Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
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Gravity and Magnetic Maps of the States
Scale: 1 pixel = 1 km. 10-degree grid; Lambert Conformal Conic Projection, Center 32N 88W
US highways are red, interstates are gold. 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.
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.
Periods | Units of undivided age |
Q Quaternary 2 Ma - 0 | ms schist and phyllite |
Qh Holocene 10 Ka - 0 | m4 granite gneiss |
Qp Pleistocene 2 Ma - 10 Ka | m3 migmatite |
T Tertiary 65 - 2 Ma | m2 amphibolite |
Tp Pliocene (Neogene) 5-2 Ma | m1 felsic gneiss and schist |
Tm Miocene (Neogene) 23-5 Ma | cat cataclastic rocks |
To Oligocene (Paleogene) 33-23 Ma | um ultramafic rocks |
Te Eocene (Paleogene) 55-37 Ma | Prefixes |
Tx Paleocene (Paleogene) 65-55 Ma | u upper |
K Cretaceous 145 - 65 Ma | l lower |
J Jurassic 208 - 145 Ma | Suffixes |
Tr Triassic 245 - 208 Ma | Numbers and numbers with letters (2, 3a, etc) represent subdivisions of periods. See the Geologic Map of the United States for specific stratigraphy. |
Pm Permian 286 - 245 Ma | a anorthosite (precambrian); andesite (Tertiary) |
P Pennsylvanian 320 - 286 Ma | b pillow basalt |
M Mississippian 360 - 320 Ma | c continental deposits |
D Devonian 408 - 360 Ma | e eugeosynclinal deposits |
S Silurian 428 - 408 Ma | f felsic volcanic rocks |
O Ordovician 505 - 428 Ma | g granitic rocks. May have number suffixes to distinguish different ages. |
C Cambrian 570 - 505 Ma | gn gneissic rocks |
Z Precambrian Z 900 - 570 Ma | i undivided intrusive rocks |
Y Precambrian X 1600 - 900 Ma | m undivided metamorphic rocks |
X Precambrian Y 2500 - 1600 Ma | mi mafic intrusive rocks |
W Precambrian W 4600 - 2500 Ma | 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 | |
Combinations of symbols refer to transitional or undivided units. For exampleDS refers to undivided Devonian and Silurian rocks. | ' Phanerozoic metamorphic overprint, usually amphibolite grade or above; Paleozoic in Appalachians, Mesozoic and Tertiary in the West. |
Principal sources for data. Links, even those of government agencies, go out of date quickly and are mostly not provided. Search by title and publication number to find links to data.
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Created 23 July 2001, Last Update 11 January 2020