As discussed in a previous chapter, there are established conventions for naming rock formations and groups in almost every part of the world. When creating a geological map it is mandatory to follow these conventions so that others will be able to understand what you are saying when you mention a particular formation from a specific group that, of course, is from the Upper Tertiary Period. This all means something to other geologists and so it is best to stick with their program.

Generally, most governments have sent geologists into the field in order to prepare regional geological maps of a particular area of the world. These are the people who describe and name the new formations that are found and the names generally pertain to geographic features already named in the area. The maps are color coded with each color referring to a specific rock formation. While there is always a key to the colors, the person doing the map will also name the formation using a sequence of letters such as: K_{ch which could mean Chimu Formation from the Cretaceous Period. Note that the upper case letter is always the age of the rock formation and the formation name is given in the subscript.}

The age of rock formations is determined by reference to certain marker horizons and is based on the assumption that newer rocks are on top of older rocks. In particular situations this assumption is sometimes challenged but generally it is intuitively followed. By convention the ages of rock types follow these general names and I have shown a table taken from Jeff Poling (1997). There are are lots of sub-classifications as well but these can be regional and so I won’t go into them here. There are monographs specific to the area of interest which will describe the sub-groups.