Caliche underlies the soils of the northeastern Llano Estacado as single, double, or, in a few places, multiple layers, each consisting of relatively unindurated caliche grading upward into the indurated cap-rock. Laboratory studies show that the caliche is composed of varying but roughly equal parts of calcite and/or, rarely, silica gel, silt-size quartz particles, and small water-filled vacuities, all arranged isotropically in space with the elastics being largely non-adjoining. Physiographic and theoretical considerations indicate that the caliche is a product of long continued deposition and results from subsurface evaporation of soil moisture in an eolian aggrading soil profile. The ultimate source of the appears to have been fine eolian elastics. The caliche, with interruptions, apparently has been forming continuously since its inception in the Pliocene, and its multiple occurrence is a reflection of climatic variations in the Pliocene and Pleistocene.