less horizontal, the river will enter at a swallow hole and follow bedding planes and joints down to the water table where resurgence occurs. This is the situation in much of the Yorkshire Dales.

Impermeable beds

If an impermeable rock (perhaps shale or slate) hinders the downward progress of the water through a cave system, it will result in resurgence. An example of this is the White Scar cave system near Ingleborough, where impermeable Ordovician slates underlie the Carboniferous limestone.

Dipping beds

Some beds of limestone dip gently downwards as a result of earth movements in the geological past. The river enters a swallow hole and then takes the shortest route by following a single bedding plane down to the resurgence. In some cases the cave system may end below the water table, but the high hydrostatic pressure forces the water back to the surface through a joint.

Fossil caves

Stump Cross Cavern

So far we have considered only caves containing water, but in some cases the cave is dry. This is because rivers above the water table widen joints to form new cave systems and abandon the old ones. The abandoned caves are sometimes called 'fossil caves'.

Caves below the water table may become dry if the water table drops, such as during a period of glaciation or where erosion has lowered the resurgence.

In the Stump Cross cave system there are several tubular passages that originally developed below the water table, but are now dry because the water table has dropped below the level of the cave.

Fossil caves often have a lot of rubble from roof collapse in them because there is no water to dissolve or carry it away, and they are often characterised by calcite precipitation and the development of stalactites and stalagmites.