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60.6: The map's "explanation"

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    It’s not a “legend.” It’s not a “key,” though those are common terms for similar features. On a geologic map, we call the area where the various units are named, described, and their colors and symbols defined by the name “explanation.”

    Consider this example, which is a close-up look at one of the different units depicted on the geologic map of Pennsylvania: Ss is the symbol for the Shawangunk Formation, depicted on the map in a dark brown color. You can read a description for the formation, including its four sub-units, called members, as well as the other geologic formations with which they have been correlated (and scroll around to see a few more examples):

    Figure \(\PageIndex{1}\): 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.

    Various abbreviations are used for the various named portions of geologic time:

    Table \(\PageIndex{1}\)

    Geologic time unit (era or period) Abbreviation Geologic time unit (era or period) Abbreviation
    Archean A Pennsylvanian PP
    Paleoproterozoic X Permian P
    Mesoproterozoic Y Triassic* T\(_R\)
    Neoproterozoic Z Jurassic J
    Cambrian* C Cretaceous* K
    Ordovician O Tertiary* (archaic) T
    Silurian S Paleogene Pg
    Devonian D Neogene Ng
    Carboniferous* C Quaternary Q
    Mississippian M    

    *A few of these are a little confusing: Carboniferous got the “C” first, so Cambrian got a C with a slash through it, “C,” and poor Cretaceous was stuck with “K.” As far as Ts go, Tertiary got the T first, which left Triassic to be symbolized with a T that had a subsidiary R coming off its stem. However, when “Tertiary” was officially stricken from the ICS geologic timescale in 2003, to be partially replaced by the Paleogene, that freed up the solo T, which is now officially designated as symbolizing Triassic. However, a lot of old maps use “T” to mean Tertiary, not Triassic. So that’s confusing! There were already a bunch of Ms and Ps by the time it came to subdivide the Proterozoic, so that’s the reason for the random X, Y, and Z there.

    Various symbols are used to show all these features on the geologic map:

    A disaplay of 15 common symbols from geologic maps. Orientation of strata are shown with a T shape. The top bar of the T is parallel to the strike, and the "stem" of the T is the dip-direction. At its tip, a two-digit number is written: the angle of dip. Vertical strata are indicated with a similar strike line, but it has two smaller dip-tick lines coming off either side of it. Overturned strata also show a similar strike line, but their dip-tick line starts on one side, then loops over to point at hte other side. Horizontal strata are shown by a circle with a cross/plus in the middle of it. Foliation is shown by a strike line plus an open triangular tab in the dip-direction (instead of a straight "tick" line). Vertical folation is shown with a similar strike line but two equal open triangular tabs on either side. The hinge of an anticline is a line (straight or curved) with two arrows projecting away from it. The hinge of a syncline is also shown with a line, but now the two flanking subsidiary arrows point inward, to the line. If the anticline or syncline is plunging, a big arrowhead is added to the end of the main line, but the flanking arrows remain the same. Linear features are shown with a small arrow to show their trend direction and a two-digit number at the showing the angle of plunge. Strike-slip faults are shown by a heavy line, with half arrows on either side, pointing in opposite directions. Reverse or thrust faults are shown by a heavy line with filled-triangular "teeth" on one side - the upthrust side or hanging wall. Normal faults can be shown by a heavy line with straight tick-marks coming off one side (the down-thrown side or hanging wall) or by tick marks with a small "ball" on the end, or with U and D labels (U is for "upthrown" side and D is for "down-thrown" side). Finally, geologic contacts are shown with heavy lines, solid where known for sure, and dashed where approximate or inferred.
    Figure \(\PageIndex{2}\): A summary of some different symbols used on geologic maps.

    This page titled 60.6: The map's "explanation" is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Callan Bentley, Karen Layou, Russ Kohrs, Shelley Jaye, Matt Affolter, and Brian Ricketts (VIVA, the Virginia Library Consortium) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.