Is it ensure, insure, or assure?
iron-rich duricrust, an indurated, or hardened, layer in or on a soil. Soil particles are cemented together by iron oxides (such as Fe2O3) precipitated from the groundwater to form an erosion-resistant layer. Often the soil covering is eroded from the surface of the ferricrete layer, which is exposed as a rock surface; parts of old ferricrete layers may remain as remnants of old erosion surfaces. Extensive ferricrete formations, such as occur in West Africa and Western Australia, may provide rich deposits of limonite or hematite, iron ores. The term laterite is often substituted for ferricrete but technically refers to a soil rich in iron oxides and aluminum.