Explain the formation of bays and headlands (6 marks)

Some areas of coast, such as the 90 kilometer Purbeck coastline on the South coast of England are known as discordant coastlines. This means they have bands of rocks with alternating resistance, in layers perpendicular to the shore. The more resistant bands of Portland Limestone are interspersed with less resistant Eocene clay. Erosive processes are more noticeable in the less resistant rock type, which erode at a greater rate, leaving the more resistant rocks protruding out as headlands, and the more eroded, less resistant rock in bays that have retreated further from the initial coastline. This is known as differential erosion. As the waves hit the headland, they refract around it and concentrate the energy of the orthogonal waves (perpendicular to the wave crest direction) onto the sides of the headland. This not only further erodes the headland but also decreases the energy reaching the bays, hence bays experience little wind or wave energy and thus erosion. An example is the exposed coast from Durlston Head to White Nothe, and the sheltered Poole Bay. Because the headlands are eroding, and the bays are not, they become level again and the coast returns to its straight profile. The processes of erosion continue, and the bays become more exposed again eroding at a relatively faster differential rate. Much like a gorge this feature means over time the headlands and bays protrude and retreat, with a resultant erosion.

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