2B.1C Rocky Coasts and Coastal Plains
Rocky coasts (high and low relief) result from resistant geology (to the erosive forces of sea, rain and wind), often in a high-energy environment, whereas coastal plain landscapes (sandy and estuarine coasts) are found near areas of low relief and result from supply of sediment from direct terrestrial and offshore sources, often in a low-energy environment.
Rocky Coasts
- Rocky coasts occupy about 1,000 km of the UK's coastline, mainly in the north and west.
- Cliffs vary in height from high-relief areas,
- e.g. 427 m Conachair Cliff on the Isle of Hirta in the Outer Hebrides
- to low-relief
- e.g. 3m cliffs at Chapel Porth in Cornwall
- Rocky coasts usually form in areas of geology that is resistant to the erosive forces of the sea, rain and wind. Their lithology and structure means they erode and weather slowly
See 2B.3A for how lithology affects resistance.
Rocky coastlines form in a high-energy environment where erosion > deposition.
Erosion is continuously moving transported and deposited sediment as well as slowly eroding the cliff.
Coastal Plains - (sandy and estuarine coastlines)
- Coastal plain landscapes are relatively flat, low relief areas adjacent to the sea.
- They often contain freshwater wetlands and marshes due to the poor drainage of the flat landscape.
- Their littoral zone is composed of sand dunes, beaches, mud flats and salt marshes.
Coastal plain landscapes form in low-energy environments where deposition > erosion, so they experience a net accumulation of sediment. They form through coastal accretion (a continuous net deposition of sediment.) This comes from:
- offshore sources (transported by waves, tides or current)
- terrestrial sources (transported by rivers, glaciers, wind or mass movement)
Coastal plains may be
- sandy coasts, composed of sands, shingles and cobbles.
- estuarine (alluvial) coasts composed of mud (clays and silts)
They form most of the UK's south and east coastline.
Other Information
- Weathering is the breakdown of rock in situ, and may be a mechanical, biological or chemical process.
- Erosion is the breakdown of rock and its immediate transport by the eroding agent.
- Coasts are attacked by marine processes (e.g. wave erosion) and by terrestrial processes (e.g. weathering, mass movement, wind erosion, river erosion)
- Mass movement is the downslope movement of material due to the force of gravity.
Coastal Plain Formation
- They usually form by coastal accretion, where continuous net deposition causes the coastline to extend seawards. This is often extended biologically as plants colonise shallow water, trapping sediment and forming organic deposits when they die.
- They also form by sea level change, when the falling sea level exposes a flat continental shelf. e.g. the Atlantic coastline of the USA.