Several thousand meters of clay from the Cretaceous fill the Tunisian Trough, laden with ammonite fossils and calpionellids, an extinct species of single celled organisms, as well as sand and calcareous deposits. In central Tunisia, the Meloussi Formation is made up of marine sandstone and carbonates. The Boudinar Formation comprises fluvial sandstones with poor sorting and the Gafsa Group contains clay, sand and carbonation sequences.