The zone of saturation occurs because water infiltrating the soil reaches an impermeable layer of rocks so that it is not able to penetrate any further into the earth (this impermeable layer is known as an "aquitard" or.    From Wordnik.com.  [Featured Articles - Encyclopedia of Earth] Reference

The zone of saturation occurs because water infiltrating the soil reaches an impermeable layer of rocks so that it is not able to penetrate any further into the earth (this impermeable layer is known as an “aquitard” or “aquiclude”).    From Wordnik.com.  [Aquifer] Reference