The mineralogy and geochemistry of the clay fraction ( <2µ) of the Quaternary deposits in the Numedal area, southern Norway, have been studied. The Numedal clays contain considerable amounts of illite, but chlorite, vermiculite, mixed layer minerals, quartz, microcline, and acid plagioclase are also usually found. Amphibole is present in most of the samples. Traces of montmorillonite are occasionally found. During weathering the amount of vermiculite and mixed layered illitevermiculite increases at the expense of trioctahedral illite and chlorite. The average chemical composition of the clay fraction ( <2µ) corresponds to that of coarse clays. The clay material is only slightly weathered, and the residual indexes correspond to those of fresh rocks. Comparison of the mineralogical and chemical data shows the physicochemical variations during weathering, transport and sedimentation, and the composition of the parent material to be of significant importance to the clay composition.