# 60.6: The map's "explanation"

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It’s not a “legend.” It’s not a “key,” though those are common terms for similar features. On a geologic map, we call the area where the various units are named, described, and their colors and symbols defined by the name “explanation.”

Consider this example, which is a close-up look at one of the different units depicted on the geologic map of Pennsylvania: Ss is the symbol for the Shawangunk Formation, depicted on the map in a dark brown color. You can read a description for the formation, including its four sub-units, called members, as well as the other geologic formations with which they have been correlated (and scroll around to see a few more examples):

Various abbreviations are used for the various named portions of geologic time:

Table $$\PageIndex{1}$$

Geologic time unit (era or period) Abbreviation Geologic time unit (era or period) Abbreviation
Archean A Pennsylvanian PP
Paleoproterozoic X Permian P
Mesoproterozoic Y Triassic* T$$_R$$
Neoproterozoic Z Jurassic J
Cambrian* C Cretaceous* K
Ordovician O Tertiary* (archaic) T
Silurian S Paleogene Pg
Devonian D Neogene Ng
Carboniferous* C Quaternary Q
Mississippian M

*A few of these are a little confusing: Carboniferous got the “C” first, so Cambrian got a C with a slash through it, “C,” and poor Cretaceous was stuck with “K.” As far as Ts go, Tertiary got the T first, which left Triassic to be symbolized with a T that had a subsidiary R coming off its stem. However, when “Tertiary” was officially stricken from the ICS geologic timescale in 2003, to be partially replaced by the Paleogene, that freed up the solo T, which is now officially designated as symbolizing Triassic. However, a lot of old maps use “T” to mean Tertiary, not Triassic. So that’s confusing! There were already a bunch of Ms and Ps by the time it came to subdivide the Proterozoic, so that’s the reason for the random X, Y, and Z there.

Various symbols are used to show all these features on the geologic map:

This page titled 60.6: The map's "explanation" is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Callan Bentley, Karen Layou, Russ Kohrs, Shelley Jaye, Matt Affolter, and Brian Ricketts (VIVA, the Virginia Library Consortium) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.