The small letters indicate either the name of the unit, if it has one, or the type of rock, if the unit has no name. The place where two different geologic units are found next to each other is called a contact , and that is represented by different kinds of lines on the geologic map. The two main types of contacts shown on most geologic maps are depositional contacts and faults . 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.