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REVIEWING MAPS AND CROSS SECTIONS

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					    REVIEWING MAPS AND CROSS SECTIONS

    R      EVIEWING GEOLOGIC MAPS AND CROSS                  Look for obviousproblems, suchas dangling contact
           SECTIONS is much like making them: The                lines, questionably  placedlines, erratic or un-
    reviewer and the author are concernedaboutthe                explainedchangesin formation thickness,and
    samethings. A first concernis that mapsand sections          doubtful fits of contactsand faults to topography
    have all their neededparts-the simple matter of com-         and beddingattitudes.
    pletenessthat can be verified with a checklist. A sec- Make somenotes to yourself in soft pencil on the
    ond, more important concerndeals with accuracyand            review copy-reminders to look more closely.(Do
    logic-why lines are where they are, what they show           not mark on the colored copy.)
    and if they show it correctly, and what interpreta-
    tions these lines suggest.Maps and sectionsmust be         The first step servestwo importants purposes:
    geometricallyconsistentand logical.
                                                                 It gives you a general feeling for the map area-a
                                                                 feeling to be expandedas you review the map and
                                                                 sectionsmore closely.
    RESPONSIBiliTIES                                         2. It helps you gain somelevel of confidencein the
                                                                 map and sections.For example,you gain an im-
       Reviewingmaps from time to time is expectedof all         pressionthat
     Surveygeologists,just as eachof us expectsothers to
    review maps that we have prepared ourselves.The                  The map has few problems,or
    principal responsibility of the technical reviewer is to         It is a good map but needssomeattention, or
    help the author and, so, ultimately to help the Survey            It is a fair map but has many problems, or
    and the users of Survey products. With different eyes             It is seriouslyflawed.
    and different insights, you as reviewer can ferret out
                                      in
    failures of logic or consistency mappingor inter-          Maps and sectionssubmitted for review shouldbe
     pretation and other problemsthe author might fail to    able to stand by themselveswithout reference to
    see. If an interpretation is flawed, tactfully suggest   others, publishedor unpublished.They must be
    reasonable   alternatives.                               legible, neatly drafted on a clearly legible base, and
       The author is obliged to considereachreview           printed on a clearly legible review copy.
     commentand to make any necessary        changes.If sug-
                                         the
    gested changesare unacceptable, author must
     explain why in a marginal note or memorandum.      The
     responsibility of the author to respondto every com-     WHAT TO CHECK NEXT
     ment shouldbe kept in mind by the reviewer in
     decidingwhether or not to commenton minor points        Marginal data
     of contention.                                            Most but not all of the following apply to all maps:

                                                             Spellingand punctuationare correct.
    STARTING         THE REVIEW                              Latitude and longitude, townshipsand ranges are
                                                                   correctly placedand nurnbered.
      As a first step, you as reviewer shouldscanthe         Scaleis appropriatelygiven:
    map, sections,and explanationto get familiar with            Ratio scale.
    the map units and their sequence,  generallithologies,       Rake or bar scale.
    and thicknesses. Find out why the map was made;              Vertical scaleon sections;if exaggerated,must
    learn its intendedpurpose.                                       sayso.
                                                                  Contourinterval.
     Get a general overview of the structure, both from      Magnetic declinationand year, if needed,are correct.
        the map and from the sectionsas interpreted by       Sourceof base map is credited, and projection of base
        the author.                                                map is provided with the credit.

     230      Reviewing maps and cross sections




1
Mapping credit is given:                                                  UNITS
                                                           REVIEWINGTHE MAP
    Authors, assistants, time(s) of mapping, acknowl-
         edgment of work by others are included.             As you go over the map you will be working back
Location or index map is included.                         and forth constantly from the map and sections to
A sketch map showing published maps of contermi-           the explanation. You should carefully consider the
       nous areas is included if appropriate.              "Description of Map Units" and "Correlation of Map
A sketch map showing the reliability of data if appro-     Units," therefore, early in the review. In this way,
       priate. Such a map would show the area for          you will quickly learn the sequence of units, their
       which data were assembled from photogeologic        ages, thicknesses an$ variations, lithologies, sequence
       interpretation, areas of reconnaissance mapping,    of events, and other characteristics that you must
       areas originally mapped at different scales, and    know to adequately evaluate the map and sections.
       areas mapped by different contributing authors.
Title is appropriate and definitive. Includes quad-
       rangle or area name, county (if needed), and        WHAT LOOK FOR IN THE EXPLANATION
                                                              TO
       State or country.                                   Units and symbols for rock units on the map and
Authorship line and year of publication are correct            cross sections are in the explanation and are
       and positioned below title.                             clearly and consistently labeled on map and cross
                                                               sections, in the correlation of map units, and in
Correlation of map units                                       descriptions of map units. Symbols are kept to
                                                               four letters or less.
Unit boxes are complete and accurate in relation to        Descriptions of map units are clear and concise and
   adjacent units and time boundaries.                         follow a consistent format, including
Age brackets are clearly and carefully drawn.                  distinguishing characteristics such as rock type(@,
                                                               color, grain size, bedding, and thickness. Also
Description of map units                                       given are any other distinctive characteristics that
All units shown in boxes are actually on the map.              the map user should know about, such as
All units shown on the map and sections are listed             mineralogy and fossils.
    and described.                                         Descriptions are complete enough to enable a map
Map, sections, and explanation all use the same unit           user to idenlxFy the rock unit in the field and to
    symbols.                                                   permit a reader to compare each unit with the
                                                               same unit elsewhere.
                                                           If a map unit is present only in a small part of the
Map symbols                                                    map area, its description might usefully say
All conventional and special structural symbols shown          where.
    on the map and sections are included in the expla-     If a description includes several units or members
    nation and are adequately explained.                       within a formation, they are described in sequence
Any dashed or dotted lines on the map are explained.           from youngest to oldest, from the top down.
Faults shown on the map are adequately explained as
    to kind and attitude.
Symbols for fault displacements are explained.
Traces of axial surfaces of folds (or symbols for crests   General Guidelines
    and troughs) are accurately plotted and explained.
                                                             Maps and cross sections must be completely and
                                                           clearly drafted. Poorly prepared copy reflects on the
References                                                 author, and the reviewer should not be asked to do
References are complete and in Survey style.               what the author has not done in terms of compilation,
                                                           thoughtful interpretation, and cross-checking.
                                                             Maps and cross sections must be internally consist-
Other                                                      ent. Interpretations shown on maps or on cross sec-
All enclosed areas on the map and sections are             tions must be logical, reasonable, and compatible with
    labeled by symbol or are distinctively colored.        mapped data. Maps and cross sections should fit with
Line weights on the map and sections are distinctive       adjacent, recently prepared maps and cross sections;
    enough to be clearly identified by the illustrator     structural interpretations, new concepts, or serious
    for final drafting.                                    misfits should be explained.

                                                                                Reviewing maps and cross sections   231
What to Look For on the Map and Sections                             4. Normal, reverse, and strike-   Direction and amount of dip
                                                                        slip faults-Continued           (three-point method).
  After reviewing the guidelines that follow, look at                                                  Direction of throw versus drag.
every line on the map and sections. Ask yourself the                                                   Agreement of throw symbols
following questions:                                                                                    and unit offsets.
                                                                                                       Offset of intersecting faults.
Are all lines properly located? Are they topographi-                 5. Thrust faults.                 Same as 4, plus:
   cally compatible?                                                                                   Variation of foot-wall and
Are they compatible with other stratigraphic and                                                        hanging-wall units versus
   structural data?                                                                                     attitude of beds.
Are they logical?                                                                                      Vertical sequence of plates
                                                                                                        versus joints.
                                                                                                       Ramps versus ramp folds or
  Every line on the map is related to other lines and                                                   faults.
to other data; taken together, all must make sense.                                                    Direction of apparent transport
  You cannot review a map well or consistently by                                                       versus erosional trace of
random spot checking. You must work your way                                                            thrust.
systematically over the map area bit by bit, checking                6. Folds.                         Justification of trace of axial
back and forth from map to sections to explanation                                                       surface, crest, or trough as
to make sure that everything fits. Only then, will you                                                   deduced from bedding
have answered the critical questions: Is this a disci-                                                   attitudes.
plined, thoughtful, consistent geologic map; are the                                                   Plunge of fold from bedding
                                                                                                         attitudes and map patterns.
sections properly constructed and are the interpreta-                                                  Dip of axial surface.
tions they show logically supported by the map; is                                                     Offset on faults.
this map a credit to the author and to the Geological
Survey; and is it an appropriate addition to a long                  7. Does the geology of the map    External consistency or expla
                                                                        area make regional sense?       nation for variation.
line of quality geologic maps made by Survey field
geologists?


CHECKLISTS REVIEWING MAPS
          FOR                                                        -Is topographic profile plotted correctly?
                                                                     -Are the lines of section selected to best show the
AND SECTIONS                                                             structure?
                                                                      - Do map lines correctly intersect the profile?
                                                                     -Are apparent dips correctly plotted?
                                                                     -Are all structural features in the line of section
Feature                           Check against                          shown on cross sections?
1. Sequence of stratigraphic      Correlation of map units.
   units and tongues.             Description of map unit.            .Are data near the line of section correctly pro-
                                  Text.                                  jected into the cross section?
                                                                      - Are unit thicknesses compatible with surface
   Thicknesses of stratigraphic   Thickness as measured from             geology?
   units.                          the map.                           - Are folds correctly shown?
                                  Thickness given in text and
                                                                      - Do kinds of structural features in section match
                                   explanation.
                                                                         kinds of structural features on map?
   Contacts.                       Strike and dip on topography       - Do structural features that intersect on the map
                                     for consistency with bedding        near a section also properly intersect on section
                                     attitudes.                           and at correct along-strike or along-plunge
                                   Offsets on faults (map units
                                     move up dip on downthrown
                                                                          depths?
                                     side).

   Normal, reverse, and strike-    Justification by offset, struc-
   slip faults.                      tural deflections, or sheared
                                     rock symbol.
                                   Offset shown by symbols.
                                   Amount of throw along trace
                                     for consistency or merge
                                     into fold.

232      Checklists for reviewing maps and sections
    If two or more sections accompany the map, are          bit of available data, will carefully preserve every at-
      thicknesses of units, dips and throws on faults,      titude, and will clutter the map in the belief that the
      and depths to geologic intersections compatible       more profuse the data the more accurate the map, no
      from section to section? (At this point, a com-       matter how repetitious. A sufficiently detailed geolog-
      plex section might be cut apart and taped back        ic map is one that adequately (1) portrays the geology
      together to restore the section as it was before      of a region at the selected map scale, (2) provides the
      faulting.)                                            factual basis for interpretations shown in accompany-
    Do hanging wall and foot wall match across              ing sections, and (3) reflects disciplined, thoughtful
      faults and is direction of throw the same as in-      fieldwork. If more detail is required in an especially
      dicated by map symbol? By pattern of units off-       complex area, an inset map may be prepared at an
      set on map?                                           appropriate larger scale.
    In restored (cut apart) sections, can holes or             The accuracy and logic of a map reflect the care,
      overlaps be explained reasonably? (Area or            thought, and discipline of the mapper. Carefully
      volume balance.)                                      restored but erroneous cross sections can be drawn
    Do bed lengths in highly folded areas or between        from a map that is wrong.
      major thrust faults match from top to bottom             Even the simplest geologic map shows rock rela-
       in the sequence? Do they match properly be-          tions that have resulted from complex sequences of
       tween restored structural features?                  events. Such sequences must agree on the map and
    If two sections intersect, are all lines correctly      on all accompanying cross sections. A fault that
      plotted on both sections at the vertical line of      moved only in Proterozoic time cannot be shown cut-
       intersection?                                        ting Paleozoic rocks. A fault that is buried beneath
    Does the map support the interpretation shown           alluvium should be shown by a dotted line if the
       on the sections? Are the sections consistent         alluvium appears on the map.
       with what is known regionally?                          Some suggestions for reviewing large-scale maps of
    If the vertical scale is exaggerated, is the exag-      small areas do not apply to small-scale maps of large
       geration warranted?                                  areas because both rocks and structures can and do
                                                            change significantly across large areas, but the fore-
                                                            going suggestions for map mechanics apply equally.
                                                            Beyond that, the map and any accompanying cross
  Maps occasionally are submitted for publication at        sections must be logical, and interpretations in cross
scales larger than needed to adequately portray the         sections must be supported by mapped data. A second
geologic detail. Unless a particular scale is required to   map may be appropriate for showing structural data.
match some local map series, such as the uniform
1:24,000-scale series for Kentucky, the reviewer
should recommend whatever scale best fills the need.
  Appropriate map scales and data density are gener-
ally determined by the kind of geologic map needed to
solve anticipated problems and by the available topo-
graphic base. Some mappers will doggedly plot every




                                                                          Checklists for reviewing maps and sections   233

				
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