Marine refers to well-stratified rocks mostly (but not entirely) of marine origin. Eugeo refers to eugeosynclinal rocks, that is, deep-water sedimentary rocks of continental slope or trench origin. Other headings are self-explanatory.
There are about 160 lithologic units on the Geologic Map of the United States by King and Beekman, counting units with metamorphic overprint. The 256 colors on an 8-bit color palette are more than enough to show these, but many of the colors are very hard to distinguish by eye. Colors were chosen to minimize confusion as much as possible, but inevitably there will be adjacent colors that are hard to tell apart. To improve contrast, a few colors have been duplicated for units widely separated in space and time. For example, colors for early Paleozoic volcanic units (found only in the Appalachians) have also been used for some units in the far West.
Some periods are divided in some locations and undivided in others. Undivided periods generally use the middle color for the period. In practice this seems to result in little confusion. If adjacent units are other divisions of the period, the color represents a subdivision. If adjacent units are different periods, the unit is undivided.
Symbols
Periods
* Q Quaternary 2 Ma - 0
o Qh Holocene 10 Ka - 0
o Qp Pleistocene 2 Ma - 10 Ka
* T Tertiary 65 - 2 Ma
o Tp Pliocene
o Tm Miocene
o To Oligocene
o Te Eocene
o Tx Paleocene
* K Cretaceous 145 - 65 Ma
* J Jurassic 208 - 145 Ma
* Tr Triassic 245 - 208 Ma
* Pm Permian 286 - 245 Ma
* P Pennsylvanian 320 - 286 Ma
* M Mississippian 360 - 320 Ma
* D Devonian 408 - 360 Ma
* S Silurian 428 - 408 Ma
* O Ordovician 505 - 428 Ma
* C Cambrian 570 - 505 Ma
* Z Precambrian Z 900 - 570 Ma
* Y Precambrian X 1600 - 900 Ma
* X Precambrian Y 2500 - 1600 Ma
* W Precambrian W ???? - 2500 Ma
Units of undivided age
* ms schist and phyllite
* m4 granite gneiss
* m3 migmatite
* m2 amphibolite
* m1 felsic gneiss and schist
* cat cataclastic rocks
* um ultramafic rocks
Combinations of symbols refer to transitional or undivided units. For example DS refers to undivided Devonian and Silurian rocks.
Prefixes
* u upper
* l lower
Suffixes
* Numbers and numbers with letters (2, 3a, etc) represent subdivisions of periods. See the Geologic Map of the United States for specific stratigraphy.
* a anorthosite (precambrian); andesite (Tertiary)
* b pillow basalt (Washington)
* c continental deposits
* e eugeosynclinal deposits
* f felsic volcanic rocks
* g granitic rocks. May have number suffixes to distinguish different ages.
* gn gneissic rocks
* i undivided intrusive rocks
* l lake beds; Green River and related units of Wyoming region.
* m Precambrian metamorphic rocks
* mi mafic intrusive rocks
* q quartzite (Cambrian and Precambrian Y only) Precambrian Y quartzite is not shown on the Geologic Map of the United States and is used for the Sioux, Barron, Baraboo and related quartzites of the upper Midwest.
* s syenite
* v mafic and intermediate volcanic rocks
* ' Phanerozoic metamorphic overprint, usually amphibolite grade or above; Paleozoic in Appalachians, Mesozoic and Tertiary in the West.
