Distinctive associations of third-order, erosional landforms indented into second-order structural forms such as plains and plateaus. They are produced by aqueous dissolution, either acting alone or in conjunction with (and as the trigger for) other erosion processes. Karst is largely restricted to the most soluble rocks, which are salt (360,000 mg/liter), gypsum and anhydrite (2400 mg/liter), and limestone and dolostone (30–400 mg/liter). The numbers denote effective solubility in meteoric waters under standard conditions.

Being so soluble, salt is seen only in the driest places (for example, Death Valley, California), where it displays intensive dissolution topography. Surface gypsum karst is also comparatively rare in humid regions, but its dissolution at depth may produce widespread collapse and subsidence landforms (covered karst); it is often well developed in arid and semiarid areas such as the Pecos River valley of New Mexico. Pure limestones (CaCO₃) are the principal karst rocks, hosting the greatest extent and variety of features. Dolostone (the double carbonate mineral, CaMg (CO₃)₂) is significantly less soluble: the type and scale of dissolutional forms developed in it is normally less than in limestone, giving rise to topographies that are transitional to fluvial (stream-derived) landscapes.