caliche

A hardened layer of calcium carbonate (calcite) that forms at or near the surface in arid and semi-arid soils, where evaporation draws dissolved minerals upward and deposits them as a cement-like crust. Caliche can be inches or feet thick, soft and chalky or hard enough to require a jackhammer. It is the defining subsurface feature of much of the desert Southwest — the reason fence posts won't go in, trees won't root, and water won't percolate. In construction and archaeology, caliche is the layer you hit when you stop digging.