The Cretaceous fuller's earth of Southern England has been studied using sedimentological, petrographical and X-ray diffraction techniques, and by 40Ar-39Ar radiometric dating. The results indicate that the ultimate sources of the fuller's earth were basic to intermediate, alkaline pyroclastic materials, perhaps stemming from volcanicity associated with the Mesozoic rifting of the North Atlantic. The Wolf Rock phonolite is considered as a potential pyroclastic source. Deposition occurred under normal sedimentary processes, with partial re-working of land-fall ash, and incorporation of orthoclastic debris.