Formation—a [rock unit] useful for mapping and distinguished primarily on the basis of lithologic characters.
Fossil—the remains or traces of organisms buried by natural causes and preserved in the earth’s crust.
[Guide fossil]—a [fossil] which, because of its limited vertical but wide horizontal distribution, is of value as a guide or index to the age of the rocks in which it is found.
Fossiliferous—containing fossilized organic remains.
Fusulinid—a spindle-shaped [foraminifer]: [test] shaped like a grain of wheat.
Gastrolith—highly polished well-rounded pebbles found associated with certain reptilian fossils; “stomach stones.”
Gastropod—a terrestrial or aquatic invertebrate, typically possessing a single-valved, [calcareous], coiled shell; member of class Gastropoda, [phylum] Mollusca: snails and slugs.
Geologic age—the age of an object as stated in terms of geologic time (e.g., a [Pennsylvanian] fern, [Cretaceous] dinosaur).
Geologic map—map showing distribution of [rock] outcrops, structural features, mineral deposits, etc.
Geologic range—the known duration of an organism’s existence throughout geologic time (e.g., [Cambrian] to Recent for brachiopods).