16.3: What is a Geologic Map Legend?

What Is a Geologic Map Legend?

The map legend to a geologic map is usually printed on the same page as the map and follows a customary format (Figure 16.3). 

The symbol for each formation, or unit, is shown in a box next to its name with a brief description. The symbols typically follow superposition and are stacked in an age sequence from oldest at the bottom to youngest at the top. This allows the map reader to quickly assess the relative age of each unit. The geologic age (typically the geologic period) is listed for each unit in the key and uses standard symbols (Table 16.1). The map legend also contains an explanation of the symbols shown on the map, such as the symbols for different types of faults and folds (Table 16.2). The explanations of rock units often follow the map symbol on the map, but for very large maps are often given in a separate pamphlet that accompanies the map. The explanations include descriptions with enough detail for any geologist to be able to recognize the units and learn how their ages were determined.