The water table is highest under the highest land areas , and lowest towards the valleys , where it may appear at the surface close to streams, lakes, and marshes. The reason why this occurs is because the water that percolates through the unsaturated zone raises the water table, while seepage in low-lying areas lowers the water table. Sedimentary rocks often exert a strong influence on the amount of soil contained underground. Impermeable rock or sediment is called an aquiclude . Sandstone is an example of a rock type that is capable of transmitting and storing water, while shale is an example of a rock type found in an aquiclude.
Farmers everywhere take advantage of an aquifer locate between two aquicludes.
The pressure found in the water can create free-flowing wells known as artesian wells . In areas where limestone is found below the surface, water may dissolve some of the calcium carbonate to create carbonic acid which further eats away the limestone. The most common subterranean feature found in such areas are caverns , which are interconnected cavities in limestone bedrock formed by the corrosive action of circulating groundwater. On the surface, the dissolving of limestone below can create areas with numerous surface collapses known as sinkholes . These are common features found in southern Georgia and northern Florida.