The water table rises and falls seasonally and from year to year, depending upon the amount of precipitation. Sometimes the water table may fall below the bottom of the well or below the pump. Sometimes overpumping the well or pumping too many wells in a small area lowers the water table sufficiently to check the yield of the well. WHAT IS AN ARTESIAN WELL? An artesian well is one in which pressure forces water to rise in the well above the level where it was found. In some artesian wells water flows out at the surface. Conditions for artesian wells are illustrated in figure 19. The shale above the sandstone and limestone aquifers in the figure is “tight” and does not permit water to escape upward to the level at which it enters the aquifers in the intake areas. The water is under natural pressure. When a well is drilled through the shale and into the aquifers, water rises, seeking its own level. Most of the deep wells in northern Illinois are artesian wells, though few of them flow at the surface anymore. WILL YOU ALWAYS FIND WATER IF YOU GO DEEP ENOUGH? This is generally true. However, in much of the southern two-thirds of Illinois the deeper waters are quite salty. Therefore, only the upper few hundred feet of rocks are worth exploring for water. In the northern third of Illinois, where the deepest water wells are located, fresh water extends in some places to more than 2,000 feet. HOW CAN GROUND WATER BE FOUND? The most effective way of locating a ground water supply is by using knowledge of the geology and ground water conditions—gained partly from study of existing well records—to determine the most favorable areas and depths to drill to. An additional tool that has been used successfully in Illinois is the electrical earth resistivity survey. The resistivity survey attempts to locate buried sand and gravel layers that commonly are sources of ground water. Test drilling is recommended at sites that appear to be underlain by sand and gravel. There is no known method that will positively “find” ground water without drilling. ILLINOIS STATE GEOLOGICAL SURVEY WHY IS GEOLOGY IMPORTANT? Geology and allied sciences are being called upon increasingly to aid in more intelligent and efficient discovery and utilization of mineral wealth. Our national life and welfare are more and more dependent upon the further discovery and proper use of the earth’s natural resources. Illinois is widely recognized by scientists, industrialists, and educators as being outstanding in its promotion of research and industrial development. WHAT ARE SOME OF THE SERVICES AVAILABLE FROM THE SURVEY?