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Chalk is a soft, white, porous, sedimentary carbonate rock.It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor.
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. The converse of a discordant coastline is a concordant coastline.
The chalk outcrop at Flamborough Head in the north produces a headland relatively resistant to coastal erosion whilst the coastline south of this at such places as Mappleton and Hornsea with their soft glacial deposits are vulnerable.
An escarpment (or scarp) is a type of cliff formed by the movement of a geologic fault, a landslide, or sometimes by rock slides or falling rocks which change the differential erosion of the rock layers. Most cliffs have some form of scree slope at their base. In arid areas or under high cliffs, they are generally exposed jumbles of fallen rock.
Coastal erosion may be caused by hydraulic action, abrasion, impact and corrosion by wind and water, and other forces, natural or unnatural. [3] On non-rocky coasts, coastal erosion results in rock formations in areas where the coastline contains rock layers or fracture zones with varying resistance to erosion.
Although somewhat resistant to erosion, erosion of both coasts has created the famous white cliffs of Dover in the UK and the Cap Blanc Nez in France. The Channel Tunnel was bored through solid chalk – compacted remains of sea creatures and marine-deposited, ground up calciferous rock/soil debris.
The chalk often contains larger fossils, including ammonites, belemnites, and brachiopods. Flint occurs throughout in both nodule and tabular layers. [57] The iridium-rich layer of sediment, found around the world at the K-Pg boundary, is missing in Dorset. This may be due to the uplift and erosion that removed the upper layers of the chalk. [58]
Geological resistance is a measure of how well minerals resist erosive factors, and is based primarily on hardness, chemical reactivity and cohesion. [1] The more hardness, less reactivity and more cohesion a mineral has, the less susceptible it is to erosion.