Special line symbols indicate faults. One is shown by the strainght heavy line cutting across geologic units on the left side of themap. These and the thinner lines for other contacts between geologic units are solid, dashed, or dotted depending on how completely the contacts are exposed. Symbols give the strike and dip of layers in rock, two measurments that describe their orientation in space. Symbols show the shapes of folded rock layers.
Geologic mapping is much more than a description of the rocks and materials.
Geophysics - Describing the physical condition of rocks and the Earth by determining gravitational and magnetic fields and monitoring seismicity. Isotope geology - Determining the ages of rocks by checking the decay of natural radioactive isotopes and tracing the ancestry of rocks by isotopic characteristics. Paleontology - Fossils are used to determine ages of rocks and to describe the environment where the organisms once lived. Paleomagnetism - Uses weak magnetic fields imparted to rocks when they formed to describe their ages and how they have moved since. The National Cooperative Geologic Mapping Program is responsible for developing geologic maps and coordinating geologic mapping across the country.