Other types of surficial deposits and rocks have poorly interconnected pores and do not allow ground water to flow freely through them. These relatively impermeable deposits are known as aquitards. The top of the zone of saturation is known as the water table. The elevation of the water table controls the direction and velocity that ground water flows. Even beneath the surface of the earth, water flows downhill.
Ground water flows from areas where the water table is elevated towards areas where it is lower.
Ground water flows down water table, perpendicular to the contours of equal elevation of the water table. The topographic contours of the water table are known as piezometric contours and map of these contours is known as a piezometric map. The rate of underground water flow is a great deal slower than the rate of flow of surface water moving down the same slope or gradient. Amherst's Water Supply The Town of Amherst, like many New England towns gets its municipal water by pumping ground water from a sand and gravel aquifer that is located within the boundaries of the town. The sands and gravels in the aquifer, known locally as the Lawrence swamp aquifer were deposited as the glaciers retreated northward at the end of the Pleistocene.