Prospecting with Geological Maps


February 16, 2012

The search for potential aggregate sources begins with the proper framework.

By Bill Langer

When I was studying geology in college, one of my professors had us prepare a geologic map of a rather small area near campus. The main goal was to learn some of the elements included on geologic maps. Very little field work was involved, and the accuracy of the map was not important.

On a whim, I included on my map a small, hot pink, irregularly shaped area. I described the map unit as: An imaginary Holocene (modern) deposit — included on the map to liven up an otherwise boring part of the field area.

The professor returned the map to me. He had redrawn the area in the shape of a dinosaur and commented that the imaginary rocks were actually Jurassic age. I responded with a thrust fault (a line with saw teeth on one side) to give the dinosaur a mouth. The professor retaliated with strike and dip symbols (looks like a short capital letter T) as texture on the skin. A few mines and quarries (crossed picks), a swamp, a railroad and highway, and a bunch of buildings, and we had an imaginary dinosaur that would make any third grader proud.

This leads me to the subject of this column — prospecting with geologic maps. Prospecting for potential sources of aggregate commonly begins with desktop studies, and a geologic map commonly provides the framework for those studies.

Geologic maps of the bedrock typically have map units that portray geologic ages and rock types by use of colors and patterns. Geologic age may seem unrelated to aggregate, and map unit names may be confusing. However, geologists familiar with the engineering properties of rocks can convert some geologic maps into more easily understood products that are specifically designed for prospecting for aggregate.

The process is dominantly an exercise in transformation in which the original mapped units are assigned new attributes that relate to the aggregate industry, and even geologic age comes into play. For example, large amounts of high-quality limestone and dolomite are produced in the midcontinent of the United States from rocks between the ages of 505 million and 286 million years old.

Some of the best information is contained in the map unit descriptions of the rock types. Map units representing rocks such as gneiss and limestone might be classified as potential sources of aggregate, whereas shale and marl might be classified as unsuitable for use as aggregate. But not all gneiss and limestone make good aggregate, and not all shale and marl make poor aggregate. Furthermore, geologic map units often have multiple rock types. Some geologic map units range from gneiss to shale or limestone to marl, and these gradational changes usually are not identified by internal contacts within a single map unit on the geologic map.

Finally, inference cannot replace data, and map units cannot arbitrarily be classified or subdivided. The success of the map transformation depends on how closely the properties of the geologic map units relate to the use of the mapped materials as aggregate, how heterogeneous the geologic map units are, and what degree of accuracy and reliability are required. Thus, map transformation requires judgment, and success also depends on the experience, skill, and knowledge of the person transforming the map.

Small-scale geologic maps are available for the entire conterminous United States and may be acquired from the U.S. Geological Survey or state geological surveys. Geologic maps can also be found at some college departments of geology, perhaps even at my alma mater.

Bill Langer is a research geologist who spent 41 years with the U.S. Geological Survey. He can be reached at

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