Mapping What's Under Illinois

By mapping resources that lie under the surface of the Prairie State, ISGS geologists give direction to Illinois' environmental and economic future

by Anne Mueller

Getting where you want to go often requires the use of a map. Dr. E. Donald McKay and his staff are working to comprehensively map what's below Illinois' surface to give direction to the state's environmental and economic course.

As senior geologist and head of the Geologic Mapping and Framework Studies Group for the Illinois State Geological Survey, McKay has the labor-intensive task of mapping all of Illinois, 100 feet down, on a large scale of 1: 24, 000. Because only 4 percent of the state has undergone such geological scrutiny, McKay sees a largely unexplored frontier in the midst of the Illinois heartland.

"We estimate it takes a geologist a full year to map a 7.5-minute quadrangle, which is an area about 55 square miles in size," McKay says. There are 1, 071 such quadrangles in Illinois, each extending 7.5 minutes in latitude (north to south) and 7.5 minutes in longitude (east to west). Forty-three 7.5-minute quadrangles have been mapped on a 1: 24, 000 scale so far, four of them within the last year.

Production could more than triple to as many as 15 mapped quadrangles a year if a federal program created in 1992 were to operate at full funding levels. However, the National Geologic Mapping Program, which matches state funds used for geologic mapping, has allowed states to share only about $1 million a year out of the $25 million it now is authorized to distribute.

At current funding levels, it's expected to take about 50 years to map the entire state on a 1:24,000 scale. Consequently, the Illinois Geological Mapping Advisory Committee, weighing environmental and societal factors, has determined the order in which areas will be mapped. Such factors as groundwater resources, groundwater protection, lake shore and stream erosion, landfill and waste disposal and coal development planning were given the highest rankings, followed by earthquake risks, coal, oil and gas, sand and gravel, and limestone, dolomite and sandstone resources. The rankings and their distance from population centers were the basis for IGMAC's priorities.

Statewide mapping on a 1:24,000 scale is one of thousands of geologic projects undertaken by the Illinois State Geological Survey since it was established by the state legislature 90 years ago to study and report on the state's geology. In addition to maps, the Survey provides basic geologic data to individuals and agencies that have the responsibility for formulating policies for land use, waste disposal, pollution and construction. Its information is also used by teachers, students and individuals interested in finding out what's below the earth's surface.

Some geologic maps, however, are limited in the amount of detailed information they can convey because very generalized mapping is done on a small scale, such as 1:500,000. Map scale defines the relationship between a measurement on the map and the actual measurement on the ground. Scale is generally stated as a ratio or fraction—1:100,000 or 1/62,500, for example. The numerator, usually 1, represents map distance, and the denominator represents horizontal ground distance. So the scale

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1:100,000 says that any unit, such as 1 inch or 1 centimeter on the map, represents 100,000 of the same unit on the ground. For example, on a 1:500,000 scale, 1 inch on the map would represent 500,000 inches on the ground, or about 8 miles. On a 1:24,000 scale, 1 inch would equal 2,000 feet.

Different scales serve their purposes. Small-scale maps, such as 1:500,000 and 1:250,000, are useful for regional planning or for comprehensive views of large projects. Bedrock has been mapped for the entire state on a scale of 1:500,000, as has Quaternary (glacial) geology, possible groundwater yield, pesticide contamination potential and other maps soon to be used for low-level radioactive waste screening.

Illinois' "stack-unit" map uses a 1:250,000 scale in which 1 inch equals about 4 miles. It depicts the horizontal and vertical distribution of earth material for the entire state down to a depth of 50 feet. It is useful for such applications as assessing the contamination potential of waste disposal practices.

Maps ranging from 1:50,000 to 1:100,000 are considered intermediate in scale and are especially suited to land management and planning. Maps of mineral resources and coal structures are examples of some of the maps available in this range.

Only a small portion of the state—4 percent—is mapped on a large scale of 1:24,000. Beyond determining the type of rock in a given location or where coal or sand and gravel resources are located, McKay says having the entire state mapped in that amount of detail could yield much in the way of environmental benefits.

"Since the 1960s, there's been greater emphasis on the global aspects of geology," McKay says. "Today, people use geological mapping to know where to install septic tanks, find suitable locations for landfills, anticipate foundation conditions for construction projects and to understand what's underground when they're cleaning up waste spills."

In order to create a geologic map, Survey geologists need to put in some long hours. They begin with a search through existing maps and literature, including the hundreds of thousands of well records on file at ISGS. The search will involve looking at well logs, water well borings and any borings that were done for coal tests or by the highway department. The remarks of the individual who did the original field work give the geologists an understanding about subsurface geology of the area being mapped.

If any aerial photographs are available, the geologists will bring them to the field to trace features seen in the photos and confirmed with on-site observations. They'll look for rock outcroppings and exposures in eroded stream banks to determine the kinds and ages of rocks. Where things are concealed by vegetation, buildings or even cities, geologists can use a drill rig to determine the kinds of rocks beneath the surface, where the rocks are dipping and where the movement of a fault has taken place. The goal is to get as much information as possible within the upper 100 feet or so of the earth's surface.

Back in the office, geologists will put their pencils to thin, strong polyester sheets of Mylar and roughly mark in boundaries containing the information they've gathered. Other individuals will put the geologists' information into publishable form. In the case of 1:24,000 scale, 7.5-minute quadrangle maps, the format is 18 inches wide by 24 inches high. McKay says it's a manageable size for publication purposes and the same size and scale used by the U.S. Geological Survey for a series of topographical maps popular among hunters and other outdoor recreationists.

McKay says the amount in detail contained on a 1:24,000 scale map is analogous to an inset map of a city contained on a state highway map. While a state road map depicts interstates, state highways and secondary roads, individual city maps are on a larger scale and capable of depicting major streets and local landmarks.

"Currently, all we have is a kind of 'highway map' for all of the state," McKay says. "What we're trying to reveal to people who need to know is what's really there. Plus people in general have a curiosity about what's beneath their lawn."

Anne Mueller is a staff writer for the Illinois Department of Natural Resources.

48 * Illinois Parks & Recreation * September/October 1995