Geologic maps usually contain a lot of text. As you can see from this example map of New Jersey, nearly half the area of the map will be used to describe all the rock units shown on the map:
Let’s take a closer look at a key.
This is an example of a surficial geology key. All of the units are arranged in chronological order, with the oldest unit at the bottom and the youngest unit at the top. The diagram shows the relative ages of the units and their colors. Units at the same level are of similar age, while units above or below a certain unit are younger and older, respectively. In this case most of the units consist of unconsolidated deposits – this is because a surficial geology map doesn’t focus on bedrock. However, some bedrock does crop out in the field area of the map. Bedrock units are described in a separate section of the map from unconsolidated deposits.
Images: “Geologic Map Key” by USGS FGDC Digital Cartographic Standard for Geologic Map Symbolization, 2006; “Bedrock Geologic Map of Northern New Jersey” by USGS