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:
It gives you a general feeling for the map area-a
feeling to be expandedas you review the map and
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
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 If an interpretation is flawed, tactfully suggest others, publishedor unpublished.They must be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-
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.
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
Mapping credit is given: UNITS
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
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
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
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
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
Normal, reverse, and strike- Justification by offset, struc-
slip faults. tural deflections, or sheared
Offset shown by symbols.
Amount of throw along trace
for consistency or merge
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