Analysis of bimodal orientation data

Statistical models underlying the analysis of orientation data commonly assume a unimodal symmetric population, such as the circular normal distribution. If the sample distribution is skewed or bimodal, standard procedures usually produce misleading results. Where such situations occur, a mixture of two or more circular normal distributions may be used as the population model. The parameters describing each mode and the mixing proportion may be estimated by the method of maximum likelihood using numerical techniques. This approach is applied to a distinctly bimodal set of cross-bedding data from the Mississippian Salem Limestone of central Indiana.