dunes

storm beach

 

Chatter-marked beach cobbles

 

Braewick

Shingle cusps giving way inland to a berm and to storm beach ridges

 

Key sites

Stead of Culswick

 

Bay-head beach

Cusps and horns form where incoming waves divide, depositing fine gravel as horns and scouring out sand to form cusps. This forms the uneven edge of a sandy beach.

Beach

Definition: a strip of land between high and low water mark and made up of deposits of mud, sand or gravel. Bascom (1980) has argued that a beach includes the entire system of sediment set in motion by waves to a depth of ten meters or more off ocean coasts.

Flinn estimates that there are over 100 sandy beaches on Shetland. Many are small voe- or bay-head beaches. The beaches show a mix of shell and silicate sand, with shell content ranging between 10 and 50% in south Dunrossness.

There are several elements to a beach, each of which relates to the processes that form and shape it. The part mostly above water and influenced by the waves is termed the beach berm. The berm has a crest and a face to seaward. At the base of the face, there may be a trough, and further seaward one or more longshore bars: slightly raised, underwater embankments formed where the waves first start to break.

Imbricate storm beach gravels, Bight of Stavaness

The sand or gravel deposit may extend well inland from the berm crest, where there may be evidence of one or more older crests (the storm beaches) resulting from storm waves and beyond the influence of the normal waves. The elongate beach stones are often stacked like bricks facing the waves. At some point the influence of the waves (even storm waves) on the material comprising the beach stops, and if the particles are small enough, winds shape the feature. Where wind is the force distributing the grains inland, the deposit behind the beach becomes a dune.

The shape of a beach depends on whether 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. The combination of the very coarse calibre of storm beaches on Shetland with the high energy wave environment means that storm beach faces here can be very steep indeed.