Part 7

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?

The occurrence and distribution of rock and mineral deposits in Illinois are studied by the State Geological Survey Division of the Department of Registration and Education (fig. 20). In seeking new information concerning the state’s natural mineral resources, physical and chemical studies of useful rocks, minerals, and mineral products are made. Techniques and methods are developed and experiments are conducted to find and prepare Illinois’ mineral substances for use.

The results of the Survey’s field and laboratory findings are published and made available to all citizens upon request. Much information that has not been published is in the open files of the Geological Survey where it may be examined by the public. Members of the Survey’s scientific and technical staff answer requests for information received by letter, telegram, telephone, or personal interview.

Since the main ideas of geology are easy to understand and greatly increase one’s enjoyment and appreciation of the world in which we live, the Geological Survey publishes popular and educational booklets designed for the use of Illinois teachers and students. The Survey also distributes a labeled collection of rocks and minerals for class use in Illinois schools. Six geological science field trips are conducted each year throughout various sections of the state for teachers and interested laymen. Members of the Survey staff give illustrated lectures to organized groups about the geology and mineral resources of the state.

WHAT IS THE CHARGE FOR THESE SERVICES?

All of the Survey’s publications are distributed free to schools and teachers. Only topographic maps and certain base maps, which are prepared and printed by the U.S. Geological Survey in Washington, are sold at nominal prices.

Era General Types of Rocks Period or System and Thickness Epoch

CENOZOIC “Recent Life” Age of Mammals Quaternary 0-500′ Pleistocene or Glacial Age Recent—alluvium in river valleys Glacial till, glacial outwash, gravel, sand, silt lake deposits of clay and silt, loess and sand dunes; covers nearly all of state except northwest corner and southern tip Tertiary 0-500′ Pliocene Chert gravel; present in northern, southern, and western Illinois Eocene Mostly micaceous sand with some silt and clay; present only in southern Illinois Paleocene Mostly clay, little sand; present only in southern Illinois MESOZOIC “Middle Life” Age of Reptiles Cretaceous Mostly sand, some thin beds of clay and, 0-300′ locally, gravel; present only in southern Illinois PALEOZOIC “Ancient Life” Age of Amphibians and Early Plants Pennsylvanian Largely shale and sandstone with beds of coal, 0-3,000′ limestone, and clay (“Coal Measures”) Mississippian Black and gray shale at base; middle zone of 0-3,500′ thick limestone that grades to siltstone, chert, and shale; upper zone of interbedded sandstone, shale and limestone Age of Fishes Devonian Thick limestone, minor sandstones and shales; 0-1,500′ largely chert and cherty limestone in southern Illinois Age of Invertebrates Silurian Principally dolomite and limestone 0-1,000′ Ordovician Largely dolomite and limestone but contains 500-2,000′ sandstone, shale, and siltstone formations Cambrian Chiefly sandstones with some dolomite and shale; 1,500-3,000′ exposed only in small areas in north-central Illinois ARCHEOZOIC and PROTEROZOIC Igneous and metamorphic rocks; known in Illinois only from deep wells

Illinois State Geological Survey Educational Series 9

Transcriber’s Notes

—Silently corrected a few typos.

—Retained publication information from the printed edition: this eBook is public-domain in the country of publication.

—In the text versions only, text in italics is delimited by underscores.