3 Rocks and weathering
Limestone scenery
Revised
Limestone scenery is unique on account of its: l permeability l solubility in rain and ground-water Limestone consists of mainly calcium carbonate. Because of their permeability, limestone areas are often dry on the surface and are known as Karst landscapes. Carboniferous limestone has a distinctive bedding plane and joint pattern, described as massively jointed. These features act as weaknesses allowing water to percolate into the rock and dissolve it. One of the main processes affecting limestone is carbonation–solution. The process is reversible, so under certain conditions calcium carbonate can be deposited in the form of speleothems (cave deposits such as stalactites and stalagmites) and tufa (calcium deposits around springs). Limestone is also affected by freeze–thaw, fluvial erosion, glacial erosion and mass movements.
Expert tip
Surface features
Now test yourself
As the joints and cracks are attacked and enlarged over thousands of years, the limestone’s permeability increases. Clints and grikes develop on the surface of the exposed limestone. Large areas of bare exposed limestone are known as limestone pavements. Depressions can range from small-scale swallow holes (or sinks) to large dolines up to 30 m in diameter. These are caused by the solution of limestone but can also be formed by the enlargement of a grike system, by carbonation or fluvial activity, or by the collapse of a cavern. Rivers can disappear into these holes, hence the term ‘sink’. Resurgent streams arise when the limestone is underlain by an impermeable rock, such as clay.
Stalactites develop from the top of the cave, whereas stalagmites are formed on the base of the cave.
16 What are the two main theories about tor formation? 17 What is equifinality? 18 What are the main processes affecting limestone? 19 Explain the formation of swallow holes.
Answers on p.215 Tested
3.3 Slope processes and development Slope development Rock type
Geological structure is an important influence on slope development. This includes faults, angle of dip and vulcanicity: l Faulting can produce steep valley sides, as in a rift valley. l Folding can produce either steep or gentle slopes depending on the angle of dip. l Vulcanicity produces intrusions of resistant igneous rock. For example, Great Whin Sill is a harder and more resistant rock than the surrounding dolomite, and so has produced a steep slope. Rock type and character influence whether a rock is affected by weathering, and to what extent it can resist the downslope force of mass movement. Resistance is largely physical. Regular jointing can increase the risk of movement, as well as the amount of water that enters a rock.
Revised
A slope is an inclined surface (hillslope). It can also refer to the angle of inclination.
Expert tip Slopes that comprise many different types of rocks are often more vulnerable to landslides due to differential erosion. The softer rocks get worn away and can lead to the undermining of harder rocks.
Climate
Many slopes are shaped by climate, which affects the types and rates of processes that operate in a region and when they occur: l In arid regions, slopes are jagged or straight due to mechanical weathering and sheetwash.
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Cambridge International AS and A Level Geography Revision Guide
