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60.7: Examples to help train your eye

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    22865
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    Example \(\PageIndex{1}\): Horizontal strata

    When strata are horizontal, you only see the top layer unless erosion has cut down through it (in stream channels, say), exposing deeper layers. The overall pattern is that the geologic contacts follow the contour lines, and don’t vary much in elevation. In areas of dendritic drainage, such as the Atlantic Coastal Plain, the Great Plains, or the Colorado Plateau, this produces a distinctive branching pattern to the geologic map, as in these two examples:

    Figure \(\PageIndex{1}\): Bick, K.F., and Coch, N.K., 1969, Geology of the Williamsburg, Hog Island, and Bacons Castle quadrangles, Virginia: Virginia Division of Mineral Resources, Report of Investigations 18, scale 1:24,000. Explore more here.
    Figure \(\PageIndex{2}\): Sawyer, J.F., and Fahrenbach, M.D., 2011, Geologic map of the Lemmon 1-degree x 2-degree quadrangle, South Dakota and North Dakota: South Dakota Geological Survey, Geologic Quadrangle Map GQ250K-1, scale 1:250,000. Explore more here.
    Example \(\PageIndex{2}\): Angular unconformities

    Angular unconformities are as distinctive on geologic maps as they are in cross-section: two sets of formations meet each other at an abrupt angle. The lower, older formations will typically be folded or tilted, while the upper, younger formations will typically be close to horizontal.

    Figure \(\PageIndex{3}\): Haley, B.R., Glick, E.E., Bush, W.V., Clardy, B.F., Stone, C.G., Woodward, M.B., and Zachry, D.L., 1993, Geologic map of Arkansas: U.S. Geological Survey, scale 1:500,000. Explore more here.
    Example \(\PageIndex{3}\): Folds

    So how do you recognize anticlines vs. synclines on geologic maps? There are two main techniques: (1) look for strike and dip symbols, and (2) look for age patterns. In an anticline, the strata dip outward away from the axis (hinge) of the fold. The anticline’s oldest strata will appear in the middle, and younger strata will appear on the limbs. In a syncline, the strata dip inward toward the axis (hinge) of the fold. The syncline’s youngest strata will appear in the middle, and older strata will appear on the limbs.

    A diagram showing an anticline plunging into the Earth, before and after erosion. The anticline's hinge plunges to the right. After erosion, this makes a "V" shape that opens to the left.
    Figure \(\PageIndex{4}\): Plunging folds, like this anticline, produce V-shaped outcrop patterns.

    When a fold plunges into the Earth at some angle, its outcrop pattern changes from a series of parallel stripes to a series of nested “V” shapes. As with non-plunging folds (those that have horizontal hinges), in a plunging anticline, the oldest strata are in the middle, and the strata dip outward on the two limbs. And as with a with non-plunging synclines, plunging synclines also have the oldest strata on the limbs, and the strata dip inward toward the youngest strata in the middle.

    In this example, you can tell it’s an anticline, because it has older strata in the middle (Devonian, D) and younger strata on the limbs (Mississippian and Pennsylvanian, M and PP):

    Figure \(\PageIndex{5}\): Berg, T.M., Edmunds, W.E., Geyer, A.R., Glover, A.D., Hoskins, D.M., MacLachlan, D.B., Root, S.I., Sevon, W.D., and Socolow, A.A., 1980, Geologic map of Pennsylvania (2nd ed.): Pennsylvania Geological Survey, Map 1, scale 1:250,000. Explore more here and here.

    In contrast, here (same map, different fold), you can see the pattern is reversed. This allows you to deduce that it’s a syncline, because Mississippian strata are the middle (younger) and Devonian and Silurian strata occur on the limbs of the fold (older).

    Figure \(\PageIndex{6}\): Berg, T.M., Edmunds, W.E., Geyer, A.R., Glover, A.D., Hoskins, D.M., MacLachlan, D.B., Root, S.I., Sevon, W.D., and Socolow, A.A., 1980, Geologic map of Pennsylvania (2nd ed.): Pennsylvania Geological Survey, Map 1, scale 1:250,000. Explore more here and here.

    In this example, you can see the strike and dip symbols more or less parallel to the contacts between the beds, and dipping westward on the west limb, and eastward on the east limb. This tells you it must be an anticline:

    Figure \(\PageIndex{7}\): Reynolds, M.W., and Brandt, T.R., 2006, Preliminary geologic map of the Townsend 30′ x 60′ quadrangle, Montana: U.S. Geological Survey, Open-File Report OF-2006-1138, scale 1:100,000. Explore more here.
    Example \(\PageIndex{4}\): Faults

    Here are two examples showing normal faults...

    The first one is marked with D/U notation showing which side went down and which went up:

    Figure \(\PageIndex{8}\): Wilson, E.D., Moore, R.T., and Cooper, J.R., 1969, Geologic map of Arizona: Arizona Bureau of Mines, scale 1:500,000. Explore more here.

    ...And this one shows the alternate style, with a little dip-tick coming off the fault trace, on the down-dropped side. Sometimes, as with this example, the mapmaker puts a little “ball” at the end of the tick-mark, making it look like a lollypop:

    Figure \(\PageIndex{9}\): Wolfe, E.W., and Morris, Jean, 1996, Geologic map of the Island of Hawaii: U.S. Geological Survey, Miscellaneous Geologic Investigations Map I-2524-A, scale 1:100,000. Explore more here.

    With thrust faults, a saw-toothed pattern is used. The little triangular “teeth” are on the upper block of rock. There are a lot of examples in this map!

    Figure \(\PageIndex{10}\): Mudge, M.R., 1972, Pre-Quaternary rocks in the Sun River Canyon area, northwestern Montana: U.S. Geological Survey, Professional Paper PP-663-A, scale 1:48,000. Explore more here.
    Example \(\PageIndex{5}\): Structural domes & basins

    Unlike anticlines and synclines, structural domes and basins are areas where the strata are flexed around a central point, rather than a line (fold axis or hinge). In map view, they appear like “bull’s eyes” with a series of concentric rings of strata. If the strata dip outward away from that central point, then it is a structural dome. Structural domes expose their oldest strata in their center, while strata get younger toward the outer edges of the structure. The reverse is true for structural basins: they are structurally lowest in the middle, so all the strata dip inward toward that “deepest” part of the structure. This means that structural basins expose their youngest strata in their center, while strata get older toward the outer edges of the structure.

    Here is an example from Utah: the San Rafael Swell is a structural dome. If you zoom in, you’ll see the pale blue strata exposed in the middle are Permian, with strata getting younger (Triassic, then Jurassic, and then Cretaceous) as you work your way outward toward the edges.

    Figure \(\PageIndex{11}\): Hintze, L.F., Willis, G.C., Laes, D.Y.M., Sprinkel, D.A., and Brown, K.D., 2000, Digital Geologic Map of Utah: Utah Geological Survey, Map 179DM, scale 1:500,000. Explore more here.

    Another example, of the opposite situation: here, in northern New Mexico, is a structural basin, with the youngest strata (Tsj) is surrounded by progressively older and older strata (Tn, Toa), into the Cretaceous series (they start with K):

    Figure \(\PageIndex{12}\): Anderson, O.J., and Jones, G.E., 1994, Geologic map of New Mexico: New Mexico Bureau of Mines and Mineral Resources, Open-File Report 408, scale 1:500,000. Explore more here.

    As a friendly reminder, the 1746 Guettard / Buache map of the chalk outcrops around the Paris Basin was another example of a structural basin.

    Now that we’ve covered these basics, it’s time to test yourself...


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