<P> In the formation of sea cliffs, wave erosion undercuts the slopes at the shoreline, which retreat landward . This increases the shear stress in the cliff - forming material and accelerates mass movement . The debris from these landslides collects at the base of the cliff and is also removed by the waves, usually during storms, when wave energy is greatest . This debris provides sediment, which is transported through longshore current for the nearby bay . Joints in the headlands are eroded back to form caves, which erode further to form arches . These gaps eventually collapse and leave tall stacks at the ends of the headlands . Eventually these too are eroded by the waves . </P> <P> Wave refraction disperses wave energy through the bay, and along with the sheltering effect of the headlands this protects bays from storms . This effect means that the waves reaching the shore in a bay are weaker than the waves reaching the headland, and the bay is thus a safer place for water activities like surfing or swimming . Through the deposition of sediment within the bay and the erosion of the headlands, coastlines eventually straighten out . But then the same process starts all over again . </P> <P> Beaches are dynamic geologic features that can fluctuate between advancement and retreat of sediment . The natural agents of fluctuation include waves, tides, currents, and winds . Man - made elements such as the interruption of sediment supply, such as a dam, and withdrawal of fluid can also affect beach stabilization . A headland bay beach can be classified as being in three different states of sedimentation . Static equilibrium refers to a beach that is stable and does not experience littoral drift or sediment deposition or erosion . Waves generally diffract around the headland (s) and near the beach when the beach is in a state of static equilibrium . Dynamic equilibrium occurs when the beach sediments are deposited and eroded at approximately equal rates . Beaches that have dynamic equilibrium are usually near a river that supplies sediment and would otherwise erode away without the river supply . Unstable beaches are usually a result of human interaction, such as a breakwater or dammed river . Unstable beaches are reshaped by continual erosion or deposition and will continue to erode or deposit until a state of equilibrium is reached in the bay . </P>

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