The Yorkshire Dales is known for its limestone scenery, limestone geology and limestone caves. For lots of us, venturing underground is just not an option, nor is a long walk in to a remote location. This EarthCache gives the EarthCacher the chance to learn about, and gain some understanding about how caves are formed.
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This EarthCache arose out of a walk from the parking at Ribblehead to look at the limestone pavement with some other EarthCachers, and as is per the occasion, then a discussion was had about how the subject was formed.ย
You do not need to go up onto the limestone pavement to examine this location.ย You need to be stood on grass.
ย Two visitors from the Burnley area, that I met at the location, who were on a geology tour.ย
We are here to learn about cave formation, without having to put on a headtorch, and venture into the underworld.ย ย
The geology.ย ย
The rock that you see is limestone, which is a sedimentary rock. It was originally an accumulation of lime mudย and / or the shells of marine organisms, which built up layers, was then compacted and eventually limestone was formed.ย Limestone is mostly formed of calcium carbonate. Look around, and you will notice lots of limestone, with varying shapes and shades of grey.ย
Right where you are stood is a feature in the limestone, a small tunnel in the limestone.ย We shall use this to learn about cave development.ย ย
What is a cave?
A cave is a natural underground void or hollow in the ground. Now caves cover a large range of geological features. Some examples are :
Sea caves. These form at the coast, and are the result of the constant erosion by seawater and tidal action. However sea levels can rise and fall, and the west coast of Scotland are sea caves, which are now not on the coast, due to sea level change and uplift of the landscape.ย
Talus caves. These are voids inbetween rocks or boulders often at the bottom of slopes or cliffs.ย These can be found in areas such as Crookrise crags near Skipton, in the Lake District in Langstrath, and in Galloway on the slopes of Cairnsmore of Fleet.ย
Lava tubes/caves. These are voids left by volcanic action. Lanzarote and Hawai is a place where these are found.ย ย
Solutional caves. These are caves where rainwater dissolves a rock, as the water absorbs carbon dioxide, becomes acidic, and then slowly erodes theย rock.ย
Cave Formation.ย ย
We shall refer to the process relating to Solutional caves for the purpose of this EarthCache.
Limestone may haveย weaknesss, which rainwater will exploit. These weaknesses could be due cracks such as styolites, bedding planes, cracks and scrapes formed by glacial action, fault lines, sedimentation change and strata change.ย ย
Above is an example of change in strata, which is a specific layer of rock. We see limestone strata, on top of Horton Formation Siltstone strata . The limestone is more permeable to water so will erode more easily, and be more prone to cave formation.ย This is Moughton Scar, near Austwick.ย ย
Sedimentation change. This is Little Scar. It is all limestone, but there are different layers of limestone sediment, some more resistant to erosion. Cave formation will occur in layers of sediment which are easier to erode.ย
Bedding planes. You will notice horizontal lines going across the rock face, these are bedding planes. Groundwater will exploit these as they are a weakness, thus caves can form along bedding planes at first. This is in the Ribblehead area.ย
Stylolites. In the bottom of the crag, there is a wavy line. This is a stylolite, which is formed by pressure and stress during the formation of the rock. Water can use these as channels of movement, which are then eroded, and voids firm, which then turn into caves. The straighter lines are bedding planes. This is a limestone outcrop near Little Stainforth.ย
Fault line. In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of rock-mass movement.A fault line can create a weakness, which groundwater will exploit. This is Dow Cave in Wharfedale, where the cave has formed along the fault. The picture has been taken from the Web for educational purposes.ย ย
Stages of cave formation.ย
Solutional caves do not form suddenly. They form in stages.ย
A. Cave formation starts with a weakness in the limestone.ย The black line represents a crack. The water which exploits it, saturated the area, causing erosion to start by the dissolution of calcium from the limestone.ย This basically means removing the calcium which causes the limestone to dissolve eventually.
B. As erosion occurs, so the crack widens and deepens. It always occurs at the level of the water table in the limestone.ย Where there is greater concentration of water, so there is greater saturation of the rock and erosion.
C. Eventually a flow of water can occur as the crack becomes a void. This flow of water may concentrate in places, with greater turbulence (flow) causing greater erosion. As this happens so it starts to erode where this happens.ย
Here you will notice the water has eroded a channel.ย ย
D. As the water erodes down, so the flow can increase, due to weather conditions, it can mean a greater amount of water. This further erodes the walls of what now is a cave. Debris can also be washed downstream, which can scour the walls, and cause more erosion. Caves can frequently fill to the roof, which can therefore affect all angles of the cave.ย Caves which do this frequently often have less speleothems, which are formations such as stalagmites and stalactites, as there is not enough time for them to form, due to water current.ย ย
As water erodes, it can form a vadose pattern, such as above, the water basically cuts down through erosion, and then widens. Think of a V.ย
E. As erosion further processes at tines you can have erosion on all angles.ย In reality at times, some caves may not look like this due to the way that they have formed.ย
In some cases, water levels and flow and current can be reduced, abd then the process of stalagmites and stalactites formation can occur, but that process is for another EarthCache.ย ย
This being an EarthCache, in order to log it, I ask that you answer some questions. Please send them to me, and do not include them in your log. You can send them to me by using the message facility or email, both of which can be found by looking at my profile. Remember, you are looking for a hole in the rock, it may be large, it may be small, it may be rough and it may be smooth, it is for you to find out.
1. Using the above information and pictures, please explain at what stage this feature is at. What is particular about it...you will have to put your arm in?
2. Please explain why it appears as it does, making reference to how caves are formed.ย
3. What do you think will eventually happen to this feature?