2B.2C Geological Structure and Cliff Profiles
Geological structure (jointing, dip, folding, faulting) is an important influence on coastal morphology and erosion rates, and also on the formation of cliff profiles and the occurrence of micro-features, e.g. caves.
Cliff profiles are influenced by geology, especially the resistance (to erosion) of the rock, and the dip of rock strata in relation to the coastline.
Joints
Joints are fractures in rocks created without displacement.
They occur in most rocks, often in regular patterns, dividing rock strata up into blocks with a regular shape.
Stair Hole (a small cove adjacent to Lulworth cove)
Purbeck Limestone is intensely folded, known as the Lulworth Crumple. The folding created heavy jointing making the Purbeck Limestone erode much more rapidly that the adjacent Portland Limestone.
Bantry Bay
Carboniferous Limestone is heavily jointed, leading to more rapid fluvial and marine erosion than in adjacent sandstones, contributing to the formation of headland and bay coastal morphology.
Faults
Faults are major fractures in rock created by tectonic forces, with displacement of rocks either side of the fault line. They are often large scale, extending many kilometres. It significantly increases rate of erosion, since zones of faulted rock are much more easily eroded.
Huge forces are involved in faulting and displacing them. Because of this, either side of the fault line, rocks are often heavily fractured and broken, which is easily exploited by marine erosion.
Bantry Bay
In Bantry Bay in Cork in the Republic of Ireland, a major fault runs SW-NE down the centre of the bay. The Carboniferous Limestone is significantly weakened by faulting, allowing rapid fluvial erosion, and the resulting river valley was inundated to form a ria when sea levels rose. Faulting also allowed more rapid marine erosion than in unfaulted Old Red Sandstone, contributing to headland and bay coastal morphology.
Folds
Folds are bends in rocks. They are produced by sedimentary rock layers being squeezed by tectonic forces. The two main types are anticlines and synclines.
Folded rock is often more heavily fissured and jointed, meaning they are more easily eroded.
It also increases erosion rates by increasing angle of dip, and by causing joint formation as rock is stretched along anticline crests and compressed in syncline troughs.
Dip
Dip is the angle of inclination of the rock strata from the horizontal. It's a tectonic feature. Sedimentary rocks are deposited horizontally, but can be tilted by folding and faulting by tectonic forces. It can have dramatic effects on cliff profiles.
Horizontal dip produces a vertical, or near-vertical profile, with notches reflecting weathering and small scale mass movement of strata that are jointed or more easily eroded.
High angle of seaward dip (>45) produces a sloping, low-angled profile with one rock layer facing the sea; vulnerable to rock slides down the dip slope when uppermost strata are attacked by sub-aerial processes. The profile slopes corresponding to that of strata dip. Bedding planes between strata are weakly bonded and readily loosened by weathering.
Low angle of seaward dip (<45) produces a steep profile, that may even exceed 90 degrees, creating areas of overhanging rock; very vulnerable to rock falls. Frequent small-scale mass movement of material weathered from cliff face. Major cliff collapse when undercutting by marine erosion makes overhang unsustainable.
Landward dipping strata produces steep profiles on 70-80' as downslope gravitational force pulls loosened blocks into place. Very stable profile with few rock falls.
cliff profile: the height and angle of a cliff face, plus its features such as wave-cut notches or changes in slope angle.
Cliff profiles are influenced by geology, especially the resistance (to erosion) of the rock, and the dip of rock strata in relation to the coastline.
Joints
Joints are fractures in rocks created without displacement.
They occur in most rocks, often in regular patterns, dividing rock strata up into blocks with a regular shape.
- In igneous rocks, cooling joints form when magma contracts as it looses heat.
- In sedimentary rocks, joints form when rock is subject to compression or stretching by tectonic forces or weight of overlying rock.
- When overlying rock is removed, underlying strata expand and stretch, creating unloading joints parallel to the surface.
Stair Hole (a small cove adjacent to Lulworth cove)
Purbeck Limestone is intensely folded, known as the Lulworth Crumple. The folding created heavy jointing making the Purbeck Limestone erode much more rapidly that the adjacent Portland Limestone.
Bantry Bay
Carboniferous Limestone is heavily jointed, leading to more rapid fluvial and marine erosion than in adjacent sandstones, contributing to the formation of headland and bay coastal morphology.
Faults
Faults are major fractures in rock created by tectonic forces, with displacement of rocks either side of the fault line. They are often large scale, extending many kilometres. It significantly increases rate of erosion, since zones of faulted rock are much more easily eroded.
Huge forces are involved in faulting and displacing them. Because of this, either side of the fault line, rocks are often heavily fractured and broken, which is easily exploited by marine erosion.
Bantry Bay
In Bantry Bay in Cork in the Republic of Ireland, a major fault runs SW-NE down the centre of the bay. The Carboniferous Limestone is significantly weakened by faulting, allowing rapid fluvial erosion, and the resulting river valley was inundated to form a ria when sea levels rose. Faulting also allowed more rapid marine erosion than in unfaulted Old Red Sandstone, contributing to headland and bay coastal morphology.
Folds
Folds are bends in rocks. They are produced by sedimentary rock layers being squeezed by tectonic forces. The two main types are anticlines and synclines.
Folded rock is often more heavily fissured and jointed, meaning they are more easily eroded.
It also increases erosion rates by increasing angle of dip, and by causing joint formation as rock is stretched along anticline crests and compressed in syncline troughs.
Dip
Dip is the angle of inclination of the rock strata from the horizontal. It's a tectonic feature. Sedimentary rocks are deposited horizontally, but can be tilted by folding and faulting by tectonic forces. It can have dramatic effects on cliff profiles.
Horizontal dip produces a vertical, or near-vertical profile, with notches reflecting weathering and small scale mass movement of strata that are jointed or more easily eroded.
High angle of seaward dip (>45) produces a sloping, low-angled profile with one rock layer facing the sea; vulnerable to rock slides down the dip slope when uppermost strata are attacked by sub-aerial processes. The profile slopes corresponding to that of strata dip. Bedding planes between strata are weakly bonded and readily loosened by weathering.
Low angle of seaward dip (<45) produces a steep profile, that may even exceed 90 degrees, creating areas of overhanging rock; very vulnerable to rock falls. Frequent small-scale mass movement of material weathered from cliff face. Major cliff collapse when undercutting by marine erosion makes overhang unsustainable.
Landward dipping strata produces steep profiles on 70-80' as downslope gravitational force pulls loosened blocks into place. Very stable profile with few rock falls.
cliff profile: the height and angle of a cliff face, plus its features such as wave-cut notches or changes in slope angle.
Micro-Features
Micro-features are small-scale coastal features such as caves and wave-cut notches which form part of a cliff profile.
They form in areas weakened by heavy jointing, which have faster rates of erosion, enlarging the joint to form a sea cave.
The location of micro-features found within cliffs, are often controlled by the location of faults and/or strata which have a particularly high density of joints and fissures.
They form in areas weakened by heavy jointing, which have faster rates of erosion, enlarging the joint to form a sea cave.
The location of micro-features found within cliffs, are often controlled by the location of faults and/or strata which have a particularly high density of joints and fissures.