The thickest deposits of phosphorite form in areas characterized by carbonaceous shale and chert. The phosphorite is usually carbonaceous and pelletal, and it is mixed with skeletal matter and phosphatic shells. Deposits may be up to one metre (about 3 feet) thick. Phosphorites also form on stable areas associated with sandstone or shale. These deposits are not carbonaceous but do contain nodules and phosphatized shells. Typical phosphorite beds contain about 30 percent phosphorous pentoxide (P₂O₅) and constitute the primary source of raw materials for most of world's production of phosphate fertilizers (fertilizer). Significant deposits of phosphorites in the United States include the Phosphoria Formation in Idaho and the Monterey Formation in California. Major deposits also occur in the Sechura Desert in Peru. Alteration of phosphorites tends to leach carbonates and sulfides and increase the percentage of phosphorus pentoxide. The Phosphoria Formation, for example, contains about 34 percent phosphorus pentoxide near the surface compared to only about 28 percent at depth.