Geological mapping is a core activity of geology. Therefore, it follows that it has been a core function of geological surveys for over 200 years (beginning with William Smith’s 1815 map of England, Wales, and Scotland and here in Illinois going back to the Worthen maps of the 1860s and 1870s). Mapping is foundational to geologic survey objectives to evaluate mineral, water, and other resources in support of economic development, public health, and environmental protection. However, as crucial an activity as mapping has been, it has often been viewed as “less than research” and more akin to a regimented and proscribed activity to “fill in the blanks” on a base map—in other words, not a scientific exercise and not worthy of academic research pursuit. I take issue with that perspective!
When a geologist decides to map a region, they review previous literature, perhaps including older geological maps. Based on this initial review, they have an idea of what might be found once field investigations commence. This is the geologist’s hypothesis. Once investigations begin, every outcrop that is observed, every drill core that is obtained, every line of geophysics that is made, every sample that is collected and analyzed, every water-well or engineering boring that is evaluated, and every iterative computer visualization and draft map that is constructed all contribute to a dynamic and ongoing progression of geological understanding.
This can be quite an exhaustive process. For example, recently completed mapping of Will County, Illinois, relied on data from ~29,000 logs from water wells and engineering borings, 8.4 miles of geophysics, 6,102 feet of exploratory drilling and sampling from 105 locations, and more than 150 investigated outcrops.
Through this process, the geologist has either confirmed or rejected that initial hypothesis, formed an additional hypothesis, or created multiple working hypotheses along the way. Field activity and data collection almost always involve evaluating portions of a mapping area where additional information is needed, and this progresses along with laboratory work and computer visualization until an acceptable measure of predictability of observations emerges. This may involve obtaining samples from a drill core in an area of sparse data and then predicting the outcome with reasonable certainty, otherwise known as successfully testing the hypothesis.
A geological map is never truly complete. Maps always can be updated with new data and interpretations and, depending on intended use, may require very detailed information. Also, a geological map can never be a true 100% representation of reality. The map is always a model of the geologist’s best interpretations based on an understanding of environments of deposition, stratigraphy, sedimentology, unit correlations and ages, paleontology, mineralogy, etc. Based on that understanding, the geologist extends mapping units into those regions of sparse or no data. Declaring “victory,” or being done, occurs when the geologist is comfortable with the outcome.
We have now come full circle. This entire process boils down to explicitly following the scientific method, from initial background research, to forming a hypothesis, to hypothesis testing and formulating often multiple working hypotheses, to analyzing data and drawing conclusions, and finally reporting results through publication of the geological map. It is pure discoverability and research that is rigorously tested and reported, often with peer-reviewed accompanying or off-shoot publications. The outcome of geologic mapping can result in new interpretations about Earth’s evolution—from the origins of the planet to changes in climate that caused the Ice Ages—and can change scientific thought. Mapping has profound influence on our economy and ability to sustain drinking water supplies and other natural resources. This activity remains a central function of the Illinois State Geological Survey.
Learn more about ISGS maps on the survey's website.
If you have a geoscience/geology question, or have a specimen of a rock, mineral, or fossil that you'd like to share, Ask the State Geologist!