Geologic Map of (Area)

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
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Geology with One-Degree Grid

Geology with County Lines

Geology with Highways

US highways are red, interstates are gold. Note that highways are bordered in black. Plain red lines are faults. Refer to a road map for route numbers - this map is busy enough without them! I recommend using the plain map for geologic interpretation, with this map used for more specific location information.

Topography

Legend

Marine refers to well-stratified rocks mostly (but not entirely) ofmarine origin. Eugeo refers to eugeosynclinal rocks, that is, deep-watersedimentary rocks of continental slope or trench origin. Other headings areself-explanatory.

There are about 160 lithologic units on the Geologic Map of the United Statesby King and Beikman, counting units with metamorphic overprint. The 256 colorson an 8-bit color palette are more than enough to show these, but many of thecolors are very hard to distinguish by eye. Colors were chosen to minimizeconfusion as much as possible, but inevitably there will be adjacent colors thatare hard to tell apart. To improve contrast, a few colors have been duplicatedfor units widely separated in space and time. For example, colors for earlyPaleozoic volcanic units (found only in the Appalachians) have also been usedfor some units in the far West.

Some periods are divided in some locations and undivided in others. Undividedperiods generally use the middle color for the period. In practice this seems toresult in little confusion. If adjacent units are other divisions of the period,the color represents a subdivision. If adjacent units are different periods, theunit is undivided.

Symbols

Periods

• Q    Quaternary    2 Ma - 0
  • Qh    Holocene    10 Ka - 0
  • Qp    Pleistocene    2 Ma - 10 Ka
• T    Tertiary    65 - 2 Ma
  • Tp    Pliocene
  • Tm    Miocene
  • To    Oligocene
  • Te    Eocene
  • 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 exampleDS 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
• 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
• 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.

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Created 23 July 2001, Last Update 18 January 2020