Usually the symbol is the combination of an initial capital letter followed by one or more small letters. The capital letter represents the age of the geologic unit . The most common division of time used in letter symbols on geologic maps is the Period. Here is a geologic map of the Yosemite national Park region in the Sierra Nevada. Remember, just because the map shows a fault doesn t mean that fault is still active and is likely to cause an earthquake.
Rocks can preserve records of faults that have been inactive for many millions of years.
But knowing where the faults are is the first step toward finding the ones that can move . The thin black lines on this map show known faults in the Yosemite area. Geological maps normally accompanied by a color-coded key to the rock types and their ages exposed on the mapped area. Because the geology of different regions can vary considerably, each map will have its own key. Clearly, the effort that goes into the creation of a sound geologic map is much more than a topographic map.