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Meagan Baalman
A geological map represents ages of rock. By knowing the ages of the rocks one can find anticlines and synclines. Not only are geological maps brightly colored and easy to look at, but those colors show the absolute ages of the rock formations that differ throughout the world. These maps show only the rocks that are outcropping on the surface of the earth.
Geological maps are able to show the subsurface geology by using the rules of tectonic features, the absolute age of rock formations, and relative position of the rocks in sequence. In order to explain what formations are one must begin with the four major time units in which all rocks are classified. These are called eras. An era is subdivided into rock systems that are also called periods. Systems are divided into what is called a series or an epoch and those are divided into groups which are finally separated into formations. Rock units are mapped as formations and groups of formations based on their fossil content or by methods known as Absolute Dating. These can be recognized in various, scattered places around the world.
A Geologic Map lists only the time periods found in the map area-- whatever place the map may be covering. It does not list the rock units in other parts of the world that are excluded on that particular map.