Discordant coastline

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Durlston Head (limestone) to Handfast Point (chalk), with Peveril Point (limestone) dividing Durlston Bay from Swanage Bay Durlston bay from durlston castle.jpg
Durlston Head (limestone) to Handfast Point (chalk), with Peveril Point (limestone) dividing Durlston Bay from Swanage Bay

A discordant coastline occurs where bands of different rock type run perpendicular to the coast.

Rock (geology) A naturally occurring solid aggregate of one or more minerals or mineraloids

Rock or stone is a natural substance, a solid aggregate of one or more minerals or mineraloids. For example, granite, a common rock, is a combination of the minerals quartz, feldspar and biotite. The Earth's outer solid layer, the lithosphere, is made of rock.

Perpendicular property of being perpendicular (perpendicularity) is the relationship between two lines which meet at a right angle (90 degrees). The property extends to other related geometric objects

In elementary geometry, the property of being perpendicular (perpendicularity) is the relationship between two lines which meet at a right angle. The property extends to other related geometric objects.

Coast Area where land meets the sea or ocean

The coast, also known as the coastline or seashore, is the area where land meets the sea or ocean, or a line that forms the boundary between the land and the ocean or a lake. A precise line that can be called a coastline cannot be determined due to the Coastline paradox.

The differing resistance to erosion leads to the formation of headlands and bays. A hard rock type such as granite is resistant to erosion and creates a promontory whilst a softer rock type such as the clays of Bagshot Beds is easily eroded creating a bay.

Both headland and bay are two coastal features that are related and often found on the same coastline. A bay is a body of water—usually seawater and sometimes fresh water— mostly surrounded by land, whereas a headland is surrounded by water on three sides. Headlands are characterized by breaking waves, rocky shores, intense erosion and steep sea cliffs. Bays generally have less wave activity and typically have sandy beaches. Headlands and bays form on discordant coastlines, where the land consists of bands of rock of alternating resistance that run perpendicular to the coast.

Granite A common type of intrusive, felsic, igneous rock with granular structure

Granite is a common type of felsic intrusive igneous rock that is granular and phaneritic in texture. Granites can be predominantly white, pink, or gray in color, depending on their mineralogy. The word "granite" comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a holocrystalline rock. Strictly speaking, granite is an igneous rock with between 20% and 60% quartz by volume, and at least 35% of the total feldspar consisting of alkali feldspar, although commonly the term "granite" is used to refer to a wider range of coarse-grained igneous rocks containing quartz and feldspar.

Promontory prominent mass of land that overlooks lower-lying land or a body of water

A promontory is a raised mass of land that projects into a lowland or a body of water.

Part of the Dorset coastline running north from the Portland limestone of Durlston Head is a clear example of a discordant coastline. The Portland limestone is resistant to erosion; then to the north there is a bay at Swanage where the rock type is a softer greensand. North of Swanage, the chalk outcrop creates the headland which includes Old Harry Rocks.

Dorset County of England

Dorset is a county in South West England on the English Channel coast. The ceremonial county comprises the non-metropolitan county, which is governed by Dorset County Council, and the unitary authority areas of Poole and Bournemouth. Covering an area of 2,653 square kilometres (1,024 sq mi), Dorset borders Devon to the west, Somerset to the north-west, Wiltshire to the north-east, and Hampshire to the east. The county town is Dorchester which is in the south. After the reorganisation of local government in 1974 the county's border was extended eastward to incorporate the Hampshire towns of Bournemouth and Christchurch. Around half of the population lives in the South East Dorset conurbation, while the rest of the county is largely rural with a low population density.

Swanage coastal town and civil parish in the south east of Dorset, England

Swanage is a coastal town and civil parish in the south east of Dorset, England. It is at the eastern end of the Isle of Purbeck and one of its two towns, approximately 6 14 miles (10 km) south of Poole and 25 miles (40 km) east of Dorchester. In the 2011 census the civil parish had a population of 9,601. Nearby are Ballard Down and Old Harry Rocks, with Studland Bay and Poole Harbour to the north. Within the parish are Durlston Bay and Durlston Country Park to the south of the town. The parish also includes the areas of Herston, just to the west of the town, and Durlston, just to the south.

Chalk A soft, white, porous sedimentary rock made of calcium carbonate

Chalk is a soft, white, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. Calcite is an ionic salt called calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells (coccoliths) shed from micro-organisms called coccolithophores. Flint (a type of chert) is very common as bands parallel to the bedding or as nodules embedded in chalk. It is probably derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction. Flint is often deposited around larger fossils such as Echinoidea which may be silicified (i.e. replaced molecule by molecule by flint).

The converse of a discordant coastline is a concordant coastline.

Concordant coastline

A concordant, longitudinal, or Pacific type coastline occurs where beds, or layers, of differing rock types are folded into ridges that run parallel to the coast. The outer hard rock provides a protective barrier to erosion of the softer rocks further inland. Sometimes the outer hard rock is punctured, allowing the sea to erode the softer rocks behind. This creates a cove, a circular area of water with a relatively narrow entrance from the sea.

Examples

See also


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Natural arch A natural rock formation where a rock arch forms

A natural arch, natural bridge, or rock arch is a natural rock formation where an arch has formed with an opening underneath. Natural arches commonly form where inland cliffs, coastal cliffs, fins or stacks are subject to erosion from the sea, rivers or weathering.

Coastal erosion The loss or displacement of land along the coastline due to the action of waves, currents, tides. wind-driven water, waterborne ice, or other impacts of storms

There are two common definitions of coastal erosion. It is often defined as the loss or displacement of land along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms. In this case, landward retreat of the shoreline, measured to a given spatial datum, is described over a temporal scale of tides, seasons, and other short-term cyclic processes. Alternatively, it is defined as the process of long-term removal of sediment and rocks at the coastline, leading again to loss of land and retreat of the coastline landward. Coastal erosion may be caused by hydraulic action, abrasion, impact and corrosion by wind and water, and other forces, natural or unnatural.

Jurassic Coast World Heritage Site on the coast of southern England

The Jurassic Coast is a World Heritage Site on the English Channel coast of southern England. It stretches from Exmouth in East Devon to Studland Bay in Dorset, a distance of about 96 miles (154 km), and was inscribed on the World Heritage List in mid-December 2001.

Lulworth Cove cove

Lulworth Cove is a cove near the village of West Lulworth, on the Jurassic Coast in Dorset, southern England. The cove is one of the world's finest examples of such a landform, and is a World Heritage Site and tourist location with approximately 500,000 visitors every year, of whom about 30 percent visit in July and August. It is close to the rock arch of Durdle Door and other Jurassic Coast sites.

Isle of Purbeck peninsula in Dorset, England

The Isle of Purbeck is a peninsula in Dorset, England. It is bordered by water on three sides: the English Channel to the south and east, where steep cliffs fall to the sea; and by the marshy lands of the River Frome and Poole Harbour to the north. Its western boundary is less well defined, with some medieval sources placing it at Flower's Barrow above Worbarrow Bay. According to writer and broadcaster Ralph Wightman, Purbeck "is only an island if you accept the barren heaths between Arish Mell and Wareham as cutting off this corner of Dorset as effectively as the sea." The most southerly point is St Alban's Head. Its coastline is suffering from erosion.

Durdle Door natural limestone arch

Durdle Door is a natural limestone arch on the Jurassic Coast near Lulworth in Dorset, England.

Hoodoo (geology) A tall, thin spire of relatively soft rock usually topped by harder rock

A hoodoo is a tall, thin spire of rock that protrudes from the bottom of an arid drainage basin or badland. Hoodoos typically consist of relatively soft rock topped by harder, less easily eroded stone that protects each column from the elements. They generally form within sedimentary rock and volcanic rock formations.

Cuesta A hill or ridge with a gentle slope on one side and a steep slope on the other

A cuesta is a hill or ridge with a gentle slope on one side, and a steep slope on the other. In geology the term is more specifically applied to a ridge where a harder sedimentary rock overlies a softer layer, the whole being tilted somewhat from the horizontal. This results in a long and gentle backslope called a dip slope that conforms with the dip of resistant strata, called caprock. Where erosion has exposed the frontslope of this, a steep slope or escarpment occurs. The resulting terrain may be called scarpland.

Geology of Dorset

Dorset is a county in South West England on the English Channel coast. Covering an area of 2,653 square kilometres (1,024 sq mi); it borders Devon to the west, Somerset to the north-west, Wiltshire to the north-east, and Hampshire to the east. The great variation in its landscape owes much to the underlying geology which includes an almost unbroken sequence of rocks from 200 Ma to 40 Ma and superficial deposits from 2 Ma to the present. In general the oldest rocks appear in the far west of the county, with the most recent (Eocene) in the far east. Jurassic rocks also underlie the Blackmore Vale and comprise much of the coastal cliff in the west and south of the county; and although younger Cretaceous rocks crown some of the highpoints in the west, they are mainly to be found in the centre and east of the county.

Worbarrow Bay

Worbarrow Bay is a large broad and shallow bay just to the east of Lulworth Cove on the Isle of Purbeck, Dorset, England.

Peveril Point human settlement in United Kingdom

Peveril Point is a promontory on the east-facing coast of the Isle of Purbeck in Dorset, England, and is part of the town of Swanage. It forms the southern end of Swanage Bay. It is located at 50°36.43′N 01°56.69′W OS Grid Ref: SZ 041 787.

Caprock or cap rock is a harder or more resistant rock type overlying a weaker or less resistant rock type. Common types of caprock are sandstone and mafic rock types. An analogy of caprock could be the outer crust on a cake that is a bit harder than the underlying layer. Common caprock locations are escarpments, mesa formations, and salt domes. In the petroleum industry, caprock is generalized to any nonpermeable formation that may prevent oil, gas, or water from migrating to the surface.

Worbarrow Tout

Worbarrow Tout is a promontory at the eastern end of Worbarrow Bay on Isle of Purbeck in Dorset on the south coast of England, about 6 kilometres (3.7 mi) south of Wareham and about 16 kilometres (10 mi) west of Swanage. Immediately to its east is Pondfield Cove.

Cow Corner

Cow Corner is the north-western end of Worbarrow Bay, a small secluded bay on the south coast of the Isle of Purbeck, in Dorset England.

Flowers Barrow Iron Age hill fort in Dorset, England

Flower’s Barrow is an Iron Age hillfort, built over 2500 years ago, above Worbarrow Bay in Dorset on the south coast of England.

Gad Cliff Dorset, England

Gad Cliff is a south-facing cliff face, immediately to the east of Worbarrow Tout and Pondfield Cove, on the south coast of the Isle of Purbeck in Dorset, England. Behind it is Gold Down, part of the Lulworth Ranges.