Geological Concepts and Timescale

At a number of places in these descriptions you will find a number of Geological terms used. I have hyperlinked key words to further sources of reading that you may find interesting or useful. If you are not familiar with some of the concepts involved I would recommend these resources for your edification.

Geological timescale

In order to fully appreciate the time that it has taken for things to happen, either the creation of the caves or the creation of the much more ancient rocks that they are in you will need to know something of the Geological timescale. To that end I provide here a summary geological timescale with the names as commonly used in the UK and the age of the top of each section in Millions of years old (with the exception of the top of the Pleistocene which is in thousands of years instead)

This has (unfortunately in many ways) been revised significantly in the last few years as a much greater mastery of British and International stratigraphy has allowed international and intercontinental correlations and new type sections have been agreed. This means that some of the standard British terms for ages of rocks are not now used in the more modern texts.

If you do want the detail, rather than repeat and have to update a constantly shifting set of names I provide a link to the Standard British Geological Survey Timescale . Here you can browse through the various ages and see the latest terminology.

You can also find links to the Stratigraphical Framework documents if you wish to get information on old and new terms for specific beds of rock

It is my intent to provide a summary of some of the key old and new terms as they specifically relate to UK Cave geology here when I get some time

Rocks & Structures

Type of rock

The main aspect of geology that effects a "caving" region is the type of rock that is found there. These can be broadly categorized into Cave Bearing Rocks & Mined Rocks

Cave bearing rocks are basically soluble in some way, Mined rocks can be almost anything that has a commercial reason for mining it

Rocks that are of speleological interest in Britain are mainly limestone as described below, but include some of the rocks that have been mined. These include a number of Sandstones that have been either quarried for building stone or use in other industrial processes. Examples of these that are accessible to cavers are the Dinas Silica Mines South Wales, the Box Stone Mines near Mendip, and the Alderly edge Copper Mines in Cheshire.

Limestone is made of Calcium Carbonate (CaCO₃), either in the form of the shells of microscopic and larger animals or precipitated from sea water directly. Most Limestones are laid down in warm tropical/subtropical seas such as the Bahama's and the Great Barrier reef where large amounts of limestone are being created today, so it's a good indicator that the areas with caves were once in these sorts of climates, and by looking at these climates today we cab understand things about the limestone which help us find more caves

Limestone is dissolved by slightly acidic water as well as by the normal processes of physical erosion (i.e. the scraping away by the abrasion of harder minerals such as sand grains when washed against it). This means that even slowly moving water passing through the rock can enlarge any existing weaknesses and turn them into caves.

Dolomite is Calcium Magnesium Carbonate (CaMg(Co₃)₂) the additional Magnesium makes it a slightly heavier and harder mineral and one that is less soluble in acids. Therefore caves in Dolomite tend to be smaller.

Marble is a term that is used for either of these rocks when it has been recrystallized by heat and pressure.

There are other rocks that contain caves Dolomite and Marble are too types of rocks that are very closely related to Limestone and they will be described below. Other evaporate rocks such as gypsum and salt can have caves formed in the same way, but these rocks are very soluble and you need a much drier climate than the UK for them to be usual at the surface and have caves formed in the same way.

Aspects of Limestone that effect caves

There are many factors that effect the cave that will be formed. The specific nature of the Limestone, the amount of water passing through, how acid the water is and how much sediment it's carrying, the dip of the rock and whether it's cut through by faults or has been folded. The following sections go through these in some more detail.

Solubility/Permeability

One of the Key aspects that effects the way caves will be formed in limestone is the solubility of the rock. In fact calcite is not much more soluble than Quarts, it requires a chemical reaction to change it into a soluble form. Solubility is therefore a chemical reaction with the Calcium Carbonate (CaCO₃) being dissolved in a number of acids based on dissolved Carbon Dioxide Calcium Carbonate (CO₂).

The Carbon dioxide gets into the cave as dissolved gasses in the water, and then the resulting acid reacts with the limestone to form bicarbonate which is dissolved into the water and washed away. There are a number of reactions that take place, but these can be summarized as

CaCO₃ + CO₂ + H₂O     becomes     Ca²⁺ + 2HCO₃-     becomes     Ca(HCO₃)₂(aq)

The reaction has in fact made calcite more than 5 times more soluble than the original calcite.

Some limestones are very porous and the water can flow easily through these pores. However such rocks are usually very soft and don't form very good caves as they collapse so easily. An example of this in the UK is the Cretaceous age rock, the Chalk of South and Eastern England which is very porous.

The lack of such porosity is however no limiter to cave formation. The most popular cavernous rock in Britain is the Carboniferous limestone. This is a poorly porous rock, very hard and crystalline but it has many bedding planes and has been fractured by many joints as you can see in the picture which allow the water to flow through the rock and therefore the dissolution takes place.

The details of how dip and strike effect cave formation will be described below

Limestone that has been turned to Marble by heat or pressure will generally not dissolve as easily as a softer rock. Small sharp caves in older limestones such as the caves of the Isle of Skye, Scotland, which are in very old Dalradian Limestones (600 Million Years Old) are examples of this. However it's not right to assume all older or marble

A limestone can also be made harder because of chemical impurities. As described above, a good example of this is caves that are formed in limestones that have a high proportion of Dolomite, Calcium Magnesium Carbonate (CaMg(Co₃)₂). This is often seen where there is a partially dolomitized limestone and you get some beds with more dolomite and some with more calcite. In these occasions there is often a clear difference with smaller passages in the dolomite and large in the calcite In South East Wales there are dolomitic layers in the sequence and the passages in these layers tend to be smaller and sharper.

Thickness and bedding of Limestone

The thickness of limestone has a major effect on the type of caves that are formed.

Some limestones are very thin bedded because they were formed in an environment where there was some interruption in sedimentation whereas some were formed in an environment with a more constant input of sediment, or there was some form of reworking of the sediment such as bioturbation There are series of limestones that contain both thin and thick beds such as the one from Burrington Coombe shown here. This thin bed of rock is seen across a wide area and shows up as a significant marker bed

Thin limestones that are inter-bedded with other rocks such as shales may be totally or partially sealed from having water flow into them. This will prevent cave formation if there is a lot of the other rock, or if the water has managed to enter the limestone, it can give rise to a level passage at a water table. It is also not easy for long pitches to be formed where there are thin limestone layers with non-soluble layers between.

In such areas small "flat" systems are usually developed along the bedding planes are formed. I have personally explored some very small flat grotty holes in the Yoredale Series rocks of Dentdale, North Yorkshire

Depth of Limestone

Caves can form in any limestone, but deep vertical caves will only be formed where there is sufficient depth of limestone for them to be formed in. In the UK there are different depths of limestones in the different regions with Yorkshire having the greatest vertical depths of rock and therefore not unsurprisingly the greatest concentration of deep vertical caves. However because caves are not just vertical it is actually in South Wales where the greatest depth within a UK cave 308m is found in South Wales. It is Ogof Ffynnon Ddu.

OFD has caves right through the limestone beds from the topmost in the upper areas of the cave right down to the lower layers just above the Old Red Sandstone that is below the cave. It is not a steeply vertical cave the depth is because of a relatively gently downward slope over a long horizontal range. Very deep sections of limestone can be found in mountainous regions such as the Pyrenees in Spain and the Vercors area of France that contains the Gouffre Berger.

Reefs

Reefs are areas of limestone that were once algal and/or coral reefs, just like the Great barrier reef today. These tend to be thicker, unbedded areas of limestone that may have different properties to the limestone around them. The fore reed (ocean side) will tend to be thinner bedded with fragments of corals etc. that have been broken off and washed down, whereas the back reef (sheltered between reef and coast) will often be very thinly bedded or have layers of sediment in it depending on whether river input was close or not. They can either be areas that are good for cave development or areas with poor cave development depending on the surrounding limestone and their physical properties The classic area for reef limestones in the UK is Derbyshire where the Castleton Reef limestones contain some of the regions best and most visited caves.

Relation to other rocks

At places where a thicker layer of limestone is found beneath a layer of non permeable rock (a rock that does not allow water though), entrances are commonly found because the run-off has been concentrated into one place by the streams instead of seeping in over a wider area. One of the most classic examples of this sort of cave development is Kingsdale in Yorkshire where Simpsons', Swinsto & Yordas caves are found where the water flows off the Yoredale rocks onto the limestone. In all of these caves the water has cut down through the Limestone aggressively and in that way creating the classic SRT through trips that are enjoyed by so many of us.

Folding

All of the British limestones have been folded and uplifted from where they were formed at the bottom of warm tropical seas.

There are 2 types of folds that are most common. Anticlines (upfolds) and synclines (downfolds)

The different caving regions have been subjected to different degrees of folding with the South Wales region being formed out of one large synclinal basin, and Derbyshire being a large anticlinorium (an anticline with smaller folds on the sides of it) they therefore have different dips and this has a major effect on the caves of each region as will be described in more detail in the regional descriptions below.

Folding also gives rise to joining where the relatively brittle limestone is cracked without movement (compare with faulting below). This is show in the blue limestone layers in the diagrams. Jointing usually occurs in 90 degree grid patterns when viewed from above. These can easily be seen on limestone pavements which are flat beds of limestone which have been scraped clean by ice and then weathered to the point where you can see the join patterns easily.

Water will tend to flow either along a horizontal plane (at or below the water table) or will try and descend to a lower level (either above or below the water table).

The limestones of the Yorkshire Dales are almost horizontally flat, with almost no folding visible in large areas of limestone. Combined with their thickness this leads to water flowing along the bedding until it can find a way down and therefore a series of horizontal and vertical passages are formed.

Around the "North Crop" of the South Wales region the limestones have been folded to a moderate extent. This combined with the way that valleys have been formed that often mean that water has some distance to flow before it can resurge. The result is systems that gently work their way down a vertical height, without having to form vertical pots.

The resulting long stream passages are of course famous are superb trips, but as you go along it is interesting to look at the detailed shapes to the passages and relate them to the geology. The OFD stream way whilst generally trending down dip as a vadose canyon showed evidence where it has taken a path based on the join patterns in many places

In Mendip the rocks are folded to much a steeper angle as can be seen in the picture from Burrington Coombe. Here the water often has to flow deep below the water table before it can find a way up again. This gives rise to deep sumps and caves with steeply sloping passages

Faulting

There are 2 types of faults and both are seen in caves. The type of fault that occurs when rocks are under tension is called a Normal Fault and that caused when rocks are under compression is called a reverse fault.

if you look at the blue layers on these diagrams you can see that on the normal fault there is a downwards step across the fault and a bed of rock will not repeat whereas with a reverse fault the rock is pushed up and over itself and it is possible for a rock to appear twice once above another. If the Reverse fault is at a low angle to the rocks that it is in, it is called a Thrust

Faults are lines of fracturing that cut through layers of limestone, just as they cut through any other rock. They may bring a layer of impervious rock against a layer that allows water through and this may have a major affect on cave development.

If on the normal fault diagram the blue is the limestone and water is flowing along it, and if the grey is an impervious mudstone then the water will not be able to glow further. if on the reverse fault we consider the green to be an impervious later the same could happen, but look carefully and on this case you can see that the limestone touches at corners and this could be a situation where water could flow upwards if it was under enough pressure by coming from a higher level above and to the left (a U bend could be formed)

Even if there has not been sufficient movement to bring rocks of different types together faults will generally be areas of weakened rock that are exploited by water to form caves. An example of that is shown here where cave development is occurring along a minor mast of the Moine thrust complex in the Traligill Valley, in Assynt, Scotland. The steeply sloping section to the left of the picture is one part of the limestone that has been over thrust by the layers now above which may have come from the same layer, or even a later beneath that on the left depending on how far they have moved. As at this specific location both sides of the thrust are limestone it's not east to tell

In South wales there is a pervasive NNW - SSE faulting pattern, and this is reflected in the orientations of the passages in such caves as Agen Allwedd, Daren Cilau, Craig y Ffynnon and is probably most clearly seen in Ogof Draenen where the fault line that forms the main stream passage has been such a strong feature that it has captured all of the water and to present there is no cave significant discovery to the West of the fault line in any area other then the highest parts of the cave. Some of these faults in South Wales have almost no movement from one side to the other, and some have movements of many meters.

Veins

Areas where hot water has been passing through the rock Fault lines (such as Derbyshire) often have other minerals deposited into them, these are called veins. These minerals include such things as galena ,lead ore, and are often sought after as they are economically valuable. These are useful to the caver as the mining of these minerals creates interesting passages that can be visited, and these passages often intercept caves that cannot be seen at the surface.

Veins can be quite thick and there is frequently a combination of minerals which are mirrored on each side of the vein with metal minerals in certain bands within the overall vein. The thickness of the valued bands will obviously encourage or discourage miners to follow it and where the valued mineral is something like Gold which was mined at Dolaucothi Gold Mines it doesn't take much to make it worthwhile

Mined out veins often contain spectacular formations because the metal ores are oxidized, dissolved and then re-precipitated as secondary minerals such as oxides and carbonates which tend to be spectacular colours such as these in Coniston Copper Mines , Cumbria, UK. The bedrock for these mines is not limestone. The common primary and secondary minerals found in UK caves are described in the Cave Minerals section below

Topography & Sediments Top

Aspects of Topography that effect caves

Water and Water Table

The key factor that influences the cave development and the passage shapes of caves is whether the passage is formed above or below water. The level of limestone that is filled up with water is known as the Water Table. This is mostly influenced by the permeability of the rock and the shape of the land above and in some cases below the caves (although level of rainfall and layers of impervious rock can also have an effect)

Where water goes into the ground it will try and descend as fast as possible. To do this it makes use of the joints and faults described above and if the bedding is sloping it will use this as well. Once it reaches an impervious layer it will not be able to descend as fast and it will run along that layer until it reaches a surface outcropping where it will form a spring.

This is the classic model and is a simple model that only really applies in very straightforward valleys. What happens in many places is that the water table influences flow in other ways. The Water can flow horizontally along strike in a section of rock that is filled with water towards the lowest point where the water can escape. This can be some distance away and examples of long main river passages that run for many miles are known.

The escape point can be in a different valley all together such as in Ogof Draenen South Wales where the risings are 6 miles south of the entrance. The hydrology of this cave is very interesting because it shows that the water table was once much higher and flowed to a northern valley whereas it now flows a much further distance to a southern valley. This is considered to be a case of water capture when the southern valley was deepened (probably in the Ice Age) and the sudden change from northerly flow to southerly flow can be seen in the cave with a dramatic change in roof line at the capture point at the junction of main stream passage and Gilwern passage.

Understanding these aspects is important in discovering caves because in this case a major passage that was considered an inlet was suddenly recognized as an outlet passage which had much more chance of remaining as a large passage. This spurred exploration in that part of the cave and subsequently many more kilometers of passages were found

As described above water will flow down until it hit's the water table or an impervious layer. If there is an impervious layer of some sort that is not widespread through the rock then a feature called a Perched Water Table can be formed. This is a part of the rock where the water table is above an otherwise bottom position within the strata. it can be just above the impervious layer as shown or if the input is sufficient it can be higher again.

Hill / valley shapes

Water tables usually mimic land surface to some extent, examples will be valley sides where the water table can run along the side of the valley and if there is impervious rock at the bottom of the valley you will get springs and a river running at the bottom

Water will flow out of the rock if the ground is very steep which will mean that the shape of the water table will be very steep as well.

Steep Sided valleys created by glaciation may not give enough outcrop of limestone, and the angle of dip may not be right for it to move a long way underground, and therefore for long caves to form, but may have a steep water table within them and may have deep caves in them. Examples of such caves are to be found in Kingsdale, Yorkshire as described on the previous page. Here there are a series of cave entrances right at the point where the overlying Yoredale series rocks finish and the water runs onto the massive limestone beds. The caves are steep as they rapidly move water down to the valley floor level where the water table is situated right at the point of the Kingsdale master cave

Sediments in water

The gravel and sand that has been picked up by rivers as they flow are a major factor in eroding limestone that is often forgotten against the more unusual water soluble properties. Simple abrasion accounts for a variable amount of the erosion the amount will depend on the relative solubility and acidity of the water vs the sediment content and water flow. Studies have indicated that abrasion is not linear with water speed, but a more complex model including water speed, sediment type and sediment grain size is needed as sediment can promote erosion at low supply rates by providing the means for abrasion, but it in fact inhibits erosion at high supply rates by burying underlying bedrock beneath more or less transient deposits. Maximum erosion rates occur at a critical level of coarse-grained sediment supply where the bedrock is only partially exposed. Fine-grained sediments provide poor abrasive tools for lowering bedrock river beds because they tend to travel in suspension

As mentioned above, where sediment is deposited on the floor of a cave it may protect the floor of the cave from erosion. This in the cave environment where acid erosion can also take place this gives rise to a mode of cave formation called paragenesis as will be described below

Types of water

Not strictly topography, but influenced by it is the concept of water mixing.

Caves are now also known to be initiated at inception horizons that occur at the water table, or at a layer of mixed water types such as sea and freshwater. This is because the mixing of chemistries can give a more aggressive erosional capability

Cave Development Top

Above or below the water

Passages that are formed below the water table will have the capability of eroding the limestones on all sides. They can produce what is know as a Phreatic Tube which is round to elliptical in shape. Phreatic tubes can form either along the horizontal strike or they can be down dip and back up again such as in the Mendip caves where phreatic loops form the sumps in places like Wookey Hole

In practice phreatic passages tend to be influenced by bedding which is a plane of weakness which water can erode, and therefore a modified version of the elipe with extensions along the bedding is found. The modification does not need to be horizontal as show in this diagram. If the bedding is on an angle then the modification will be along the same angle. If the bedding is not a significant feature bu another feature such as a fault plane is more significant then the extension of the phreatic passage will be along that plane of weakness

Passages that are formed above the water table are called Vadose passages. The pure form of a vadose passage is one where the floor of the passage is being eroded by an active river running in the passage. As the river meanders it will generally create a deep narrow passage which is called a vadose canyon

It is quite normal for a phreatic passage to form and then become a vadose passage. This is either because the whole regional water table has dropped, or more frequently the passage was stated as a small phreatic passage, but as water created larger passages it became possible for water to move downwards sooner

Whatever the reason the resulting passage shapes are often quite distinctive. If the Phreatic passage had formed to a sufficiently large size then a passage shape known as a keyhole will form as show in the picture to the right

If however the phreatic passage was only small before vadose development took over it will appear as just a rounded roof, or in some cases a small snaking feature on an otherwise flat roof. These are inception passages and may be a matter of a few inches or up to a meter before some larger form of erosion took hold and developed the passage in some other way. Such roof tubes can often be seen in the roofs of chambers and from these the direction and nature of original water flow can be determined which can be an extremely important aspect of determining the overall cave development. There are many excellent examples of this in the entrance passages and the general area of Big Chamber near the Entrance in Ogof Fynnon Ddu in South Wales

Vertical passages

One of the most sought after features in cave exploration is a pitch or vertical shaft. These can be formed in either the vadose or the phreatic zone, and can in fact be formed by water moving upwards or downwards when they are in the latter Phreatic zone.

In the vadose zone the mode of formation is straightforward as they can only form from the top down. Here the normal processes of erosion take place with sand and gravel in the water having a significant effect, with the impact of water and sediments clearly being an erosional factor

Formed from the Bottom Up

Another mode of cave passage formation was identified in the late 1960's and is becoming increasingly widely recognized. This is called Paragenesis and it can only take place in the phreatic zone because the mode of formation is for the floor of the passage to get covered with a layer of sediment which stops it being eroded and then the cave to develop upwards from that impervious layer.

For this to happen there must be enough water flow for acidic waters to erode the roof or walls, but not so much water flow that the protective sediment layer is removed. If the layer of sediment stays intact this can lead to very large canyons that look at first the same as vadose canyons, but with very close inspection the migration of any curves in the passage wall will be upwards instead of downwards.

Another feature of paragenetic passages are notching. Notching will occur where the water is flowing in such a way that sideways weaknesses are exploited, rather than the roof of the passage or maybe of the passage was not filled with water. A passage can have one or more sections above the floor that are widened giving shelves of rock between the notches. One of the best examples of notching in the UK is to be found in Megadrive in Ogof Draenen.

Collapse passages

Where a passage is in rocks that do not have significant inherent strength, such as a thin bedded rock where there are softer muddier layers between the harder pure lime layers, the roof of any passage that is formed will be weak and may collapse. If the material that is on the floor continues to be eroded, this can occur in a repeated fashion so that the passage becomes larger and squared off rather than the rounded features described above These passages become very squared off in some cases such that they almost look man made.

A good example of this is the Ogof Draenen Main passage at the point it is entered. If you take the old route down this main passage the floor is literally strewn with boulders and the walls are straight and vertical. In some places it is possible to see a boulder on the floor that can be visibly matched to a section of roof. Such floors are usually very unstable when freshly found and can be dangerous to walk on. The first significant accident in Ogof Draenen was when a caver was standing on what was obviously a weak boulder which snapped beneath him and he fell and broke a finger

Boulder Chokes

If this cycle of collapse continues it can lead to a catastrophic failure where the passage becomes filled with boulders (this can also happen in vertical shaft). The ultimate state for a boulder choked cave is of course one that is filled back in again, but in fact most boulder chokes have gaps and spaces between the rocks which cavers are able to work their way through very carefully ( and often with a lot of digging and scaffolding)

Deposits & formations Top

What do we mean by Deposits and formations

To describe cave deposits and formations some understanding of the terminology is needed.

Deposits are primarily anything that has been brought into the cave since it formed; mud, sand, stalactites and stalagmites are all cave deposits.

Some deposits such as boulders and fragments of shale etc. lie where they fell and are known as autochthonous, others have been moved to their current location and are known as alochthonous

The shaped deposits that are found in caves such as stalactites, stalagmites, columns, curtains and helictites are given the name formations.

Similar shaped formations are normally created by similar processes regardless of the materials involved; e.g.. both mud and calcite stalactites are formed by dripping water.

The term speleothem is reserved for formations that have been laid down in a cave from water and are typically crystalline formations with banding only. Mud formations are not usually included in the term speleothems: meaning a calcite stalactite is a speleothem, but a mud stalactite is not.

There are many terms for speleothems, some are different in different parts of the world and some are subdivisions of more common type. In the sections below some of the more common terms are described. If you want to know more, good places to start are the Wikipedia Speleothems Page and the Virtual Cave Solution Caves Page

Types of formations and how they form

Calcite speleothems outnumber those of other minerals by a very large amount.

The main reason that we have calcite speleothems is that the reaction that dissolved the limestone has become reversed. This occurs because the cave environment has a lower level of CO₂ than that in the rock and can come out of the carbonate solution.

This is as follows

Ca(HCO₃)₂(aq)

becomes

Ca²⁺ + 2HCO₃-

becomes

CaCO₃ + CO₂ + H₂O

The reactions for other carbonate minerals are similar and this can include minerals that form even when they were not dissolved from a single source mineral. In other words copper and carbonate dissolved from Chalcopyrite and limestone can become a copper carbonate rather than a calcium carbonate when they are deposited together. This can give some incredible colours as will be described in the minerals section below

Stalactites - C for Ceiling

Stalactites are formations which hang from passage roofs, wall projections and undercuts. They form many of the most spectacular formations in all caves.

There are 2 types of stalactites, Straw stalactites are relatively self explanatory. They are hollow tubes typically of calcite, about 5mm across and up to many meters in length. Straws form by the deposition of crystals around the edge of a drop of water and it is the maximum possible size for a drop of water that gives them their regular size. Some Straws are very pure calcite, colourless and transparent, but most have some form of impurities in them which make them translucent or opaque. Overall straws are normally white, but there are coloured ones in various caves

They occur when the speed of water being delivered into the straw is enough for it to grow, but not too fast that the water is just washing the dissolved calcite away. In fact they will also not form when the incoming water is moving too slowly as, in that condition the central canal blocks up with crystals, and the water flows down the outside and create a a massive stalactite as described below.

Straw stalactites are interlocking crystals which can be seen in close up macro pictures of the ends of them.

Some straws, of which there are good examples in Travertine Passage Ogof Craig a Fynnon (picture on the left), become blocked and the water percolates through the sides of the straw, depositing material on the outside, either as fine crystals or more massive layers.

Massive stalactites are not always as large as the name might imply, such as the ones to the right which are only about a foot in length, but they can give some impressive displays. They may be formed by deposition on the exterior of a straw type deposit, but they also form where water runs over an edge and flowstone develops into pendant formations.

Massive stalactites are often banded inside with growth layers where the amount of deposition has changed from season to season. In some places these layers act almost like tree rings with significant interruptions being visible in drought years. This gives an ability to correlate environmental information from cave to cave and is now an expending field of interest as we try and understand more of how climates change.

Where water flows out into a large chamber or passage and into an environment with less CO₂ you tend to get a lot of stalactites. The Hall of the Mountain King in Ogof Craig a Fynnon is a good example of this.

Stalagmites - G for ground

 Stalagmites form on the passage floors and upright block and boulder surfaces, such as these in Ogof Fynnon Ddu, South Wales (left) and Shatter Cave. Mendip (right)

They are often, but not always, formed below stalactites, indeed in some cases it can be hard to see where the corresponding stalactite is or the water that is forming the stalgamite is coming from. They  are normally broader than their associated stalactite, but not always so as you can tell from the picture from Coventosa in Matienzo, Spain (below). They can be up to many meters (even 10's of meters) high, and are in the main formed from massively crystalline calcite.

Just like stalactites they are often layered and can be used for environmental and dating purposes.

The rate of growth and the size of the stalagmite will depend on the amount of carbonate saturated water arriving at the floor of the cave and how it reacts when it gets there. This can be effected by things such as the ability of air to evaporate the water and the shape of the ground that is being deposited on.

Some small stalagmites form what are called Poached Eggs or Fried Eggs where the central portion looks darker (and often yellowed) compared to the outer edges. This is due to different crystal shapes

Columns

Columns are formations that reach from floor to ceiling, they are normally formed when a stalactite and stalagmite reach each other and coalesce.

Any stalactite and Stalagmite pair that are above and below each other can become a column and there are many examples where there is very little gap between the two

Columns are often an even thickness along their length with some thickening at the top and bottom. unless they are recently joined such as the ones to the left above in Shatter Cave, Mendip which are clearly a stalactite and stalagmite in the recent past, and a clear comparison to the one from Coventosa, Matienzo, Spain to the right which has clearly been joined for a more substantial time.

In some places it is postulated that columns have been formed in situations where there has been artificially high carbonate input because of lime kilns on the surface above the cave. The famous Columns in Ogof Fynnon Ddu are said be some to be formed for this reason, although I have yet to see definitive evidence

Flowstone

Flowstone is formed when layers of minerals are deposited from water slowly flowing down cave walls, across a roof or along the passage floor.

It is often rippled and coloured by mud and iron and is sometimes built into large 'bosses', which if they are over an edge will have stalactites dripping from them.

In the picture from Shatter cave above the area to the right of the large Stalagmite can be considered to be flowstone

Curtains

Curtains are formed where water runs down a sloping roof or ledge and deposits a line of crystals. These are then developed in a downwards direction, often with bands of colour which are formed by the amount of any impurities varying from time to time.

The typical curved and folded shape of curtain formations is often attributed to the curving path taken by the original water finding a route down the rock just as droplets take an erratic path when moving down a window.

However as many curtain formations have an increase in folding as they lengthen this seems to be unlikely as a complete answer. It may be that the out edges dry more or something to do with the way crystals grow, but to date I have not read a totally satisfactory answer for this phenomenon

Helictites & Heligmites

helictites are unusually shaped formations which grow from the passage walls or roof and heligmites are those which grow from the passage floor. Helictites have fine feeder channels for the water just like straw stalactites, but this is generally much finer such that there is not a water droplet, but the shape of the growth is more influenced by crystal directions which means they can develop in strange directions. Helictites can develop into straws and more massive stalactites, and vice verse as the amount of water feeding the stalactite changes.

Anthodites

Anthodites are radiating groups of crystals or needles which unlike helictites are radiating out from a central base. These may occur at the end of a straw, or directly radiating from a cave wall. They are usually aragonite, but Anthodites made from gypsum are known. Anthodite crystals are quite small 1mm -10mm which gives the formations a feathery appearance similar to the anthers of a flower which is where the name comes from. 

Some excellent examples that I had the pleasure of studying are to be found in Urchin Oxbow and surrounding areas in Ogof Daren Cilau, South Wales. These were the first Aragonite formations to be reported from a Welsh cave, and since their discovery and description Aragonite has become more widely recognized in many other caves in the area.

Splash Formations

Where lime rich waters abound there are the possibilities for splash formations, These form around the outside of waterfalls and splash pools and are often characterized by many small bulbous formations.

Excellent examples of these can be seen in Gilwern Passage in Ogof Draenen. Part way up the passage there is a water inlet from above and the flowstone around the pool is covered in splash deposits

Needles & Crystals

Some minerals such as gypsum grow a single or twinned crystals which can develop into very large crystals. This is possible because these minerals are soluble at normal temperatures and there is a steady flow of the mineral in a soluble form to add to the crystal. Gypsum has an unusual growth form compared to most speleothems as it grows from the bottom of the crystal (i.e. that still in the mud or attached to a cave wall) rather than on the outer surfaces or edges as is normal. What this means is that Gypsum crystals can push things away from the mud or wall surface and it is not unusual to find patches of dried mud suspended on "legs" of crystals

Cave Fossils Top

What are fossils & where to find them

In previous sections we explored the aspects of geology that give us the varied types of cave that we find in the British Isles. Often when we venture underground into these caves we pass strange marks on the cave walls. Many of these are fossils, a wide variety of which are found in the Limestones that we're caving in.

Fossils are the remains of dead animals or plants or the tracks and traces caused by such. They are usually animals with hard outer shells rather than soft creatures such as jellyfish, and also vertebrates such as fish, but there are exceptions as you will read below.

The chance of finding specific types of fossils depends on the type and age of the limestone. For example the Metamorphosed and very early limestones of the parts of Scotland like the Isle of Skye are not going to be as richly fossiliferous as the Carboniferous Limestone that we find in most of the rest of the UK caving regions, and even that varies with reef areas such as are found in Derbyshire being much richer than the shelf carbonates found in parts of Wales and Yorkshire.

Metamorphism for the none geologists is the process of changing one rock into another by heat and pressure. This normally includes recrystallizing anything that is there into other minerals and in that process the original structures are often destroyed to such an extent that it is hard to recognize that they were there of the limestone will no-longer break at the line of weakness that is the surface of the shell wall and therefore they are hard to extract and identify

Identifying Fossils in Caves

It is often difficult to make an exact identification of the fossils present in caves, as all that we see are cross sections where the fossils are cut through by the cave passage, sometimes at odd angles. What I hope to give here is an insight into the various main types of fossils and the habits and forms of the living creatures that these fossils once were. It is often difficult to make an exact identification of the fossils present in caves as all that we see are cross sections where the fossils are cut through by the cave passage, sometimes at odd angles.

The classic sectional shapes of fossils will be described below and illustrated which should aid in their identification

The types of fossils

Brachiopods

Some of the most common fossils seen in cave walls are crescent shapes of white calcite showing up in the darker limestone. These are usually the remains of a group of shells called Brachiopods. Two shelled creatures that spent most of their lives resting on the sea floor, supported by spines, or holding themselves down with a small fleshy foot called a pedicle. This pedicle stuck out of the shell through a small hole called a foramen.

Brachiopods are marine, shelled invertebrates that look superficially very much like mussels (see bivalves below). They are actually quite different in their anatomy, and they are not closely related at all.

They have two shells (valves) of calcium carbonate or combination of calcium phosphate and chitinous organic substance

The two shells of brachiopods are of unequal size (inequalvalved). The valve that has the attachment for the pedicle called the pedicle valve and is usually the lower and somewhat larger valve. The other valve is called the brachial valve and holds the a feeding tentacles, the lophophore or brachia. The brachidium is a long ribbon or loop-shaped calcified support for the lophophore which is attached to this shell.

Sometimes the brachiopods are seen with both shells in place and the right way up, and other times they are seen broken up and washed into layers where there are many broken and / or stacked shells. These are called Life position and Death Assemblage respectively.

Brachiopods are found in rocks of all Phanerozoic ages because as a group they have existed from the very beginning of the Cambrian radiation to the present day. However it is fair to sat that their peak time was in the Paleozoic with a small resurgence in the Jurassic where rhynconellids are quite common. The Brachiopods are now relatively insignificant. Most surviving brachiopods are mostly found in very cold parts of the Pacific Ocean, either in polar regions or at great depths. There are about seventy living genera and about 300 living species.

Some of the most common brachiopods in the Carboniferous Limestone are the Productids  (Example from near County Pot, Yorkshire left) which are strophomenids. Occasionally, and particularly common in Derbyshire. Very large specimens of the genus Gigantoproductus can be seen in roof in Carlswalk Cavern, Stony Middleton, Derbyshire. I have seen these misidentified on a number of caving websites as Oysters which are actually bivalves. They can reach up to 30cm across and are seen as sharp edged usually white curves in the dark limestone. The life position of the productid brachiopods is with the curved shell cupped upwards and a second more flattened shell above with the animal living in the space between, and the whole animal held in life position by a multitude of spines rather than the animal being held down by it's fleshy foot as described above. 

Another common brachiopod seen in Carboniferous Limestone are the Spiriferids ( example from Burrington Coombe, Mendip right). These were actually more dominant in the previous Devonian period, but still very common in the Lower Carboniferous which is the age of our best caving limestone in Britain

Crinoids

Another fossil that is at least as common in limestone as the brachiopods is the Crinoid or "sea lily". These fossils often go unnoticed because they are usually fragmented into unspectacular pieces. This is a shame as the complete animal is quite spectacular.

Although the creature resembles a plant, even to its "roots", it is however an animal (an echinoderm or relative of the sea urchins and having the same five sided symmetry) which feeds by filtering its food from the water. The circular fragments that we usually find, once again often in layers, are from the stem or arms of the animal and are called ossicles. The stem leads from a hold fast attached to the ground (or possibly a floating object such as a log) or in sediments up to a cup shaped body (calyx). Some layers of limestone are made up almost entirely of Crinoid ossicles.

Crinoids are known from Cambrian aged rocks right until the present day, but as with the Brachiopods above their times of largest diversity both in numbers and in species diversity were the Upper Palaeozoic and again in the Jurassic where the five pointed star shaped Pentacrinites is common

Some limestones in the Dentdale area of Yorkshire are wonderfully dark grey with white crinoids. These are wonderful when cut and polished

Corals

A third group of fossils often seen in cave walls are the Corals. These are the predecessors of the animals which form the Great Barrier Reef and the reefs in the Bahamas.

Corals are Anthozoa which are Cnidarians (Coelenterata in some older books), a group that includes jellyfish and other polyp type creatures. Cnidaria are the simplest animals, and do not even possess organs , all they have is a stomach and a mouth which is surrounded by tentacles

One of the main features of corals that makes them of interest to cavers is their ability to lay down huge volumes of limestone. There are two key groups of corals that are found in Carboniferous Limestone, the Tabulate corals and the Rugose corals

The Tabulate corals are all colonial and grow upwards as a colony secreting a new floor or tabula every so often for the active polyp to sit on.

They were most common in pre-Carboniferous times and good examples have been collected from Wren’s Nest near Dudley in the English West Midlands which is the remains of a Silurian coral reef system. The most common species that are seen are Halysites which in plan view resembles a set of chains as the polyps grew beside each other in rows of elliptical tubes, Favosites which has closely packed hexagonal corralites (where the polyps live). Michelinia is another similar coral often seen in carboniferous limestone and Syringopora which is a tubular branching form that where the tubes are round in cross section

The Rugose Corals (referring to their wrinkled appearance) are also known as "horn corals" because the solitary forms look like animal horns

Common solitary rugose corals include Zaphrentis a small horn shaped form, Caninia a large (up to 60cm/2 foot) curved form and Dibunophylum a medium sized solitary form/p>

The limestones of Derbyshire are made up in many areas of reef knolls built up from algae and from the skeletons of these corals, but they can be seen in other caving areas as well, one particularly spectacular example being in the lower entrance to Dan-yr-Ogof, just beyond the climb to the cavers gate. About 20m further on there is a band of coral about a meter thick that extends for about 15m along the wall of the cave.

Gastropods

Some rarer, but still occasionally seen fossils are the Gastropods.

Although Gastropods that are cut through so you can see a clear spiral seem clearly identifiable, this is not always the case, as they can be confused with the Goniatites described below. Both have representatives that spiraled in the same way, and it is only the presence of chambers in the shell of the goniatites that enables then to be distinguished.

The gastropod's shell however is not chambered and their soft parts occupy the whole shell. Some were grazers living on algae and other plant material whilst others active hunters of other marine creatures (well as active as a snail can get).

Gastropods Goniatites

Even more rarely identified are the Goniatites.

Goniatites are related to modern Nautilus and squids, and also to the extinct ammonites, they have a shell made up of chambers with the animal living in the latest and largest, or "body" chamber. They were active swimmers and hunters, using the empty chambers for buoyancy.

Like the Gastropods, those that are cut through so you can see the spiral have a chance of being clearly identifiable, but also like them, this is not always the case. Both have representatives that spiraled in the same way, and it is only the presence of chambers in the shell of the goniatites that enables then to be distinguished. However the thin walls of the chambers are not always well preserves and there's one in the right hand wall of the passage as you start up Salubrious Passage that I have always been unsure of.

Bivalves

Bivalves are the most common group of 2 shelled seashells alive today. Thay are things such as mussels, clams and oysters.

Most of them, such as mussels and clams have the same sahpe shalls both side, but others have differently shaped shells on each side as do th eoysters

Both types of shapes are very common right through geological time and both forms can be found in the Carboniferous.

Back that old they were however less common than the Brachiopods, but it is still worth lookin gclosely when you find a shel to try and work out whether it was a Bivalge or Brachiopod

Bryozoa

Bryozoa are small collonial animals that are often mistaken for corals. The most common forms are as a thin mat of encrustation on the surface of another shell, rock or even a plant, and as small braching tubules that are self supporting.

Bryozoa are another creature that is still in existance and they can be found encrusting seaweeds on many parts of the UK coast.

Sharks

Most of the Fossils mentioned so far were gentle bottom dwelling filter feeders, or at least hard shelled creatures. These are the most common, because it is these types of animals that are the most likely to become fossilized.

However in some places the remains of more active and more aggressive creatures can be found. In Ogof Draenen for instance Shark teeth and fin supporting and defensive spines can be seen. These are so common and so spectacular that they can be used to identify the particular layer of limestone that they occur in and map it around the cave.

Some of these fins from a fish called Ctenacanthus major are so large (about 3" across) that we can even see the holes where the blood vessels passed through the bone.

Cave Minerals Top

A Mineral is defined in the text books as "a substance of fixed chemical composition and structure formed by the inorganic processes of nature."

Normally each speleothem is made from only one mineral, but some are made up from more. Each mineral has a unique chemical composition and structure which means that it will have certain properties which can be used to identify it. To test these properties geologists use a number of methods which include scratching the mineral or treating it with acid. Cavers should learn to rely on appearance and try and avoid resorting to these tests.

This page covers a number of minerals that are of interest to British Cavers, some being "Cave Minerals", and some being those found in British mine workings that are commonly visited by cavers.

In addition there is some discussion of various "biominerals" being those mineral like substances formed by the biological processes of nature.

Limestone Minerals

Calcite - CaCO₃

Calcite is the commonest mineral found in caves, being the main mineral of Limestone. It is dissolved by mildly acidic waters, but re-precipitated to form the majority of stalagmites and stalagmites that decorate caves so wonderfully.

Not surprisingly, as calcite is the commonest limestone mineral it is also the commonest mineral of cave formations. there are many forms of calcite crystals in caves. The size and shape of the crystals is extremely variable and depends on a number of factors that effect their speed of formation

An excellent example of this can be seen in "Poached Egg Passage" in Giant's hole in Derbyshire. The Poached eggs are "proto stalagmites". The inner region is frequently wet and in this region the crystals grow relatively slowly, whereas the outer edges dry out more often and as a result the crystals in this region tend to be smaller. These latter micro crystalline areas are more opaque and this gives the poached egg appearance

Calcite can appear as white or colourless crystals when pure, but many speleothems are coloured by iron and mud into a range of red/brown colours and some such as the Blue Greenies behind Leonie Jayne's Playpens in Daren Cilau are coloured pale green by copper.

Dolomite - CaMg(CO₃)₂

Dolomite is found in different amounts in many of the limestones of the British caving regions. In South Wales it is notable because layers that are highly dolomitised forms smaller sharper passages

Formations

Aragonite - CaCO₃

Aragonite is a polymorph of calcium carbonate, that is they have the same chemical composition, but different crystal structures (i.e.. their atoms are arranged differently). The most stable form at cave temperatures and pressures is calcite and so there are many questions as to why aragonite should form at all. It is thought that the presence of manganese ions and the rate of evaporation have some effect.

Aragonite was thought to be restricted to the Mendip caving region, but has now been found to be widely distributed in the Llangattwg caves

Gypsum (Selenite) CaSO₄

Gypsum is often found growing from mud deposits and can form fine needles that stick up for a foot or more (30 cm to the none imperial amongst us). Outside of caves it can be found as Massive or Desert Rose forms

Moonmilch

"Vein" Minerals

Galena - PbS

Galena is the primary lead ore that was sought in Derbyshire and elsewhere where Lead mining has taken place. It is an unmistakable heavy grey mineral that breaks with cubic habit and is shiny when fresh.

Galena is one of the last minerals to be deposited in the veins and is generally found in the center of the veins surrounded by various gangue minerals

Fluorite (Fluorspar)

Barite (Barites)

This heavy mineral is not as heavy as galena, but then it's not metallic and shiny either. The best way of identifying barite is that it seems like "heavy calcite". This is not strictly correct in many scientific ways, but it's a good rule of thumb. Barite can be pinkish or white in colour and is not transparent.

Malachite

Azurite

Mud Minerals

Clay mineralogy is a subject for the specialists. There are many varied clay minerals and you need specialist equipment to identify them. To the rest of us this remains a wonderfully slimy or heavy and sticky substance that we have to deal with on a routine basis.

British Caving Regions Top

These descriptions are not exhaustive, they provide some information on how the geology of the areas relates to the caves. If you need more detailed information on any of these areas a good starting point is the Caving Regions of the UK and Eire section of the UK caving.com wiki

South Wales

The south Wales caving region is the British caving region that has seen the largest amount of cave discoveries in the last 20 years. The 1960's to 1980's being the Llangattwg years with the discovery and exploration of Agen Allwedd, Ogof Craig a Fynnon, and Ogof Daren Cilau respectively and the 1990's being now the Draenen years.

The South Wales caving region is bounded on it's inner edge by the South Wales Coalfield and on it's outer edges by a series of mainly Devonian aged rocks above which sits the basal conglomerate. It has been described as a bowl of limestone in which that coal sits. The bowl is not a regular shape though being a somewhat distorted oval shape as it has suffered somewhat over geological time, being cracked through by a series of faults that have often been eroded to give large valleys cutting into the interior.

These Valleys allow a longer and deeper section through the limestones, and are the areas where a lot of cave exploration has focused. It is only in more recent years as extensive dye tracing work has been carried out that the water flows through some of the more inaccessible areas have been determined and now the caves that support these flows are being discovered.

The shallow dipping limestone with run-off from higher rock layers and the relatively deeply incised valleys means that water can move a long way in South Wales from the point it goes underground to the point it comes out again. Given this it is not unsurprising that South Wales has 4 of the top 5 longest caves in the UK with the following published lengths as of November 2010 according to the UK caves Database website which maintains a close eye on such things

Derbyshire

The Derbyshire caving region is a tale of water and heat as well as the limestone. Cracked through by a series of faults it has been flushed through by heated mineral rich waters that have left immense mineral deposits as the waters cooled and their ability to carry strange mineral salts was diminished.

Derbyshire is an area where caves have been created, filled in again by these mineral rich waters leaving behind their deposits, and being rediscovered as the minerals were removed by "The Old Man".

A number of Derbyshire caves have only been discovered because of the mining activity and conversely some of the old mines of the area have only been rediscovered because of the activities of modern cavers.

Yorkshire Dales

Yorkshire is the largest UK Caving region and because of that it is also in many ways the most diverse. Carboniferous Limestone covers (or underlies) many square miles and so there is immense potential for cave development.

The scenery of the Yorkshire dales is one that has been shaped by the rocks and by Ice. The amount of ice covering the area at times was immense. Even the famous Three Peaks which stuck out of the ice as probably harsh bare rock at times were sometimes covered.

It is the former presence of these Ice sheets that gives us the spectacular areas of limestone pavement that dominate the landscape places like Chapel le Dale.

The Yorkshire Dales are characterized by beds of almost horizontal rock. The main Cave bearing rock is the 200 m thick Great Scar Limestones overlain by shales and sandstones with thin limestones within. These layers above the Great Scar Limestone are the Yoredales, formed in shallower waters. The thin limestones of the Yoredales do contain some caves, e.g. Nettle Pot in Dentdale. but are most important because the boundary between these rocks and the thicker limestone below is the first chance that water has to sink rapidly through the limestone and is therefore the site where many of the caves start.

Mendip

Mendip is a fairly complex piece of limestone geologically speaking. The rocks are more strongly folded than in other "major" caving regions. This gives the caves an unusual character of steeply dipping passages and deep sumps.

Ireland

County Clare has to be one of the most stunning places in the British Isles to go caving - even if you never go underground. The magnificent expanses of carboniferous limestone pavement and the friendly and warm reception that you get wherever you go means that this is a personal favorite for just a break.

The caves tend to be small, but there are a number of systems that do link, but only for the divers. This can be explained by the relatively flat bedding and low relief that means that the water table is fairly close to the surface

The "Minor" Caving Regions

Dudley

There is and unexpected and little visited area of speleological interest in the West Midlands can be found in the area of the Wrens Nest Nature Reserve. The Place to visit is the Mines that are in the Silurian (Wenlock) Limestone.

The mines have been mined in pillar and Stall fashion, but are quite extensive. There is a significant depth to these systems in places, but the lower levels are flooded and are accessible only to divers.

The site is a well-preserved Silurian coral reef with more than 700 types of fossil which have been discovered there. Probably the most famous (especially locally is Calymene blumenbachi, a trilobite nicknamed the Dudley Bug or Dudley Locust by 18th century quarrymen. For a geologist it is a wonderful place to visit as you can see a real reef ecosystem of corals, tabulate & Rugose, sponges, bryozoans, brachiopods, gastropods, bivalves, crinoids, criconarids, nautiloids, etc.

When quarrying finished in the early 20Th century, the site was left with a substantial network of underground caverns many of which are now flooded to a considerable depth. The site was originally studied by the Sir Roderick Murchison, whose work in defining the Silurian System was mainly based on fossils and rock formations found at sites in Dudley.

The Wren’s Nest is such an important paleontological site, that in 1956 it became a National Nature Reserve (NNR). Wren’s Nest was actually Britain’s first geological NNR.

In October 2004, Wren’s Nest and Castle Hill were declared a Scheduled Ancient Monument in recognition of them having the best surviving remains of the limestone quarrying, mining and processing industry in Dudley. This includes the last remaining surface opening limestone cavern in the world – known as the Seven Sisters. There was a major roof collapse in October 2001, which, if left unchecked, could have resulted in the loss of the Seven Sisters so there have been programmes to look at stabilization work. They are also looking to increase the tourism capability of the site which is probably not good for wild caving, but if it preserves the site it's got to be worth it.

Devon

Devon has some interesting if small caves and some interesting mines. Some of the latter are to be found near the wonderful village of Beer in East Devon where the Beer Quarry caves complex is open to the public.

Yorkshire Moors

Although the Yorkshire dales is the classic British caving region, there are caves also to be found on the moors and the East coast.

North Wales

North Wales has a number of Carboniferous limestone outcrops in the area around the Great Orme and continuing down past Ruthin and down to Oswestry. Classic sites include the Minerva Mine Complex and the Great Orme Mines themselves.

There is a decent amount of limestone here (about 400m on the Great Orme), but it has been subjected to dolomitization and mineralization.

The Great Orme mines are famous as copper mines, but also include some earlier emplacement of Lead and zinc minerals.

Skye/Scotland

The caves of Skye are small and sharp, having been formed in very old and marbleized Durness limestone.

The limestones have been marbleized by the emplacement of the tertiary volcanics (the rocks of the Red and Black Cullins which were emplaced as molten rocks). They include dykes of igneous rocks in places (although I'm not sure if this has been seen in any caves).

Useful Links Top

Cave Conservation Top

At all times you should strive to remember the cavers motto...

Take nothing but pictures.

Leave nothing but footprints.

Kill nothing but time