Sediment size distributions commonly plot, on probability paper, as zigzag lines. The normal (Gaussian) distribution (straight-line plot) is here adopted as the basic type, and zigzag curves are considered as combinations or modifications of normal distributions. Possible modifications include: simple mixing (adding two or more basic components), censoring, truncation, and filtering. Bimodal shallow shelf sands possibly result from mixing of sediments, either from two different sources or carried by two different agencies. It is most important to examine the detailed plot, rather than two or three statistical parameters, such as the mean and the standard deviation, which do not represent the zigzag curve very well.