42 Stratigraphic nomenclature and description 
STRATIGRAPHIC NOMENCLATURE AND DESCRIPTION 

T HE ROCKS OF THE UNITED STATES are 

classified for mapping purposes by a complex 

scheme of named material and temporal or geologic 

time terms. Most of the time terms (for example, 

Cretaceous) were defined and first used by European 

geologists and have since been accepted by geologists 

in most other parts of the world. As parts of the 

United States were mapped geologically, local 

divisions of rocks were recognized and named in 

individual study areas. Lithology, rather than faunal 

assemblages, is the basis of this local classification, 

principally to aid in recognizing and mapping the 

units so as to summarize the geologic history of each 

area and evaluate the geologic resources. 

The stratigraphic units discussed in this chapter are 

classified into categories and ranks. The first category 

includes "material units" that are called lithostratigraphic, lithodemic, allostratigraphic, magnetopolarity, or pedostratigraphic units in the North American 

Stratigraphic Code (NASC) by the North American 

Commission on Stratigraphic Nomenclature (1983, p. 

852). Lithostratigraphic unit is a term much used outside the United States (International Subcommission 

on Stratigraphic Classification). Some lithostratigraphic units may have faunal boundaries that coincide with lithologic ones. Uncertainties in interpreting 

original definitions of lithologic units, or in correlating 

them with units near or far, have led to many local 

geologic names and to increasing numbers of units 

each year. The second category is the "temporal 

units" which are chronostratigraphic, geochronologic, 

geochronometric, polarity-chronostratigraphic, 

polarity-chronologic, and diachronic units. 

The term "geologic name" has evolved in its application by the Survey since the late 19th century. At 

that time the Geologic Names Committee (or GNC) 

not only considered names for lithologic units and the 

geologic age terms applied to them but also adjudicated (1) the correct use of petrologic terms and 

structural terms and their symbols, and (2) the aptness of colors, patterns, and all other symbols used on 

geologic maps and reports published by the Survey. 

The term "geologic name" as used in STA is the 

name of a defined rock body, or the local name applied to a mapped rock unit. The map unit is recognized by its lithologic content (homogeneous or 

heterogeneous) and its boundaries. It (1) is assigned a 

place within the geologic age sequence, (2) has a 

stratigraphic rank, and (3) is mappable. The unit may 

be formal: that is, it may be defined by standards of 

the presently (1983) accepted code, or may have been 

defined under a previously accepted (1933, 1961, 

1970) code, or it may have been named and found! 

useful before the writing of any code and has gained 

acceptance through common usage. 

Formal geologic names are those that have been 

defined according to the standards of the time in 

which they were introduced. Units named and found 

useful before 1983, the publication date of the most 

recent code, are not discarded just because they were 

not defined according to current standards. Map wits 

may also be informal and not defined by any standard. Any author using informal names should clearly 

distinguish them from formal names. 

GEOLOGIC NAMES UNITS AND THE 

Because the Geological Survey examines various 

aspects of geology throughout the United States, all 

its publications adhere to broadly uniform procedures 

in dealing with the nomenclature and classification of 

rock masses. The responsibility for this conformity is 

delegated to a group of Survey geologists assigned to 

one of the Geologic Names Unit (GNU) staffs in 

Reston, Va., Denver, Colo., and Menlo Park, Calif., 

under the technical guidance of the Geologic Names 

Committee (GNC). 

The Geologic Names Committee was organized in 

1899 to consider all names of geologic formations and 

other divisions of rock classifications, to determine 

whether the names comply with nomenclature used in 

previously published Survey reports, and to reconnmend policy on stratigraphic nomenclature. Members 

of the committee are appointed by the Chief Geologist 

and are responsible to him through authority delegated by the Director. The members of the GNC are 

chosen for their experience and knowledge in the 

science. In addition to the chairman, the committee 

currently consists of geologists chosen from the three 

centers (Reston, Va., Denver, Colo., and Menlo Park, 

Calif .). 

The Geologic Names Committee defines and 

recommends policy and rules governing stratigralphic 

nomenclature and classification in all manuscript$; and 

Stratigraphic nomenclature and description 43 
maps originating in the U.S. Geological Survey and 

concerning the geology of the United States. The 

actions of GNC apply whether these reports and maps 

are to be published by the Survey or by an outside 

organization, and whether they result wholly or partly 

from the work of Geological Survey employees. 

Except for papers intended as abstracts for talks or 

for some papers in the Open-File Report series, every 

manuscript that is written by a Survey author and 

that contains stratigraphic names is read and 

approved by a member of a GNU staff before its 

publication is authorized by the Director. Technical 

problems, such as differences of opinion among 

Survey authors or nonconformance to Survey policy 

or to the North American Stratigraphic Code, are 

referred to the committee for recommended solution. 

The basis of the committee's action is the North 

American Stratigraphic Code (the code) and its 

predecessors. The North American Commission on 

Stratigraphic Nomenclature currently has more than 

20 members chosen from 8 geological societies and 

State and Federal surveys in North America: 

American Association of Petroleum Geologists, 

Geological Society of America, Geological Survey of 

Canada, Geological Association of Canada, Canadian 

Society of Petroleum Geologists, U.S. Geological 

Survey, Association of American State Geologists, 

Asociaci6n MBxicana de Ge6logos Petroleros, Sociedad 

Ge6logica Mexicans, and Instituto de Geologia de la 

Universidad NacionAl Autdnoma de MBxico. Several 

editions of the code have been printed since the first 

one was prepared in 1933; the most recent edition, 

published in 1983, is for sale by the American Association of Petroleum Geologists, Box 979, Tulsa, OK 

74101. Survey authors can get copies from one of the 

GNU offices. Amendments and additions to the code 

are proposed from time to time. They are published 

as Notes or Reports, usually in the American Association of Petroleum Geologists Bulletin. Before proposed changes are adopted by the commission for 

inclusion in the code, comments and discussions are 

invited from the geologic profession. 

The classification and nomenclature of rock units in 

manuscripts resulting from cooperative investigations 

with State geological surveys, or other outside organizations or individuals, may follow the scheme of the 

cooperating organization, with an appropriate statement of explanation. Any classification scheme used 

must have been described in a published report or the 

description must be included in the proposed report. 

Although the Geologic Names Units do not judge 

the validity or use of any name outside the publications of the U.S. Geological Survey, GNU records of 

stratigraphic names do include all formal geologic 

names of the United States as they are used in 

publications. These records are available for reference 

at all times to all geologists inside and outside the 

Survey. A geologist who plans to name a forinal 

stratigraphic unit may reserve the geographic name 

with one of the GNU staffs in Denver, Menlo Park, 

or Reston so that other geologists who may inquire 

about the name can be informed of the first author's 

intention. If that geographic name is already reserved, 

an author then has an opportunity to select another 

suitable name. Proposals for new nomenclature and 

revisions of existing nomenclature are made in 

reports that are published in recognized scientific 

mediums as defined by the 1983 code. Proposals will 

be more quickly accepted if they are discussed with 

peers who work for State surveys, academia, and 

industry as well as with those who work for the 

Survey during the preparation of a report. 

In reviewing manuscripts, GNU staff members 

depend on a file of annotated records, on the framework of policy and objectives as set forth by the GNC 

and its chairman, and on the North American Stratigraphic Code. Each staff member is expected to bring 

significant departures from these guides to the attention of the local committee members or to the chairman of the committee so that problems may be 

resolved expeditiously at a local level. The full committee formulates general policy and advises on 

specific nomenclatural problems. 

CONFORMANCE TO THE CODE 

AND MODIFICATION TO 

STRATIGRAPHIC UNITS 

Four codes (Ashley and others, 1933; American 

Commission on Stratigraphic Nomenclature, 1.961, 

1970; North American Commission on Stratigraphic 

Nomenclature, 1983) have been written to provide 

guidance to scientists using stratigraphic terminology 

and to provide the profession with standards for 

naming, defining, and classifying "rock units and their 

spatial and temporal relations" (North American Commission on Stratigraphic Nomenclature, 1983, p. 847). 

Each code reflects advances in scientific knowledge 

and technology at the time of its preparation. Some 

recommendations in the 1983 code are similar to 

those drawn up in 1903 and published in the :24th 

Annual Report of the Director. Parts of the 1983 

code serve researchers who work with discontinuitybounded sequences (allostratigraphic units), with 

intrusive, deformed, and highly metamorphosed rocks 

(lithodemic units), and with rocks distinguished by 

remanent-magnetic properties (magnetostratigraphic 

44 Geologic Names Units and the Geologic Names Committee 
units). Pedostratigraphic units replace the classification termed "soil-stratigraphic units" in the 1961 

and 1970 codes. The term "lithostratigraphic unit" 

replaces the term "rock-stratigraphic unit" of the 

1961 and 1970 codes. 

Accuracy and clarity are the two main objectives in 

all written and graphic presentations of stratigraphic 

data. A common procedure of the GNU staff and one 

recommended to authors and technical reviewers of 

stratigraphic reports is to place the geologic-map 

explanation and the stratigraphic chart alongside the 

geology section of the text. As the geology section is 

read, it can easily be compared with the other two 

parts. An assessment may be made at that time concerning stratigraphic changes and conformance with 

the code. Questions that arise may be addressed to 

the GNU reviewer and resolved before turning the 

report in for review. 

The stratigraphic nomenclature record file is updated whenever a report that adequately explains the 

reasons for a stratigraphic change is submitted for 

publication. Obviously, many of the changes added to 

the file as accepted by the Survey are those recommended by Survey authors on the basis of their work. 

Because the Survey prefers to have a uniform nomenclature for any one area, a person proposing a change 

should discuss it with a review staff member and with 

peers working near the report area before the report 

is completed. However, preference for uniform 

nomenclature does not preclude differences of interpretations as long as the differences of opinion are 

recognized and clearly documented. 

The primary responsibility of the GNU staff is to 

ensure conformance to the code and proper use of 

formal stratigraphic nomenclature. When necessary, 

biostratigraphic nomenclature is checked by specialists 

in the Survey's Branch of Paleontology and Stratigraphy, and isotopic dates are reviewed by specialists 

in the Branch of Isotope Geology. GNU staff members carefully read the comments by technical reviewers of all major stratigraphic reports. In proposing or 

modifying nomenclature, authors should give careful 

attention to previous and current work of others, both 

within and outside the Survey. Proposals will be more 

quickly accepted if they are discussed with peers. 

The published lexicon volumes should not be quoted 

as the authority in stratigraphic discussions; reference 

should be made to the original article upon which the 

lexicon data are based. 

TEMPORAL AND RELATED UNITS 

Temporal units are used "to establish a time 

framework for the discussion of geologic history" 

(NACSN, 1983, p. 849). The terminology applied to 

temporal units is used beyond the local area; in fact, 

the boundaries of most temporal units have been 

accepted by international agreement. The terms 

"time-stratigraphic" and "geologic time" units of the 

1960 code are designated only as chronostratigraphic 

and geochronologic units respectively in the 1983 

code. 

Units of time may be expressed in years (y or yr) as 

my, m.y., m.yr. for millions of years and as by, b.y., 

b.yr. for billions of years for an interval of time; or in 

annum as ka for kilo-annum, Ma for mega-annum, Ga 

for giga-annum for ages. For example, boundaries of 

the Late Cretaceous Epoch currently are calibrated at 

66 Ma and 96 Ma, but the interval of time represented by this epoch is 30 m.y. (art. 13c of the oode). 

Chronostratigraphic units. These units designate 

the position of the material unit in geologic time. 

Examples of chronostratigraphic terms (listed in order 

of decreasing stratigraphic rank) are Phanerozoic 

Eonothem, Mesozoic Erathem, Cretaceous System, 

Upper Cretaceous Series, Maastrichtian Stage. 

Geochronologic units. Geochronologic units (listed 

in order of decreasing stratigraphic rank) are used to 

designate the uge of the material units within geologic 

time, such as Phanerozoic Eon, Mesozoic Era, Cretaceous Period, Late Cretaceous Epoch, and Maastirichtian Age. Boundaries used for geochronologic terms 

correspond to the time span of a chronostratigraphic 

unit. 

Geochronometric units. Rocks older than 570 Ma 

are divided into geochronometric units (fig. 15). !rheir 

boundaries are usually internationally agreed-upon 

ages. No type localities have been designated. 

Examples are Archean Eon, Late Archean Era. 

Polarity-chronostratigraphic units. The reco'gnized polarity-chronostratigraphic units (listed in 

order of increasing age) are Brunhes, Matuyama, 

Gauss, and Gilbert. 

Polarity chronologic and diachronic units are additions to the 1983 code; no named units have yet been 

designated. 

Lithostratigraphic, lithodemic, allostratigraphic, 

magnetopolarity, and pedostratigraphic units are encompassed in the general term "stratigraphic unit" in 

this text section. References to all articles are those 

of the 1983 code. 

FORMAL NAMES 

The 1983 code carefully lists the requirements for a 

formal geologic unit name (article 3) and states that 

Formal names 45 
the first letter in all words used in the formal geologic unit name is capitalized (arts. 3-16, 22-47, 

55-60 of the code). 

New Names 

Authors planning to define new formal lithostratigraphic, lithodemic, allostratigraphic, magnetostratigraphic, or pedostratigraphic units should read the 

appropriate articles of the code that list the requirements for new names. All formal geologic names are 

compound; they consist of a geographic name and a 

rank term or descriptive term. The geographic 

name-a river, town, or other permanent geographic 

feature-is at or near the place where the geologic 

unit is typically developed. The geographic name for a 

new unit should be referable to an established geographic name printed on a topographic map or on a 

State map, county map, Forest Service map, or other 

map that shows names approved by a national board 

for geographic names (art. 7a of the code). In the 

United States, the appropriate board is the Board on 

Geographic Names (p. 83). The author should determine that the geographic name has not been used for 

another previously named stratigraphic unit. 

If stratigraphic units are to be named in areas 

without named geographic features, a new geographic 

name can be proposed through the Board on Geographic Names. Instructions and application forms for 

proposing new geographic names are available, and 

these forms should be completed by the author at an 

early stage in the preparation of a report. 

The report in which the new stratigraphic name is 

defined should contain 11 basic elements: 

1. Intent to designate or modlfy a formal unit 

2. Designation of category and rank of unit 

3. Selection and derivation of geographic name 

4. Specification of type locality, section, or area 

(preferably on a geologic map with a topographic 

base showing location and mappability of units) 

5. Description of unit 

6. Definition of boundaries 

7. Historical background 

8. Dimensions, shape, and other regional aspects 

9. Geologic age 

10. Correlations 

11. Genesis (where applicable) 

"These requirements apply to subsurface and offshore, as well as exposed units" (North American 

Commission on Stratigraphic Nomenclature, 1983, art. 

3, p. 851). None of these elements can be used as a 

single valid criterion for establishing a new name. 

Previously Defined Names 

When using stratigraphic names that have not been 

adopted by the Survey or that have a complex history 

of controversial or varied use, authors should include 

citations that clanfy their use of the name. The name 

could be referred to, for example, as the Deer Valley 

Limestone of Flint (1962) or Deer Valley Limestone 

(Flint, 1962), or an alternative statement could read, 

"Flint in 1962 named the Deer Valley Limestone." 

To adopt a stratigraphic name that has been defined 

previously but has not been evaluated by the Survey 

for compliance with the code, an author should (a) 

confirm that the 11 elements listed above are attributed to the unit; if they are not, the author should 

complete them and include a statement of intent, such 

as, "This unit, named by Smith in 1970, is here 

adopted," or "here accepted," or "here used" and 

(b) briefly summarize Smith's description of the 

stratigraphic unit. 

Modification of Existing Names 

Existing formal names may be modified if evidence 

for a change is presented in a published report. A 

name is said to be revised if the boundary of the unit 

is changed slightly, if stratigraphic rank is changed, if 

the unit is reassigned to another unit, if a unit is 

placed in a unit of higher rank, or if a unit is divided 

into units of lesser rank. The rank or descriptive term 

may be changed locally or regionally, for example, 

from "sandstone" to "formation." An abandoned 

term should be replaced by another stratigraphic 

name that may or may not be formal; an abandoned 

term may be reinstated. A change in age designation 

could result from new data, such as fossil identifications, isotopic ages, or a physical correlation with a 

dated unit or with a unit between two dated units. 

The areal limits of a unit may be extended by surface 

or subsurface mapping. The following modifications 

are listed in alphabetical order. 

Abandoned names (art. 20 of the code) 

If an author proposes to abandon the name of 

a stratigraphic unit, the author should state why 

(improperly defined or obsolete) and should give the 

name of the unit or units replacing the abandoned 

name. The old name is then referred to in subsequent 

reports as being obsolete, abandoned, or of former 

usage, or it is preceded by a dagger. An author may 

be more prudent to not use a term rather than abandon it, for other workers mapping at different scales 

may find the term useful. By stating the reasons for 

not using a term, the author will allow time to determine if subsequent workers agree that the name is 

invalid (art. 5a). The word "abandoned" should not be 

46 New names 
applied to the name of a unit that is being areally 

restricted. An abandoned name is understood to be 

abandoned everywhere, including its type section, 

locality, or area. 

Areal limits (art. 12 of the code) 

Some stratigraphic units can be recognized and 

mapped over wide areas, such as several States; 

others are limited to very small areas. The name for 

a defined unit may be extended to separately exposed 

rock bodies if they are demonstrably continuous, if 

they merge in the subsurface, or if they were formerly connected. Stratigraphic units should be extended 

away from the type locality (area or section) only as 

far as the significant lithologic features of the unit 

can be recognized. 

Assignment to another stratigraphic unit 

(art. 19c, d of the code) 

Regionally, a named member may extend from one 

formation into another, especially in areas of intertonguing. The component formations of a group may 

also change regionally. An author's discussion of the 

reassignment of a stratigraphic unit should include 

the geographic limits and the reasons for the new 

assignment. 

Change in age assignment 

A report that changes the age of a stratigraphic 

unit must include the evidence for the change. The 

author is reminded that the definition of a stratigraphic unit is "independent of time concepts" (art. 22d, e). 

Thus, a stratigraphic unit can be assigned to two or 

more systems, or to two or more series. The author 

should explain the evidence for age assignments that 

differ from ages reported by previous workers. 

Change in stratigraphic rank 

(art. 19b, c, d of the code) 

Laterally, a formation may become a member of 

another formation, and a formation may become a 

group or vice versa. If a change in stratigraphic rank 

is required, the author should clearly give the areal 

limits of the rank change as well as the reasons for 

the change. Units of group rank may be divisible into 

named units of formation rank or they may be divisible into named and unnamed units. A formation need 

not be divisible into formally named parts. When a 

unit is divided, the original name should not be 

applied to any of the divisions. 

Redescription or change in lithologic designation 

(arts. 17 and 18 of the code) 

Changes in lithologic designation may be necessary 

where the rocks between the upper and lower contacts of a formally defined stratigraphic unit vary 

areally in lithologic composition. Lithologic changes 

over great distances are due to either depositional or 

postdepositional causes. In a sedimentary rock that 

has been metamorphosed, for example, a sandstone 

(Dakota Sandstone) may become a quartzite (Dak:ota 

Quartzite) or a limestone (Leadville Limestone) may 

become a dolomite (Leadville Dolomite); a unit that is 

predominately sandstone may pass laterally into an 

interbedded sandstone and shale sequence (Dakota 

Formation). The lithologic description in the text is 

the place to delineate these changes. A lithologic 

designation is more meaningful than the word "formation"; if, for example, a lithostratigraphic unit is 

predominately sandstone, it should be called a saindstone and not a formation. However, if a unit is a 

heterogeneous mixture of lithologies (for example, 

sandstone and shale beds), the term "formation" can 

be used. Formally named lithodemic units whose 

mineralogic content changes over a large area may be 

changed in designation from place to place (Boulder 

Creek Granodiorite to Boulder Creek Quartz Monzonite). 

Reinstatement of an abandoned name 

(art. 20e of the code) 

An abandoned name rarely needs to be reinstated, 

but the procedure is acceptable if (1) the reinstatement will not cause confusion, (2) the original defhition of the name is still valid, and (3) the geographic 

name has not been applied to another stratigraphic 

unit. These points should be discussed with other 

geologists knowledgeable about the area. A check: of 

the name by GNU staff is also essential. 

Revisions of boundaries or contacts 

(arts. 19a and 23 of the code) 

Revision of the boundaries or contacts of a stratigraphic unit is a complex and difficult change for an 

author to make and for others to accept. Many units 

have been revised several times during their histories 

(fig. 10). Authors must then become acquainted with 

each revision and must state in their reports whose 

revision is followed. If revisions become extremely 

complex, abandonment of the name may be better 

than another revision or, if a unit is valid elsewhere, 

authors may restrict it from their areas of study or 

may simply not use it in their reports rather than 

abandon it. 

The intended purpose of the 1983 code was to 

describe how to name formal geologic units. Only one 

section in the code @. 850-851) compares formal and 

informal geologic unit nomenclature. For all informal 

stratigraphic units, custom dictates that the first 

letter of the first word may be capitalized, but the 

first letter of the second word is always lowercase. 

Informal names 47 
Informally designated geologic units are widely used 

in the geologic literature. Many informal names have 

very important local and, in some instances, regional 

use. A clear distinction, therefore, must be retained 

between these two types of nomenclature if we are 

to communicate ideas to other scientists. If a first 

author clearly states that a term is informal, all 

subsequent users of that nomenclature have a responsibility to follow the first author's designation unless 

another author modifies the term in a published 

report. To convert an informal unit to formal status 

requires all the information about the unit outlined in 

article 3. 

Informal names are those names applied to stratigraphic units that were not defined according to 

standards in use at the time of their first publication. 

In the past, some stratigraphic names that should 

have remained informal because they were not adequately defined and described in a publication have 

become formal through common usage. Failure to 

clearly categorize a name as formal or informal, 

to define and describe a new name, or to fully 

explain the revision of a previously introduced 

name results in difficulty in communication of 

stratigraphic ideas. 

Informal units may have local extent or economic 

value, such as aquifers or coal beds, or they may be 

useful for correlation. Some are designated unranked 

terms within a formally named unit. An informal 

unit may be named for (1) color (Mahogany bed, 

orange marker, red marker); (2) position (lower member, upper unit); (3) lithology (shale member, sandstone unit); (4) type of deposit (terrace gravel, 

alluvium, Anderson coal); (5) letter (J sandstone, 

member A); (6) number (unit 2); or (7) locality 

(Dupuyer Creek unit, Sauk sequence). 

Authors occasionally wish to link informal units 

with geographic localities. If so, the informal units 

must be clearly distinguished from formal units, which 

are always named for geographic localities. If a place 

name is combined with a stratigraphic rank or a 

descriptive term, the word order should be inverted 

to read, for example, sandstone at Pine Creek, member of Pine Creek, or bed near Pine Creek; the need 

for a clear distinction between formal and informal 

nomenclature prohibits the use of an uncapitalized 

rank or descriptive term following the place name. 

However, the place name can be used without 

inverted word sequence if the second word of the 

informal unit is not in the formal nomenclatural 

hierarchy. For example, combine a place name such 

as Pine Creek with an informal term such as assemblage, unit, sequence, interval, layer, rocks, strata, or 

deposits. 

An unnamed part of a formally named stratigraphic 

unit such as the Pierre Shale may be designated informally by using a lithologic term with the geographic 

name (Pierre sandstone) after the formal name of the 

unit has been given. 

Names of intrusive masses of igneous rocks, such 

as dike, sill, stock, pluton, batholith, and laccolith, are 

considered to be form terms and not part of the 

formal stratigraphic classification used by the U.S. 

Geological Survey, though many such features have 

been assigned names. In the descriptive material of 

map explanations and texts, lithology should be emphasized, rather than structural form. The 1983 code 

(art. 40) recognizes such masses as formal lithodemes 

if the requirements of a formally named stratigraphic 

unit are met. 

The terms "facies" and "sequence" are not parts of 

the formal stratigraphic classification scheme and are 

not capitalized, even though some geologists use them 

for names of rocks in local and regional areas. Terms 

such as Catskill facies and Sauk sequence are 

informal. 

The glacial-climate unit classification defined in the 

1960 code is informal. The initial letters of terms such 

as interglaciation and glaciation are lowercase (for example, Wisconsin glaciation, Olympia interglaciation). 

Summary of Formal Versus Informal Usage 

The distinction between formal and informal stratigraphic nomenclature must be kept clear throughout 

reports and maps. Follow guidelines in the North 

American Stratigraphic Code (1983) for formal 

nomenclature. Describe informal nomenclature and 

the style of its use early in a report. For informal 

nomenclature, do not use a geographic name as a subject nominative until the informal status is made clear 

in a report. Stratigraphic rank terms-such as supergroup, group, formation, member, bed, supersuite, 

suite, and complex-should never be used after a 

place or geographic name in informal nomenclature. 

In reports describing informal stratigraphic units, 

rock names must be connected indirectly with geographic names in the abstract, introduction, stratigraphic description, and conclusion of the text and 

in tables, illustrations, and plates. 

Examples of Informal Nomenclature 

Correct use (indirect tuff of Stony Point 

connection): limestone of Doe (1956) 

granite of Baldy Peak 

Correct after informal Stony Point tuff 

status is defined and Baldy Peak granite 

described: 

48 Summary of formal versus informal usage 
Technically acceptable, but formation of or at 

use not encouraged Perry Canyon 

(indirect connection): 

Incorrect use (direct Stony Point member 

connection): Baldy Peak formation 

In a geologic report the stratigraphy of an area is 

usually discussed chronologically, the oldest formation 

first and the youngest last, like the geologic history. 

This usage does not necessarily apply to the order of 

discussing strata penetrated in wells. For all nomenclature changes in all reports, the steps outlined in 

the 1983 code must be followed. In an accompanying 

abstract, a summary of the proposed stratigraphic 

changes may benefit the readers, and persons who 

read the abstract first may be enticed to read the 

full report. 

Terms should be used consistently throughout the 

report. In paragraph headings, on maps, on illustrations, in tables, and in the first use of the name in a 

paragraph, the full formal name should be used. 

The Survey format for a typical geologic-map explanation includes a correlation diagram or sequence 

of map units and a description or a list of map units. 

Design is dictated by modern preparation procedures, 

especially by the use of word processors. 

Correlation or Sequence of Map Units 

The "Correlation of Map Units" (figs. 7, 8) or 

"Sequence of Map Units" is a chart that shows the 

general interrelations of all the mapped units. Lithostratigraphic, lithodemic, allostratigraphic, and 

pedostratigraphic units in correlation diagrams are 

arranged in vertical columns and in chronologic 

sequence, the youngest unit at the top and the oldest 

at the base. This arrangement contrasts with the 

recommended arrangement of texts of geologic 

reports, wherein the oldest rocks are discussed first. 

In the vertical columns, boxes for individual map 

units are usually joined top to bottom, but if the 

individual map units are unconformable, the word 

"unconformity" is inserted between boxes. If two or 

more rock units have the same age, they are placed in 

separate boxes in separate columns but in the same 

horizontal position. The size of an individual box is 

determined by its relation to other units and not by 

its stratigraphic rank or thickness. Where several vertical columns are necessary, the author may wish to 

separate and title the columns by geography (West of 

Pine Creek) or geologic association (Pine Creek batholith) or another scheme. Boxes should be drawn as 

simply as possible, so users can readily distinguish 

map units by symbol and color and can recognize 

relationships of units to one another. Complex intertonguing of stratigraphic units can be shown in a 

separate diagram below the explanation, 

Braces to the right of the map-unit boxes delimit 

the group or supergroup, complex, suite or supersuite, 

series or epoch, system or period, and occasionally, 

erathem or era (in that order, from left to right). The 

series or epoch and system or period braces are 

essential. For consistency, one of these classification 

schemes should be selected-either the position terms 

(series, system) or the time terms (epoch, period)-for 

an individual map explanation. If a relationship is 

unknown or uncertain, a map unit may be enclosed by 

a brace and the uncertainty indicated by a query in 

parentheses at the right side of the brace-for example, Pleistocene(?). For a few units, the designation 

"age uncertain," "age unknown," or "lithologic 

sequence and age uncertain (or unknown)" may best 

describe the placement of such units in the correlation 

diagram. These phrases are placed to the right of the 

boxes, next to the brace. 

The lettering associated with all the braces is placed 

horizontally. Group, supergroup, complex, suite, 

supersuite, and series or epoch terms are in capital 

and lowercase letters (fig. 7). The system (and era, if 

used) is in capital letters (fig. 7). The words "system" 

and "series" or "period" and "epoch" are not shown 

on map explanations (Jurassic, not Jurassic System; 

Upper Jurassic, not Upper Jurassic Series), but the 

words "Group" and "Suite" are given (Glen Canyon 

Group; Routt Plutonic Suite). 

Description of Map Units 

The "Description of Map Units" is an abbreviated 

account of the lithology, color, and thickness of the 

rocks in each unit. Each unit is described in order of 

increasing age and from left to right. The upper 

member of a formation is thus described before the 

middle or lower members. Boxes showing the mapunit symbols are in the left column. The name of the 

stratigraphic unit is followed by its position (usually 

the series term) or age in parentheses to the right; 

for example: 

I Raritan Formation (Upper Cretaceous) 

Epoch terms rather than series terms are suitable 

on map explanations especially on those showing nonStratigraphic unit descriptions 49 
CORRELATION OF MAP UNITS 

Hdocene 

Pleistocene } QUATERNARY 

Lower Jumssic and JURASSIC AND t up,,, TRlASSlC 

i 

I I ( TRlAsslC 

} Lower Triassic 

:* M&Pumwyivm!an I 

I msnvmm 

i Lower Pennsylvanian 

Upper Mississippian 

} MlsslSSrPPlAN 

) Lower Mississippian 

) Upper Cambrian } CAMBRlAN 

Chinle Formation (Upper Triassic) 

Main Met top. dark-red shale and siltstone about 

15 m thick, pale-pink medium-dried sandstone about 5 

) Middle Proterozoic } PROTEROZOIC F' m 

. . ... 

Prv 

DESCRIPTION OF MAP UNITS 

Alluvium (Holocene)-Possibly includes some Pleistocene 

deposits. Pdy sorted silty sand and gravel, ldy cobbly and bouldey. In some places contains appreciable 

organic matter. Maximum thickness a few meters 

Landslide Deposits (Holocene and Pleistocene)-Vey 

heterogeneous mixtures of rock fragments and soil. 

Maximum thickness several tens of meters 

Talus and other colluvium (Holocene and Pleistocene)- 

Talus includes angular hagments of mk below cliffs and 

steep slopes. Colluvium includes slope wash. 

Heterogeneous mixtures of silt, sand, gravel, cobbles, 

and boulders; inteqrades with talus. Maximum thickness 

a few tens of meters 

Bishop Conglomerate (OligocenejLight-gray to pinkishgray very poorly sorted loosely cemented pebbly, cobbly 

to bouldery conglomerate and sandstone. Some boulders 

exceed 2 m in diameter. Boulders and roundstones derived from nearby Paleozoic and Precambrian tenanes. 

Tuffaceous interbeds. Thickness highly varied; may locally exceed 60 m 

Glen Canyon Sandstone (Lower Jurassic and Upper Triassic)-Pink, highly crossbedded fine-grained eolian 

sandstone. About 230-240 m thick, but exposed only in 

a partial section at south boundary of quadrangle 

m thick in Gddk, and varicolo;ed red, yellow, and lavender shale and sandy shale about 52 m thick at base 

Gartm Member4ray to tan to red medium- to coarsegrained pebbly conglomeratic crossbedded sandstone. 

Pebbles mostly gray quartzite and chert Forms cliffs and 

dipslopes. Thickness about 10-18 m 

Moenkopi Formation (Lower Triassic)-Varicolored red 

(predominant), brown, green, and gray (subordinate) 

shale, mudstone, and siltstone. Near base contains lightgray gypsiferous siltstone and shale. As much as 230 m 

thick, but on the poorly exposed flank of the Mud 

Springs monocline may be thinned tectonically or faulted 

to only about 110 m 

Park City Formation (Permian)-Undifferentiated on cross 

sections only 

Upper unit-Mostly soft light-gray, light-greenish-gray, and 

light-yellow thin-bedded shale, siltstone, fine-grained 

sandstone, dolomite, and limestone; poorly exposed. 

Boundary with overlying Moenkopi Formation (Tm) is 

placed at the color change from overall tawny gray below 

to light red above. As mapped, upper unit is as thick as 40 m but may include some beds that could be assigned 

to the Moenkopi 

Lower unit-Light-gray to brownish-gray unevenly bedded 

fine-grained sandstone, sandy cherty limestone, and 

claystone. Forms caprock on cliffs and dipslopes. Thickness about 15-18 m 

Weber Sandstone (Middle Pennsylvanian)-Light-gray to 

yellowish-gray very thick bedded crossbedded finegrained sandstone. Foms massive cliffs and steep bare 

slopes. Thickness about 300 m 

Morgan Formation (Middle Pennsylvanian) 

Upper member-Red fine-grained crossbedded to planarbedded sandstone and interbedded gray to pale-lavender 

cherty fossiliferous limestone. Individual beds less than 

1 m to several meters thick. Pink to red chert nodules 

and lenses. Forms cliffs and ledgy slopes. Total thickness 

about 210-230 m, but not My exposed 

Lower memberShown in cross section only 

Round Valley Limestone (Lower PennsylvanianWhown in 

cross section only 

Doughnut and Humbug Formations (Upper Mississippian)-Shown in cross section only 

Madison Liimestone (Lower Mississippian)-Shown in cross 

section only 

Lodore Formation (U~wr Cambrian)--Shown in cross section only 

- Uinta Mountain Group (Middle Proterozoic)-Shown in cross section only 

- Contact-Dashed where approximately located Fault--Approximately located. Arrows indicate relative movement. Shown in cross sections only 

Fold--Showing trace of axial surface. Dashed where approximately located; dotted where concealed 

,+ Anticline, showing crestline 

,- 

Asymmetrical antidine--Shorter arrow indicates steeper limb 

,+4 Syncline, showing troughline and direction of plunge 

----.f--. Asymmetrical synclindhorter arrow indicates steeper limb 

Synclinal bend of monocline 

.+ Anticlinal bend of monocline 

3- Strike and dip of beds 

Strike of vertical beds 

\\\\\ Trace of master joints 

Figure 7. A simple explanation for a Geologic Quadrangle Map. 

50 Explanations for geologic maps 
CORRELATION OF MAP UNITS 

Figure 8. A moderately complex correlation diagram from the explanation of a ldegreex 

2degree geologic map. 

Explanation for geologic maps 
layered rocks, such as lithodemic units (art. 13d), 

some volcanic rocks, and glacial deposits. 

Repetition of the position or age term with generally 

designated map units-for example "Cretaceous sedimentary rocks" or "Upper Cretaceous sedimentary rocks"- 

should be avoided if the age designation is shown to 

the right-side brace@) of the correlation diagram. 

Symbols for Map Units 

Letter symbols for map units are considered to be 

unique to each geologic map in that adjacent maps do 

not necessarily use the same symbols for the same 

map unit. Some parts of the symbols, however, are 

standardized for all maps. Symbols consist of capital 

and lowercase letters that convey two kinds of information. The capitalized first letter of the symbol 

stands for one or several of the chronostratigraphic, 

geochronologic, and geochronometric units, as shown 

in parentheses on figure 7. 

The lowercase second part of the map-unit symbol 

stands for the rock-unit name. The letter or letters 

are usually chosen from the initial letters of the 

map-unit name, formal or informal, applied in the 

"Description of Map Units." If the map unit has a 

formal name, the geographic or place name is a key 

for part of the symbol. For example, the map symbol 

for the Sevier River Formation where it is of Pliocene 

and Miocene age could be Tsr or Nsr (T for Tertiary 

or N for Neogene). If the map unit has an informal 

name, such as "sandstone at Pine Creek," of Tertiary 

age, the map symbol would be Tpc. The lithologic 

term is usually keyed into the map symbol only when 

a place name is not associated with the map-unit 

name. Quaternary alluvium would be labeled Qa or 

Qal. Jurassic limestone would be labeled JI. Series or 

epoch terms are not indicated in the symbol. Group 

names are seldom indicated either. The total number 

of letters applied to one map symbol should not be 

less than two nor more than four. 

Two or more systems. If rocks in a single map 

unit belong to two systems, the symbol for the younger system is listed first. For example, a Quaternary 

and Tertiary assignment would be shown as QT. If 

one map unit is assigned to more than two systems, 

the symbols for the youngest and oldest systems, in 

that order, can be selected. For example, a Quaternary, Tertiary, and Cretaceous unit could be QK. 

Informal units. Suitable lithologic terms should be 

applied to a map unit that has no formal name and is 

not associated with a specific geographic locality. The 

first letter of the first word of that lithologic term 

should be selected as part of the map symbol; for 

example, the map symbol for Cretaceous sedimentary 

rocks should be shown as Ks or Kr. 

Map symbols in text. Map symbols should not be 

used in the text of a report as an abbreviated method 

of designating a unit. For example, Je should not be 

used in a text as a substitute for the "Jurassic Entrada Sandstone." Inclusion of map symbols in the 

text, in parentheses after the unit name, is useful to 

aid the reader in referring from text to map, and is 

justifiable when several similarly designated mapped 

units are being compared-for example, metasedimentary rocks (units Xd, Xsc, Xcg, and Xsi). 

Units spanning rocks of several ages. If a map 

unit includes rocks of diverse kinds that span a large 

amount of time, the map symbol can be constructed 

by selecting the symbol for the youngest and oldest 

system, and a single lowercase letter for the rock. 

For example, D€r could be used as a symbol for 

Devonian, Silurian, Ordovician, and Cambrian rocks. 

The letter "s" can replace "r" if "sedimentary" is 

part of the map-unit designation. Some circumstances 

may require use of a letter for an era or for several 

eras and a lowercase letter for the rock. Examples 

include CzPzr for Cenozoic, Mesozoic, and Paleozoic 

rocks, and R for Proterozoic rocks. 

Superscripts. Superscript letters and numbers are 

used in some map symbols, especially for Tertiary and 

Quaternary terrace deposits. The youngest and lowest 

terrace deposit is assigned number 1. Older and higher terrace deposits are assigned numbers 2, 3, and so 

forth in order of increasing age. 

Dropped letters. If a mapped area consists entirely of rocks belonging to one system, such as Quaternary or Precambrian, the letter (Q or p€) for the 

system may be dropped as part of the map symbol; for 

example, fg may be used for a Quaternary fan gravel 

or rnb for a Precambrian migrnatitic biotite gneiss. 

CORRELATION CHARTS AND 

The designs of correlation charts (fig. 9) and 

stratigraphic tables (fig. 10) are complicated because 

columns are read in both horizontal and vertical directions. Hierarchy or stratigraphic rank is shown by 

placing the largest or highest rank at the left side of 

each column. The units are listed in order of increasing age, the youngest at the top and the oldest at the 

bottom. If the diagram is to be spread across two 

pages, the author should indicate the top alignment 

of the diagram and should select the position of the 

gutter between the pages. 

Correlation charts differ from stratigraphic tables, 

as the terms are used here, in that a chart usually 

shows an author's interpretation of rock units and 

52 Symbols for map units 
L lnformnl WaMrm lntarior mollu8csn forsil zones 

B nurnbert reprarent tones identified nt vl substage outcrop sections (figs. 29, and 14) 

upper ( 23 ~cophites mntrtmus 

.! ~iddh 1 a inarumus dejormi~ 1 ". 7 

18 Scophrtss nfgrleollens~s 

Yppsr 17 SPphEtm whltflsldt 

16 &aphlteo wamE 

E 

.?! - 

15 Prlonocyclua mmbf 

0 Pl~lamnth~~ wyomingense I 

B 5 Acanthocams amphlbo!um 

E, 

Middle 4 Acanthocema almmd~nss 

3 Amnthecems muldoonense 1 1 -r 1 2 Amntb-gramnse 1 I alvoocrmr srlbartr 

No mollu8can fodt neord 

1 Age of basal contact of Niobrara Formation from Evens (1976, p. 121). 

Figure 9. A correlation chart, showing stages, ages, 

lines indicate a hiatus in the sequence of beds. 

K-Ar ages (m.y.1 1 Osngl area, (Obrndwicb and 

Cobban, 1975) Weston County ' 

.,- 

molluscan zones, and formations at selected localities. Ruled 

This article 

Twist Gulch Formation 

Arapien Shale 

Figure 10. A stratigraphic table showing varied terminologies used for the same rocks. 

Correlation charts and stratigraphic tables 53 
their ages as related to units that other workers have 

recognized elsewhere. A tabIe usually lists the rocks 

in the report area in increasing age from top to 

bottom or gives the historic development of nomenclature use by author and year. The sequence of vertical columns in tables may give the age or position, 

name, thickness, lithology, and other pertinent information on each unit in columns to the right of the 

rock-unit name. 

Time terms are usually placed in the left columns of 

the chart or table (fig. 9). For readability, they may 

have to be repeated on the right side of a large chart 

or table. Diagonal or vertical rules or shading usually 

connote missing rock; wavy lines connote unconformities. All boxes for rock units should be identified by 

name, whether the names are formal (Frontier Formation) or informal (unnamed member). The names 

are usually shown by capital and lowercase letters, 

but the first letter of the first word in each entry, 

whether formal or informal, is capitalized. Abbreviations should be avoided, but if space is a problem, a 

standard abbreviation may be used, or the box size 

may be enlarged to accommodate the lettering, or a 

footnote may be used to identify the rock unit. Rarely, in the columns of large stratigraphic tables, rock 

units are identified by map symbols. Such symbols are 

best explained by headnotes in which the stratigraphic 

units are listed in proper stratigraphic sequence, the 

youngest first. 

If a chart or table is a compilation of age and rockunit assignments from several sources (fig. lo), the 

author may use individual headings at the top of each 

column. If the stratigraphy to be shown is extremely 

complex or poorly understood, the author may generalize by titling the chart or table as "A list of * * * 

(correlation of units shown in each column not 

implied)." 

The stratigraphic order of published measured sections, like tables and map explanations, is opposite 

that of the sequence as measured in the field. The 

youngest unit is listed first, and any divisions of it 

are indented under it or are set in a different type, 

descending in order of increasing age. The beds are 

numbered if needed for reference elsewhere in the 

text or on illustrations (fig. 11). The rock term is 

given first, followed by the descriptive terms (figs. 11, 

12), to emphasize the rock type of each unit rather 

than other attributes, but the device is effective only 

if the emphasized term comes first in the line. Directions for formatting measured sections on a word 

processor are given on page 259. 

BURKE FORMATION (LOWER PART) AND 

PRICHARD FORMATION (UPPER PART) 

Section of lowermost Burke and up rmost Prichard Formations 

measured by ce and wm ass Em outcro s along Mont. 

Highway 28 in 21 N., R. 24fV., and T. 21 N.. 8. 25 W. Sanders 

County, Mont. Base of section is in SW'/rNE% sec. 31, T. 21 N., R. 

24 W., 565 in northeast of Welcome Spring; top of section is op- site entrance to campground at east end of Rainbow Lake. 

Eeasured by E.R. Cressman in August. 1980. 

Thickness CwnuJative 

(meters) thickness 

(meters) 

Burke Formation (lower 

37. Siltite and ara &%:laminated 

and very thinly interbedded; medium 

gray to olive gray (weathers mostly greeeh gray, moderate brown along some jomts); siltite and ar even layers 0.5-5 cm thick; s fPb t layers exhibit loadcasts at base and grade into overlying ar te layers ------ 36. Poorly expos4 &few small expo- sures are similar to unit 33 -------- 35. Quartzite, medium- a to olive-gray (weathers mostly '&it o~ve gray. moderate brown along some joints) m even beds mostly 0.2-0.4 m, thick; 

contains some faint wavy to planar 

laminae ....................... 34. Similar to unit 33; ripple marks com- mon in lower half, mud cracks in upper half. Interval about 50 percent covered ........................ 35. Siltite and argillite, interlaminated; 

siltite is dfvk gray (weathers pale olive), argdhte 1s greenish (weathers moderate browb rG 

joints); lam-e from less than 1 mm to 1 cm thck; siltite laminae wm- monly lenticular and channel into 

under1 g argillite; mud cracks com- mon. d? tcrop ap s massive; s lits 

along planar Zg surfaces aLut 

0.6 m apart ..................... 32. Quartzite. medium-gray (weathers 

light brown) in even beds 0.1-0.3 m thick; faint planar and trough 

lamination ..................... 31. Covered ........................ 30. Siltite, medium-gray (weathers green- ish gray); in beds mostly 5-20 cm thick, contains some inconspicuous wavy laminae. Weathers to blocky fragments. Interval about 50 percent covered ........................ 29. Covered; hillside exposures su gest interval is mostly argillite s& to 

upper part of umt 28 ------------- 28. Argipte and siltite; u per one-third is 

mertum-gray well&aved argillite containing a few planar siltite laminae 1-2 mm thick and a few 

cross-laminated siltite lenses as much as 1 cm thick; middle one-third 

is interlaminated siltite and argillite containin some planar to broadly 

lensing sftite beds several cm thick; 

lower one-third is similar to upper one-third. Partly covered ---------- 27. Mostly covered, hillside float and a 

few exposures near road su gest that u per one-fourth is interkmmated 

sxtite and argillite with irregular laminae and small-scale scour-and-fill 

features and that lower three-fourths 

is wellcleaved argillite similar to that 

of unit 23 ...................... 

Not n: 

19 

9 

320 

22 

10 

35 

16 

44 

36 

103 

54 Measured sections 
4 Figure 11. A part of a measured section showing bed 

numbers, bed thicknesses, and cumulative thicknesses. 

Color terms used to describe rocks should be as 

specific as possible, and if colors are given in both 

wet and dry states, other workers will more easily 

recognize the rocks in the field. Exact color images 

are effectively conveyed from author to reader by 

reference of the "Rock-Color Chart" of the National 

Research Council (Goddard and others, 1948). This 

chart permits most rock colors to be accurately identified as to hue, value, and chroma by comparing the 

outcrop or hand specimen with the color chips on the 

chart. If the chart is used, it should be used consistently. If it is not used, color terms must be carefully 

chosen to clearly and consistently convey the author's 

meaning to the reader. Dark red and brick red, for 

example, are inexact terms; most rocks described in 

the literature as dark red are in fact light red, and 

red bricks come in many hues, values, and chromas. 

CROSS SECTIONS AND 

STRATIGRAPHIC COLUMNAR SECTIONS 

Stratigraphic limits of complexly intertongued units 

can be shown by pattern or by solid lines and arrows 

(fig. 13). Locations of nomenclature changes, sometimes arbitrarily limited, can be identified by vertical 

dashed lines. Note Chandler-Corwin limit on 

figure 13. 

Columnar sections can be compiled for large areas 

to show the stratigraphic unit name, its position 

by geologic system, its rock type, and its thickness 

(fig. 14). Standard patterns indicate rock types. Standard outcrop form is used for cliff, ledge, and slope. 

STRATIGRAPHIC STYLE AND EXPRESSION 

Stratigraphic terms should rarely be abbreviated, 

but on charts, tables, graphs, and maps, the abbreviations listed below are acceptable if space is tight. 

Periods are used after the abbreviated term on 

charts, tables, and graphs, but generally not on maps. 

Term or lithology Group ----------------- Formation ----------- Member -------------- Sandstone ----------- Siltstone ------------- Shale ------------------- Limestone ----------- Dolomite ------------- 

Conglomerate ----- 

Abbreviation 

GP. 

Fm. 

Mbr. 

Ss. 

Slts. 

Sh. 

Ls. 

Dol. 

Cgl. 

Term or lithology Abbrewiution 

QUa&ite ------------- Qzt. 

Volcanics ------------ Volc. 

Claystone------------ C1 st. 

Mudstone ------------ ~&t. Granite --------------- Gr. 

Gneiss ----------------- Gn. ~h~~lite -------------- Rhy. 

System, period, Use map or era. 

SECTIONS OF THE STURGIS FORMATION 

Upper part of the section 

[This art of the composite section includes 61 m of strata de- scribes) from the core taken at the core hole Oil-30 locality and at the 

stratigraphic test hole, drill hole CS-1801, locality. It corresponds to 

the 21.3- to 82.3-111 portion of the u r art of the section of the 

Sturgis Formation (Kehn, 1973, p. Kl-%11). The systemic boundary between the Permian and the Pennsylvanian Systems may be 

within this unit] 

Unit 

Pennsylvanian System 

Upper Penns lvanian Series 

Sturgis Xrmation (in part): 

Shale, green to gray, calcareous; clayey and brown to reddish near middle and 

silty at base ......................... Sandstone, medium-gray, very fine grained; 

argillaceous ......................... Shale, dark-gi ---- ------------------- Shale, very gray to black; argillaceous; scattered limestone bands and nodules; 

carbonaceous in upr-{art ------------- Coal, bright- and ull anded; fine pyrite and calcite-filled veins at top ----------- Shale, medium-gray, clayey, nonbedded ---- Co*, dull to bright, fusain partings; bony with carbonaceous shale partings at top and base; calcite laminat~ons in bony coal 

at base ............................. Shale, medium-gray, clayey, nonbedded; 

plant impress~ons at top; limestone 

nodules m lower part ------------------ Shale, medium-gray, and limestone, tan, 

dense; silty toward base --------------- Shale, dark-gray; with laminations of lightgray siltstone and light-gray, fine 

pruned sandstone at base -------------- Sandstone, light- to medium-gray, fine- gralned ............................. Shale, greenish-gray to black; carbonaceous 

at base; pyrite nodules and laminations --- Coal, dull to bright, much fusain; pyrite bands on pyrite or on cleat; much bony coal and carbonaceous shale bands ------ Shale, medium-gray, nonbedded; calcareous, 

with white limestone nodules ----------- Limestone, light-gray to gray, finely crystalline to dense ------------------- 

Depth 

(meters) 

118.9-120.7 

120.7-122.7 

122.7-124.9 

124.9-134.3 

134.3-134.8 

134.8-135.1 

Figure 12. A part of a measured section derived from a 

drill core. 

The initial letters of formal geologic names have 

been capitalized by the Survey since 1961. Geologic 

names in material quoted directly from sources written before that time should follow the usage of the 

original author, but names in paraphrased-material 

should be capitalized in manuscripts, tables, charts, 

and map explanations, even though the names were 

not capitalized by the original author. When three or 

more stratigraphic units are included as a sequence in 

one sentence, an author may prefer listing them with 

the general correct rank term, rather than the specific term. For example, Twin Creek Limestone, 

Morrison Formation, Dakota Sandstone, and Mancos 

Stratigraphic style and expression 55 
Tunalik Kaolak 

No. 1 Meade No. 1 Oumalik 

No. 1 No. 1 

~-.43 km a - 107 km, -1-62 km--+;; 

EXPLANATION 

Dominantly nonmarine facies 

I l,'I::-'i Dominantly shallow marine ..-. sandstone and shale 

~oiok 

Formation I 

Figure 13. A stratigraphic cross section showing complex intertonguing. 

56 Cross sections and stratigraphic columnar sections 
East 

Oumalik Titaluk Wolf Creek Umiat Umiat McCulloch Gubik 

No. 1 No. 1 No. 3 No. 1 No. 2 Col. U. 2 No. 1 

-*-57 km-40 km-46 km -1. 10 9 18 -Ikm - km -'- 35 km 

Seabee Formation. Colville Group SP R SP R SP R 

SP R SP R 

- .'I, . I ' , 

.-. .- . . 

. _". 

167.6 m of fault 

6!-. , 

Torok 

Formation 

Foresets projected from ! seismic line 6BE-77 \ 

\ in syncline to south \ 6:~ I 

\ \ 

Bottomsets of westerly derived Torok deposited on 

bonomset turbidite facies of southerly derived 

Torok and Fortress Mountain Formations at some 

horizon below 

Bottomsets 

(basinat) 

Base of pebble shale unit from 

well and seismic data and 

adjusted for tectonic thickening 

of Torok Formation 

B. P. ltkillik 

No. 1 

Cross sections and stratigraphic columnar sections 57 
Figure 14. A stratigraphic columnar section. 

Shale, though correct, could be listed simply as Twin 

Creek, Morrison, Dakota, and Mancos Formations. 

Such a listing enhances readability. Note that 

"Formations" is capitalized. 

The major divisions of geologic time, the symbols 

used on geologic maps, and the age estimates of their 

boundaries are shown in figure 15. The age estimates 

assigned to the boundaries resulted from a meeting of 

the Geologic Names Committee in 1980. Other published 

schemes may be followed instead (for example, Harland and others, 1982; Palmer, 1983; Snelling, 1985); 

in any event, specify what scheme is being followed. 

Divisions of Precambrian time (fig. 15) are based or 

isotopic or radiometric ages expressed in millions of 

Divisions of geologic time 
'Ranges reflect uncertainties of isotopic and biostratigraphic age assignments. Age boundaries not closely bracketed by existing 

data shown by -. Decay constants and isotopic ratios employed are cited in Steiger and Jager (1977). Designation m.y. used for an 

inte~al of time. 

'~odifiers (lower, middle, upper or early, middle, late) when used with these items are informal divisions of the larger unit; the 

first letter of the modifier is lowercase. 

3Rocks older than 570 Ma also called Precambrian (pC), a time term without specific rank. 

41nformal time term without specific rank. 

Figure 15. Divisions of geologic time--major geochronologic, chronostratigraphic, and 

geochronometric units. 

Subdivisions (and their symbols) 

Divisions of geologic time 59 

Age estimates 

of boundaries 

in mega-annum 

(Ma) ' 

0.010 

1.6 (1.6-1.9) 

5 (4.9-5.31 

24 (2%26) 

38 (34-383 

55 (54461 

86 63-66] 

96 (95-97) 

138 (135-1411 

Eon or 

Eonothem 

I 

205 (20&215) 

-240 

~hanerozoic~ 290 (29C305) 

-330 

360 (360-365) 

Era or 

Erathem 

~snozoic 

(Ct) 

Proterozoic 

ce) 

(A) 

pra-Arehaan (PA) ' 

~aleoroic 

tPd 

he 

PmtomzoiC ID MI~~I~ 

Prolsrozolc tYI 

Early Pmtarozolc (XI 

Late Archsan 1WI 

~tdd~s Archean IVf 

Arc!:; tU1 

Epmh or Series 

Holocene 

Pleistocene 

- Pliocene 

Miocene 

Oligocene 

Eocene 

Paleocene 

Late 1 Upper 

Early I ~ower 

Period, System, 

Subperiod, Subsystem 

Quaternary 

(a) 

reniary 

m 

(Dl 

Silurian 

61 

Ordovician 

(0) 

Cambrian 

K3 

Neogenez Subperiod or Subsystem (N) 

Subwrrod ~sleogene~ or Subsystem (R) 

Cretaceous 

(K) 

Middle 

Early 

Late 

Middle 

Early 

Late 

Middle 

Early 

Late 

Middle 

Early 

Norm dsfined 

None defined 

None defined 

None defined 

None defined 

None def~ned 

-570 

900 

1600 

2500 

3000 

3400 

3800 7 

Middle 

Lower 

Upper 

Middle 

Lower 

Upper 

Middle 

Lower 

Upper 

Middle 

Lower 

410 (405-415) 

435 (435440) 

500 (495410) 
years (m.y.) or billions of years (b.y.) for an interval 

of time, or mega-annum (Ma) or giga-annum (Ga) for 

a date. In Survey reports, divisions such as Azoic, 

Archeozoic, and Algonkian have been replaced by 

Archean and Proterozoic. The name Precambrian is 

considered to be informal and without specific 

stratigraphic rank. 

Some geologists use the informal position terms 6 6 lower," "middle," and "upper" (or "early," "middle," and "late") Precambrian. These terms, however, 

have only local application, and lower (or early) Precambrian rocks in one area may not be equivalent to 

other rocks assigned to the same position (or age) in 

another area. 

The Archean Eon is divided into the Early Archean, 

Middle Archean, and Late Archean Eras. The Proterozoic Eon is divided into the Early Proterozoic, 

Middle Proterozoic, and Late Proterozoic Eras. Position terms such as upper, middle, and lower are used 

with these divisions only informally; for example, 

upper part of the Proterozoic. The scheme of these 

geochronometric divisions has been devised simply to 

aid in the understanding of the Archean and Proterozoic history of the United States. The time boundaries 

have been chosen to split as few as possible known 

episodes of sedimentation, orogeny, and plutonism. 

Intentionally, the boundaries do not correspond to 

geologic events. The scheme is intended as an interim 

measure, pending development of an internationally 

accepted chronostratigraphic standard. 

The Phanerozoic Eon (or Eonothem) consists of 

the Paleozoic, Mesozoic, and Cenozoic Eras (or Erathems). No map symbol has been reserved for the 

Phanerozoic. Alternatives to the term include use of 

all three terms listed above or the informal term 

"post-Proterozoic." 

The term "Carboniferous Systems" is used when 

the Mississippian and Pennsylvanian Systems are not 

differentiated, but the term is seldom used in the 

United States. Reports concerned with European 

nomenclature divide the Carboniferous into Upper and 

Lower Carboniferous, or Early and Late Carboniferous. The boundary between the two parts does not 

equate with the Mississippian and Pennsylvanian 

boundary in the United States, but the interval 

between the base of the Lower Mississippian and 

the top of the Upper Pennsylvanian composes the 

Carboniferous. 

PROVINCIALAND-MAMMAL AGES OF THE TERTIARY 

Wood and others (1941, p. 8-13) devised a provincial time scale for the Tertiary of North America. 

Each name is based on a stratigraphic unit (Wasatch. 

ian from Wasatch Formation) or the name of a wellknown local fauna (Blancan from Blanco local fauna). 

A suffix "an" or "ian" is added to distinguish landmammal ages from the lithostratigraphic units. The 

1983 edition of the code, however, specifically states 

that it is undesirable to convert a lithostratigraphic 

term to a time term by adding such endings (art. 7b). 

These provincial ages, though used in many reports, 

are not part of the formal stratigraphic hierarchy; 

specific reference should be made to the chart of 

Wood and others (1941) or to the designator of a 

more recently named land-mammal age when the 

terms are used. 

Provincial series terms are accepted as part of the 

formal nomenclature (fig. 16). The Cretaceous is 

divided into three provincial series-Coahuilan, Comanchean, and Gulfian-only in the U.S. Gulf Coast 

area. The Permian, Pennsylvanian, and Mississippian 

Systems have been divided into provincial series in 

most of the United States. The initial letters of each 

word are capitalized, as Gulfian Provincial Series. 

"Neoglaciation" is an informal term used to desig 

nate glacial expansions that are younger than the 

Holocene climatic optimum. 

HOLOCENE ANDRECENT 

"Holocene" is the formal term for the epoch of 

the last 10,000 years. It replaces the former term 

"Recent." The term "recent" (lowercase r) informally 

connotes any very recent time of unspecified duration. 

EARLY, MIDDLE, AND LATE VERSUS 

LOWER, MIDDLE, AND UPPER 

The 1983 code and the Survey carefully distinguish 

between terms of time (geochronologic or geochronometric) and position (chronostratigraphic), especially 

in discussions of layered rocks. Many divisions of 

these two classification schemes are recognized internationally and have type or standard sections outside 

the United States. The initial letters of the formal or 

60 Divisions of geologic 
'Ranges reflect uncertainties of isotopic and biostratigraphic age assignments. Age boundaries not closely bracketed by existing 

data shown by -. Decay constants and isotopic ratios employed are cited in Steiger and Jager (1977). Designation m.y. used for an 

interval of time. 

'Modifiers (lower, middle, upper or early, middle, late) when used with these items are informal divisions of the larger unit; the 

first letter of the modifier is lowercase. 

3Rocks older than 570 Ma also called Precambrian (6). a time term without specific rank. 

41nformal time term without specific rank. 

Age estimates 

of boundaries 

in mega-annum 

(Ma) ' 

0.010 

1.6 (1-6-1.9) 

5 (4.9-5.3) 

24 (23-26) 

38 (34-38) 

55 (54-56) 

66 (63-66) 

96 (95-97) 

138 (135-141) 

Subdivisions (and their symbols) 

Figure 15. Divisions of geologic time--major geochronologic, chronostratigraphic, and 

geochronometric units. 

205 (200-215) 

290 (290-305) 

360 (36W65) 

410 (405415) 

435 (435-440) 

500 (495-510) 

Eon or 

Eonothem 

Divisions of geologic time 59 

Era or 

Erathem 

Cenozoic2 

(Cz) 

Epoch or Series 

Holocene 

Pleistocene 

Pliocene 

' Miocene 

Oligocene 

Eocene . 

Paleocene 

Late I upper 

Early I ~ower 

Period, System, 

Subperiod, Subsystem 

Quaternary 

(01 

~~~i~~ 

m 

Neogene 2 

Subperiod or Subsystem (N) 

Subper~od paleoeene2 or

Subsystem (R) 

Cretaceous 

(K) 
years (m.y.) or billions of years (b.y.) for an interval 

of time, or mega-annurn (Ma) or giga-annum (Ga) for 

a date. In Survey reports, divisions such as Azoic, 

Archeozoic, and Algonkian have been replaced by 

Archean and Proterozoic. The name Precambrian is 

considered to be informal and without specific 

stratigraphic rank. 

Some geologists use the informal position terms (6 lower," "middle," and "upper" (or "early," "middle," and "late") Precambrian. These terms, however, 

have only local application, and lower (or early) Precambrian rocks in one area may not be equivalent to 

other rocks assigned to the same position (or age) in 

another area. 

ARCHEAN AND PROTEROZOIC 

The Archean Eon is divided into the Early Archean, 

Middle Archean, and Late Archean Eras. The Proterozoic Eon is divided into the Early Proterozoic, 

Middle Proterozoic, and Late Proterozoic Eras. Position terms such as upper, middle, and lower are used 

with these divisions only informally; for example, 

upper part of the Proterozoic. The scheme of these 

geochronometric divisions has been devised simply to 

aid in the understanding of the Archean and Proterozoic history of the United States. The time boundaries 

have been chosen to split as few as possible known 

episodes of sedimentation, orogeny, and plutonism. 

Intentionally, the boundaries do not correspond to 

geologic events. The scheme is intended as an interim 

measure, pending development of an internationally 

accepted chronostratigraphic standard. 

The Phanerozoic Eon (or Eonothem) consists of 

the Paleozoic, Mesozoic, and Cenozoic Eras (or Erathems). No map symbol has been reserved for the 

Phanerozoic. Alternatives to the term include use of 

all three terms listed above or the informal term 

"post-Proterozoic." 

The term "Carboniferous Systems" is used when 

the Mississippian and Pennsylvanian Systems are not 

differentiated, but the term is seldom used in the 

United States. Reports concerned with European 

nomenclature divide the Carboniferous into Upper and 

Lower Carboniferous, or Early and Late Carboniferous. The boundary between the two parts does not 

equate with the Mississippian and Pennsylvanian 

boundary in the United States, but the interval 

between the base of the Lower Mississippian and 

the top of the Upper Pennsylvanian composes the 

Carboniferous. 

PROVINCIAL LAND-MAMMAL ACES OF THE TERTIARY 

Wood and others (1941, p. 8-13) devised a provincial time scale for the Tertiary of North America. 

Each name is based on a stratigraphic unit (Wasatchian from Wasatch Formation) or the name of a wellknown local fauna (Blancan from Blanco local fauna). 

A suffix "an" or "ian" is added to distinguish landmammal ages from the lithostratigraphic units. The 

1983 edition of the code, however, specifically states 

that it is undesirable to convert a lithostratigraphic 

term to a time term by adding such endings (art. 7b). 

These provincial ages, though used in many reports, 

are not part of the formal stratigraphic hierarchy; 

specific reference should be made to the chart of 

Wood and others (1941) or to the designator of a 

more recently named land-mammal age when the 

terms are used. 

Provincial series terms are accepted as part of the 

formal nomenclature (fig. 16). The Cretaceous is 

divided into three provincial series-Coahuilan, Comanchean, and Gulfian-only in the U.S. Gulf Coast 

area. The Permian, Pennsylvanian, and Mississippian 

Systems have been divided into provincial series in 

most of the United States. The initial letters of each 

word are capitalized, as Gulfian Provincial Series. 

"Neoglaciation" is an informal term used to designate glacial expansions that are younger than the 

Holocene climatic optimum. 

HOLOCENE AND RECENT 

"Holocene" is the formal term for the epoch of 

the last 10,000 years. It replaces the former term 

"Recent." The term "recent" (lowercase r) informally 

connotes any very recent time of unspecified duration. 

EARLY, MIDDLE, AND LATE VERSUS 

LOWER, MIDDLE, AND UPPER 

The 1983 code and the Survey carefully distinguish 

between terms of time (geochronologic or geochronometric) and position (chronostratigraphic), especially 

in discussions of layered rocks. Many divisions of 

these two classification schemes are recognized internationally and have type or standard sections outside 

the United States. The initial letters of the formal or 

60 Divisions of geologic time 
Altmate usage and notes 

Cornandwan includes Washiia (Upper and Lower Cretaceous), 

Fredericksburg, and Trinity (Lower Cmtaceous) Groups 

Desmoimian: spelling recently cham from Des Moinesian. 

Lower Cambrian I w-m I waucom is ud In southern Great Basin, CalBornia-Nevada. I 

Rocks in Ibex area, Utah, represent a continuous sequence from Late Cambrian into Ordovician. A stratotype for the 

Cambrian-Ordovician boundary has not yet been agreed upon. 

Figure 16. Major series and provincial series terms used in the United States. 

Divisions of geologic time 61 
defined terms are capitalized; those of informal terms 

are not. Figure 15 shows formal terms only. Mesozoic 

Era, Jurassic Period, Late Jurassic Epoch, for example, are formal time terms. The corresponding formal 

position terms are Mesozoic Erathem, Jurassic System, Upper Jurassic Series. Divisions of the following 

time and position terms are informal: Precambrian, 

Phanerozic, Paleozoic, Mesozoic, Cenozoic, Tertiary, 

Quaternary. For example, the early Mesozoic, late 

Paleocene (geochronologic) and upper Quaternary, 

lower Oligocene (chronostratigraphic) are considered 

to be informal because their boundaries have not been 

defined and adopted; the first letter of the modifying 

word is lowercase (fig. 15). 

Fossil ages are usually expressed in geochronologic 

terms. For example, fossils of Early Devonian age are 

Early Devonian fossils, not Lower Devonian. 

Age terms rather than position terms are always 

used for lithodernic units (art. 13d), and they may be 

better applied also to Tertiary volcanic rocks from 

a caldera, glacial deposits of the Pleistocene, and 

coastal or alluvial terrace deposits of the Quaternary. 

In some volcanic deposits, for example, the youngest 

layer may be preserved at the base of the volcano, or 

many miles from the volcano. The oldest Pleistocene 

deposit may be highest on the mountain. Therefore, 

the designations "early volcanic rocks" or "oldest 

till" may be preferred to "lower volcanic rocks" or 

"upper till." 

European epoch and age terms that are commonly 

used as divisions of the Ordovician to Holocene, as 

follows, are based on (1) Van Eysinga, 1987; (2) 

Geological Society of America, 1983; (3) Snelling, 

1985; and (4) B.A. Skipp, oral commun., 1988. The 

"ian" endings are not used by all stratigraphers. 

Epoch 

Age (Sources) 

Holocene 

Versilian (1) 

Pleistocene (1) 

Tyrrhenian 

Miiazzian 

Sicilian 

Emilian 

Calabrian 

Pliocene (1, 2) 

Piacenzian 

Zanclean 

Miocene (1, 2) 

Messinian 

Tortonian 

Serravallian 

Langhian 

Burdigalian 

Aquitanian 

Epoch 

Age (Sources) 

Oligocene (1, 2) 

Chattian 

Rupelian 

Eocene (1, 2) 

Priabonian 

Bartonian 

Lutetian 

Ypresian 

Paleocene (1, 3) 

Thanetian 

Montian-also spelled 

Monian 

Danian 

Cretaceous, Late (1, 2, 3) 

Maastrichtian \ 

J senonian Santonian 

Epoch Epoch 

Age (Sources) Age (Sources) 

Cretaceous, Late (1, 2, 3)-Con. Permian, Late (2, 3) 

Turonian Tatarian 

Cenomanian Kazanian 

Cretaceous, Early (1, 2, 3) 

Albian 

Aptian 

Barremian 

Hauterivian 

Berriasian 

Jurassic, Late (2, 3) 

Tithonian 

Portlandian 

Kimmeridgian 

Oxfordian 

Jurassic, Middle (1, 2, 3) 

Callovian 

Bathonian 

Bajocian 

Aalenian-not used 

by Imlay (1980) 

Jurassic, Early (1, 2, 3) 

Toarcian 

Pliensbachian 

Sinemurian 

Hettangian 

Triassic, Late (1) 

Rhaetian (1,3)--included 

as upper part of 

Norian by Tozer 

(1984); abandonment 

as an age or retention 

as a subage of the 

Norian not decided 

(Ager, 1987). 

Norian 

Carnian-also spelled 

Karnian 

Triassic, Middle (1, 2) 

Ladinian 

Anisian 

Triassic, Early (1, 2) 

Scythian 

ufimian 

Permian, Early (2) 

Kungurian 

Artinskian 

Sakrnarian-also 

spelled Samarian 

Asselian 

Pennsylvanian, Late 

and Middle (4) 

Stephanian 

Pennsylvanian, Middle 

and Early (4) 

Westphalian 

Pennsylvanian, Early and 

Mississippian, Late (4) 

Namurian 

Mississippian, Late 

and Early (4) 

Visean 

Mississippian, Early (4) 

Tournaisian 

Devonian, Late (1, 2) 

Famennian 

Frasnian 

Devonian, Middle (1, 2) 

Givetian 

Couvinian-also called 

Eifelian 

Devonian, Early (1, 2) 

Emsian 

Siegenian 

Gedinnian 

Silurian (2) 

Pridolian 

Ludlovian 

Wenlockian 

Llandoverian 

Ordovician (1, 2) 

Ashgillian 

Caradocian 

Llandeilian 

Llanvirnian 

Arenigian 

Tremadocian 

EXPRESSIONS FOR DEGREES OF DOUBT 

"Probably," "presumably," "may be," and "(?)" are 

used to express doubt about stratigraphic, geochronologic, or geochronometric unit assignments. Designation of a term in doubt must be easily understood. 

For example, a unit described vaguely as "probably 

of Late Mississippian age" could be either Late(?) 

Mississippian or Late Mississippian(?). 

If the identification of a geologic unit is doubtful, 

the query follows in parentheses after the geographic 

part of the name, as Morrison(?) Formation. 

62 European stage terms 
Authors sometimes use quotation marks around 

stratigraphic names to indicate abandonment or misapplication. Because of their varied uses and implications, quotation marks used in a stratigraphic context 

should be briefly explained. 

Certain shortened or abbreviated terms should be 

avoided in geologic manuscripts: 

Permo-Penn, for Permian and Pennsylvanian 

Cambro-Ordovician, for Cambrian and Ordovician 

Map symbols in text as shorthand versions of formation names, such as "Je" for Entrada Sandstone 

Mid-Cambrian for Middle Cambrian 

Westwater Member, for Westwater Canyon Member 

The local or commercial names of stratigraphic 

units of economic interest, such as oil sands, coal 

beds, and construction or ornamental stone, are considered to be informal names (articles 22g, 26a, 30h 

of the code). Only the first letter of the first word is 

capitalized; for example, Felix coal bed. The text 

should state that the names have local or economic 

interest only, or are informal units, subsurface units, 

or local drillers terms. 

If a term is equivalent to a formal name or has 

been replaced by a formal name, the formal name has 

preference. The economic term can be shown in 

parenthesis as follows: 

Leadville Dolomite (Yule marble) 

Greenbrier Limestone (Big lime) 

Saltsburg Sandstone Member (Little Dunkard sand) 

When planning a report, the author should consider 

the publication medium best suited for a paper containing stratigraphic information. Content, length, 

complexity of figures, tables, or stratigraphic changes, 

report area, and subject matter should be assessed in 

relation to the intended readership. For example, 

large maps and correlation charts may not fit the size 

limits of a given medium; an International Geological 

Congress volume might not be the best publication for 

name changes for rocks in a small area of West 

Virginia or Connecticut; a report on Quaternary 

terminology might reach the largest number of interested people in a journal devoted to Quaternary 

research. Certain kinds of reports, because of their 

format or lack of availability (open-file reports or 

abstracts, for example), are not proper vehicles for 

nomenclature changes (art. 4 of the 1983 code). 

New stratigraphic names or significant stratigraphic 

changes should not be introduced in an abstract that 

is to be published separately from a more complete 

report. The essential conciseness of an abstract excludes the full definition that is specified by article 4 

of the 1983 code. An informal designation such as 

limestone of, at, or near Hudson should be used in the 

abstract for the stratigraphic unit that is to be named 

and described later in a more complete report. 

New stratigraphic units should not be named in 

guidebooks that are limited in distribution to only the 

field-trip participants (code, art. 4b). Definition of 

new nomenclature and revision of previously used 

nomenclature must be widely available to the scientific community. 

Use of a name in a thesis or in "Dissertation 

Abstracts" does not constitute publication. A thesis 

prepared in conjunction with Survey work may be 

placed in open file, but the use of new or revised 

nomenclature in a thesis does not constitute publication. In preparing the thesis, the author should check 

proposed stratigraphic changes with a GNU staff 

member, and after the thesis is accepted by the 

concerned college, the new nomenclature should be 

prepared for publication as soon as possible. 

U.S. GEOLOGICAL SURVEY PUBLICATIONS 

An areal-geological report having important stratigraphic information can be published in a Survey 

Bulletin or a Professional Paper. The series rests 

largely on the subject matter and not on the size of 

the illustrations, charts, and tables. 

A Bulletin series titled "Contributions to Stratigraphy" is designed especially for stratigraphic papers. 

One number is assigned each Bulletin each year. One 

chapter, "Stratigraphic Notes," includes all short 

papers regardless of area or subject matter. Longer 

papers are published as alphabetically designated 

chapters in the order received, carrying the assigned 

Bulletin number of the year. 

All stratigraphic nomenclature changes shown on 

Geologic Quadrangle (GQ) Maps, Miscellaneous Investigations (I) Series Maps, and Miscellaneous Field 

Publication of stratigraphic data 63 
Studies (MF) Maps should be described either in the 

accompanying text or as a footnote to the explanation 

of the unit in the "Description of Map Units." If a 

new name is used on a map, it should be introduced 

either in the accompanying text or as a footnote to 

the "Description of Map Units." Take care to include 

all the required information, because the map may be 

the only published definition of the name. Just the 

"mention" of a name on a map explanation is not 

adequate explanation for a new name (code, article 

4(a), p. 852). 

64 Publication of stratigraphic data