Two Young Girls Sunbathing On The Beach

The shape of a beach depends on whether or not the waves are constructive or destructive, and whether the material is sand or shingle. Constructive waves move material up the beach while destructive waves move the material down the beach. On sandy beaches, the backwash of the waves removes material forming a gently sloping beach. On shingle beaches the swash is dissipated because the large particle size allows percolation, so the backwash is not very powerful, and the beach remains steep. Cusps and horns form where incoming waves divide, depositing sand as horns and scouring out sand to form cusps. This forms the uneven face on some sand shorelines.
There are several beaches which are claimed to be the "World's longest", including Praia do Cassino (254 km), Cox's Bazar, Bangladesh (120 km), Marina Beach in Chennai, India, Fraser Island beach, 90 Mile Beach in Australia and 90 Mile Beach in New Zealand (88 km), Troia-Sines Beach (63 km) in Portugal and Long Beach, Washington (which is about 40 km).