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Information Sheet No CLIMATE CHANGE Water and the Burren 18 Karst landscapes What is karst? Most of Ireland has been covered by ice sheets at some time and glacial landscape features are quite common. As the ice sheets, moved across the Burren, they eroded the overlying soil, weathered rock and weak bedrock, exposing fresh surfaces of limestone. What makes the Burren landscape different from other glaciated landscapes is that these surfaces of bare limestone were then and are to this day, exposed to rainwater, which dissolves the limestone. Landscapes which are dissolved in this way are termed “karst” after the Kras region in Slovenia where this type of landscape was first described. e Burren is most correctly termed a “glaciokarst” region. is is an area where the development of karst features (karstification) has been influenced by the effects of glaciation. How does the rainfall create the karst? When rain falls through the atmosphere it picks up molecules of carbon dioxide. is creates a weak carbonic acid (the substance that makes sparkling water fizz) which can dissolve the calcite in limestone. Rainwater run-off, seeping through soil picks up even more carbon dioxide, making the runoff even more acidic. e slightly acidic water that falls and flows on and through the fractured limestone of the Burren, has dissolved the rock over time, creating a karst landscape. On a karst landscape, most of the water seeps into the limestone through small channels on the limestone surface and streams disappear into sinks or swallow holes. Water also enters the subsurface through closed depressions or dolines. e water moves underground through a network of subterranean channels and caves, eventually emerging through a spring. Examples of how water enters a subterranean karst landscape SECTION MODULE D 8 Water enters through closed depressions & dolines River water enters through sinkhole Water enters through soil or limestone pavement Shale Limestone To spring

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Page 1: Information Karst landscapes Sheet No - GeoNeedgeoneed.org/wp-content/uploads/downloads/2011/05/Module-D8-Information-Sheets.pdftermed “karst” after the Kras region in Slovenia

InformationSheet No

CLIMATE CHANGE Water and the Burren

18Karst landscapes

What is karst?Most of Ireland has been covered by ice sheets at some time and glacial landscape features are quite common. As the ice sheets, moved across the Burren, they eroded the overlying soil, weathered rock and weak bedrock, exposing fresh surfaces of limestone.

What makes the Burren landscape different from other glaciated landscapes is that these surfaces of bare limestone were then and are to this day, exposed to rainwater, which dissolves the limestone.

Landscapes which are dissolved in this way are termed “karst” after the Kras region in Slovenia where this type of landscape was first described.

The Burren is most correctly termed a “glaciokarst” region. This is an area where the development of karst features (karstification) has been influenced by the effects of glaciation.

How does the rainfall create the karst?When rain falls through the atmosphere it picks up molecules of carbon dioxide. This creates a weak carbonic acid (the substance that makes sparkling water fizz) which can dissolve the calcite in limestone. Rainwater run-off, seeping through soil picks up even more carbon dioxide, making the runoff even more acidic.

The slightly acidic water that falls and flows on and through the fractured limestone of the Burren, has dissolved the rock over time, creating a karst landscape.

On a karst landscape, most of the water seeps into the limestone through small channels on the limestone surface and streams disappear into sinks or swallow holes. Water also enters the subsurface through closed depressions or dolines. The water moves underground through a network of subterranean channels and caves, eventually emerging through a spring.

Examples of how water enters a subterranean karst landscape

section Module

D 8

Water enters through closeddepressions & dolines

River water entersthrough sinkhole

Water enters through soilor limestone pavement

Shale

Limestone

To spring

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InformationSheet No

CLIMATE CHANGE Water and the Burren

19Features of a karst landscape

Limestone pavement is only found in places that were covered by ice during the last ice age. Therefore it is thought that pavements owe much of their existence to the weathering and removal of overlying rock and soil by ice sheets.

Some beds of limestone are more likely to form pavements than others: (1) beds that don‘t contain many joints (fractures), (2) beds that are more resistant to dissolution by rainwater (because they are made of purer limestone), and (3) beds that are mechanically stronger (because they were hardened by geological processes). Approximately 20% of the Burren is limestone pavement, with an additional 30% consisting of a combination of pavement and rendzina (an organic-rich, calcareous soil). In the Burren, the most pristine pavements occur near the boundary between the limestones and overlying shales.

Grikes are vertical or near-vertical fissures in limestone pavement. Initially, the limestone contains only microscopic fractures or cracks. Rainwater seeps into the cracks and dissolves the limestone along the crack, making it wider.

By the time you can see it with the naked eye, it is called a grike (or scailp in Irish). Grikes can be up to 80 cm wide and 2 metres deep. The grikes divide up the limestone pavement into blocks called clints. Most clints are 1-5 m2 in area.

Kamenitzas are small, shallow, round, flat-bottomed depressions or pools on the surface of limestone pavement. They are usually a 5 – 30 cm wide. They form as the limestone is dissolved by standing water. The limestone does not contain any microscopic holes that let water drain through, and so any water (from rainfall) will sit in hollows on the limestone surface. This water dissolves the limestone underneath, enlarging the hollow.

Runnels are channels formed where water drains from a clint and dissolves the limestone. Runnels occur on the top surface of the limestone and along the sides of grikes. On the surface, the runnels can develop a branching pattern and often appear to start at a kamenitza or link up several kamenitzas. Runnels are usually 5 – 15 cm wide, but can be wider along

section Module

D 8

Kamenitzas (dissolution hollows)

Runnels

Limestone Pavement

Clints and grikes

Clint

Grike

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the grikes. As runnels and kamenitzas continue to become larger over time, the limestone pavement becomes more dissected or broken up.

Dolines are bowl-shaped, enclosed depressions on the landscape that can be several metres to several hundreds of metres wide. They can form by the dissolution of limestone from the surface downwards, or by the collapse of overlying rock into a cave, or by a combination of both. There are at least 1,500 dolines in the Burren that have an area greater than 100 m2. Most of these larger dolines occur in the east of the Burren. This is the eastern part of the Burren has been affected by dissolution longer than the west and large dolines take tens of thousands of years to form.

It is thought that some of the larger enclosed depressions in the Burren began to form when the area was still covered by shale (see illustration below). A river or rivers flowing over the shale eventually cut down into the limestone underneath. Once even a small section of limestone was exposed, it would have been gradually enlarged by dissolution from the runoff and the river would disappear underground.

The Carran Depression is one of the oldest features in the Burren landscape and may have begun to develop millions of years before the last ice age (which began 2 million years ago).

shaleerosion

shale

river valley

erosion

limestone

shale

runoff

shale

runoff

depression becomesdeeper and wider aslimestone dissolves

Horizontal runnels on the pavement surface

Dissected pavement that has been affected by

dissolution for a long time

Doline

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The formation of terraces in the Burren

CLIMATE CHANGE Water and the Burren

20 One of the most distinctive features of the Burren hills are the stepped, terraced sides (Photo A). These terraces formed because the limestone layers are actually separated by thin bands of mudstone (clay) that are 15 - 50 cm thick. It can be difficult to see the bands of mudstone in the hills and valleys of the Burren as they are very soft and are easily washed away by rainwater. These mudstone layers are often visible in places where the rock has been broken recently (Photo B).

Rainwater gradually washes away the layers of mudstone, eating into the hillside (Photo C and graphic below). You can see this happening on the west coast of the Burren, such as at the cliffs at Ailladie (Photo D and graphic B). This leaves the layer of limestone on top of the mudstone unsupported (Graphic C). Eventually, so much of the mudstone layer is washed away that the overlying layer of limestone layer on top becomes unstable and eventually collapses under its own weight (Graphic D). This often leaves piles of loose blocks and boulders at the base of the terrace (Photo D).

A: Terraces. B: Layer of mudstone in between the limestones. C: Layers of limestone that were separated by mudstones, now eroded away. D: The notch in this cliff is forming where the seawater is wearing away the layer of mudstone.

section Module

D 8

A: Layers of limestone separated by thin horizontal bands of soft mudstone. B: Water attacks the mudstone and washes it away. C: The limestone is now left unsupported and is unstable. D: The limestone collapses, forming a terrace.

A

B

C

D

A B C d

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21Caves and springs in the Limestones

CLIMATE CHANGE Water and the Burren

Springs occur throughout the Burren wherever underground streams meet a layer of rock that is impermeable. When they meet this layer, the water cannot flow downwards and instead it flows on top of the layer until it reaches the surface. Examples of such layers include the thin bands of mudstones that lie in between the limestone beds, or layers of chert.

A minor or “perched” water table will exist above the impermeable layer. The Carran ‘turlough’ (in the Carran Depression), is an example of a perched lake. Data gathered from a borehole shows that the main water table lies 100 metres below the floor of the Carran depression. This is because the floor of the depression is underlain by limestones that contain many bands of impermeable chert. The chert does not let water seep through it.

Springs in the Burren

Perched water table

Caves form when water dissolves through the limestone - and over time enlarges the dissolution cavities to form caves and caverns. As water dissolves through the roof of the cave, stalactites, stalagmites, flowstones and columns form from calcite. These features are called speleothems. Where water exits from underground, we find springs.

Cave development

section Module

D 8

Water enters through closeddepressions & dolines

River water entersthrough sinkhole

Water enters through soilor limestone pavement

Shale

Limestone

To spring

Bedding Plane

Joints

Clint Grike

Subterranean solution channel

Cave

SwallowHole

Stalactite

Stalagmite

CavernColumn

Spring

Collasped roof

Slightly acidic rainfall and water moving through the subsoil

Gradual dissolution of limestone by water

Dissolution occurs preferentially along joints and bedding planes

Early stages of cave development

Water seeps through small fissures and deposits CaCO3

Stalagmites and stalactites form from CaCO3 deposits

Mature cave development

Outflow vis springs

Chert Layer

Spring

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CLIMATE CHANGE Water and the Burren

22Freeze-thaw action on Limestone

Many of the cliffs and terraces in the Burren limestones have loose blocks or boulders at their bases. This is because the cliffs are constantly being weathered back by freeze-thaw action.

Water percolating into the joint freezes during winter and expands, placing pressure on the rock outside the joint (graphic below). Repeated freezing and thawing weakens the rock and expands the joint to the point where it fails and the rock to the exterior of the joint breaks off and falls. Erratic boulders are also affected by freeze-thaw action – often causing them to split.

section Module

D 8

Limestone blocks at the base of a

cliff in the Burren

Limestone erratic boulder in Burren National Park. This boulder has split due to freeze-thaw action

Break-up of rocks by freeze-thaw action

A B C