Grain settling velocity distributions of sediments from modern fluvial channel, eolian barchan dune, beach, and tidal flat environments of the United States and Mexico were analyzed using a photo-extinction settling tube. The results revealed the occurrence of bimodal distributions in several specific places: (1) trough cross stratification in fluvial channels, (2) the lower slip face, lower stoss side, and crest of barchan dunes, (3) eolian mega-ripples, (4) lower beach foreshores, (5) beach storm layers, and (6) inner parts of tidal fiats. Possible sorting mechanisms for generating bimodal characteristics of these deposits are suggested and are classified into three categories: a) Mixing of two sorting processes which have different sorting tendencies. For example, mixing of avalanche sorting and projection sorting on the dune slip face is responsible for bimodal sediments of types (1) and (2). b) Special hydraulic circumstances yielding high mobility of the coarse fraction; type (3) and (4) bimodal sands are examples. c) Unusual transportation events. Storm transportation of a coarse fraction into ordinarily coarse-sediment-free environments may be responsible for type (5) and (6) distributions. This study has demonstrated that bimodal distributions of grain settling velocity of sediments could be interpreted by simple and basic sorting processes.

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