Moment summation is the most satisfactory method for computing skewness of distributions which are mixtures of two or more normal populations. Beach and dune sands are often characterized by skewed grain-size distributions because two or more normal populations are present. It is shown that the failure of some workers to discriminate between these two groups of sand, in terms of skewness, is partly a result of the method used to calculate this parameters. Measurements of New Zealand sands show beach sands to be characteristically negatively skewed, and dune sands to have positive skew. When the test was applied to Pleistocene strand deposits, it was found that the resolving power fell off as the degree of lithification increased.