Using the information on the next slide you should come up with no more than 3 sentences that explain how limestone is formed and exposed to the elements.
It doesn’t have to be as detailed but needs to show an understanding of the origins of limestone.
Limestone is formed from the fossilised remains of countless marine plants and animals, such as corals, which lived during the Carboniferous period, 345 - 280 million years ago.
These former sea beds were moved from the Equator due to continental drift and forced above sea level 280 - 230 million years ago.
During Glaciation the topsoil is scoured from the landscape exposing the limestone underneath to weathering.
Formation
Carboniferous limestone landscapes are the result of:-
The rock is divided into blocks as a result of breaks between
the rock beds (bedding planes)
Vertical cracks (joints) are created as the limestone dries out and
pressure is released.
bedding plane
joint
permeability - water passes easily through the rock by following the bedding planes and joints.
Choose any one Carboniferous Limestone feature described in your answer to part (a) and, with the aid of annotated diagrams, explain how it was formed.
Marks 6
Probably the most obvious (sensible!) Carboniferous Limestone feature to choose
would be a limestone pavement although some candidates may focus on limestone
caves and their associated underground landforms such as stalactites, stalagmites
and rock pillars.
Answers which fail to make use of diagrams should score a maximum of 4.
Sufficiently well annotated diagrams ought to be able to earn full marks.
In explaining the formation of a limestone pavement, for example, candidates
could refer to such points as:
• the part played by glacial erosion (abrasion) in scraping away any overlying
soil cover and thus exposing the horizontally-bedded, rectangular blocks of
limestone.
• joints formed in the limestone as it dried out and pressure was released.
• these joints/lines of weakness are more prone to chemical weathering than the
surrounding limestone. The limestone is dissolved over time by rainwater
(weak carbonic acid) leaving deep gaps (grykes) and intervening blocks
Max 2 marks for describing features if no explanation given. 6 marks
Swallow Holes and Pot Holes
Because limestone is a permeable rock, there are few surface streams.
Streams that flow onto limestone quickly fall into one of the many enlarged joints on the surface and disappear underground.
Where a river goes underground it is called a swallow hole or a pot hole.
Swallow holes and pot holes
The stream disappears into the ground
Limestone features
Swallow Hole
Limestone features
Caverns
Caverns form where some of the underground limestone is dissolved more quickly than the rock around it. This happens when the rock has many joints and bedding planes close together.
These cracks allow through lots of water, which dissolves away the rock completely and a cavern forms.
Limestone features
Stalactites
Form in the following way:
The water that drips into the cavern is laced with calcium carbonate that has dissolved on its passage through the rock.
The water drips from the cavern roof very slowly and some of it evaporates leaving behind calcite deposited on the cavern roof.
The water continues to drip, evaporating as it does so, and the deposits build up to form fingers of calcite that grow downwards into the cavern.
Stalactites
They grow by only a few millimetres a year.
They grow slowly, partly because the water cannot hold much dissolved limestone and partly because the caverns are cool and not much evaporation takes place.
Stalagmites
Some of the water drips onto the cavern floor where it may also evaporate.
It leaves behind calcite here as well, which is deposited on the cavern floor.
As more water drips down, more is deposited forming fingers of dripstone that grow upwards from the cavern floor.
These are called Stalagmites.
Stalagmites
Sometimes stalagmites and stalactites join together to form a limestone pillar.
Caves of Drach
Limestone gorge
Stalactite revision
Limestone landscapes
Limestone landscapes
Carboniferous limestone features
Key ideas
Limestone solution is the key to understanding all types of limestone features. It is a form of chemical weathering.
Land uses in limestone areas include tourism, quarrying, farming, forestry and military use. Many of the groups that use the land are in conflict with each other.
Carboniferous limestone is a particularly tough form of limestone which formed about 350 million years ago and is found in the UK mostly in the north of England and Ireland.
