When people describe Illinois as being flat, what exactly do they mean? What is flat? Is there a way to measure flatness? In fact, there are two ways flatness can be measured. 1) It can be measured by the amount of relief, defined here as the vertical difference in elevation between the summits and the lowlands of a given region. 2) It can be measured by the average slope, which is measured with the equation (rise/run) x 100.

Because there are two ways to measure flatness, there are four possible combinations of slope and relief, as illustrated in the following example profiles:

                  Low relief and low average slope                                               High relief and low average slope

                    Low relief and high average slope                                       High relief and high average slope

The landscape of Illinois resulted from the action of glaciers, streams, rivers, and wind. Glaciers eroded the previously existing landscape, and deposited the material elsewhere. Glacial moraines account for many of the high places in Illinois but the highest point in Illinois is a bedrock exposure in an area the glaciers never reached. The low places are generally in the river and stream valleys of the state and the lowest point in the state is at the southern tip, where the Ohio and Mississippi Rivers meet. The following list of county names contains links that will give you much more information about the specific county.