All geologic units are formed over, under, or beside other geologic units. For example, lava from a volcano flows over the landscape, and when the lava hardens into rock, the place where the lava-rock rests on the rocks underneath is a depositional contact . Faults can cut through a single geologic unit. These faults are shown with the same thick line on the map, but have the same geologic unit on both sides. 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. Special geologic maps of the faults known to be still moving are constantly being upgraded here at the United States Geological Survey, as well as by State geological surveys and university researchers. Another kind of line shown on most geologic maps is a fold axis . In addition to being moved by faults, geologic units can also be bent and warped by the same forces into rounded wavelike shapes called folds . A line that follows the crest or trough of the fold is called the fold axis.