Under the force of gravity, the remaining water percolates down and through the soil. How fast water can travel through the soil depends on the soil itself. Materials that allow water to pass through are permeable . Because sandy soils are permeable, water travels quickly, but clay soils do not let water move that fast. The extremely small size of the openings between microscopic clay particles create friction that effectively halts water movement.
Saturated clay is virtually impermeable .
As water travels through the soil it passes through the zone of aeration . This may also be referred to as the unsaturated zone . This region may be moist, but the pores are only partly filled with water. As water continues down through the soil it eventually reaches a zone where all the interconnected openings are filled with water, the saturated zone . The top of the saturated zone is called the water table .