We use a sedimentological approach to examine the formation and deformation processes associated with the accumulation of shell deposits in two major clusters of shell mounds, the Weipa group in the monsoonal environment of Albatross Bay in the Cape York Peninsula of northern Queensland, Australia, and the Farasan Islands group in the semi-arid environment of the southern Red Sea sector of Saudi Arabia. The comparison of such disparate case studies is deliberate, intended to highlight generic issues of shell accumulation and degradation irrespective of the taxonomic composition of the shells or cultural and environmental histories. It also reflects recent fieldwork in both regions conducted in parallel with collaborative arrangements for sharing of ideas and approaches and exchange of personnel in order to establish a common baseline for comparison. Comparative analysis of shell composition, fragmentation, and accumulation highlights similarities despite the different cultural and environmental contexts of the two case studies. These similarities suggest that the size and form of shell deposits are altered by a combination of processes reflecting ongoing changes in deposit composition unrelated to human actions of shell discard. Even where large shell deposits are visible and available for sampling, what is preserved is neither a static reflection of initial deposition nor of undisturbed or “completed” form. We consider the influence of such processes on assessments of rates of deposition and the interpretation of variations in the shape and size of shell deposits.