The colors of the ochres of the Provençal Colorado of Rustrel are entirely natural and linked to the presence of glauconite, a specific green clay rich in reduced iron. Emerged and exposed to the hot and humid tropical climate of the Cenomanian (between -100 and -94 million years), glauconite underwent a series of chemical reactions. It was first transformed into goethite, which gives its red color to sand, and kaolinite (a white clay).