Aquifers can be thought of as vast underground porous rocks that hold water and allow water to move through the holes in the rock. Aquifers can be composed of different types of earthen materials such as sand, shell, and limestone. Fresh and salt water fill the various sized holes in the rock. Freshwater generally fills the uppermost part of aquifers while salt water is present at greater depths.
In some areas, water in an aquifer is confined or overlain by a thick layer of clay followed by sandy soil extending up to the land’s surface. Where water in an aquifer is confined, the water is under pressure. This pressure allows water to rise in a well above the top of the aquifer and, in some places, above the land’s surface without a pump which results in an artesian well