Strictly speaking a boulder beach is one where the mean clast size meets the formal definition of “boulder” in the terms of the Wentworth grade scale, that is with a mean particle dia-meter of >256 mm (−8 phi) (Wentworth 1922). However, the term is also sometimes used in a general sense to describe beaches where the sediment is a mixture of boulders and large cobbles.
Boulder beaches are found in high wave-energy environments where clasts of these large dimensions are released directly by erosion of bedrock, or where material is delivered to the shore zone by slope movements such as rockfall. In both cases sediment size is a function of joint spacing. Preformed boulders may also be supplied by erosion of Quaternary deposits such as glacial till, and by infrequent high-magnitude river floods.
While a considerable body of literature exists on gravel beaches, (usually concerned with pebbles and small cobbles), beaches of large cobbles and boulders have been neglected. This is partly...