The transformation of the ferruginous clay (glauconia) at the base of the ochres of the Provençal Colorado of Rustrel, its coloring, and its chromatic fixation are of vegetal origin. Glauconia, first transformed into kaolin (white clays), was indeed colored by hydroxides and iron oxides originating from plants. They were formed by humic acids linked to the decomposition of the leaves of the mangroves that covered the deposit following the rise of the seabed.