An alkali sink is a salty basin land form. The term may also refer to a North American desert vegetation type (biome) characteristic of that landform. Rainwater drains to the basin and collects in areas where it cannot penetrate the soil due to a layer of clay or caliche, producing a pond or lake. When the water evaporates, it leaves behind increasing amounts of salts in the soil. Plants that tolerate the extreme salt concentrations are known as halophytes. It is generally below the saltbrush scrub vegetation type, which is typified by less salt tolerant species than alkali sink types.