Abstract

The expanded Udden-Wentworth (U-W) grain-size scale is an important tool for classifying the size of sedimentary particles. However, some of the existing terminology at the coarse end of the scale is problematic. In particular, the current terms “block” and “slab” are used to define size ranges, despite having recognized shape connotations and simultaneous application in particle form (shape) diagrams. The dual usage such of terminology is undesirable and may have contributed to the lack of strict adherence by the earth science community to the upper end of the U-W scale, resulting in part in the plethora of alternative expressions that have appeared in recent years for describing very large clasts. Our solution to this conundrum is to abandon shape-related terms and replace them with an incremental system based on the prefixes meso- and macro- to classify very large boulder size ranges. In this way, both size and shape classification of very coarse sediments can be accomplished satisfactorily without unnecessary overlap or confusion in nomenclature. The term “megaclast” is appropriate to remain as a universal descriptor for all clasts greater than boulder size (i.e., b axis > 4.1 m), as previously defined.