A map overlaid with the distribution of geological unit (the rock that lies under vegetation and soil) indicated in different colors (and/or patterns) is called a geological map. Geological maps on a small scale often depict lithologies(rock types) or formations (sets of rock units), while larger scale maps may color code by into groups by geological age, or, at an even larger scale, indicatesuper-groups. The colors used to depict each type of rock are standardized to a certain extent. Typically, plutonic rocks like granite or diorite are shown in red, sedimentary rocks like sandstone and mudstone are yellow to brown, metamorphic rock (greenstone, greenschist or amphibolite) are green, and peridotite is depicted in purple. While there are exceptions, a knowledge of that color coding system should allow you to determine the makeup of a region’s geology with a quick glance at a geological map.

Formations and rocks are created slowly, over the course of the earth’s long history. In geology, history, beginning with the Earth’s formation, is divided into a number of period. You are probably familiar with the Jurassic or Cretaceous periods, for example. When their age can be accurately determined, formations can be defined as being a certain number of “million years old” (in fact, that usage is common). Because geologic periods are defined by a significant geologic change (the extinction of a group of species, for example), it is often more descriptive to refer to a formation as “of the Jurassic period.” Histories of human society often take a similar tack—in some situations, “early Renaissance period” is more informative than “in the 14th century.”