**GEOLOGIC MAP**

**OF THE**

**SOUTH SIDE**

**OF THE MOON**

By

D.E. Wilhelms, K.A. Howard,

and H.G. Wilshire

1979

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--1162, as part of the Geologic Atlas of the Moon, 1:5,000,000 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 MAP UNITS**

**MATERIALS OF PRIMARY IMPACT CRATERS**

**AND THEIR SECONDARY CRATERS**

Cc YOUNGEST CRATER MATERIAL---Sharply textured and bright, mostly
surrounded by bright rays

Csc SECONDARY-CRATER MATERIAL

Ec CRATER MATERIAL YOUNGER THAN MOST MARE MATERIAL---Similar to unit Cc
but less sharp and bright; queried where could be unit Ic2

Esc SECONDARY-CRATER MATERIAL

Ic2 UPPER IMBRIAN CRATER MATERIAL---Younger than Orientale basin and
part or all of unit Iml, older than parts of unit Im2; small craters
lack fine textures, large craters have numerous small superposed
craters; queried where could be units Ec, Icl, or Ioc

Ic1 LOWER IMBRIAN CRATER MATERIAL---Younger than Imbrium basin but older
than Orientale basin; morphologically subdued, commonly difficult to
distinguish from units Ic2 and Nc

Isc SECONDARY-CRATER MATERIAL

Nc CRATER MATERIAL YOUNGER THAN NECTARIS BASIN BUT OLDER THAN IMBRIUM
BASIN---Few rim or interior textures visible except in largest craters;
queried where could be units pNc or Nbc

Nsc SECONDARY-CRATER MATERIAL

pNc CRATER MATERIAL OLDER THAN NECTARIS BASIN---Rim relief normally the
only feature visible except in largest craters; queried where could be
unit Nc

**BASIN MATERIALS**

> ORIENTALE GROUP---Includes material of satellitic craters and Hevelius
> Formation

Ioc MATERIAL OF ORIENTALE-BASIN SATELLITIC CRATERS---Grouped in clusters
and chains peripheral to Orientale basin and in some outlying areas.
*Interpretation:* Secondary impact craters of Orientale basin; queried
where could be primary craters

HEVELIUS FORMATION---Divisible into inner, nonlineated, and outer facies

Iohi Inner facies---Continuous deposit with ridges and grooves radial to
Orientale basin centered north of area (concentric in crater Inghirami);
thickly mantles subjacent cratered terrain. *Interpretation:* Ejecta of
Orientale basin that flowed long distances along surface

Iohn Nonlineated facies---Deposit with relatively smooth, wavy surface
gradational with inner facies; forms raised lobes in places.
*Interpretation:* Primary or secondary ejecta emplaced by fluidlike
surface flow; may include impact melt

Ioho Outer facies---Discontinuous deposits adjacent to Orientale
secondary craters (mapped and unmapped) commonly having basin-radial
ridges and grooves. *Interpretation:* Mostly ejecta formed by closely
spaced secondary impacts

Iic MATERIAL OF IMBRIUM-BASIN SECONDARY-IMPACT CRATERS---Similar to
better developed secondary craters closer to Imbrium basin (Wilhelms,
1976); queried where could be primary craters

Nbc MATERIAL OF NECTARIAN-BASIN SATELLITIC CRATERS---Grouped in
clusters, chains, and groovelike chains mostly peripheral and
approximately radial to Nectaris and other Nectarian basins; also
includes more distant, radially oriented groups. *Interpretation:*
Secondary impact craters of basin to which groups are radial or
peripheral; origin of Humorum basin-radial groups at lat 56° to 64° S.,
long 0° to 30° E. and of queried groups doubtful

Nb BASIN MATERIAL---Rim and wall material of double-ring basins.
*Interpretation:* Impact materials formed near edge of excavated cavity

Nbl BASIN MATERIAL, LINEATED---Lineated or otherwise textured material
on basin flank. *Interpretation:* Basin deposits corresponding to
Hevelius Formation of Orientale basin

Nbm BASIN-MASSIF MATERIAL---Forms inner rings and parts of rim of
basins. *Interpretation:* Blocks of crustal material uplifted during
basin excavation

Nbh HUMMOCKY BASIN MATERIAL---Hummocky terrain in floor of Schrödinger
basin. *Interpretation:* Impact melt, perhaps draped over toes of slump
structures (Howard and others, 1974)

Nj JANSSEN FORMATION---Deposit around Nectaris basin similar to outer
facies of Hevelius Formation. *Interpretation:* Ejecta of Nectaris-basin
secondary impacts (includes primary ejecta north of mapped area)

pNb BASIN MATERIAL, UNDIVIDED---Rim, wall, and inner-ring materials.
*Interpretation:* Same as for corresponding features of Nectarian basins

pNbm BASIN MASSIF MATERIAL---Large mountainous landforms commonly lying
along arcs; gradational with generally finer topography of unit pNbr.
*Interpretation:* Strongly uplifted parts of basin rims and inner rings

pNbr BASIN MATERIAL, RUGGED---Forms diverse rugged, mostly elevated
terrain; intermediate between unit pNbm and generally less rugged, lower
unit pNt. *Interpretation:* Parts of South Pole-Aitken and Australe
basins including main topographic rims, inner rings or other interior
materials, and possible South Pole-Aitken ejecta near craters Clavius,
Moretus, and Boussingault

**PROBABLE BASIN-RELATED MATERIALS**

Ip PLAINS MATERIAL---Light-colored, smooth, mostly flat-surfaced
deposits having superposition relations and crater densities indicating
Imbrian age. *Interpretation:* Primary and secondary ejecta of Orientale
and Imbrium basins and of craters

Ntp TERRA-MANTLING AND PLAINS MATERIAL---Light-colored, wavy, rolling,
or planar surfaces more heavily cratered than unit Ip. *Interpretation:*
Primary and secondary ejecta of Nectarian basins and large craters
equivalent to units Iohn, Ioho, and Ip, lacking their distinctive
textures because of degradation by cratering or other aging processes

INt TERRA MATERIAL---Moderate to weak smoothing of irregularities around
Orientale basin; some basin-radial lineations and small pits like those
of unit

Ioho. *Interpretation:* Outer secondary deposits of Orientale basin
superposed on deposits of Humorum (east) and Apollo (west) and on
Nectarian craters

IpNt TERRA MATERIAL---Similar to unit INt but slightly rougher, as
includes more crater rims. *Interpretation:* Outer deposits of Orientale
and Antoniadi superposed on pre-Nectarian craters

NpNt TERRA MATERIAL---Diverse but mostly low-lying, smooth-appearing
terrain gradational with lower and smoother unit Ntp and higher and
rougher pre-Nectarian basin and crater materials. *Interpretation:*
Probably composed of outer ejecta of Nectaris, other Nectarian basins,
and crater Clavius, thinly mantling pre-Nectarian terrain; similar
Nectarian material presumably superposed on adjacent mapped
pre-Nectarian materials but not visible

pNt TERRA MATERIAL---Forms moderately rugged to rugged, diverse terrain,
including degraded partial crater rims and lower intervening tracts,
gradational with lower and smoother unit NpNt and higher and rougher
units pNbr and pNc. *Interpretation:* Degraded equivalents of Imbrian
and Nectarian basin, terra, and plains materials and of younger crater
materials

IpNg GROOVED MATERIAL---Irregular grooves superposed on pre-Nectarian
and Nectarian crater materials *Interpretation:* Age of grooves
uncertain but morphology suggests Imbrian; possibly produced by ejecta
or seismic shaking from Imbrium basin impact

Ifc FRACTURED CRATER FLOOR MATERIAL---Raised floors with fissures.
*Interpretation:* Shrunken impact melt deposits or tectonically uplifted
and dilated crater floors

**MARE AND OTHER DARK MATERIALS**

EIm YOUNGEST MARE MATERIAL---Appears dark and lightly cratered.
*Interpretation:* Basaltic lavas

Im2 INTERMEDIATE MARE MATERIAL---Similar in dark color and crater
density to typical near-side mare material of late and middle Imbrian
age. *Interpretation:* Basaltic lava; exact age range and composition
unknown

Im1 OLDEST MARE MATERIAL---Light color and high density of superposed
craters unusual for mare. *Interpretation:* Old basaltic lava, perhaps
as old as Orientale basin

Imd MARE DOME MATERIAL---Dark, smooth, slightly elevated terrain in
maria. *Interpretation:* Basaltic intrusions or extrusions; queried
where could be terra mantled by lavas or pyroclastic deposits

Id DARK MANTLING MATERIAL---Halos around irregular craters along
fissures in floor of Schrödinger basin. *Interpretation:* Thin
pyroclastic deposits

Contact---Queried where inferred in poorly photographed areas

Crest of buried crater rim

Crest of basin ring structure---Dashed where poorly known

Fissure or narrow fault graben

Sinuous ridge

Sinuous scarp---Barb on uphill side

INTRODUCTION

This map of the lunar region below lat 45° S. is the sixth part of a
complete geologic reconnaissance mapping of the Moon at 1:5,000,000
scale. The upper half of the map portrays the geology of the
earth-facing or near side, and the lower half shows terrain that was
unknown before spacecraft photography. The near side was well
photographed at resolutions of 0.1 to 0.5 km by Lunar Orbiter IV, but
coverage of the far side is of uneven quality (fig. 1). The geology of a
small area near the south pole that was in shadow during the Orbiter
missions can be partly inferred from the nature of adjacent units.

Mapping methods and conventions are not discussed in detail here because
they are still perceived much as described earlier on a partly
overlapping reconnaissance map (Wilhelms and McCauley, 1971). Many
interpretations, however, have changed since preparation of that map. In
particular, Apollo data indicate that terra (highland) volcanism is
unimportant on the Moon and that most deposits of the terrae were
emplaced by impact processes. This interpretation has influenced our
delineation of some map units and their assembly into provinces on a
summary map (fig. 2), which shows 16 ringed impact basins in and near
the area, terra materials related or believed related to these basins,
and volcanic mare materials that postdate the basins.

BASIN MATERIALS

Most lunar impact excavations larger than 260 km in diameter and some
smaller ones possess one or more raised inner rings and so are called
multiring basins (Hartmann and Kuiper, 1962). Other circular features
this size and larger probably also originally had multiple rings, but
the inner rings are now obscured. Many terra materials mapped here can
be identified with specific basins, and the remainder are also believed
related to basins or craters. Formation of two large, ancient
(pre-Nectarian) basins (South Pole-Aitken and Australe) raised the
highest mountains; three Nectarian basin impacts and the later Orientale
basin impact outside the map area deposited extensive materials within
it; and other pre-Nectarian, Nectarian, and Imbrian basins in and
outside the area contributed to its geomorphic and stratigraphic
framework.

RING IDENTIFICATION AND DATING OF BASINS

The largest and oldest basin so far documented on the Moon is South
Pole-Aitken (Stuart-Alexander, 1978), also called Big Backside basin or
Aitken-Backside basin. The basin rim is defined by a discontinuous
mountain ring 2200 km (Stuart-Alexander, 1978) to 2500 km (this report)
in diameter. The larger figure is based on an interpretation that
massifs (map symbol pNbm) and adjacent rugged material (map symbol pNbr)
between lat 75° S., long 40 W. and the unphotographed zone are part of
the rim, and that other massifs and rugged material entirely on the far
side constitute a second ring 250 to 500 km inside the rim.
Alternatively, all the basin massifs and rugged basin material may
constitute a very broad rim whose mean diameter would be about 2200 km.
The rugged basin material as far north as lat 63° S. on the near side
may also be part of the rim flank. The near-side massifs were discovered
by Earth-based telescopic observations, and their connection with a
large basin beyond the lunar limb was predicted by Hartmann and Kuiper
(1962). The South Pole-Aitken basin is also expressed as a giant
depression detected by Zond 6 and 8 photographic altimetry (Rodionov and
others, 1971, 1977) and Apollo laser altimetry (Wollenhaupt and Sjogren,
1972; Bills and Ferrari, 1975; Kinsler and others, 1975). We estimate
the basin center to be located at about lat 56° S., long 180°. Numerous
small mare patches, partly localized in superposed basins and craters,
occupy the basin inside the inferred inner ring.

At least four other basins centered in the map area are also
pre-Nectarian in age. Planck, Poincaré and Schiller-Zucchius (\"basin
near Schiller\" of Hartmann and Kuiper, 1962) are

small, double-ring basins identified as pre-Nectarian on the basis of
degraded superposed craters. The fourth, Australe, is named for many
small, discontinuous patches of mare material collectively known as Mare
Australe. The circular pattern of the mare patches and an enclosing
arcuate chain of massifs near long 120° E. led to recognition of
Australe as a basin (Stuart-Alexander and Howard, 1970). This chain
apparently indents the rim of South Pole-Aitken, a relation which
suggests that Australe is the younger basin; exceptionally large massifs
in this chain may have been uplifted by both impacts. Additional massifs
and elevated tracts of rugged basin material and other units appear to
define two interior Australe rings.

Four additional pre-Nectarian basins have been tentatively identified. A
single ring about 700 km in diameter delineates a heavily cratered
probable basin here named Mutus-Vlacq according to the convention of
designating basins after superposed (unrelated) features (Wilhelms and
El-Baz, 1977). This postulated rim accounts for small mounds of basin
massif material (map symbol pNbm) and other elevated basinlike terrain
(map symbol pNb) near lat 62° S., long 10° to 35° E., and the basin may
have acted as a catchment for an otherwise puzzling concentration of
Nectarian plains materials (map symbol Ntp). The smaller
Sikorsky-Rittenhouse and Amundsen-Ganswindt basins show traces of inner
rings (not mapped), but their identity as basins and their ages are
uncertain because of burial by deposits from the Schrödinger basin.
Parts of their postulated rings---especially those of
Sikorsky-Rittenhouse ("unnamed A" basin of Hartmann and Wood, 1971,
later withdrawn from the list of basins by Wood and Head, 1976)-could
instead be part of the South Pole-Aitken basin. Finally, an indistinct
possible basin named Pingré by Hartmann and Kuiper (1962) and
Pingré-Hausen in this report underlies deposits of the crater Hausen and
of the basins Orientale, Bailly, and Mendel-Rydberg.

Three basins centered in the map area are believed to be Nectarian in
age ---younger than the Nectaris basin and older than the Imbrium basin
on the northern near side. The three differ greatly in degree of
degradation. An indistinct, large basin, here named Mendel-Rydberg, was
called the "SE limb basin" by its discoverers, Hartmann and Kuiper
(1962), who identified on telescopic photographs two of the three rings
mapped here. The nearby Bailly basin is smaller and younger. Both Bailly
and Mendel-Rydberg are severely degraded as a consequence of burial by
deposits from Orientale and the crater Hausen but are thought Nectarian
in age because no pre-Nectarian craters appear to be superposed. The
Schrödinger basin is younger still, and its deposits cover much of the
map area. The density of superposed craters too small to map here (\<20
km) suggests an age of late Nectarian, but this fresh-appearing basin
could be lower Imbrian as no definite Nectarian craters are superposed.
The inner ring consists of rugged crags similar to those of lunar
central peaks (Hartmann and Wood, 1971) and is believed related in
origin to such peaks although its exact formative process is uncertain
(Hodges and Wilhelms, 1978). The other, terraced ring resembles the rims
of craters (Howard and others, 1974, fig. 4) and must similarly be the
limit of the excavated cavity modified by slumping.

INTERPRETATION OF CIRCUM-BASIN DEPOSITS AND SATELLITIC CRATERS

Deposits from basins centered outside the map area dominate more than
half of its northern border zone. Emplacement processes of basin
deposits have been deduced from the well-exposed, well-photographed
Orientale materials. The inner facies of the Hevelius Formation (map
symbol Iohi) is a continuous, strongly lineated, apparently thick
deposit that extends back to the Orientale basin rim (Montes Cordillera)
north of the region and that, accordingly, must contain much primary
basin ejecta. This deposit apparently flowed along the surface at least
part of the distance from the rim, for parts of it piled up as
transverse ridges against the southeast wall of the crater Inghirami and
other obstacles (McCauley, 1968; Scott and others, 1977). The relatively
smooth, nonlineated facies of the Hevelius

Formation (map symbol Iohn) is gradational with the coarsely textured
inner facies and in part forms raised lobes bounded by distinct scarps
(Moore and others, 1974, figs. 6,7) that suggest flowage of a viscous,
fluidlike material.

Secondary impact apparently emplaced most deposits beyond the inner and
nonlineated facies of the Hevelius Formation and beyond comparable,
though less well preserved materials of other basins (Moore and others,
1974; Oberbeck and others 1974, 1975; Morrison and Oberbeck, 1975;
Oberbeck, 1975; Wilhelms, 1976). Satellitic craters of Orientale (map
symbol Ioc) so closely resemble secondary impacts of large lunar craters
(Offield, 1971; Wilhelms and McCauley, 1971; Oberbeck and Morrison,
1974; Wilhelms, 1976) that their identity as Orientale basin secondaries
seems certain. Basin-radial ridges and smooth deposits commonly lie on
the distal side of the secondary craters and constitute a discontinuous
unit called the outer facies of the Hevelius Formation (Scott and
others, 1977); most of these deposits presumably consist of secondary
ejecta. East of long 20° E. are corresponding, though less fresh,
features of the Nectaris basin, including secondary craters (part of map
unit Nbc) and ridged deposits (Janssen Formation, Stuart-Alexander,
1971). The Nectarian basin deposits (map symbol Nbl) and clustered
secondary craters (map symbol Nbc) near the map border between long 142°
and 176° W. are related to the Apollo basin centered 300 km north of the
area. We have mapped the Apollo-related units as Nectarian based on
their morphology but the basin may instead be pre-Nectarian
(Stuart-Alexander, 1978).

Other probable secondary craters scattered throughout the map area,
though not closely satellitic to basins, have been traced to their
sources by orientation. Secondaries of Schrödinger (Nectarian) are
easily identified as far as three basin diameters from its rim by
linear, groovelike shapes and the concentration in few azimuthal
directions that characterize this basin. A few probable Nectarian
secondary craters in the vicinity of the Apollo deposits arc more
closely radial to the distant Hertzsprung basin, 1400 km north (lat 1°
N., long 129° W.), than to Apollo. Numerous large chains east of the
crater Clavius, also mapped as Nectarian basin secondaries, are radial
to the distant Humorum basin (lat 24° S., long 39° W.) and may be parts
of its secondary field although their nature is problematic. Finally,
outliers of the large field of secondary impact craters of the Imbrium
basin (map symbol Iic) occur in the north part of the map area.

PROBABLE BASIN-RELATED MATERIALS

Additional units interpreted as basin related occur among and beyond the
secondary craters and lineated deposits. The inner, nonlineated, and
outer facies of the Hevelius Formation grade in places into
smooth-surfaced plains-forming material of Imbrian age that occupies
depressions (map symbol Ip). Gradations of the smooth plains material
with primary and secondary basin materials suggest that the plains
material here consists of ejecta and not volcanic material, as has been
proposed for some plains (for example, Wilhelms and McCauley, 1971;
Neukum, 1977). Similar plains of Imbrian age that are distant from the
Orientale basin may be ejecta of Imbrium-basin secondary craters or of
large Imbrian primary craters. Nectarian terra-mantling and plains
material (map symbol Ntp) is probably similar in origin to these Imbrian
deposits, for it is concentrated around Nectarian basins (Nectaris,
Schrödinger, Mendel-Rydberg, and Apollo), large craters (Clavius), and
satellitic and distant secondary craters of Nectarian basins including
those tentatively ascribed to Humorum. Age assignment of plains and
terra-mantling materials depends partly upon crater counts made on
photographs of uneven quality, so that some patches mapped as Nectarian
could be older or younger, and some mapped as Imbrian could be
Nectarian.

The outer facies of the Hevelius Formation grades outward into less
heavily mantled terrain with fewer lineations and pits that is mapped as
composite units---Imbrian and

Nectarian terra (map symbol INt) where the units visible under the
Orientale textures are mostly Nectarian, and Imbrian and pre-Nectarian
terra (map symbol IpNt) where the underlying material is mostly
pre-Nectarian. Much terrain outside the distinctive materials of
Nectarian basins, though mostly lacking conspicuous lineations and pits,
also seems subdued by a mantle that is gradational with the Nectarian
basin materials and accordingly is mapped as Nectarian and pre-Nectarian
terra (map symbol NpNt). Visibility of textures and therefore mapping of
many lunar material units depend both on photographic quality and degree
of degradation. For example, the outer facies of the Hevelius Formation
and the Orientale-related composite units may be more extensive than
mapped, especially on the far side where photographs are poorest. Also,
time-related degradation has probably blurred Nectarian textures that
once resembled those of Orientale, so that the well-defined
basin-related province is less extensive relative to the probable
basin-related province (fig. 2) for the Nectarian. System than for
Orientale. The degradation process has proceeded even further among
pre-Nectarian units, where the rugged basin material (map symbol pNbr)
and an even more poorly defined, generally low-lying unit called
pre-Nectarian terra material (map symbol pNt) occupy the positions of
the much more distinctive and varied younger basin units that they
presumably once resembled. Contributions to the pre-Nectarian terra unit
from the various pre-Nectarian basins and craters are not distinguished
on this map because of inadequate photography and exposure.

Two more distinctive but less extensive units may also be related to
basins although their origin is uncertain. The exposure of grooved
material (map symbol IpNg) in the area near long 160° E. and the better
exposures immediately to the north (Stuart-Alexander, 1978) are
antipodal to the Imbrium basin and have been interpreted as the products
of the impact of converging Imbrium ejecta (Moore and others, 1974) or
shaking induced by seismic waves from Imbrium (Schultz and Gault, 1975).
Most fractured crater floors (map symbol Ifc), the other areally
restricted unit, are probably products of extensive floor uplift, a
common phenomenon in and near basins (Pike, 1971; Schultz, 1976), but
some peripheral to the Orientale deposits could be Orientale impact
melt, as observed elsewhere in similar positions (Moore and others,
1974).

CRATER MATERIALS

Primary impact craters with a complete range in ages dot the map area,
as they do all lunar highlands. The materials of these craters are still
perceived much as described on earlier maps such as that by Wilhelms and
McCauley (1971).

The unusual Imbrian crater Antoniadi (lat 69.5° S., long 172° W.), 150
km wide, deserves special mention for two reasons. First, it is ringed
by an unusually extensive and dense array of secondary impact craters
(map symbol Isc). Second, it has both a central peak and an inner ring
of peaks, making it transitional between craters and multiring basins
(Hartmann and Wood, 1971; Hodges and Wilhelms, 1978).

In summary, the geology of the terra can be accounted for by a
succession of large and small impacts that excavated basins and craters,
deposited the ejected material as a thick blanket near the rim crest,
and flung it over a much wider distance where it cratered and
redistributed the preimpact terrain. Basins were also favorable sites
for subsequent internal activity, such as crater-floor uplift and mare
volcanism.

MARE AND OTHER DARK MATERIALS

Although, in general, the near side of the Moon has more extensive mare
material than the far side, the reverse is true in this southern area.
Except near the east limb, that part of the near side within the map
area is nearly devoid of mare material, whereas the far-side part

contains dozens of mare patches. This concentration results from
depressions afforded by the Australe and South Pole-Aitken basins and
the smaller basins and craters superposed on them.

Three age groups of mare material have been mapped (two in fig. 2), each
of which represents stratigraphically complex units with a range of
ages. The oldest group (map symbol Iml) was early recognized as
exceptionally old for lunar mare material (Stuart-Alexander and Howard,
1970). It is older than many upper Imbrian crater materials (map symbol
Ic2) and than all known mare materials of the central near side. It is
lighter in color than most other mare units and is gradational in color
and crater density with the terra and plains materials. The intermediate
mare material mapped here (map symbol Im2) substantially overlaps
typical near-side mare material in age, although its full age range has
not been determined. The distinction between mare units is clear over
most of Mare Australe but can be made only tentatively in the poorly
photographed area in the South Pole-Aitken basin. The youngest unit (map
symbol EIm) is partly or entirely Eratosthenian. Small mare domes (map
symbol Imd) and similar swellings that may be domes or mantled terra
(map symbol Imd?) also are mapped, as are small patches of dark mantling
material (Id) surrounding their apparent source vents in the Schrödinger
basin.

A few large wrinkle-ridge structures traverse part of Mare Australe but
wrinkle ridges and fault grabens are rare in the map area, in contrast
with their abundance in and near the large nearside lunar maria. This
distribution supports an inference that most nonimpact compression and
extension of the lunar crust relate to crustal loading by mare basalts
(Solomon and Head, 1979).

GEOLOGIC HISTORY

The detectable history of the region begins with the largest event
inferred from visible landforms on the Moon, the impact that. raised the
massive South Pole-Aitken rim, excavated most of the far-side part of
the map area, and probably covered the near-side part with excavated
debris. A second large impact formed the adjacent Australe basin, and
smaller pre-Nectarian impacts excavated at least three basins (Poincaré,
Planck, and Schiller-Zucchius), possibly four others tentatively
identified, and perhaps still others that have been obscured. The
Nectarian Period began with the formation of the Nectaris basin, which
is centered outside the map area but strongly influenced it by covering
and cratering many pre-Nectarian craters and basin deposits. Later in
Nectarian time, the Apollo and Humorum impacts also deposited materials
and formed secondary craters in the north part of the area, and the
Mendel-Rydberg, Bailly, and Schrödinger basins and the large crater
Clavius were formed within it. Schrödinger, with its distinctive, linear
secondary-crater chains and pronounced double-ring structure, strongly
affected the appearance of the southern far side. In the Imbrian Period,
ejecta from the distant Imbrium basin formed scattered secondary
craters, Orientale basin deposits blanketed a huge region on the west
limb including the Mendel-Rydberg and Bailly basins, and Orientale
ejecta formed many secondary craters beyond this blanket. Mare volcanism
left still-visible traces in the lowlands of the South Pole-Aitken and
Australe basins beginning about the time of the Orientale impact,
continuing through the Imbrian Period, and locally extending into the
Eratosthenian. Impact events, which in Imbrian time produced the
crater-basin Antoniadi and other large craters, continued at a low rate
through the Eratosthenian and Copernican Periods.

REFERENCES CITED

Bills, B. G., and Ferrari, A. J., 1975, Lunar topography: Proceedings
6th Lunar Science Conference, Geochimica et Cosmochimica Acta, suppl. 6,
v. 3, frontispiece, pl. 2.

Cummings, David, 1972, Geologic map of the Clavius quadrangle of the
Moon: U.S. Geological Survey Miscellaneous Geologic Investigations Map
I-706, scale 1:1000000.

Hartmann. W. K., and Kuiper, G. P., 1962, Concentric structures
surrounding lunar basins: University of Arizona Lunar and Planetary
Laboratory Communications, v. 1, no. 12, p. 51--66.

Hartmann, W. K., and Wood, C. A., 1971, Moon: Origin and evolution of
multi-ring basins: The Moon. v. 3, no. 1, p. 4--78.

Hodges, C. A., and Wilhelms, D. E., 1978, Formation of lunar basin
rings: Icarus, v. 34, no. 2, p. 294--323.

Howard, K. A., Wilhelms, D. E., and Scott, D. H., 1974, Lunar basin
formation and highland stratigraphy: Reviews of Geophysics and Space
Physics, v. 12, no. 3, p. 304--327.

Karlstrom, T. N. V., 1974, Geologic map of the Schickard quadrangle of
the Moon: U.S. Geological Survey Miscellaneous Geologic Investigations
Map I-823, scale 1:1000000.

Kinsler, D.C., Merrill, R. B., and Srnka, L. J., 1975, Apollo laser
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v. 1, p. 71--100

Morrison, R. H., and Oberbeck, V. R., 1975, Geomorphology of crater and
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Lunar Science Conference, Geochica et Cosmochimica Acta, suppl. 6, v. 3,
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Mutch, T. A., and Saunders, R. S., 1972, Geologic map of the Hommel
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Investigations Map I-702, scale 1: 1 000 000.

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I-691, scale 1:1000000.

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U.S. Geological Survey Miscellaneous Geologic Investigations Map I-713,
scale 1:1000000.

Rodionov, B. N., Isavnina, I. V., Avdeev, Yu. F., Blagov, V. D.,
Dorofeev, A. S., Dunaev, B. S., Ziman, Ya. L., Kiselev, V. V., Krasikov,
V. A., Lebedev, O. N., Mikhailovskii, A. B., Tishchenko, A. P.,
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\_\_\_\_\_\_\_\_1978, Geologic map of the central far side of the Moon:
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of the Moon: U.S. Geological Survey Miscellaneous Geologic
Investigations Map I-948, scale 1:5 000 000.

Wilhelms, D. E., and McCauley, J. F., 1971, Geologic map of the near
side of the Moon: U.S. Geological Survey Miscellaneous Geologic
Investigations Map I-703, scale l:5,000,000.

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Geochimica et Cosmochimica Acta,

suppl. 7, v. 3, p. 3629--3651.

\[Authors' notes on geologic mapping:\]

Geology mapped 1973-77. Near side mapped by D.E. Wilhelms and H.G.
Wilshire, far side by K.A. Howard and D.E. Wilhelms. Data sources: Lunar
Orbiter photographs (fig.1 ) courtesy of National Aeronautics and Space
Administration. Earlier maps of parts of area by Offield (1971),
Stuart-Alexander (1971), Wilhelms and McCauley (1971), Cummings (1972),
Mutch and Saunders (1972), Pohn (1972), Scott (1972), Karlstrom, (1974),
and Saunders and Wilhelms (1974). Prepared on behalf of National
Aeronautics and Space Administration under Contract No. W-13,130.

\[base map notes\]

Shaded-relief base chart: Lunar Polar Chart (LMP-3), 2d ed., October,
1970, prepared by Defense Mapping Agency, Aerospace Center (formerly
Aeronautical Chart and Information Center, U.S. Air Force), St. Louis,
MO, 60318. Dots refer to center of named feature. Some feature names
shown on published base chart have been changed in accordance with
rulings of the International Astronomical Union and others have been
deleted for clarity.