In the Waipaoa River catchment, clay minerals were used to identify the source rocks contributing appreciable quantities of fine sediment to the floodplains of the Lower Waipaoa River valley. The conclusion arrived at is that all easily erodible rocks contribute material and that sediment is not all derived from rocks showing spectacular forms of erosion. The clay mineral, montmorillonite, is present in all rocks of the catchment and is partly responsible for the high degree of erosion in the crushed argillites at the headwaters of Waipaoa River.