REVIEWING MAPS AND CROSS SECTIONS

R EVIEWING GEOLOGIC MAPS AND CROSS

SECTIONS is much like making them: The

reviewer and the author are concerned about the

same things. A first concern is that maps and sections

have all their needed parts-the simple matter of completeness that can be verified with a checklist. A second, more important concern deals with accuracy and

logic-why lines are where they are, what they show

and if they show it correctly, and what interpretations these lines suggest. Maps and sections must be

geometrically consistent and logical.

Look for obvious problems, such as dangling contact

lines, questionably placed lines, erratic or unexplained changes in formation thickness, and

doubtful fits of contacts and faults to topography

and bedding attitudes.

Make some notes to yourself in soft pencil on the

review copy-reminders to look more closely. (Do

not mark on the colored copy.)

The first step serves two importants purposes:

1

RESPONSIBiliTIES

It gives you a general feeling for the map area-a

feeling to be expanded as you review the map and

sections more closely.

It helps you gain some level of confidence in the

map and sections. For example, you gain an impression that

2.

The map has few problems, or

It is a good map but needs some attention, or

It is a fair map but has many problems, or

It is seriously flawed.

Maps and sections submitted for review should be

able to stand by themselves without reference to

others, published or unpublished. They must be

legible, neatly drafted on a clearly legible base, and

printed on a clearly legible review copy.

Reviewing maps from time to time is expected of all

Survey geologists, just as each of us expects others to

review maps that we have prepared ourselves. The

principal responsibility of the technical reviewer is to

help the author and, so, ultimately to help the Survey

and the users of Survey products. With different eyes

and different insights, you as reviewer can ferret out

failures of logic or consistency in mapping or interpretation and other problems the author might fail to

see. If an interpretation is flawed, tactfully suggest

reasonable alternatives.

The author is obliged to consider each review

comment and to make any necessary changes. If suggested changes are unacceptable, the author must

explain why in a marginal note or memorandum. The

responsibility of the author to respond to every comment should be kept in mind by the reviewer in

deciding whether or not to comment on minor points

of contention.

WHAT TO CHECK NEXT

Marginal data

Most but not all of the following apply to all maps:

STARTING THE REVIEW

As a first step, you as reviewer should scan the

map, sections, and explanation to get familiar with

the map units and their sequence, general lithologies,

and thicknesses. Find out why the map was made;

learn its intended purpose.

Spelling and punctuation are correct.

Latitude and longitude, townships and ranges are

correctly placed and nurnbered.

Scale is appropriately given:

Ratio scale.

Rake or bar scale.

Vertical scale on sections; if exaggerated, must

say so.

Contour interval.

Magnetic declination and year, if needed, are correct.

Source of base map is credited, and projection of base

map is provided with the credit.

Get a general overview of the structure, both from

the map and from the sections as interpreted by

the author.

230 Reviewing maps and cross sections
Mapping credit is given: 

Authors, assistants, time(s) of mapping, acknowledgment of work by others are included. 

Location or index map is included. 

A sketch map showing published maps of conterminous areas is included if appropriate. 

A sketch map showing the reliability of data if appropriate. Such a map would show the area for 

which data were assembled from photogeologic 

interpretation, areas of reconnaissance mapping, 

areas originally mapped at different scales, and 

areas mapped by different contributing authors. 

Title is appropriate and definitive. Includes quadrangle or area name, county (if needed), and 

State or country. 

Authorship line and year of publication are correct 

and positioned below title. 

Correlation of map units 

Unit boxes are complete and accurate in relation to 

adjacent units and time boundaries. 

Age brackets are clearly and carefully drawn. 

Description of map units 

All units shown in boxes are actually on the map. 

All units shown on the map and sections are listed 

and described. 

Map, sections, and explanation all use the same unit 

symbols. 

Map symbols 

All conventional and special structural symbols shown 

on the map and sections are included in the explanation and are adequately explained. 

Any dashed or dotted lines on the map are explained. 

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 

and troughs) are accurately plotted and explained. 

References 

References are complete and in Survey style. 

Other 

All enclosed areas on the map and sections are 

labeled by symbol or are distinctively colored. 

Line weights on the map and sections are distinctive 

enough to be clearly identified by the illustrator 

for final drafting. 

REVIEWING THE MAP UNITS 

As you go over the map you will be working back 

and forth constantly from the map and sections to 

the explanation. You should carefully consider the 

"Description of Map Units" and "Correlation of Map 

Units," therefore, early in the review. In this way, 

you will quickly learn the sequence of units, their 

ages, thicknesses an$ variations, lithologies, sequence 

of events, and other characteristics that you must 

know to adequately evaluate the map and sections. 

WHAT TO LOOK FOR IN THE EXPLANATION 

Units and symbols for rock units on the map and 

cross sections are in the explanation and are 

clearly and consistently labeled on map and cross 

sections, in the correlation of map units, and in 

descriptions of map units. Symbols are kept to 

four letters or less. 

Descriptions of map units are clear and concise and 

follow a consistent format, including 

distinguishing characteristics such as rock type(@, 

color, grain size, bedding, and thickness. Also 

given are any other distinctive characteristics that 

the map user should know about, such as 

mineralogy and fossils. 

Descriptions are complete enough to enable a map 

user to idenlxFy the rock unit in the field and to 

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 area, its description might usefully say 

where. 

If a description includes several units or members 

within a formation, they are described in sequence 

from youngest to oldest, from the top down. 

General Guidelines 

Maps and cross sections must be completely and 

clearly drafted. Poorly prepared copy reflects on the 

author, and the reviewer should not be asked to do 

what the author has not done in terms of compilation, 

thoughtful interpretation, and cross-checking. 

Maps and cross sections must be internally consistent. Interpretations shown on maps or on cross sections must be logical, reasonable, and compatible with 

mapped data. Maps and cross sections should fit with 

adjacent, recently prepared maps and cross sections; 

structural interpretations, new concepts, or serious 

misfits should be explained. 

Reviewing maps and cross sections 231 
What to Look For on the Map and Sections 

After reviewing the guidelines that follow, look at 

every line on the map and sections. Ask yourself the 

following questions: 

Are all lines properly located? Are they topographically compatible? 

Are they compatible with other stratigraphic and 

structural data? 

Are they logical? 

Every line on the map is related to other lines and 

to other data; taken together, all must make sense. 

You cannot review a map well or consistently by 

random spot checking. You must work your way 

systematically over the map area bit by bit, checking 

back and forth from map to sections to explanation 

to make sure that everything fits. Only then, will you 

have answered the critical questions: Is this a disciplined, thoughtful, consistent geologic map; are the 

sections properly constructed and are the interpretations they show logically supported by the map; is 

this map a credit to the author and to the Geological 

Survey; and is it an appropriate addition to a long 

line of quality geologic maps made by Survey field 

geologists? 

CHECKLISTS FOR REVIEWING MAPS 

AND SECTIONS 

Feature 

1. Sequence of stratigraphic 

units and tongues. 

Thicknesses of stratigraphic 

units. 

Contacts. 

Normal, reverse, and strikeslip faults. 

Check against 

Correlation of map units. 

Description of map unit. 

Text. 

Thickness as measured from 

the map. 

Thickness given in text and 

explanation. 

Strike and dip on topography 

for consistency with bedding 

attitudes. 

Offsets on faults (map units 

move up dip on downthrown 

side). 

Justification by offset, structural deflections, or sheared 

rock symbol. 

Offset shown by symbols. 

Amount of throw along trace 

for consistency or merge 

into fold. 

4. Normal, reverse, and strike- Direction and amount of dip 

slip faults-Continued (three-point method). 

Direction of throw versus drag. 

Agreement of throw symbols 

and unit offsets. 

Offset of intersecting faults. 

5. Thrust faults. 

6. Folds. 

Same as 4, plus: 

Variation of foot-wall and 

hanging-wall units versus 

attitude of beds. 

Vertical sequence of plates 

versus joints. 

Ramps versus ramp folds or 

faults. 

Direction of apparent transport 

versus erosional trace of 

thrust. 

Justification of trace of axial 

surface, crest, or trough as 

deduced from bedding 

attitudes. 

Plunge of fold from bedding 

attitudes and map patterns. 

Dip of axial surface. 

Offset on faults. 

7. Does the geology of the map External consistency or expla 

area make regional sense? nation for variation. 

- Is topographic profile plotted correctly? 

- Are the lines of section selected to best show the 

structure? 

- Do map lines correctly intersect the profile? 

- Are apparent dips correctly plotted? 

- Are all structural features in the line of section 

shown on cross sections? 

. Are data near the line of section correctly projected into the cross section? 

- Are unit thicknesses compatible with surface 

geology? 

- Are folds correctly shown? 

- Do kinds of structural features in section match 

kinds of structural features on map? 

- Do structural features that intersect on the map 

near a section also properly intersect on section 

and at correct along-strike or along-plunge 

depths? 

232 Checklists for reviewing maps and sections 
If two or more sections accompany the map, are 

thicknesses of units, dips and throws on faults, 

and depths to geologic intersections compatible 

from section to section? (At this point, a complex section might be cut apart and taped back 

together to restore the section as it was before 

faulting.) 

Do hanging wall and foot wall match across 

faults and is direction of throw the same as indicated by map symbol? By pattern of units offset on map? 

In restored (cut apart) sections, can holes or 

overlaps be explained reasonably? (Area or 

volume balance.) 

Do bed lengths in highly folded areas or between 

major thrust faults match from top to bottom 

in the sequence? Do they match properly between restored structural features? 

If two sections intersect, are all lines correctly 

plotted on both sections at the vertical line of 

intersection? 

Does the map support the interpretation shown 

on the sections? Are the sections consistent 

with what is known regionally? 

If the vertical scale is exaggerated, is the exaggeration warranted? 

Maps occasionally are submitted for publication at 

scales larger than needed to adequately portray the 

geologic detail. Unless a particular scale is required to 

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 generally determined by the kind of geologic map needed to 

solve anticipated problems and by the available topographic base. Some mappers will doggedly plot every 

bit of available data, will carefully preserve every attitude, and will clutter the map in the belief that the 

more profuse the data the more accurate the map, no 

matter how repetitious. A sufficiently detailed geologic map is one that adequately (1) portrays the geology 

of a region at the selected map scale, (2) provides the 

factual basis for interpretations shown in accompanying sections, and (3) reflects disciplined, thoughtful 

fieldwork. If more detail is required in an especially 

complex area, an inset map may be prepared at an 

appropriate larger scale. 

The accuracy and logic of a map reflect the care, 

thought, and discipline of the mapper. Carefully 

restored but erroneous cross sections can be drawn 

from a map that is wrong. 

Even the simplest geologic map shows rock relations that have resulted from complex sequences of 

events. Such sequences must agree on the map and 

on all accompanying cross sections. A fault that 

moved only in Proterozoic time cannot be shown cutting Paleozoic rocks. A fault that is buried beneath 

alluvium should be shown by a dotted line if the 

alluvium appears on the map. 

Some suggestions for reviewing large-scale maps of 

small areas do not apply to small-scale maps of large 

areas because both rocks and structures can and do 

change significantly across large areas, but the foregoing suggestions for map mechanics apply equally. 

Beyond that, the map and any accompanying cross 

sections must be logical, and interpretations in cross 

sections must be supported by mapped data. A second 

map may be appropriate for showing structural data. 

Checklists for reviewing maps and sections 233