This soil, typically 6 inches or so deep, was formed over long stretches of geological time as new soil formation exceeded the natural rate of erosion. (treehugger.com)
These encourage soil fauna and flora, improving soil formation and structure and creating more stable systems. (ifoam.bio)
But sometime within the last century, as human and livestock populations expanded, soil erosion began to exceed new soil formation over large areas. (treehugger.com)