The sands are underlain by calcareous hardpan. An area can have layers of sandy soil, fractured bedrock, clay, or hardpan. A red clay loam layer above the hardpan was recognized. A cemented hardpan a few feet beneath the surface restricts roots and water percolation. Strong trade winds blew away most of the topsoil, leaving behind red hardpan dirt. Redder in the surface and denser hardpan than associated Madera soil. Synonyms include ferruginous duricrust, hardpan and ironpan. Winter rains may be necessary to loosen the hardpan soil where some nests are deposited. Its roots break up hardpan surfaces, while simultaneously holding together the soil to prevent erosion. Part of this soil profile includes an underlying clay or iron hardpan. Over-ploughing can lead to the formation of hardpan. When the lakes dry up, they leave a crust or hardpan behind. They are classified as northern hardpan vernal pools. The depth to hardpan ranged from 1 foot to 6 feet. The results are primarily determined by how extensive and / or intractable the hardpan is.