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. Where the original depositional contact between geologic units is preserved, it is shown on the geologic map as a thin line (location 4).

Information provided by: http://geology.wr.usgs.gov