They show the distribution and extent of particular rock types or groups of rock types. Depending on size and scale, the maps may cover large or small areas. Most have brief descriptions of the rock types. Most geologic maps are issued by public or private scientific agencies. The booklets also list libraries in the subject State where USGS reports and maps may be consulted.
Older catalogs are available free of charge at the locations listed above.
Geologic organizations of many States also publish geologic maps, as do many universities and scientific journals. Geologic maps may be located through public or university libraries. Comparing one's own specimens with those in a museum collection can help in identifying them. Most large rock collections are well labeled. Small rock collections abound in libraries, schools, public buildings, small museums, and private homes.