It s not just sandstones that are permeable. Chalk, made up of the remains of countless tiny shells is also permeable mainly because there are cracks in the rock. The percentage of the rock that is made up of spaces is known as the rock s porosity. Water, that has passed through the soil will continue to move down through the underlying rock until it reaches a level where all the pore spaces are full of water. This level is called the water table.
Water moving in the rock, below the water-table is called groundwater.
Layers of rocks that are permeable and allow a significant amount of water to flow through them are called aquifers. Chalk and particular sandstones, make up the 2 most important aquifers. About a third of Britain s public water supplies come from aquifers, but in some regions of the country it s as much as three quarters. The shape of the water-table follows that of the ground surface. Groundwater flows from areas where the water-table is high, to areas where it s low.