Two major standards for geologic map unit symbolization are the International Commission on Stratigraphy and the American standard. In general, younger map units are given light colors and older map units are given dark colors. However, not all organizations apply this methodology. Traditionally the legend is displayed with the youngest map unit at the top and the oldest map unit at the bottom. Often, the agency from which the data are received has a color and symbolization standard that should be asked for in conjunction with the data.
Lithology (type of rock) codes will have optional subscripts. These should be separated into their own attribute column so that they can be displayed on the map (if labels are desired) as subscripts. The subscript labels require a bit of advanced labeling knowledge.
See the USGS Suggested Colors for Geologic Maps (pdf) for more information as well as the International Commission on Stratigraphy.
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