**GEOLOGIC MAP**

**OF THE**

**NEAR SIDE**

**OF THE MOON**

By

Don E. Wilhelms and John F. McCauley

1971

Prepared for the National Aeronautics and Space Administration

by

U.S. Department of the Interior, U.S. Geological Survey

(Published in hardcopy as USGS Miscellaneous Investigations Series Map
I--703, as part of the Geologic Atlas of the Moon, 1:5,000,000 Geologic
Series. Hardcopy is available for sale from U.S. Geological Survey,
Information Services, Box 25286, Federal Center, Denver, CO 80225)

> Please direct questions or comments about the digital version to:
>
> Richard Kozak
>
> U.S. Geological Survey
>
> 2255 N. Gemini Drive
>
> Flagstaff, AZ 86001
>
> e-mail: rkozak@flagmail.wr.usgs.gov

**DESCRIPTION OF UNITS**

**DARK MATERIALS**

Materials of the extensive and conspicuous dark plains known as maria
are the principle constituents of this class, but less extensive dark
domes, dome-studded plateaus, and terra mantles, all of which occur
within or near the maria, are also included.

**MARE MATERIALS**

Mare materials cover about 40 percent of the area mapped. Most are
located in the centers and concentric troughs of the large approximately
circular multi-ring depressions such as the Imbrium basin and within
large irregular depressions such as Oceanus Procellarum. Smaller patches
are in terra depressions such as those east of Mare Vaporum and in
craters such as Archimedes.

Relative age is determined principally from superposition and embayment
relations with adjacent units, from density and age of main-sequence
craters (see below) superposed on and overlapped by the maria,l and to
some extent from albedo2 and color3 properties. The maria are clearly
younger than almost all terra materials, including those of the
crater-like multi-ringed basins. They are older, however, than almost
all ray craters, such as Copernicus, and non-ray craters such as
Eratosthenes, both of which show symmetrical rim deposits and fields of
satellitic craters superposed on the mare. Crater frequency counts on
telescopic photographs (Shoemaker and others, 1962)4 suggested that the
maria are approximately contemporaneous, and the material at the present
mare surface was used to define informally the top of the Imbrian System
(Wilhelms, 1970b, p. 23), although the probability of substantial local
variations in age was generally recognized (McCauley, 1967b; Wilhelms,
1970b, p. 30--32; Offield and Pohn, 1970). Lunar Orbiter photographs
confirm that mare surfaces differ in density and age of superposed and
overlapped craters, and an attempt has been made on this map, as on
recent larger scale maps (Trask, 1970a), to divide the maria according
to relative age. Contacts between mare units as observed on Orbiter IV
photographs are, however, gradational and locally diffused by ray cover.
In the absence of synoptic coverage at the required higher resolution,
the two-fold subdivision used here should be regarded as a preliminary
and incomplete portrayal of age variations within maria.

Several lines of evidence indicate that the maria are volcanic (in the
sense that they solidified from molten rock probably formed beneath the
surface). Their flat smooth surfaces generally terminate against the
edges of the confining basins in abrupt contacts along elevation
contours, and this indicates emplacement as fluid flows. Their
distribution is controlled by the basins, but because the maria are
younger they are not considered to be the direct and immediate products
of basin formation. A predominantly basaltic composition is suggested by
the low albedo, fluid-like emplacement relations, local flow-front
morphology, Surveyor analyses, and the recent Apollo 11 and 12 results
(Lunar Sample Analysis Planning Team, 1970; Lunar Sample Prelim. Exam.
Team, 1970).

Em, *mare material*

Type area: Latitude 3°S. to 4°S., longitude 36°W. to 37.5°W., Wichmann
CA region (Cummings, 1971; West and Cannon, 1971) (See Lunar Orbiter III
frames 173--180; Orbiter IV, 137; Orbiter V, 169--176).5 Other typical
occurrence: Surveyor I site, 2.5°S., 43.2°W., northeast of crater
Flamsteed (Offield, 1972 ) (Orbiter I, 184--215; Orbiter III, 181--212;
Orbiter IV, 143)

Includes mare materials assigned other ages on 1:1,000,000-scale maps

Less extensive than Imbrian mare, constituting about 30 percent of the
dark plains; commonly localized near mare-terra contacts. Distinguished
from Imbrian mare material by at least two of following criteria: (1)
lower albedo2 than adjacent mare; (2) truncation of rim materials of
Eratosthenian craters (such as Manilius and Bullialdus); (3) superposed
craters no older than Eratosthenian; (4) lower frequency of craters
except where clusters of satellitic craters present; (5) well-preserved
primary structures such as smooth-rimmed pits (as in an embayment north
of Mare Vaporum), small domes (as at type area), very sharp ridges (as
at type area and southeastern Mare Serenitatis), and recognizable flow
fronts (in Mare Imbrium---Kuiper, 1965, p. 29--63; Fielder and Fielder,
1968; Schaber, 1969) (Orbiter V, 159--162); (6) relatively strong color3
contrasts with adjacent mare (usually bluer, but redder along west edge
of Oceanus Procellarum)

Probably relatively thin cover of young flows or pyroclastic materials
over thicker accumulations of Imbrian mare material. Regolith
(impact-produced "soil" layer) thinner than on Imbrian mare. Age of some
patches embaying Eratosthenian craters possibly Copernican

Im, *mare material*

Type area: 0.5°N. to 0.5°S., 0.5°W. to 1.5°W., Sinus Medii (Trask,
1970b; Rowan, 1971) (Orbiter II, 93, 113--136; Orbiter III, 86--101;
Orbiter IV, 101, 102, 108, 109; Orbiter V, 108--115). Reference area for
mapping at Orbiter IV scale: between craters Archimedes (overlapped by
mare material) and Eratosthenes (superposed on it) (Orbiter IV, 109,114)

First mapped as Procellarian System and later as Procellarum Group
(history of nomenclature given by Wilhelms, 1970b, p. 30--32)

Most extensive single unit, occupying about one-third of map area;
mainly dark smooth plains. Color range from relatively bluish to reddish

Mostly basaltic lavas, probably with some pyroclastic materials and
impact-produced fragmental layers of varying thickness

**DOME AND MANTLE MATERIALS**

Emd, *mare dome material*

Type area: Six domes near 7.5°N., 28°W., north of crater Hortensius
(Orbiter III, 123; Orbiter IV, 126,133)

Smooth low domes having gentle convex-upward profiles; circular or
elliptical in plan view; diameter usually \<15 km; heights of several
hundred meters. Small craters commonly at summit; small steep-sided
hills superposed on some domes

Shield volcanoes or laccoliths. Eratosthenian age implied by apparent
superposition on Imbrian mare material, but possibly contemporaneous
with uppermost mare material, as shown on most l:1,000,000-scale maps

Emp, *mare plateau material*

Type area: Marius Hills, within 150 km west and north of crater Marius
11.9°N., 50.8°W. (McCauley, 1967a) (Orbiter IV, 150, 157; Orbiter V
210--217)

Includes dark hummocky material of the Procellarum Group (Carr, 1965);
Harbinger Formation (Moore, 1965; Orbiter V, 186--193); Tacquet
Formation (Carr, 1966); Marius Group (McCauley, 1967a)

Complexes of domes and cones of varied form (bulbous, steep, or
flat-topped with ramparts around upper flanks) and associated planar
deposits, all elevated above the mare. Similar in albedo to mare

Constructional volcanic features and blanketing flows and pyroclastic
materials. Age implied by superposition on mare, generally lower density
of superposed craters, and topographic freshness of individual features

CId, *dark mantling material*

Type area: 20°N. to 20.5°N., 10°E. to 10.5°E., northwest of crater
Sulpicius Gallus (Orbiter IV, 97; Orbiter V, 90--93)

Includes Vallis Schröteri Formation (Moore, 1965, 1967; Orbiter V,
202--209); Sulpicius Gallus Formation (Carr, 1966); dark mare material
(Carr, 1966; Orbiter V, 66--69---Littrow region); Doppelmayer Formation
(Titley, 1967)

Darkest lunar material mapped except for some Eratosthenian mare
material. Partly subdues subjacent topography (at Orbiter IV scale)
within mapped occurrences; high, steep, bright hills within some
patches. Surface smooth in detail. Commonly associated with linear
troughs at margins of mare basins and irregular craters in mare and
terra. Some occurrences embayed by mare material, some superposed on it.
Relatively reddish northwest of Aristarchus (Vallis Schröteri
Formation); other occurrences relatively bluish. Terrain adjoining
mapped occurrences commonly dark but not subdued

Pyroclastic origin suggested by subduing effect; bright hills probably
exhumed older terrain. Age ranges from Copernican in fresh, sparsely
cratered patches in Copernicus region and southeastern edge of Mare
Serenitatis to Imbrian in mare-embayed occurrences

**CIRCUMBASIN MATERIALS**

This class of terra units consists of widespread, texturally distinctive
materials that lie predominantly beyond the most prominent rings of the
best preserved large multi-ring basins. Like crater rim deposits (see
below), circumbasin materials are generally uniform in character at a
given distance from the basin center but change gradationally in the
radial direction. The most prominent parts of the scarps or mountain
rings encompassing all multi- ring basins are made up of rugged,
isolated or coalescing massifs (unit pIr). The materials outside the
most prominent scarps of the Imbrium basin (Shoemaker and Hackman,
1962), the Orientale basin (McCauley, 1968), and the Nectaris basin
(Stuart-Alexander, 1971; Scott, 1972) appear to form a thick blanket on
subjacent cratered topography near the basin and to become progressively
thinner outward; the visible blanket, however, is recognizable for
hundreds of kilometers from the basins. The circum-Imbrium material
described by Shoemaker and Hackman (1962) is here divided into three
units. The two closest to the basin (the Alpes Formation and material of
Montes Apenninus) are fairly rugged and grade laterally into the
pre-Imbrian rugged material of the mountain-ring structures. The third
and outermost unit (Fra Mauro Formation) and the similar unit
surrounding the Orientale basin (Hevelius Formation) resemble the more
finely textured outer radial rim deposits of smaller young craters such
as Copernicus. These deposits are lineated with smooth-surfaced sinuous
ridges and intervening troughs whose amplitudes decrease progressively
with increasing distance from the basin. The circum-Imbrium units and
the Hevelius Formation are continuous around their respective basins,
except where buried by younger materials. The Fra Mauro grades laterally
outward into another type of lineated terrain (pIl) whose grooves and
ridges are relatively straight. The lineaments are radial or subradial
to the center of the Imbrium basin. Material with mostly subdued radial
lineaments, the Janssen Formation, occurs around the southern part of
the Nectaris basin and apparently represents the degraded remnants of a
still older circum-basin blanket, possibly including materials like
those of the circum-Imbrium lineated terrain.

The Janssen Formation is overlain by upper and middle pre-Imbrian
craters and overlies lower pre-Imbrian craters, so it probably was
formed in the early part of middle pre-Imbrian time. The Fra Mauro
Formation is earliest Imbrian in age by definition (Wilhelms, 1970b);
similarity in superposed crater densities, lateral continuity, and the
gradational nature of contacts with the Alpes Formation and the
Apenninus materials, suggest that these three circum-Imbrium units are
essentially contemporaneous. The Hevelius Formation around Orientale is
known to be younger than the Fra Mauro because of its far better state
of preservation, superposition of contemporaneous satellitic craters on
Imbrium-related structure, and the relative freshness of superposed
craters. Like the Fra Mauro, it is overlain by mare material, Imbrian
terra plains material, and some upper Imbrian craters, but it overlies
lower Imbrian craters, so is approximately middle Imbrian in age. The
rugged and lineated units (pIr and pIl) are believed to consist mostly
of prebasin rocks deformed when the nearby basin was formed. Therefore
these units, although considered structural not stratigraphic units, are
assigned the pre-Imbrian age of their component rocks and not the age of
their deformation.

Ihe, *Hevelius Formation*

Type area: 1°N. to 2°N., 68°W. to 68.5°W., on rim and western part of
floor of crater Hevelius (McCauley, 1967a) (Orbiter IV, 169)

Closely spaced ridges and troughs about 7 km long and 1 km wide alined
radially or subradially to center of Orientale basin; progressive
increase in size

from type area towards Cordillera scarp at edge of Orientale basin about
550 km to southwest (outside map area). Subdues underlying topography;
orientation of subradial structure influenced by pre-basin subjacent
topography. Associated with Orientale satellitic craters (Isc).
Gradational eastward into stringers and disconnected patches of faintly
lineated materials that become indistinguishable from underlying terra
units with increasing distance from basin

Impact ejecta deposited ballistically and by radial flowage at or near
ground surface

If, *Fra Mauro Formation*

Type area: 0° to 2°S., l6°W. to 17.5°W., north of crater Fra Mauro
(informal definition: Eggleton, 1964; formal: Wilhelms, 1970b, p. 25)
(Orbiter IV, 120, 121). Reference area: 3°S. to 4.5°S., 17°W. to 18°W.
(Orbiter III, 132-135; Orbiter IV, 120)

Sinuous to straight, smooth-textured ridges or elongate hummocks
typically 2 to 4 km across and 5 to 20 km long, oriented radially and
subradially to Imbrium basin

Impact ejecta from Imbrium basin. Original form similar to Hevelius
Formation but more degraded by erosion and mantling by younger materials

Ial, *Alpes Formation*

Type area: 47°N. to 48°N., 5°E. to 8°E., east of Montes Alpes (Page,
1970) (Orbiter IV, 110, 115; Orbiter V, 102)

Included in Fra Mauro Formation by Eggleton (1965) and Schmitt, Trask,
and Shoemaker (1967)

Blocky or knobby but smooth-surfaced, closely spaced hills 2--5 km in
diameter without conspicuous preferred orientation or lineation. Occurs
both within Apennine ring, as at type area, and beyond, as near Kepler

Related to nearby Imbrium basin; may be (1) erosionally degraded ejecta,
(2) structurally deformed pre-basin bedrock, or (3) a combination of
both

Iap, *material of Montes Apenninus*

Type area: 16°N. to 17.5°N., 3.5°W. to 4.5°W. (Orbiter IV, 109)

Included in Fra Mauro Formation, hummocky member, by Hackman (1966) and
Wilhelms (1968)

Rough coarse blocks of material having elongate rectilinear outlines
parallel to Apennine scarp bordering Imbrium basin, and smooth to
undulating interblock materials. Blocks gradational in size with Alpes
Formation (finer) and pre-Imbrian rugged material (coarser)

Probably bedrock intensely fractured at time of Imbrium impact.
Interblock materials possibly basin ejecta

pIj, *Janssen Formation*

Type area: 43°S. to 45°S., 37.5°E. to 39°E., in older crater Janssen
(Stuart-Alexander, 1971) (Orbiter IV, 76)

Rolling subdued terrain having numerous linear features including
ridges, scarps, and grooves radial to Nectaris basin

Nectaris basin ejecta blanket equivalent to, but more degraded and
cratered than, the younger Fra Mauro and Hevelius Formations

pIl, *lineated material*

Type area: 5.5°S. to 7°S., 3.5°W. to 5°W., northwest of crater
Ptolemaeus (Orbiter IV, 108)

Sharp, raised ridges, intervening level areas or deep troughs, and
smooth whaleback-shaped hills with narrow grooves; linear features
oriented radial to Imbrium basin. Distal to and gradational with Fra
Mauro Formation

Mostly prebasin rock pervasively faulted by Imbrium impact; some
structures tectonically reactivated after basin formation. Some basin
ejecta (Fra Mauro Formation) possibly present

pIr, *rugged material*

Type area: Mons Hadley, 26.5° N., 4.5°E. (Orbiter IV, 102; Orbiter V,
106)

Includes material of smooth ridges and hills (Trask and Titley, 1966;
Titley, 1967)

Rugged blocks most commonly 10 to 30 km across, generally with
rectilinear outlines; forms highest and most rugged parts of arcuate
raised ridges in and around major circular multi-ringed basins; most
extensive and rugged around Imbrium, more subdued elsewhere. Slopes
steep, smooth, and bright at Orbiter IV scale. Gradational contacts with
adjacent low terrain, or bordered by apron of material that encroaches
on adjacent materials

Prebasin rock uplifted during formation of basins by impact. Possibly
some post-impact tectonic rejuvenation. Certain more rounded blocks
possibly formed or modified by volcanism

**TERRA PLAIN, PLATEAU, AND DOME MATERIALS**

This class comprises all bright and intermediate-albedo materials of the
terrae or "highlands," except those of craters and the distinctive
circumbasin units; it includes flat or gently undulating light plains
materials that occur in depressions at about the same elevation as the
maria. The class is divided into nine distinctive and two nondistinctive
units.

Some terra materials overlap the mare materials in age but most are
older, as shown by embayment relations and crater density comparisons.
Plain, plateau, and dome materials transect and cover the circumbasin
units or modify their textures, thus are younger than nearby
multi-ringed basins.

The distinctive deposits are believed to be mostly of volcanic origin,
and the nondistinctive materials are of mixed or unknown origin.

**DISTINCTIVE MATERIALS**

These units are characterized by distinctive surface textures or
aggregates of similar distinctive landforms. The category includes (a)
plains deposits interrupted only by circular craters, and (b) deposits
containing positive landforms which resemble terrestrial volcanic
features. The units are commonly superposed on the rims and floors of
large pre-Imbrian craters, fewer of which are thereby visible within the
confines of the units than in adjacent terrain. The plains deposits are
the most extensive of these units and fill troughs concentric with the
circular multi-ringed mare basins, floors of large craters, and many
depressions of irregular shape. The plains units are easily mapped on
all photographs, whereas the other units are difficult to see or
delineate on telescopic photographs and their distinctive character was
not fully recognized until Orbiter IV photographs became available. The
units containing positive landforms are subdivided according to the
following criteria: (1) physiographic freshness of the landforms, (2)
size of the central depression of the landform relative to the rim or
flank deposit, and (3) shape of the central depression---furrowlike or
round to irregular "pits." They are further divided into: (a) "domes"
consisting of a single landform and (b) more extensive units consisting
of aggregates of landforms. (Individual occurrences and small closely
spaced aggregates in which the central crater dominates are mapped
separately as "crater materials.")

Deposits are dated by means of superposition and intersection relations,
and, less rigorously, by relative states of degradation. Pre-Imbrian
plains deposits are dated by the superposition of lowermost Imbrian
crater clusters. Imbrian deposits, the most abundant type, are commonly
embayed by Imbrian mare materials, are generally more densely cratered
than mare, and transect or overlap Imbrium or Orientale basin deposits
or structures. Some dome and all clustered hilly materials are
tentatively dated as Imbrian because of their subdued appearance.
Imbrian plains material commonly appears to embay or overlie the hilly,
hilly and pitted, and hilly and furrowed deposits and therefore is
younger Imbrian. A morphologic sharpness comparable to that of the
Gruithuisen domes, which are superposed on mare material, serves as the
basis for dating two units as Eratosthenian or Copernican. Plains
deposits in Copernican-age craters are necessarily Copernican; other
sparsely cratered plains are tentatively assigned a Copernican age. In
general, terra materials are complex and each deposit probably includes
materials of a wide age range.

All materials in this category are believed to be predominantly
volcanic; some formed from isolated sources and some from multiple
sources distributed over a wide region. The steep landforms may be
accumulations of relatively viscous lavas. The lack of distinctive
landforms in the plains deposits prevents confident interpretation; they
may be composed of fluid lavas, ash-flow tuff,

freefall tuffs, or, partly, erosional debris. The shape and
size-frequency distribution of craters superposed on the plains are
consistent with an impact origin, but some of these craters could be
volcanic and genetically related to the unit. The plains and extensive
aggregates of positive landforms accumulated to sufficient thickness in
some depressions to bury large pre-Imbrian craters, indicating extensive
pre-mare volcanism. These deposits are of higher albedo and contain
steeper landforms than the maria, suggesting a distinctive composition.
The higher albedo may be partly explained by (l) mass movement of
surficial material on slopes, exposing fresh, brighter bedrock, and (2)
repeated impact cratering, which has created many steep bright
subresolution microslopes.

Cp, *plains material*

Type area: Rim (8.9°S., 26.8°E.) and floor (10.8°S., 26.9°W.) of crater
Theophilus (Orbiter III, 78; Orbiter IV, 77, 84)

Includes Theophilus Formation (Milton, 1968); terra mantling material in
Mare Tranquillitatis (Carr, 1970; Wilhelms, 1970a) (Orbiter II, 35--42;
Orbiter III, 5, 9, 11; Orbiter V, 55--62); part of Vallis Schröteri
Formation

Smooth, relatively uncratered at Orbiter IV scale. Intermediate albedo,
similar to Imbrian terra plains material. Recognized mostly in floor and
rim depressions of Copernican-age craters; plains near Rima Bode II
(13°N., 3.5°W.; Orbiter V, 120--123), undulatory material at south edge
of Mare Tranquillitatis, and moderately dark plains near Vallis
Schröteri also included. System of fine cracks and small domes on some
crater floors and dunelike texture at Rima Bode II visible at Orbiter V
scale (5-40 m)

Occurrences in craters mostly post-impact volcanic materials, possibly
with admixture of erosional debris. Occurrences in Eratosthenian craters
possibly Eratosthenian in age. Rima Bode and V. Schröteri material
volcanic; Tranquillitatis material either volcanic material or erosional
debris

CEd, *dome material*

Type area: Gruithuisen γ and δ, 35.7°N. to 37°N., 39°W. to 41°W.
(Orbiter IV, 145,151; Orbiter V, 182--185)

Includes Cobra Head Formation (Moore, 1965; Orbiter V, 202--205)

Individual fresh-appearing bright domes or cones, some having small
craters or furrows at summits or on flanks. Round to elliptical, as much
as 30 km in diameter; height-to-width ratio approximately 1:10, steeper
than mare domes

Mostly viscous lava deposits with some pyroclastics

CEhf, *hilly and furrowed material*

Type area: 10.5°S. to 11°S., 15.5°E. to 16.5°E. north of crater
Descartes (Orbiter IV, 89)

Includes rugged, bright material of Kant Plateau (Milton, 1968)

Aggregates of closely spaced fresh-appearing hills capped by sinuous or
straight furrows. Hills mostly steep, with short furrows, like unit Ihf
but brighter and topographically sharper. Includes ridges along long
narrow fissures in floor of crater Vitello, 30.4°S., 37.4°W. (Orbiter
IV, 136, 137, 142, 143; Orbiter V, 168); high thermal anomaly6

Mixed volcanic deposits erupted from fissures

Ip, *plains material*

Type area: 3.5°N. to 4.5°N., 15.5°E. to 16.25°E., east of crater Cayley
(Orbiter II, 59--66; Orbiter IV, 90)

Includes Apennine Bench Formation (Hackman, 1966); Cayley Formation
(Morris and Wilhelms, 1967); pre-Imbrian or Imbrian plains-forming
material (Carr, 1966; Titley, 1967)

Mostly smooth and flat but some undulatory areas included. Intermediate
albedo, brighter than mare material, darker than most circumbasin and
other terra materials. In topographic lows within terrae. Craters more
numerous than on mare except for local occurrences in southern
highlands. Most contacts with higher terrain abrupt; some gradational
with mare

Probably mostly volcanic; emplacement in fluid state suggested by some
contact relations and resemblance of planar surfaces to those of mare.
Crater density indicates unit generally older than mare, but isolated
smooth occurrences possibly contemporaneous with or younger than mare.
Some smaller occurrences surrounded by high source areas possibly
derived by erosion

Id, *dome material*

Type area: 4.2°S., 14.7°E., on east rim of crater Taylor B (Orbiter IV,
89)

Individual subdued domes or cones mostly without summit craters or
furrows. Some similar to Eratosthenian or Copernican domes but smoother;
others irregular to arcuate (as at type area); height-to-width ratio
apparently less than for younger domes; as much as 25 km in diameter.
Intermediate albedo

Similar to Eratosthenian or Copernican domes but more eroded. Arcuate
domes emplaced over sites of older impact or volcanic crater rims. Some
may be extensively eroded structural blocks like those of unit pIr

Ih, *hilly material*

Type area: 1.3°S. to 1.7°S., 6.4°E. to 7.7°E., north of crater Pickering
(Orbiter IV, 96, 97)

Aggregates of closely spaced, subdued to moderately steep domes or cones
with or without small summit craters or furrows. Circular or elliptical,
mostly 1 to 5 km in diameter. Similar to some occurrences of Alpes
Formation, but hills more regular in size and plan

Volcanic constructional features or eroded blocks of other origin

Ihf, *hilly and furrowed material*

Type area: 19°S. to 20°S., 50.5°W. to 51°W., northwest of crater
Mersenius (Orbiter IV, 149)

Includes rugged material of Kant Plateau (Milton, 1968)

Aggregates of closely spaced, subdued hills and ridges 3 to 15 km long
and 2 to 6 km wide capped by distinctive sinuous furrows. Irregular
craters and chain craters abundant (where \>10 km, mapped individually).
Includes low ridges along long narrow fissures in crater floors

Densely packed array of pyroclastic or composite cones like those in
some youthful terrestrial volcanic provinces

Ihp, *hilly and pitted material*

Type area: 18°S. to 19°S., 60°W. to 61°W. (Orbiter IV, 161)

Gently rolling to hilly terrain containing aggregates of subdued
irregular to circular craters and hills. Craters mostly narrow-rimmed or
rimless, 2 to 10 km in diameter. Hills equidimensional to elongated by
3:1, mostly \<10 km in greatest dimension

Complex volcanic deposits, possibly predominantly pyroclastic,
accumulated to considerable thickness over older crater terrain. Pits
probably of internal origin and related to the deposits, but some may be
secondary impact craters from unrecognized sources

pIp, *plains material*

Type area: 44°S. to 45.5°S., 22.5°E, to 25°E. (Orbiter IV, 88)

Generally flat terrain with a greater population of large and older
craters than on Imbrian plains-forming material an basal Imbrian units.
Some craters circular, some irregular; many superposed craters of lower
Imbrian clusters (Icc). Intermediate albedo

Probably volcanic, representing an older variant of the Imbrian
plains-forming material

**NONDISTINCTIVE MATERIALS**

Terrain of intermediate albedo and moderately subdued nondescript
surface texture is separated on the basis of relief into two units. The
older (IpIt) has moderately high relief and forms parts of the
basin-concentric rings and upland terrain between basins. The younger
unit (It) has moderately low relief and generally occupies topographic
depressions.

These units are not readily datable, and their assigned ages are
approximations. The unit assigned a pre-Imbrian or Imbrian age is mostly
pre-Imbrian inasmuch as it contains remnants of many large, very subdued
pre-Imbrian craters (unmapped). In addition, exposures commonly coincide
with structural highs concentric with pre-Imbrian basins such as Humorum
and Crisium, so that the unit probably consists in great part of now
unrecognizable basin ejecta. Its relatively smooth surface, however,
suggests that the materials or processes responsible for smoothing are
of Imbrian age. Occurrences of the second unit, on the other hand, show
no underlying pre-Imbrian basement and seem to have a population of
underlying and superposed craters like that of Imbrian units. Also, some
of these occurrences lie in or near the Imbrium basin and interrupt
textures of its blanket or its structures. Therefore the bulk of these
materials are believed to be of Imbrian age---possibly a thicker
accumulation of the same materials as on unit IpIt. The formation of
both units probably extended over a considerable time.

Although basin ejecta is probably a major component of both units, they
also may contain complexly interbedded impact-crater and volcanic
deposits, mixed with local erosional debris. The stratigraphy of these
units may never be unraveled in detail by photogeologic methods.

It, *terra material, undivided*

Type area: 21°S. to 22°S., 45°W. to 46°W., southwest of crater Gassendi
(Orbiter IV, 143, 149)

IpIt, *terra material, undivided*

Type area: 8°S. to 9°S., 20.5°E. to 21.5°E. (Orbiter IV, 84)

**CRATER MATERIALS**

This class includes materials of all types of craters that are 10 km and
larger in rimcrest diameter, or that form clusters or chains of this
size. The term "crater" here includes rimmed depressions of diverse
shapes less than 250 km in rimcrest diameter and excludes other types of
depressions such as linear rilles and the large multi-ringed basins and
their concentric troughs. Each mapped occurrence comprises all materials
associated with the crater including, without subdivision, those of the
rim flank, wall, floor, and peak. The outer limit is drawn to include
textures such as concentric terraces and radial ridges which are
conspicuous, are clearly related to the crater, and which obscure
subjacent units; satellitic craters and rays are not mapped but some are
visible on the base chart. (On l:l,000,000-scale maps, subunits are
separated and the outer limit is drawn farther out, to include all
crater-related textures.) Materials within the crater believed to be
significantly younger than the crater, such as mare and plains material,
are separated on this map. Crater materials are divided into five major
morphologic categories believed to reflect genetic distinctions, and
these in turn are divided into from one to eight age categories.

**MATERIALS OF MAIN-SEQUENCE CRATERS**

This category comprises about 40 percent of all nearside craters ≥10 km
in diameter. It includes individual craters and chains and clusters of
dissimilar individuals, and pairs of similar individuals partly
separated by septa. The craters have a circular or subcircular outline,
a collar of rugged and terraced materials around the rim about one-half
crater diameter wide, and concentric terraces on interior walls if
rimcrest diameter ≥20 km. Floors are lower than the surrounding terrain
except where filled by younger materials. The best preserved craters
have radial ridges and grooves in the rim material beyond one-half
crater diameter from the rim crest and chains and loops of satellitic
craters beginning at about one diameter.

Relative age is determined from superposition relations where
stratigraphic markers are present: Copernican crater materials are
superposed on Eratosthenian materials; both are superposed on Imbrian
mare material; Imbrian crater materials are overlapped locally by mare
material but are superposed on the circum-Imbrium materials and
associated structures. Pre-Imbrian crater materials are overlapped or
cut by these circum-Imbrium materials or structures. Where not in
contact with a datum plane, crater materials are dated by physiographic
freshness on the assumption that within a given size class the degree of
subdual of similar topographic elements--- rim-flank structure, rim
crest, interior terraces, and central peak---is a function of crater age
as suggested by Dietz (1946), Baldwin (1949), and Hackman (1961). A
refined physiographic dating scheme by Pohn and Offield (1970) based on
Orbiter photographs is used here, with modification appropriate to the
mapping scale. A continuum of crater types is recognized, starting with
very sharp, bright-rayed craters having large thermal anomalies6 such as
Tycho and Aristarchus, here assigned a late Copernican age (Cc2), and
ending with degraded, vaguely expressed, thermally bland ring structures
or craters such as Hommel, assigned an early pre-Imbrian age (pIc1).
Coincident with this degradation is the disappearance of rays in
Eratosthenian craters, and of other associated features such as radially
ridged rim material and satellitic craters, in Imbrian craters. As a
crater becomes more degraded its membership in the sequence becomes more
uncertain; that is, it may not be a degraded equivalent of a Tycho-type
crater---it may have had a different original morphology. In addition,
geologic environment affects crater physiography. Within the terrae the
rays and rim deposits of many craters are more poorly defined than on
the maria. Many

craters in the terrae could therefore be younger than indicated, as
suggested by a deficiency of craters mapped as Eratosthenian in the
southern terrae.

An impact origin for the larger young craters seems required to account
for the energy necessary to eject the projectiles that produced the long
rays (as much as 20 times the crater diameter) and the loops and chains
of satellitic craters (Shoemaker, 1962). Smith (1966) noted that most
lunar craters have rougher, higher rims than terrestrial calderas of
comparable size. In general, the morphology of all the craters of the
sequence suggests an explosive origin (Baldwin, 1949, 1963). Their
generally random distribution and the observed cumulative
frequency-versus-diameter relations for a given geologic unit (Dodd and
others, 1963; McCauley, 1967b, p. 444--446) support an impact origin.

Cc2 , *crater material*

Type area: Tycho, 43.2°S., 11.2°W. (Orbiter IV, 119, 124; Orbiter V,
125--l28). Reference areas: Aristarchus, 23.7°N., 47.3°W. (Orbiter V,
194--201); Dionysius, 2.8°N., 17.3°E. (Orbiter V, 80--83)

Very sharp topographic detail at all Orbiter scales. High thermal
anomaly6. Extensive radial rim materials and rays with associated loops
and chains of sharply defined satellitic craters (not mapped)

Cc1, *crater material*

Type area: Copernicus, 9.7°N., 20°W. (Shoemaker and Hackman, 1962)
(Orbiter IV, 121, 126; Orbiter V, 150--157)

Sharp topographic detail at Orbiter IV resolution; subdual of all crater
elements evident at higher resolutions. Either bright rays or high
thermal anomaly, or both. Extensive radial rim material and related
fields of satellitic craters around large craters (Orbiter V, 142--145)

Ec, *crater material*

Type area: Eratosthenes, 14.5°N., 11.3°W. (Shoemaker and Hackman, 1962)
(Orbiter IV, 114; Orbiter V, 133--136)

Fairly sharp topographic detail, but without surrounding bright rays or
pronounced thermal anomaly. Extensive radial rim material and fields of
satellitic craters around large craters, but topographic detail of these
less sharp than around Copernican craters

Ic2, *crater material*

Type area: Plato, 51.4 N., 9.2°W. (Orbiter IV, 122, 127, 128; Orbiter V,
129--132---east rim flank)

Rim crest and interior terraces moderately sharp. Fairly numerous
superposed small craters. Radial rim material and satellitic craters
recognizable around largest craters (Plato, Archimedes)

Icl, *crater material*

Type area: Piccolomini, 29.8°S., 32.3°E. (Orbiter IV, 76, 77)

All topographic elements subdued. Radial rim material and satellitic
craters inconspicuous or not visible except in largest craters
(Petavius, Iridum crater). Moderately high density of small superposed
craters, often with one or more craters in the ≥10 km range

pIc3, *crater material*

Type area: Alphonsus, 13.5°S., 2.7°W. (Ranger IX, Orbiter IV, 108;
Orbiter V, 116--119)

Considerably subdued, having rim material consisting of raised ring with
little apparent textural detail except smooth external terraces.
Concentric terraces on interior wall partly coalesced and cut locally by
channels. Floors of these and older craters generally filled with
younger plains units

pIc2, *crater material*

Type area: Clavius, 58.4°S., 14.4°W. (Orbiter IV, 118, 130, 131, 136,
142)

Very subdued, with rim material consisting of low, often incomplete
ring. Wall terraces subdued, incomplete, broken by channels. High
density of superposed craters

pIcl, *crater material*

Type area: Hommel, 54.5°S., 33°E. (Orbiter IV, 82, 88, 95)

Broad, low-rimmed ring structure. Inner and outer terraces similar to
those of younger craters recognized only in largest examples

**MATERIALS OF CRATER CHAINS AND CLUSTERS**

Crater chains are of two types: (1) three or more individual craters
alined and similar in size and morphology that are overlapping,
tangential, or alined along a rille; and (2) pairs of similar
overlapping craters not separated by septa. Crater clusters are
nonlinear arrays of three or more similar overlapping or tangential
individuals (with or without septa). Individual craters are either
circular or irregular in outline, mostly ≤10 km in diameter. Rim
materials, if present, are relatively smooth. Interior terraces are
generally lacking, and floors are shallow. The chain and cluster
category is gradational with the irregular crater category in cases
where outlines merge to form a single occurrence ≥10 km in diameter.

Morphology is a less reliable age criterion than for the main-sequence
craters because most chain cluster craters are small and topographically
simple. As a first estimate, however, they are assigned an
"Eratosthenian or Copernican" age if they are sharp-rimmed, relatively
uncratered, and otherwise fresh appearing; an Imbrian age if moderately
subdued; and a pre-Imbrian age if strongly subdued and highly cratered.

Crater chains and clusters are believed to be of internal and secondary
impact origin because the alinement, grouping, distribution patterns,
and morphology argue against primary impact. Alinement along fractures
suggests internal origin. Concentration of similarly sized craters
between one and two diameters from a crater or basin and partial
alinement of clusters and chains radial to it suggest secondary impact
origin. This origin is believed certain for satellitic craters (Isc) of
the Iridum crater and Orientale basin (which resemble smaller unmapped
secondaries of craters such as Copernicus and Eratosthenes), quite
likely for crater clusters and chains (Icc) at the appropriate distance
from the Imbrium basin, and possible for certain chains and clusters
(pIch) around the Nectaris basin.

CEch, *material of crater chains and clusters*

Type area: northwest segment of Rima Hyginus, 8°N., 6°E. (Orbiter IV,
97, 102; Orbiter V, 94--97)

Tangential and overlapping craters of similar size and morphology, or
linear furrow with scalloped edges (in Pitatus). Individual craters 2--5
km at type area, l--3 km at Rima Davy I (11°S., 6°W.); topographically
sharp; bowl-shaped profile or small flat floor. Distinct thermal anomaly
at type area. Albedo high at Rima Davy and type area

Volcanic, analogous to terrestrial maars or collapse craters

Ich, *material of crater chains and clusters*

Type areas: (a) Chain extending east-southeast from 14.8°S., 13.8°E.,
south rim of crater Abulfeda (Orbiter IV, 89; Orbiter V, 84); (b)
cluster centered at 30°S., 4°W. (Orbiter IV, 107, 108, 112, 113)

Most individual craters moderately subdued. Three general types:
circular and bowl shaped, circular or irregular with narrow ridgelike
rims and broad shallow floors, or linear furrow with scalloped edges

Most are probably volcanic, but some may be clusters of secondary impact
craters, particularly of Imbrium and Orientale basins; linear chains in
central part of Moon and radial to Imbrium probably produced during
basin formation by radial faulting or secondary impact. Age range early
to late Imbrian.

Isc, *satellitic-crater material*

Type areas: Clusters and chains centered (a) at 55.5°S., 48°W., on
southeastern side of crater Schiller C (Orbiter IV, 160, 167) and (b) at
53 N., 24°W., northeast of the crater around Sinus Iridum (Orbiter IV,
140, 145)

Type (a) individual craters in 10--15 km size range with sharp rim
crests and bowl-shaped interiors without distinct floors. Alined in
closely packed chains or clusters with the walls down-range from
Orientale basin commonly breached and grading outward into a herringbone
ridge-and-trough pattern. Most but not all located beyond mappable
extent of Hevelius Formation, typically 1200 to 1500 km from center of
Orientale basin. Similar in general properties and distribution to
secondary impact craters identified around Copernicus but averaging
about three times larger. Type (b), around Iridum, similar but in 5-8 km
size range

Secondary craters formed by impact of large blocks ejected at low to
intermediate trajectory angles from (a) Orientale basin and (b) Iridum
crater, which are nearly of same age

Icc, *material of crater clusters and chains*

Type area: Cluster centered at 36°S., 27°E., on rim of crater Riccius
(Orbiter IV, 83)

Individual craters as much as 25 km in diameter; majority of each mapped
cluster and chain composed of craters \>10 km. Moderately subdued
rimcrest and bowl-shaped interior without distinct floor, except where
(in some western occurrences) filled by younger materials. Early Imbrian
age indicated by similarity of rimcrest sharpness and density of
superposed craters to oldest main-sequence craters on Imbrium blanket;
but interior more bowl-shaped than in main sequence. Concentrated in
southern and southeastern terrae, on pre-Imbrian units. Grouped in
closely packed clusters and chains of overlapping individuals; some
chains radial to Imbrium basin. Similar in general grouping and
morphology to Orientale satellitic craters but more subdued, and
individuals and clusters somewhat larger

Either secondary impact origin or volcanic. Secondary impact from
Imbrium basin favored for most on basis of distribution, size, and
apparent early Imbrian age

pIch, *material of crater chains and clusters*

Type areas: (a) Vallis Rheita, 39°S., 47°E. (Orbiter IV, 64, 71); (b)
Southeast of Schiller (Orbiter IV, 148, 154, 155)

\(a\) Large linear valleys with separated and overlapping, circular or
elliptical craters, and (b) chains or clusters of at least four
tangential or overlapping

circular individuals; some individual craters resemble main-sequence
pre-Imbrian craters, others only narrow, broad-floored rings

Linear valleys produced during basin formation by radial faulting or
secondary impact. Some craters outside fourth ring of Nectaris basin, as
at 45°S., 48°E., possibly Nectaris secondaries; others possibly primary
impacts alined by chance, or volcanic

**IRREGULAR-CRATER MATERIALS**

This category includes materials of individual craters or overlapping
craters without septa; the category may locally be gradational with the
chain and cluster category. Outlines are irregular with marked
reentrants or elongated by more than 2:1. Rim materials are relatively
smooth. Inner profiles may be like those of other classes, but most
examples lack terraces on walls and have relatively shallow floors.

Age criteria are the same as for chains and clusters.

The irregular shape suggests an internal origin. Some, however, could be
clustered secondary impact craters; overlapping, modified primary impact
craters; or primary impact craters produced by fragments of a body
broken up by tidal force before impact (Sekiguchi, 1970).

CEci, *irregular-crater material*

Type area: Crater adjacent to Rima Bode II, 13°N., 4°W. (Orbiter IV,
109; Orbiter V, 120--123)

Sharp topographic detail at all scales, distinct thermal anomaly, and
high albedo at type crater. Other occurrences also fairly sharp but less
bright and without detected thermal anomaly

Ici, *irregular-crater material*

Type area: Catharina D, 17°S., 21.5°E. (Orbiter IV, 84)

Moderately subdued. Small to large (10 to 40 km); diverse in outline and
profile, majority having narrow rims, smooth profiles, and broad floors.
Age range early to late Imbrian

pIci, *irregular-crater material*

Type area: Schiller, 51.8°S., 40°W. (Orbiter IV, 148, 154, 155, 160)

Range from deep, with steep inner walls that are terraced or radially
streaked, to shallow, with narrow rims and broad floors. Type example as
subdued as young pre-Imbrian main-sequence craters

**MATERIAL OF SMOOTH-RIMMED CRATERS**

This category is composed of individual craters having outlines similar
to those of main-sequence craters. Rim materials are smooth, without
pronounced terraces or radial rim deposits. Satellitic craters are
absent. Concentric interior terraces are present in some craters, and
floors are commonly about level with the surrounding terrain. Diameters
are mostly in the 20--40 km range.

All craters in this category are assigned an Imbrian age on the basis of
rimcrest sharpness and common embayment by Imbrian mare material;
however, some craters appear to be superposed on the mare material or
gradational with it and could be post-Imbrian. Although topographic
features are subdued to a degree that would indicate early Imbrian or
pre-Imbrian age in main-sequence craters, superposition of the type
example on middle or late Imbrian features of the Orientale basin floor
suggests relative youth and, therefore, an original form distinct from
that of the main-sequence craters.

These craters differ sufficiently from main-sequence craters to suggest
a different origin. Non-impact origin is suggested by the lack of rugged
exterior terraces. The craters resemble terrestrial ash-flow calderas in
morphology and size (Smith, 1966). They could have been formed, however,
by an unusual type of impacting body or unusual conditions (fluidity?)
in the target materials at time of impact.

Ics, *material of smooth-rimmed craters*

Type area: Crater Kopff in Orientale basin at 18°S., 90.5°W., outside
the map area (McCauley, 1968) (Orbiter IV, 187). Reference area in map
area: Lassell, 15.5°S., 7.8°W. (Orbiter IV, 113)

**UNDIVIDED CRATER MATERIALS**

Crater materials are placed in one of three "undivided" categories if
the craters are too small or too greatly modified to show the criteria
necessary for assignment to one of the major morphologic categories or
to the age categories of the main sequence. Their distribution is
similar to that of the main-sequence craters, and most are probably
degraded equivalents of craters originally like Tycho; some may be small
or modified equivalents of the smooth-rimmed category. Because their
original form is unknown, physiographic dating is precluded; individuals
can be dated, however, from local superposition relations. "Undivided"
craters constitute about a third of the nearside craters larger than 10
km.

Cc, *undivided crater material*

Type area: Guericke C, 11.5°S., 11.5°W. (Orbiter IV, 113)

Sharp-crested fresh craters and associated deposits with bright (in a
few cases, dark) rays or halos and (or) high thermal anomaly. Rimcrest
diameter mostly ≤15 km

Ic, *undivided crater material*

Type areas: (a) Large, Stadius, 10.5°N., 13.5° W. (Orbiter IV, 114,
121); (b) small, Aratus A, 22°N., 5°E. (Orbiter IV, 102); (c) atypical,
Alpetragius, 16°S., 4.5°W. (Orbiter IV, 108)

Moderately subdued form. Includes craters of two types: (1) those
difficult to assign to either the upper or lower Imbrian because of
small size, resolution limitations, extensive flooding by younger
materials, or proximity to large youthful craters; (2) those whose
central peaks and interior profiles are unlike those of main-sequence
craters. Prominent inner ring concentric with rim of some small (10--15
km) craters (such as Hesiodus A, 30.1°S., 17°W.)

pIc, *undivided crater material*

Type areas: (a) Large, Ptolemaeus, 9.3°S., 1.8°W. (Orbiter IV, 108); (b)
small, craters between 42°S. and 46°S., 27°E. and 31°E. (Orbiter IV, 83)

Subdued ring structures either deeply buried or too degraded for
assignment to a pre-Imbrian subdivision. Craters generally ≤20 km, but
larger severely degraded rings narrow relative to rimcrest diameter also
included

![](media/image1.pct){width="3.1666666666666665in"
height="1.5555555555555556in"}

Crest of basin ring structure (generalized)

*Dashed where inferred from mare ridges, widely separated terra hills,
or relatively low terra features*

NOTES TO DESCRIPTION OF UNITS

1Terms "crater(s)" and "mare" (plural, maria ) which, strictly, refer to
topographic forms, are used for brevity in many descriptions in place of
"crater materials" and "mare materials," which are the geologic units
mapped and interpreted.

2Albedo (brightness at full Moon) values are expressed in qualitative
terms which correspond approximately to normal albedos determined by
Pohn

and Wildey (1970) as follows:

> Very low albedo (very dark at full Moon) \< .074
>
> Low (typical of the maria) .074-.096
>
> Intermediate (typical of the terrae) .096-.159
>
> High(bright at full Moon) \>.159

3All lunar colors are reddish; color is expressed here as bluish or
reddish relative to a reddish average (Whitaker, 1966; McCord, 1969).

4References cited are listed in the accompanying pamphlet.

5Lunar Orbiter IV frames listed are high resolution (70--150 m
identification resolution); for other Orbiter missions the number given
includes both the high- and moderate-resolution frames (ranging from 1
to 40 m), though some high-resolution frames do not include the feature
in question.

6Thermal data are expressed as relative strength of near-infrared signal
at eclipse. Thermal anomalies above a background (Shorthill and Saari,
1969). Thermal anomalies are believed to result from a local abundance
of exposed rock.

Mapped 1968--1969. Wilhelms mapped the area east of long 10° W. and the
remaining parts of the Tycho and Clavius quadrangles; McCauley mapped
the rest of the area and prepared most of the geologic summary.

Authors relied heavily on the 1,000,000-scale maps listed in the
accompanying pamphlet

PRINCIPAL DATA SOURCES

*Topographic forms and textures:* Photographs from Lunar Orbiter IV
(11--25 May 1967), courtesy of Lunar Orbiter Project Office, Langley
Research Center, NASA; index *in* Kuiper and others (1967); U.S.
National Space Science Data Center (1969); Kosofsky and El-Baz (1970);
supplemented by near-terminator telescopic photographs (Kuiper and
others, 1967) in the eastern regions where Orbiter photographs are poor

*Albedo:* Unpublished near-full-Moon photographs 5818 and 5819 taken
with the U.S. Naval Observatory 61-inch reflector (Flagstaff, Ariz.);
published photograph L-18, Lick Observatory 36-inch refractor (Kuiper,
1960); Pohn and Wildey (1970)

*Thermal:* Shorthill and Saari (1969)

*Color:* Whitaker (1966); McCord (1969)

Lunar base mosaic LEM-1, 3rd edition, 1966, by the USAF Aeronautical
Chart and Information Center, St. Louis. Missouri 63118

GEOLOGIC SUMMARY

MARIA AND MULTI-RING BASINS

The circular basins, generally encompassed by three or more partly
complete scarps or ring systems, dominate the geology of the Moon\'s
near side. Diameters of the innermost ring range from 250 to 600 km
(kilometers), and the spacing of the three inner rings increases from
one to the next by a regular ratio (Hartmann and Kuiper, 1962).1 The
observed and inferred crest lines of the most prominent of these
structures are shown by brown lines on the accompanying map. The
depressed central parts of the basins and the concentric troughs which
lie between successive rings are completely or partly filled with dark
plains material (mare) which is younger than the basins and which
appears to have been emplaced in a fluid and probably molten state.
These basins and their subsequent mare fill have destroyed large parts
of the earlier geologic record, and in this respect have acted like
terrestrial orogenies. The historical geology of the Moon can be
understood only in terms of these dominant, primitive structures which,
if ever present on Earth, have long since been obliterated or deeply
buried and severely modified.

The general circularity of the basins, the textural similarity of the
materials of the ring structures, and the regularity of the ring spacing
suggest a common origin. If all basin-related features are considered,
particularly for the best preserved basins, an impact origin, as first
suggested by Gilbert (1893), is strongly favored. The inner rugged rings
of the two best preserved basins, Orientale (just outside the map area)
and Imbrium, are surrounded by distinctive ejecta blankets (Hevelius
Formation and Fra Mauro Formation respectively) which can be traced
outward for hundreds of kilometers. Relicts of what is probably a third,
more degraded and older ejecta blanket (Janssen Formation) can be
recognized south of the Nectaris basin. Within and beyond these
blankets, chains and clusters of craters have been identified as
probable secondary impact craters on the basis of morphology, size, and
distance from the basin (see section on crater materials). Observable
differences among basins---such as the presence or absence of a
recognizable blanket, the completeness of the concentric ring
structures, the freshness and relief of the rings, the character of the
radial structures, and the types and numbers of superposed craters---can
be explained by variations in relative age, degree of filling by younger
mare, or proximity to younger basins. Alternative explanations for the
multi-ring basins calling for an internal origin (Spurr, 1944; Green,
*in* Oriti and Green, 1967) are inadequate because of failure to account
for all of the basins' observable properties, relative ages, and
relations to subjacent and superposed units such as the maria.

A considerable body of stratigraphic evidence shows that the mare
material in the basins is not the immediate and direct product of the
basin-forming impact but is a later volcanic fill: (1) The mare
truncates all other basin-related deposits and structures. (2) It is
located in basin-related topographic lows, including troughs between the
outer rings far from the basin centers. (3) The mare surface materials
in all basins appear to have a narrower age range (determined from the
number of superposed craters) than the basins, which differ greatly in
state of preservation and apparent relative age. (4) Craters which are
superposed on the basin ejecta or basin-related structures but are
embayed or filled by mare material occur in every basin (for example,
Posidonius, Gassendi, Cleomedes, and Fracastorius in the Serenitatis,
Humorum, Crisium, and Nectaris basins respectively). The postbasin,
premare craters within the Imbrium basin (that is, Imbrian-age craters)
are

approximately equal in number to the total number of postmare craters
(Eratosthenian and Copernican ages) in the same area. Thus a
considerable interval between basin formation and final mare filling is
suggested. (5) A distinct geologic unit, light plains material (unit
Ip), lies stratigraphically between the deposits of the Imbrium basin
and postbasin, premare crater materials, including those of Archimedes,
Cassini, and Plato. (6) Features far from and unrelated to the Imbrium
basin are also of Imbrian age, demonstrating an extensive history
between the formation of the basin and the last mare fill. Orientale is
one such Imbrian-age feature, and many Imbrian craters apart from those
in the Imbrium basin both predate and postdate Orientale. (7) The
nearside basins are filled by mare to varying structural levels
independently of their size. For example, Nectaris is filled to the
first ring and Crisium and Imbrium to the second and third respectively.
Uneven filling suggests different volumes of mare material in each,
reflecting internal rather than external (impact melting) controls on
their filling.

Each basin immediately after excavation and early gravitational slumping
probably resembled the present youthful-appearing Orientale basin
(McCauley, 1967b, p. 439-446; 1968). Orientale is the key to the
internal configuration of the more degraded and deeply filled basins
because of its lack of extensive mare filling and its relative youth.
Its present shallow profile suggests that the thickness of the mare fill
even in more deeply flooded and obscured basins like Imbrium is on the
order of only 2 to 5 km. Mare material, although covering almost 30
percent of the nearside surface (and very little of the far side) is,
therefore, volumetrically a small part of the Moon (radius 1738 km). The
deeply flooded basins like Imbrium may lie in regions where potential
source material was at shallower depth than it was in the less filled
basins like Orientale and Nectaris. The existence of numerous
thalassoids (basins without mare fill) on the far side of the Moon
(Lipskiy, 1965) further supports the conclusion that the location of a
basin and local crustal conditions determine the degree of later
flooding.

Oceanus Procellarum is unusual because of its non-circular, somewhat
irregular shape. It is the largest expanse of mare on the Moon that is
not obviously controlled by a single circular multi-ringed basin. It
contains, throughout most of its extent, numerous terra islands and
large ghost or vestigial craters such as the Flamsteed ring which
collectively suggests that the mare is relatively shallow. Its overall
shape however, appears to be controlled by outer ring structure of the
Imbrium basin and two vague, ancient ring structures centered near
Copernicus and Flamsteed which were identified during this work.

During and after the period of flooding, meteorite bombardment eroded
and subdued the basin rings and blankets through direct hits, impact of
far-flung secondary ballistic fragments (Shoemaker, 1965), and seismic
shaking of the surrounding terrain triggering downslope movement of
loose materials (Titley, 1966). This process led to the eventual
disappearance of the distinctive depositional textures of the
circumbasin blankets. Catastrophic premature aging and partial
obliteration of basins is exemplified by Serenitatis, whose rings have
been disturbed structurally and mantled by ejecta from the nearby
Imbrium basin. The trend of the younger Imbrium rings has in turn been
influenced by the presence of the earlier Serenitatis structure as seen
in the change in the trend of the "Apennine" ring in the Montes Caucasus
region.

TERRA PLAIN, PLATEAU, AND DOME MATERIALS

Prior to formation of the youngest mare material, the basins were also
modified by the deposition of several types of distinctive terra units
described in detail in the accompanying explanation. The most extensive
of these are the light terra plains materials of Imbrian age (unit Ip)
which lie predominantly in circumbasin troughs and on the floors of
pre-Imbrian and lower Imbrian craters. Plains identified as pre-Imbrian
(pIp) by superposition of lowermost Imbrian crater clusters occupy more
restricted areas in the southern terra. These two marelike plains units
may represent earlier generations of volcanism and crustal flooding,
their present high albedo relative to the mare material being due either
to initial compositional differences or to more extensive churning and
consequent brightening by prolonged meteorite bombardment. The most
distinctive terra unit (hilly and furrowed material, Ihf) forms level or
shieldlike plateaus that cover basin ejecta and postbasin craters and
contain numerous small primary volcanic landforms. The two largest
occurrences lie west of Nectaris between the third and fourth rings and
northwest of Humorum athwart and beyond the third ring. These plain and
plateau units, which apparently modify earlier basin or crater deposits,
suggest a more extensive history of lunar volcanism than previously
recognized. Moreover, they differ markedly in albedo and roughness from
the dark, relatively smooth mare volcanic deposits and they may thus
represent the products of a distinct terra magma or of mare magma
contaminated by terra materials. The bulk differences in composition
from the maria could, however, be minor, as in the case of terrestrial
cone and flow materials within the same volcanic field. The distribution
of the terra volcanic units is controlled predominantly by basin ring
structures; some occur at the intersection of younger rings structures
with vague circular depressions which are the probable relicts of very
ancient basins. In addition to these more extensive deposits, several
types of domes, mostly in the 10 km size range, have been identified
both in the maria (units Emd, Emp) and in the terrae (units Id, CEd).
Many of these, including the largest and most distinctive, the postmare
terra domes of the Gruithuisen area southwest of Sinus Iridum, lie along
circumbasin ring structures.

CRATER MATERIALS

Craters of all ages, mostly circular, dominate the lunar landscape at
all scales. The circular craters, except those deeply buried or in a
poor state of preservation because of their antiquity, have
physiographic characteristics that indicate an explosive origin
(Baldwin, 1949, 1963). Their cumulative frequency distributions are
difficult to explain by a volcanic model (Green, 1962) but do fit well
with an impact model (Baldwin, 1963). The circular craters, probably of
primary impact origin, are here grouped in a "main sequence" divided
into eight age categories.

Foremost among the craters not grouped in the "main sequence" are chains
and clusters of 10-15 km bowl-shaped to irregular craters lying within
and beyond the confines of the Hevelius Formation (out to 1500 km from
the Orientale basin center) and confidently identified as secondary
impact craters formed by ejecta from the Orientale basin (unit Isc).
Similar but larger, less well preserved clusters and chains of early
Imbrian craters can be identified on the pre-Imbrian terrae as probable
secondary craters of the Imbrium basin (unit Icc). Many degraded or
partly buried pre-Imbrian chains and clusters (unit pIch) near the edge
of the Janssen Formation may be secondary craters of the moderately well
preserved Nectaris basin. Thus secondary craters of

large basins constitute an important part of the total lunar crater
population in the 10--25 km size range on the older parts of the terrae.
A few smooth, low-rimmed craters such as Lassell and Damoiseau, mostly
in the 20 to 30 km size range, lack impact characteristics and could be
calderas (unit Ics). Certain irregular craters and clusters and chains
of smooth-rimmed craters that are controlled by local or regional
structural trends are also considered to be volcanic features. Most of
these are relatively small and relatively young (units Ich, Ici, CEch,
CEci), but some large irregular craters (pIci) and chains (pIch) of
probable pre-Imbrian age could also be volcanic, suggesting a possible
early epoch of crater-forming volcanism.

A number of craters, particularly those of the older, subdued variety
such as Pitatus, exhibit numerous superposed volcanic landforms such as
domes and crater chains (mostly too small to be mapped as separate
features). The presence of these landforms suggests either that the host
craters are volcanic or that impact scars may be the loci of
long-subsequent volcanism even for features much smaller than basins.

GEOLOGIC HISTORY

From these regional observations and interpretations a geological
history of the Moon can be extensively defined. The relative ages of the
multi-ring basins can be partly established by overlap relations with
one another, relative topographic freshness, and age of superposed and
subjacent craters. Orientale is younger than Imbrium; Nectaris, Crisium,
and Humorum are older but difficult to date with respect to one another
because of their geographic separation; Serenitatis and Fecunditatis are
next in the sequence, and Nubium and Tranquillitatis are the most
ancient of the prominent multi-ring basins. Each basin is inferred to
have undergone its own cycle of early volcanic filling, impact erosion,
and mantling by thick or thin to discontinuous ejecta from nearby and
distant younger basins, and each contains mare fill in varying amounts.
Thus the stratigraphic sections in and around each of these basins are
complex and reflect variable local interplay between external and
internal lunar processes.

The most primitive appearing terrain on the near side lies in the
south-central part of the map area extending as a wedge-shaped tract
broadening from Ptolemaeus to the region of the south pole. This
province consists predominantly of closely packed 100-200 km pre-Imbrian
craters of the probable impact sequence along with chains and clusters
of Imbrium and probably Nectaris secondary craters. The preservation of
this topographically positive area, within which volcanic filling played
a role of uncertain importance, is explained by its location beyond the
range of extensive blanketing and structural disturbance of the
recognizable circular basins.

The regions south and southeast of Mare Humorum, between Serenitatis and
Crisium, and southeast of Crisium also consist of basically pre-Imbrian
terrain and represent the next most primitive part of the Moon's near
side. This essentially nondescript terrain, characterized by a moderate
deficiency of large pre-Imbrian craters, is considered to represent an
interlayered sequence of now unrecognizable crater material, volcanic
material, and basin ejecta blankets; each blanket originally resembled
that around Orientale and buried much of the earlier local geological
record.

The lunar stratigraphic record becomes clearer with the catastrophic
formation of the Imbrium basin which began the Imbrian Period. The basin
and the blanket, however, have undergone considerable modification since
their formation, and only relicts of the original depositional pattern
are now recognized. Most of this blurring of original textures is
attributed to

subsequent meteorite bombardment, but some is also due to volcanism.
Extensive light plains materials filled low areas within the basin and
in the troughs between rings, truncating the braided and hummocky
textures of the blanket and basin-radial structures. Dark blanketing
material was deposited in a broad arc midway between the third and
fourth Imbrium rings. Steep domes and other distinctive terra landforms
were also formed. These various blanketing and filling materials range
from early Imbrian to Copernican in age, further supporting the
conclusion that basin modification by volcanic processes is of long
duration.

Some time after formation of the Imbrium basin, extensive modification
of parts of the older terrae began, especially northwest of Mare Humorum
and west of Mare Nectaris. Distinctive terra landforms were formed near
these older basins in greater number than near Imbrium, a fact which may
reflect time dependence in their formative process, possibly magmatic
differentiation as the Moon evolved.

The most widespread flooding of the depressed parts of the lunar crust
occurred near the end of the Imbrian Period with the filling of most of
the nearside multi-ring basins by mare material. This was followed by a
less extensive episode of mare generation, terra blanketing, and
volcanic mare plateau formation (as in the Marius Hills), mostly in the
Eratosthenian Period.

Impact cratering, at a rate probably substantially reduced from that of
pre-Imbrian to early Imbrian time, continued simultaneously with these
events. The most recent impacts produced the prominent ray craters such
as Copernicus and Tycho, whose ejecta blankets are superposed on almost
all other units. No widespread volcanic activity appears to have
occurred since this cratering, although some activity, as in the Littrow
region at the southeastern edge of Mare Serenitatis, may be
approximately contemporaneous with it.

GENERAL CONCLUSION

Systematic regional mapping shows the Moon to be a primitive body
without the orogenic belts, mobile plates, oceanic ridges, and
widespread waterlaid sedimentary deposits characteristic of the Earth\'s
crust. The Moon does, however, exhibit a geologically heterogeneous
surface with a long and complex history which can be partly unraveled
from existing photographic data. Geologic mapping reveals a Moon that is
neither dominantly "volcanic" nor dominantly "impact" but rather one in
which both processes have been operative. Most of the volcanic units are
restricted to regionally depressed areas, the distribution of which is
controlled by ancient multi-ring basins formed by impact relatively
early in lunar history. Implicit in this historical model is that a
moderate amount of magmatic fractionation and vertical differentiation
took place throughout at least the middle parts of lunar history
(Imbrian Period). Thus most of the present surface, although less
reworked than that of the Earth, is neither primordial nor of meteoritic
composition. However, even the mare rocks, which are relatively young on
the lunar time scale, have been shown by preliminary results from
Apollos 11 and 12 to be as old or older than any presently dated
terrestrial rocks. Thus the geologic record on the Moon appears to
complement that of the Earth by covering the period of time for which
the record is missing on Earth.

1References cited are listed in the accompanying pamphlet.