Longshore size grading on a boulder beach

Andrew Green, Andrew Cooper, Leslee Saltzmann

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Longshore size sorting on boulder beaches has not previously been reported. In a boulder beach comprising beachrock slabs, we report systematic longshore clast size grading. The boulder beach at Mission Rocks, South Africa is deposited on an elevated (þ3 m MSL) shore platform and comprises imbricated clasts up to 5 m in A axis dimension (9 tonnes). The clasts are derived from adjacent intertidal beachrock and eolianite outcrops and are emplaced during high-magnitude wave events. Four distinct downdrift-fining cells are present. Each is 40–50 m long.The sorting is attributed to post-emplacement clast redistribution in which the smallest clasts are transported most frequently (by lower-magnitude storms) and therefore travel farthest downdrift. The updrift cell boundary is marked by boulders (5 m in length) that exceed the transport threshold for all but the most energetic of storms while the downdrift limit contains clasts 0.3 m in length. This mechanism of longshore size sorting does not rely on variations in longshore wave power as does cell development on sand and gravel beaches. The alongshore sorting via multiple high- (and variable-) magnitude events over a period of time, distinguishes these coarse clast deposits from those of single extreme events (whether storms or tsunamis).
Original languageEnglish
Pages (from-to)1123-1128
JournalJournal of Sedimentary Research
Issue number10
Early online date1 Oct 2016
Publication statusPublished online - 1 Oct 2016


  • Boulder beach
  • sorting
  • longshore transport
  • storm
  • tsunami


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