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 IÐ1162, 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 MATERIALÑSharply textured and bright, mostly surrounded by bright rays

 Csc 	SECONDARY-CRATER MATERIAL

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

 Esc 	SECONDARY-CRATER MATERIAL

 Ic2 	UPPER IMBRIAN CRATER MATERIALÑYounger 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 MATERIALÑYounger 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 BASINÑFew 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 BASINÑRim relief normally the only feature visible except in largest craters; queried where could be unit Nc

BASIN MATERIALS
ORIENTALE GROUPÑIncludes material of satellitic craters and Hevelius Formation

 Ioc 	MATERIAL OF ORIENTALE-BASIN SATELLITIC CRATERSÑGrouped 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 FORMATIONÑDivisible into inner, nonlineated, and outer facies

 Iohi 	Inner faciesÑContinuous 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 faciesÑDeposit 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 faciesÑDiscontinuous 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 CRATERSÑSimilar to better developed secondary craters closer to Imbrium basin (Wilhelms, 1976); queried where could be primary craters

 Nbc 	MATERIAL OF NECTARIAN-BASIN SATELLITIC CRATERSÑGrouped 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 MATERIALÑRim and wall material of double-ring basins. Interpretation: Impact materials formed near edge of excavated cavity

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

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

 Nbh 	HUMMOCKY BASIN MATERIALÑHummocky terrain in floor of Schršdinger basin. Interpretation: Impact melt, perhaps draped over toes of slump structures (Howard and others, 1974)

 Nj 	JANSSEN FORMATIONÑDeposit 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, UNDIVIDEDÑRim, wall, and inner-ring materials. Interpretation: Same as for corresponding features of Nectarian basins

 pNbm 	BASIN MASSIF MATERIALÑLarge 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, RUGGEDÑForms 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 MATERIALÑLight-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 MATERIALÑLight-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 MATERIALÑModerate 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 MATERIALÑSimilar 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 MATERIALÑDiverse 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 MATERIALÑForms 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 MATERIALÑIrregular 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 MATERIALÑRaised floors with fissures. Interpretation: Shrunken impact melt deposits or tectonically uplifted and dilated crater floors

MARE AND OTHER DARK MATERIALS 
 EIm 	YOUNGEST MARE MATERIALÑAppears dark and lightly cratered. Interpretation: Basaltic lavas

 Im2 	INTERMEDIATE MARE MATERIALÑSimilar 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 MATERIALÑLight color and high density of superposed craters unusual for mare. Interpretation: Old basaltic lava, perhaps as old as Orientale basin

 Imd 	MARE DOME MATERIALÑDark, 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 MATERIALÑHalos around irregular craters along fissures in floor of Schršdinger basin. Interpretation: Thin pyroclastic deposits


	ContactÑQueried where inferred in poorly photographed areas

	Crest of buried crater rim

	Crest of basin ring structureÑDashed where poorly known

	Fissure or narrow fault graben

	Sinuous ridge

	Sinuous scarpÑBarb 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, PoincarŽ 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 Schršdinger basin. Parts of their postulated ringsÑespecially those of Sikorsky-Rittenhouse (Òunnamed AÓ 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 PingrŽ by Hartmann and Kuiper (1962) and PingrŽ-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 Ñyounger 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 ÒSE limb basinÓ 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 Schršdinger 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 Schršdinger (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, Schršdinger, 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 unitsÑImbrian 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 Schršdinger 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 (PoincarŽ, 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 Schršdinger basins and the large crater Clavius were formed within it.  Schršdinger, 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.

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Pike. R. J.. 1971, Genetic implications of the shapes of martian and lunar craters: Icarus, v. 15, no. 3, p. 384Ð395. 
Pohn, H. A., 1972, Geologic map of the Tycho quadrangle of the Moon: 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., Nepoklonov, B. V., Samoilov, V. K., Truskov, F. M., Chesnokov, Yu. M., and Fivenskii, Yu. I., 1971, New data on the MoonÕs figure and relief based on results from the reduction of Zond 6 photographs: Cosmic Research, v. 9, no. 3, p. 410Ð417. 
Rodionov, B. N., Nefed'ev, A. A., Shpekin, M. I., Valeev, S. G., and Kiselev, V. V., 1977, Relief of the MoonÕs reverse side according to Zond 8 photographs: Cosmic Research, v. 14, no. 4, p. 548Ð552. 
Saunders, R. S., and Wilhelms, D. E., 1974, Geologic map of the Wilhelm quadrangle of the Moon: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-824, scale 1: 1 000 000. 
Schultz, P. H., 1976, Floor-fractured lunar craters: The Moon, v. l5, no. 3Ð4, p. 241Ð273. 
Schultz, P. H., and Gault, D. E., 1975, Seismically induced modification of lunar surface features: Proceedings 6th Lunar Science Conference, Geochimica et Cosmochimica Acta, suppl. 6, v. 3, p. 2845Ð2862. 
Scott. D. H., 1972, Geologic map of the Maurolycus quadrangle of the Moon: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-695, scale 1:1000000. 
Scott, D. H., McCauley, J . F., and West, M. N., 1977, Geologic map of the west side of the Moon: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-1034, scale 1:5000000. 
Solomon, S. C., and Head, J. W., 1979, Vertical movement in mare basins: Relation to mare emplacement, basin tectonics, and lunar thermal history: Journal of Geophysical Research, v. 84, no. B4, p. 1667Ð1682. 
Stuart-Alexander, D. E, 1971, Geologic map of the Rheita quadrangle of the Moon: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-694, scale 1:1000000. 
________1978, Geologic map of the central far side of the Moon: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-1047, scale 1:5000000. 
Stuart-Alexander, D. E., and Howard, K. A., 1970, Lunar maria and circular basinsÑA review: Icarus, v. 12, no. 3, p. 440Ð456. 
Wilhelms, D. E., 1976, Secondary impact craters of lunar basins: Proceedings 7th Lunar Science Conference, Geochimica et Cosmochimica Acta, suppl. 7, v. 3, p. 2883Ð2901. 
Wilhelms, D. E., and El-Baz, Farouk, 1977, Geologic map of the east side 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. 
Wollenhaupt, W. R., and Sjogren, W. L., 1972, Comments on the figure of the Moon based on preliminary results from laser altimetry: The Moon, v. 4, no. 34, p. 337Ð347. 
Wood, C. A., and Head, J. W., 1976, Comparison of impact basins on Mercury, Mars and the Moon: Proceedings 7th Lunar Science Conference, Geochimica et Cosmochimica Acta, suppl. 7, v. 3, p. 3629Ð3651.
	[AuthorsÕ 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.