Abstract Caliche is used as a genus of soil-mineral accumulations, including calcareous, siliceous, ferruginous, aluminous and nitrogenous varieties in young, mature, and aged stages. Calcareous caliche (some travertine, sinter or tufa), quartzite, chalcedony, opal, iron oxides and hydrated oxides, kaolinite, bauxite, and laterite occur as caliches. Prolonged leaching of the surface soil on young plains and peneplains, with deposition of leached minerals in a constantly descending zone 3–10 feet underground, forms horizontal layers of caliche minerals to produce a formation, the duricrust, of surficial, continental origin, transgressing older beds. Climatic zonation of caliches is noted. Soil carbonates accumulated in semi-arid, less soluble soil minerals in humid, zones. After strong leaching removes carbonates, less soluble caliches accumulate even in the arid zones. Desert caliches are tough, including the glazes. The Reynosa formation consists of 85 feet of upper caliches, alluvial sands, silts, and gravels formed on a post-Oakville or post-Lagarto plain, with several soil (caliche) beds. The Lower Reynosa is slightly thicker (not 600 or 1,500 feet, as some report), including gray sandstones, gray clays and conglomerates with gravel and tufa pisolites. Compact pisolitic tufa of the lower beds with land snails is of spring origin (associated with faults?). The Reynosa, including outliers up the dip, transgresses Lagarto to Cretaceous beds. The Pliocene-Pleistocene contact may separate the upper and lower divisions. The main body of the Reynosa, capped by caliche, holds up the Reynosa Plateau or cuesta, which has a mature karst topography of knolls and basins floored in the beds overlapped by the caliche. Few rivers cross the plateau. Porous sands and gravels of the Reynosa furnish water for wells and feed many streams which head in the plateau or at its east foot. The caliche obscures and confuses stratigraphy and interferes with geophysical prospecting. Karst basins ( palanganas ) are not diastrophic in origin but may be modified by "structure."