Permeability is the ability of a soil to transmit water or air. Permeability or infiltration rate is measured in terms of the rate of water flow through the soil in a given period of time. Sandy soils are highly permeable because water drains rapidly. But soils with relatively small particle sizes are relatively less permeable. For example, water infiltration through a clay soil is extremely slow. This can cause water to accumulate at the surface or within the body of the soil resulting in whter-logging. The amount of moisture capable of percolating completely through the soil is largely dependent upon the permeability of the middle and lower horizons of soil. When either of these is impermeable, the upper layers can quickly become saturated with water, resulting in runoff or lateral movement of water through the soil. Soil erosion is the normal consequence in sloping areas while flat sites may suffer from flooding to various degrees.