Physical Geography
Limestone, Granite Landscapes
L
K
L
L I M E S T O N E
 Sedimentary rock
 Composed of at least half carbonates – most common being calcite
 Resistant to physical weathering
 Chemically unstable – prone to carbonation and solution
 Low primary permeability
 High secondary permeability – via cracks and joints o Selective weathering at points of weaknesses
K A R S T L A N D F O R M S
Feature
Areas
Dolines, Uvalas
Non-tropical areas Related landforms
Description
Doline: Closed depressions, 1-100m deep, 10-10,000m wide
Cenotes
Uvalas: Extended networks of merged dolines
Prerequisites/Formation
Selective weathering at a group if fissures that are more open to a greater depth than its neighbors.
Subsidence creates a void towards which throughflow will be directed, causing an even greater extent of solution at this point, creating a progressively larger depression.
Diagram
Uvalas are large, enclosed areas where dolines merge to create several uneven, sub-basins with pitted floors.
Thus, in matured karst regions, the size of closed depressions increase, but the number decreases.
Feature
Areas
Description
Cenotes
Non-tropical areas. Low-lying, mature karsts. Yucatan, Mexico.
Also known as collapse dolines. Circular, smooth-walled vertical shafts floored by boulders or water
Prerequisites/Formation
Cenotes develop when a water-filled cave is located right below the surface of the ground where a doline is located. Highly fractured rock may be weakened by percolating rainwater and upward solution from cave water. When the overlying rocks are weakened sufficiently, they collapse, forming a shaft which intersects the water course below
Feature Cockpit karsts
Areas
Description
Humid tropics
Closed-depression landscapes.
Prerequisites/Formation
Related landforms Tower karsts
Selective weathering at areas where joints are concentrated.
Aided by the torrential nature of rainfall, surface gullying (erosion) by streams occurs along the joints. On futher erosion, elongated depressions extend headward along joints and interconnect, forming deep and irregular cockpits separated by conical residual hills called cones.
Diagram
Further weathering of cockpit karsts lowers the floor vertically until the base surface is reached, making lateral weathering more dominant. Steep landforms that can reach 200m in height, known as karst towers are formed.
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Physical Geography
Limestone, Granite Landscapes
Feature
Areas
Description
Karren features
Recently glaciated regions; tropical and Mediterranean regions
Microsolutional features that form on exposed limestone surfaces. Favoured by pure, homogenous limestone with well-developed and widely-spaced joints.
Prerequisites/Formation
1.
Recently glaciated regions
Glacially planed, smooth surfaces of bare limestone known has limestone pavements are found.
Formed when blocks and grooved created by joints are accentuated by chemical action of water and exposed at the surface, along a bedding plane.
2.
Tropical, Mediterranean karsts
Well-fissured, ridges known as spitzkarren formed as grykes bounding clints are widened and deepened by erosion due to the concentration of flow of rainwater in channels (runnelling).
D R A I N A G E F E A T U R E S
In general, subterranean karstification is more active in temperate regions.
Feature
Description
Swallow holes, dry valleys, blind valleys
Limestone features in karst valleys dissected by streams originating on non-limetsone rocks
Prerequisites/Formation
Swallow holes are created when a funnel-shaped opening results from the widening of joints over time. The river subsequently flows into this hole leading to the subterranean drainage system. Downcutting ceases downstream of the swallow hole, while vertical erosion continues upstream.
Immediately downstream of the swallow how, a cliff may develop, beyond which is the dry valley.
A blind valley is formed upstream of the dry valley. Blind valleys are closed at the lower end by the cliff at whose base the swallow hole is located.
Successive sinks may develop over time, causing headward retreat of the stream and a series of blind valley segments. This retreat terminates when the boundary between limestone/non-limestone rock is reached.
Features Caves Related landforms Spleothems
Prerequisites/Formation
 Groundwater moving through limestone along joints and bedding planes enlarge fissures
 Solution of rock is thus localized due to confinement of the flow of water to these fissures
 Cave conduits formed
Spleothems are depositions of calcite either in the form of stalactite or stalagmites, which may join to form columns
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Physical Geography
Limestone, Granite Landscapes
G
A
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G R A N I T E
 Intrusive igneous rock o From the solidification of granitic magma
 Mainly made up of quartz and feldspar
 Resistant to mechanical weathering
 Susceptible to chemical weathering o Especially to hydrolysis in humid climates
 High secondary permeability – via joints o Encourages selective weathering o Promotes block disintegration and spheroidal weathering
L A N D F O R M D E V E L O P M E N T I N H U M I D T R O P I C S
 High temperatures, high rainfall
 Predominantly chemical weathering
 Deep layers (>30m) of weathered material build up over million of years, masking solid rock
Ruxton and Berry’s Model
 Suggests the gradual decomposition of granite from surface downwards
 Produces four identifiable zones, from surface down, in mature granite landscapes o Zone 1: Residual debris o Zone 2: Residual debris; gruss; “floating”, rounded corestones o Zone 3: Residual debris; corestones o Zone 4: Partially weathered rock
L A N D F O R M D E V E L O P M E N T I N S E A S O N A L L Y H U M I D T R O P I C S
Regolith layer is less thick (6-25m) as compared to humid tropics
 Seasonal drought slows chemical weathering, retards regolith formation
 Higher degree of surface runoff and erosion occurs during the wet season - regolith layer is removed o Vegetation is less dense, rainwash processes are more effective o Rate of regolith formation is slower than rate of regolith removal
 Deep weathered layers may decrease in thickness, or be removed completely
 Basal surface of weathering revealed
Feature
Areas
Tors
Zimbabwe, Dartmoor
Description
Small hills, heaps of boulders that rise abruptly from the ground. 4-20m in height.
Prerequisites/Formation
Weathering of rock, followed by removal of regolith exposes the bedrock as well as rounded corestones.
Feature Inselbergs: bornhardts, blocky inselbergs, ruware, castle koppies
Description
Bornhardts: Tall (>300m) inselbergs with rounded summits. Characterized by large-scale sheet-jointing.
Blocky inselbergs: Much like large-scale tors. Characterized by rectangular jointing.
Ruware: Low, smooth and convex surfaces. May grow in height to become bornhardts
Castle koppies: Degraded forms of inselbergs, formed by prolonged subaeriel weathering and collapse.
Prerequisites/Formation
Formed from land surfaces which have been subjected to one or more phases of deep weathering, followed by removal (whether partial or complete) of regolith.
May be due to climatic changes:
 Pluvial periods represent dominant weathering
 Subsequent interpluvial periods reduces the rate of weathering, vegetation degenerates o More effective surface wash strips unprotected regolith
 Basal surface of weathering is undulating due to different rates of weathering
 As pluvial and interpluvial period alternate, ruwares get progressively higher
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Physical Geography
Limestone, Granite Landscapes o Results in an even more undulating basal surface of weathering, forming bonhardts
Diagram
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