How is a Geologic Map Made?

Title Page
Introduction
What is a Geologic Map?
What is a Geologic Map Used For?
Why are Geologic Maps Important?
Hasn’t It Been Mapped Before?
What Do All Those Lines, Colors,
  and Symbols Represent?
How is a Geologic Map Made?
References

To make a geologic map, geologists roam the landscape, plotting the location of geologic contacts, faults, folds, and other features on aerial photographs, or, occasionally, directly on a topographic base map. Aligned pairs of aerial photos give the geologist a three-dimensional, birdseye view of the landscape, greatly facilitating fieldwork.

Back at the office, information from these photos is digitized and the final geologic map is edited and printed. New field methods are being developed for computerized data capture, but in the end there is no substitute for careful geologic fieldwork.

In trying to explain the process of geologic mapping, one of my favorite analogies is to compare it to putting together a puzzle. To stretch the analogy even further, consider it a three-dimensional puzzle that is missing many pieces, and that is already out of its box (which has, much to your dismay, been thrown away!). You have no picture to refer to, so you start to scan the pieces, becoming familiar with their colors and patterns and perhaps developing an idea of what the picture may be.

In geological parlance, you are doing reconnaissance work, becoming familiar with the rocks and their distribution and perhaps even formulating a working hypothesis as to their overall three-dimensional structure.

Next comes the slow process of piecing things together until patterns and finally a coherent picture appears. To do this, the geologist of course doesn't move outcrops around like the pieces of a puzzle. Rather, geologists map distinct groups of rocks or sediments. By plotting the boundaries, or contacts, between these map units on a topographic map or aerial photographs, the geologist eventually creates a picture that shows the distribution of rocks and sediments.

This "picture" is the geologic map. It may reveal that the rocks have been faulted or folded. It shows the location and type of geologic resources or geologic hazards. Even the sequence of events that lead to the present landscape can be deciphered by examining the relationships among map units.

As with our imaginary puzzle, however, the geologic map (our "picture" of the landscape) is nearly always incomplete. Rocks are commonly incompletely exposed or may have been eroded away. In either case, the geologist must interpret the rock record that remains, making an educated guess as to the true distribution and relationships among rocks and sediments. While many pieces of the puzzle may be missing, geologic maps provide our best window into the landscape's foundation.