Meteoric water moves down through the horizon and typically leaches clay minerals, iron, and calcite from the soil. The uppermost layer is called the A horizon. A fully developed soil, or mature soil, consists of three layers, or soil horizons (Figure 1). Much of the rich black soil in the midwestern United States was deposited by melting glaciers about 10,000 years ago. Examples include sand left by retreating glaciers and the mud that is left after a flood. Transported soil is deposited by agents such as ice and water and is not derived from the underlying bedrock. When soil is developed from the weathering of the underlying bedrock it is called residual soil. Subsoil underlies the topsoil and contains less organic material. Topsoil is the upper part of a section of loam, has the highest organic content, and is considered to be the most fertile layer. Loam contains sand, silt, and clay with abundant organic material. The quartz sand grains help keep the soil porous, and the clay particles hold water and nutrients for plant growth. Soil contains mostly quartz and clay particles of varying sizes. It can take hundreds of thousands of years to form soil. For example, in Wisconsin the bedrock is covered by up to 120 meters (400 feet) of glacial gravel, sand, silt, and clay-yet only the upper few feet is considered to be soil. Soil can be all or just part of the sedimentary material that covers the bedrock. The layers of weathered particles of earth material that contain organic matter and can support vegetation are defined as
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