Block riffles

These consist of timber blocks, 8 to 12 in (20.3 to 30.5 cm) square, set in transverse rows in a sluicebox; they are arranged so that in contiguous rows the joints are staggered to prevent the development of longitudinal cracks. It is usual to separate adjacent rows by means of a strip of ordinary riffle scantling. Griffith

Source: Dictionary of Mining, Mineral, and Related Terms