**GEOLOGIC MAP**

**OF THE**

**EAST SIDE**

**OF THE MOON**

By

Don E. Wilhelms and Farouk El-Baz

1977

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--948, 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 MAP UNITS**

**CRATER MATERIALS**

**(craters 20-km rim crest diameter or larger)**

Cc MATERIAL OF RAYED CRATERS

Ec MATERIAL OF FRESH NONRAYED CIRCULAR CRATERS

Ic2 MATERIAL OF MODERATELY FRESH CIRCULAR CRATERS---Older than younger
mare material (Im2) and younger than or about the same age as the
Orientale basin (outside map area)

Ic1 MATERIAL OF MODERATELY SUBDUED CIRCULAR CRATERS---Older than the
Orientale basin and younger than the Imbrium basin (outside map area)

Nc MATERIAL OF SUBDUED CIRCULAR CRATERS---Older than the Imbrium basin
and as young as or younger than the Janssen Formation

pNc MATERIAL OF HIGHLY SUBDUED, MOSTLY NARROW-RIMMED CIRCULAR
CRATERS---Older than the Janssen Formation

Isc MATERIAL OF SECONDARY CRATER CLUSTERS AND CHAINS

Irc MATERIAL OF RINGED CRATERS---Circular; one or more inner concentric
rings; shallow floors; rims narrow and smooth

Icc MATERIAL OF MODERATELY FRESH CRATER CHAINS, CLUSTERS, AND IRREGULAR
CRATERS

Ifc MATERIAL OF FURROWED CRATER FLOORS---Domed or nearly flat,
transected by cracks

Ncc MATERIAL OF SUBDUED CRATER CLUSTERS, CHAINS, AND IRREGULAR
CRATERS---Includes long, linear, valleylike chains and irregular
depressions

pNcc MATERIAL OF HIGHLY SUBDUED CRATER CHAINS, CLUSTERS, AND IRREGULAR
CRATERS

**BASIN MATERIALS**

Nb BASIN MATERIAL, UNDIVIDED---Moderately rough surface on elevated
craterlike rings of small basins younger than Nectaris

Nbl LINEATED MATERIAL AROUND BASINS---Younger than the Janssen Formation

Nj JANSSEN FORMATION (basal Nectarian)---lineated material around
Nectaris basin; more highly cratered than Imbrium basin ejecta blanket

pNbm MATERIAL OF BASIN MASSIFS---Mountainous, high-standing,
smooth-sided blocks mostly with high albedo; associated with basins
older than Nectaris

pNbr MATERIAL OF RUGGED BASIN TERRAIN---Rugged peaks and ridges smaller
than the massifs, mostly in and peripheral to basins older than
Nectaris. Also includes two short smooth

arcs: northwest part of outer Al-Khwarizmi-King basin ring, and part of
Tsiolkovsky-Stark basin ring near crater Stark

NpNbm MATERIAL OF BASIN MASSIFS (Nectarian and pre-Nectarian) Similar to
unit pNbm but associated with basins of Nectarian age

NpNbr MATERIAL OF RUGGED BASIN TERRAIN (Nectarian and pre-
Nectarian)---Similar to unit pNbr but associated with basins of
Nectarian age

**OTHER TERRA MATERIALS**

Ip LIGHT-COLORED PLAINS MATERIAL---Level, smooth surface; density and
morphology of superposed craters indicate middle Imbrian age

INp LIGHT-COLORED PLAINS MATERIAL (Imbrian or Nectarian)---Level or
somewhat undulatory surface; superposed craters indicate early Imbrian
or late Nectarian age; plains between Petavius and Humboldt could be
middle Imbrian

Np LIGHT-COLORED PLAINS MATERIAL---Similar to unit INp but superposed
craters tentatively indicate Nectarian age

It TERRA MANTLING MATERIAL---Undulatory, moderately smooth surface;
superposed craters indicate Imbrian age

Nt TERRA MANTLING MATERIAL---Similar to unit It but superposed craters
indicate Nectarian age

pNt MATERIAL OF UNMANTLED TERRA---Moderately rugged; individual relief
elements are mostly parts of craters

NpNt MATERIAL OF PARTLY MANTLED TERRA (Nectarian and pre-
Nectarian)---Smooth or moderately rough surface, rolling to moderately
rugged overall relief; age of superposed and buried craters laterally
diverse

INfp FURROWED AND PITTED MATERIAL (Imbrian or Nectarian)---Closely
spaced irregular sinuous furrows and (or) circular, elongate, or
irregular pits; kilometers in size; furrows dominant in large patches
north of Mare Marginis and Mare Fecunditatis, pits elsewhere. Locally
superposed on Nectarian-age craters and overlain by lower Imbrian
craters; perhaps variable in age

**MARE AND OTHER DARK MATERIALS**

Im2 YOUNGER MARE MATERIAL---Similar in dark color and crater density to
typical near-side mare material of late Imbrian age; perhaps varable in
age

Im1 OLDER MARE MATERIAL---Superposed craters indicate middle Imbrian
age. Lighter color than unit Im2 and most near-side unrayed mare,
similar to near-side rayed mare as between Langrenus and west map
border, and darker than light-colored plains material (units Ip, INp,
Np)

EId DARK MATERIAL (Eratosthenian or Imbrian)---Several types of material
with albedo of mare or darker, including deposits with intrinsic
fresh-appearing textures (Mare Smythii,

especially lat 6° S., long 84° E.), cones (Cleomedes), hummocky deposits
and sharp, elongate furrows (between Scaliger and Titius, lat 26 S.,
long 102° E.), mantles without apparent relief of their own (Mare
Smythii, Petavius, Humboldt, Tsiolkovsky), and dark halos around
elongate furrows (Messala). Same age or younger than younger mare
material (Im2)

Contact---Queried where location very uncertain (in northeast quadrant)

Crest of basin ring structure (generalized)---Dashed where inferred from
mare ridges, relatively low terra features, or widely separated terra
hills\*

Buried crater rim crest

Bright sinuous markings

GENERAL FEATURES

MARIA AND MULTI-RING CIRCULAR BASINS

The map area is centered on the east limb of the moon as seen from Earth
and includes 40° of longitude of the Earth-facing hemisphere and 50° of
the far side. The geology of the whole region is controlled by the large
features of impact origin known as multi-ring circular basins (Hartmann
and Kuiper, 1962). Two basic types of terrain are present, maria and
terrae. Basin and crater materials compose most of the terrae. The
maria, of volcanic origin, are concentrated in the basins and in their
peripheral troughs.

The maria are more extensive on the near side and the limb than on the
far side (geologic map and fig. 1). On the near side (western part of
the map area) they are concentrated in and around the high-rimmed
Crisium basin, in the subdued Fecunditatis basin, and in peripheral
troughs of the Serenitatis and Humboldtia- num basins (northwest corner
of map area). Mare materials of another near-side basin, Nectaris, lie
entirely outside the map area, but the basin materials strongly affect
the area. On the limb (central meridian of the map) the maria are in the
weakly expressed Marginis basin, the subdued Smythii basin, and the
large but shallow Australe basin (south edge of map area).

The Moscoviense basin contains mare just east of the map area, and other
far- side mare patches occur in craters that are in turn superposed on
basins. Cons- picuous patches are in Tsiolkovsky, located near the
center of an ancient, incomplete basin (Baldwin, 1969), and in
Lomonosov, which lies just inside the pro- jected edge of an old basin
that has much light-plains fill. These basins are here named
Tsiolkovsky-Stark and Lomonosov-Fleming after craters superposed on
them. A conspicuous far-side patch of mare material that may not be
related to either a basin or a crater is centered at lat 27° S., long
103° E.

Three far-side basins that contain no mare material have been
identified. Mendeleev and Milne are small, craterlike, relatively young
basins. Al-Khwarizmi-King, named after craters superposed on the outer
of two mapped rings, is a larger, much older feature (El-Baz, 1973).
Other ancient basins are probably present, and the entire map area has
probably been reworked repeatedly by basin and crater formation
(Baldwin, 1969; Howard and others, 1974).[^1]\*

GEOLOGIC PROVINCES

Geologic provinces are areas in which a geologic unit or groups of units
related in age and origin are concentrated (McCauley and Wilhelms, 1971)
In the map area (fig. 1) the maria are grouped into two provinces, young
and old. At least three of six terra provinces consist mostly of basin
materials. The youngest basins are surrounded by blankets of radially
lineated ejecta and clusters of secondary impact

craters (province by). The two largest occurrences fan out from the
Humboldtianum basin, centered north of the map area, and the Nectaris
basin, west of the southwest corner; smaller blankets surround
Mendeleev, Milne, and Moscoviense. The lineated blankets and the
secondary clusters are diagnostic of the impact origin of the source
basins. A second province (bo), topographically more rugged than the
first, forms the multiple rings and some peripheral terrain of several
old basins.\* Extensive tracts of terra that appear mantled occur near
the young and old basins and form a third province (tm) . Although the
distinctive lineated textures have not been observed in this mantle, its
association with basins suggests that it is composed mostly of degraded
basin ejecta.

Other terrain, mostly on the far side, consists of densely packed
craters (province tc). This province contains little mantle or other
basin material except the ancient, subdued rings of the
Al-Khwarizmi-King, Tsiolkovsky-stark, and Lomonosov-Fleming basins, and
a few additional short arcs of rings. Thus the province owes its
preservation to lack of significant modification by young basins and is
generally the most primitive province.

The two provinces of light plains, young and old, may or may not be
related to basins. As discussed in the section on stratigraphy, they
could be either ejecta from distant basins or volcanic rock. Notable
concentrations occur in the Lomonosov-Fleming basin, near the Smythii
basin, southeast of Langrenus, and in the northwest corner of the map
area.

GEOPHYSICS AND GEOCHEMISTRY

Laser altimeters carried by orbiting Apollo spacecraft measured absolute
elevations and delineated many topographic features of the area (Kaula
and others, 1972, 1973). Two representative profiles (*A-A\'*, fig. 2;
*B-B\'*, fig 3) show that the near side in the map area averages about 3
km lower in elevation than the far side. The near-side terrain traversed
by the profiles is composed mostly of basin-related terra provinces and
maria; the higher far-side terrain consists of the cratered terra
province overlain by one younger, plains-filled basin (Mendeleev). Small
peaks and troughs in the profiles belong to crater rims and interiors.

From this and additional data, Kaula, Schubert, Lingenfelter Sjogren,
and Wollenhaupt (1973) have determined a 2-- to 3-- km offset toward the
Earth of the center of mass of the Moon from its center of figure. They
ascribe this to a variable thickness of a low-density terra crust,
thinner on the near side than on the far side. They believe that the
maria are concentrated on the near side because the mare basalts could
more easily penetrate the thinner crust. Similarly, local thinning of
crust through excavation by basins, and particularly by craters
superposed on basins, may explain the observed distribution of maria on
the far side.

X-ray fluorescence and gamma-ray spectrometers were carried in orbit on
Apollo missions 15 and 16 to extend geochemical findings from the small
spots sampled or analyzed by Surveyor, Apollo, and Luna missions to
larger areas of the Moon. The X-ray results, reported as concentration
ratios of aluminum or magnesium to silicon, are particularly significant
in the map area. Comparisons can be drawn between these ratios and those
obtained from analyses of returned rock and soil (Adler and others,
1972, 1973). Concentration ratios typical of

mare and terra rocks are observed. Ratios of Al to Si (fig. 4) and Mg to
Si (fig. 5) are, in general, inversely related. Extreme values of the
ratios are obtained over relatively uninterrupted geologic units---low
Al:Si and high Mg:Si over maria and the converse over highlands. This
inverse relation holds over Fecunditatis, Crisium, and Smythii with two
exceptions. First, Smythii has a higher Al:Si ratio than the other two
maria (fig 4); this may be a result of mixture of small terra patches of
the mantled terra province with mare patches. Second, the Mg:Si ratio
characteristic of Fecunditatis extends farther east than the Al:Si ratio
(fig. 5); this extension is over an area of Langrenus ejecta that
probably contains fragments of mare rock. The mare-free cratered terra
province has the two lowest Mg:Si ratios and the two highest Al:Si
ratios; the latter also extend west of this province. Departures from
exact inverse correlation may result from instrumental factors
particularly in concentration ratios of Mg to Si.

Intermediate values of the two element ratios may result from mixtures
of mare and terra end members caused either by lunar processes or by
limited instrumental resolution. Zones of intermediate values occur near
the large maria or correlate with regions in the central and western
parts of the map area where small patches of mare and terra are
intermixed. Impact cratering has probably mixed mare and terra materials
in the regolith of such zones Also, the instruments integrate spectra
over about 60 km2 and combine mare and terra spectra that probably would
appear unmixed if measured with finer resolution. An alternative possi-
bility for intermediate values, which remains to be tested by additional
data reduction and orbital missions, is the presence of geochemical
provinces in the terra that do not correspond to mapped geologic units
or provinces.

The gamma-ray spectrometer measurements (fig. 6) indicate that most of
the map area is relatively low in natural radioactivity (Metzger and
others, 1973). Most of the overflown areas show natural radioactivity
readings that correspond to less than l part per million of thorium. The
highest, but still moderate, readings of a few parts per million of
thorium, are over the maria and over the terra east of Langrenus.

Gravity measurements, which were made only on the near side and limb
regions, indicate gravity anomalies (mascons) in excess of 200 milligals
in Mare Crisium (Sjogren and others, 1972) and Mare Smythii (Sjogren and
others, 1974). The mascons are similar to those of other near-side
circular basins and may be inter- preted as a result of an isostatically
uncompensated disk of basaltic fill in these basins (Sjogren and others,
1972). Negative gravity anomalies occur in nine craters 100 km in
diameter or larger. These mass deficiencies are consistent with
uncompensated loss of material from the crater-forming impact
(Geochimica et Cosmochimica Acta, 1973).

Apollo 15 subsatellite magnetometer data (Russell and others, 1973)
indicate that the lunar field over most of the map area exhibits an
average radial component in the range 0.4 to -0.4 gammas. Higher
readings (0.8 to 0.4 gammas) are found within and east of the crater
Humboldt. Two lower values (-0.4 to -0.8 gammas) are southeast of
Tsiolkovsky, in the crater Subbotin and northeast of it. The sources of
the lunar magnetic field are not yet well defined.

STRATIGRAPHY AND INTERPRETATION OF UNITS

Most geologic units mapped here are defined as on a geologic map of the
near side of the Moon at the same scale (Wilhelms and McCauley, 1971).
The lower size limit for mapping units is 20 km across rather than the
10 km used on the near-side map. An important departure is the
introduction of the Nectarian System. Materials deposited before the
formation of the Imbrium basin, called pre-Imbrian on the near-side map
and on other previous lunar geologic maps, are here divided into the
Nectarian System and the informal pre-Nectarian (Stuart- Alexander and
Wilhelms, 1975). Materials of and stratigraphically above the Nectaris
basin, but below those of the Imbrium basin, are classed as Nectarian.
Materials formed before the Nectaris basin are pre-Nectarian.
Morphologic characteristics of Nectarian and pre-Nectarian craters can
be determined where they contact the extensive ejecta blanket (Janssen
Formation) and secondary-crater clusters of the Nectaris basin
(southwest map corner); elsewhere craters are correlated by morphologic
comparison

CRATER MATERIALS

In mapping crater materials, additional departures from conventions of
the near side map (Wilhelms and McCauley, 1971) are: (1) boundaries are
extended out- ward to include more faintly textured rim materials and
satellitic crater clusters; (2) most crater materials that would have
been designated "pre-Imbrian undivided" on the near side map are here
assigned to the pre-Nectarian, some to the Nectarian; (3) previously
undivided Imbrian crater materials are divided into upper Imbrian (map
unit Ic2) and lower Imbrian (Ic1) materials. (4) Copernican crater
materials are not divided because albedo data or high-resolution
photographs are lacking for part of the area. Petavius B, Giordano
Bruno, and Necho are probably late Copernican; the bright, extensive,
overlapping rays of the latter two craters were inappropriately termed
the "Soviet Mountains" (Barabashev and others, 1960; El-Baz, 1972a); (5)
the few irregular craters are not mapped separately from clusters and
chains, which they generally resemble.

Most large, detail-rich circular craters are interpreted to be of impact
origin Moreover, an impact origin is affirmed for the less distinctive
craters mapped on the near-side map as "undivided pre-Imbrian" and here
mostly as pre-Nectarian because their frequency distribution fits an
impact model. Their subdued appearance is probably due to impact erosion
and mantling. However, certain other craters that lack the rough rims,
radial ejecta, and deep floors characteristic of impact craters seem too
restricted in size range and distribution to be degraded impact craters.
These are the usually double-ringed craters (map unit Irc) of subequal
size (20--60 km) that are concentrated in the Smythii basin. They are
commonly the sites of young, dark, presumably volcanic deposits (map
units EId and Im2).

Several crater floors are traversed by cracks. Three of these are mapped
as a geologic unit (material of furrowed crater floors) because their
uniform appearance suggests that they consist of a distinct material,
probably volcanic or impact melts. Where cracks cut across units they
are more likely of tectonic origin (probably rebound), and the

floors are not mapped as a distinct unit; such floors occur in Humboldt,
Petavius, Gauss, Cleomedes, and several smaller craters.

Many craters are grouped in clusters and chains of similar, apparently
contemporaneous individuals (map units pNcc, Ncc, Icc, Isc). Most are
probably of secondary impact origin. Distribution and morphology show
that most Nectarian-age clusters and chains are secondary to the
Nectaris and Humboldtianum basins; some in the southeast quadrant are
secondary to Milne and Schrödinger (320-km diameter basin south of map
area). Chains and clusters of pre-Nectarian age are radial to the
Crisium basin and could be remnants of its secondary impact retinue, as
could a chain at lat 15° S., long 65°E. (Hodges, 1973a), here mapped as
Nectarian. Map unit Isc includes clusters that are satellitic to
recognized primary impact craters of Imbrian age and are almost
certainly their secondary impact craters. Some of these clusters are
distant from the large craters but are identified as secondary by their
morphology. Other conspicuous Imbrian- age chains, a few clusters, and
some irregular craters are mapped separately (map unit Icc) because
identification as secondary impact craters is uncertain. Most of these
are in the north, and are radial to the Imbrium basin; they could be
part of its outer secondary impact field. Some chains and clusters could
be volcanic, as could a few small irregular craters that contain
possible volcanic material (for example at lat 25° S., long 103° E.
(Ncc) and lat 39° S., long 129° E. (Icc).

BASIN MATERIALS

Young basins display two or more concentric mountainous rings, hilly
material between the inner rings, a hummocky to lineated ejecta blanket
beyond the outermost conspicuous ring, and clustered secondary craters
in and beyond the lineated ejecta blanket (Baldwin, 1963;
Stuart-Alexander and Howard, 1970; Wilhelms, 1970; Hartmann and Wood,
1971; Wilhelms and McCauley, 1971; Howard and others, 1974; Moore and
others, 1974). Basins in the mapped area show these features to a degree
dependent on basin age, size, and later cratering and other
modifications. Impact basins and craters are probably gradational
members of a size continuum in which the number of rings is proportional
to feature size (Stuart-Alexander and Howard, 1970).

Basin materials are assigned ages according to whether they are believed
to have been thoroughly fragmented and redeposited by the impact or
merely dis- placed in more or less coherent masses by uplift or
horizontal thrusting. (In both cases, radiometric ages of contained
rocks may be relicts or pre-basin impact or igneous events.)
Redeposition is believed to have produced the lineated textures of the
basal Nectarian Janssen Formation around the Nectaris basin (Stuart-
Alexander, 1971) and the probably younger lineated material (map unit
Nbl) around the Humboldtianum, Milne, Mendeleev, and Moscoviense basins.
On the other hand, the high peaks of the concentric basin rings are
believed to be massifs of prebasin rock uplifted when the basin formed,
perhaps overlain by considerable basin ejecta. Consequently these peaks
are pre-Nectarian in pre-Nectarian basins (map unit pNbm) and both
Nectarian and pre-Nectarian (map unit NpNbm) in Nectarian basins.
Gradational, but generally less coarsely structured, rugged terrain of
the rings and of circumbasin lower areas is similarly dated (map units
pNbr and NpNbr). The origin of these units is uncertain,

however, and they could consist mostly of redeposited ejecta or impact
melt. The principal concentrically structured, hummocky, craterlike
rings of the small Milne, Mendeleev and Moscoviense basins-probably
include ejecta and thrust material; this material (map unit Nb) is
gradational with the lineated ejecta and is assigned the same age. The
Nectarian and pre-Nectarian age assigned to the extensive,
nondistinctive mantled terra material (geologic map unit NpNt,
concentrated in province tm) reflects its probable origin as a mixture
of degraded ejecta of Nectarian and pre- Nectarian basins and craters.

The relative ages of the Nectaris, Humboldtianum, and Crisium basins are
only partly established. Humboldtianum is believed younger than Nectaris
because of its somewhat lower density of superposed craters (Hartmann
and Wood, 1971) and its extensive sharp-textured ejecta and secondary
crater clusters (map unit Ncc). The prominence of these features is
especially significant because Hum- boldtianum is the smaller basin (600
km diameter of the main ring versus 850 km for Nectaris). Nevertheless
Humboldtianum could be pre-Nectarian because an old-appearing crater
(Belkovich) lies within the basin north of the map area Lucchitta, in
press). Crisium displays many youthful features and is probably the
youngest of the basins here considered pre-Nectarian. Its rim (units
pNbm and pNbr) is more rugged than the other basin rims, including that
of Nectaris (out- side the area). There are some Crisium radial
lineations (in unit pNbr) and, as noted, some possible secondary impact
chains. Crisium is nevertheless considered older than Nectaris and
Humboldtianum because these lineations and chains are relatively rare in
spite of comparable basin sizes; the most conspicuous ring of Crisium is
500 km in diameter and the outermost high ring has a 680 km diameter
Wilhelms, 1973).

Other pre-Nectarian basins are also relatively dated by morphologic
freshness. density of superposed craters (Hartmann and Wood, 1971), and
presence or absence of surrounding mantled terra deposits. These
criteria suggest the following ascending order of formation of
pre-Nectarian basins:\* Tsiolkovsky-Stark, Al-Khwarizmi-King,
Lomonosov-Fleming, Australe, Marginis, Fecunditatis (or Fecunditatis,
Marginis), Smythii, Serenitatis, and Crisium, followed by the Nectarian
basins Nectaris, Humboldtianum, Moscoviense, Milne, and Mendeleev.

OTHER TERRA MATERIALS

Light-colored deposits that form plains or that mantle and subdue terra
are widespread in the area, though less common than on the central near
side. On the basis of density and morphology of superposed craters, the
plains materials are tentatively divided into three gradational units
(of middle Imbrian, early Imbrian or late Nectarian, and Nectarian age),
and the less distinct terra-mantling materials are divided into two (of
Imbrian and Nectarian age). Each deposit may consist of accumulated
layers of more than one age capped by the mapped unit.

Evidence is growing for an impact origin of plains and mantle materials,
once widely believed to be mostly volcanic. Rocks collected from plains
at the Apollo 16 site outside the map area are impact breccias (Apollo
Field Geology In- vestigation Team, 1973). They could be ejecta from the
Imbrium or Orientale basins or both (Eggleton and Schaber, 1972; Chao
and others, 1973; Moore and others, 1974). Imbrian plains and mantles in
the map area could also be of this origin

in spite of their great distance from the two basins (Chao and others,
1973; Moore and others, 1974). Extensive Nectarian-age plains and
mantles surround the lineated ejecta blankets of the Nectaris and
Humboldtianum basins and could be distal ejecta of these basins, as
could more distant patches. Extensive patches of plains material of
Imbrian and Imbrian or Nectarian age between Petavius and Humboldt could
consist partly of ejecta of these craters. The ejecta could be clastic,
or some plains could be impact melt, as proposed for young plains near
the crater King (Howard, 1972). However, a volcanic origin cannot yet be
excluded for all occurrences. For example, Imbrian-age plains in the
Lomonosov-Fleming basin seem too concentrated to be the deposits of
basins or craters.

A furrowed and pitted unit resembles hilly and pitted and hilly and
furrowed units on the near side (Wilhelms and McCauley, 1971), including
the Descartes material at the Apollo 16 site. Additional, unmapped
occurrences are suggested by widely spaced furrows near Pasteur (El-Baz
and Roosa, 1972) and by textures faintly visible on some poor
photographs. The near-side occurrences were interpreted before the
Apollo 16 mission as terra volcanic materials, but the returned rocks
apparently are impact breccias in which original igneous textures have
been severely altered (Apollo Field Geology Investigation Team, 1973).
The Soviet spacecraft Luna 20 landed in a large furrow-dominated patch
in the map area (lat 3° 22\' N. long 56°33\' E.) and apparently returned
material like that collected at the Apollo 16 site (Vinogradov, 1972;
Wilshire and others, 1974). This suggests that the furrowed and pitted
unit was emplaced by impact. It may be composed of basin ejecta and
other material that has been peppered by secondary impacts from craters
and basins. Possible sources of the secondaries north of Mare Marginis
include the craters Gauss, Lomonosov, and Neper, and the basins
Humboldtianum and Orientale; the latter is exactly antipodal to the
occurrence and a theoretically reasonable source (Moore and others,
1974). The Imbrium basin is a possible source for the
northwest-southeast belt south of Mare Crisium. Internal origin remains
possible, however, especially for occurrences with fissures and faults,
as in the Smythii basin and between Crisium and Fecunditatis.

In summary, an impact ejecta origin is preferred for the plains,
mantles, and furrowed and pitted materials because of the results of the
Apollo 16 and Luna 20 missions. However, the distribution, stratigraphic
relations, and morphology of these units remain consistent with internal
origins.

MARE AND OTHER DARK MATERIALS

Two units of mare material are mapped, older and younger. These
materials are darker than any terra plains, and some have ridges typical
of near-side mare units. Surface missions, including Luna 16 in the map
area (lat 0°42\' S., long 56° 18\' E.; Earth and Planetary Science
Letters, 1972), have established that the mare materials are basaltic.

Most of the older unit occupies the Australe basin (Stuart-Alexander and
Howard, 1970). Its age is established by superposition of the ejecta
blankets and satellitic crater fields of the craters Humboldt and
Jenner, both of which contain or are embayed by the younger mare unit
(for example see photo IV-12H at lat 36° S., long 88° E. where Humboldt
secondary craters overlie the older unit and are partly flooded by the

younger unit). This overlap of mare material by Imbrian crater materials
is rare on the Moon and suggests that the older mare unit is older than
most or all near-side mare material.

In the absence of clear stratigraphic relations, the two units are
distinguished by density of small superposed craters. The older unit is
mapped in the Marginis basin and in large craters in and near the
Lomonosov-Fleming basin (see photo IV-17H to compare the two mare units
and light-colored plains and terra units). With less certainty, it is
mapped in some patches around the Humboldtianum and Crisium basins,
where photographs are of marginal value for crater studies. The younger
unit seems to have the cratering characteristics of near-side mare
material of late Imbrian age, although many of the smaller patches could
be older or younger.

Several types of nonmare dark materials are present (unit EId) that
could be post-Imbrian. In Mare Smythii, some material has
fresh-appearing flow lobes and hummocks, and thin, sparsely cratered
dark material is superposed on the unusual multiple-ringed craters or
surrounds fresh-appearing rilles that are probably its source. Between
the craters Scaliger and Titius (lat 26° S., long 102° E.) the unit
consists of hummocky dark material rent by fresh-appearing fissures. The
surrounding mare material also appears young, being sparsely cratered
and containing sharp wrinkle ridges and distinctive lava benches. This
dark complex and Smythii may be the youngest volcanic centers in the map
area. In other places a mantle darkens and slightly subdues densely
packed Humboldt secondary craters (lat 31.5° S., long 89° E.---see
IV-12H). This dark mantle and those inside several large craters may be
thin beds of mare lava or a pyroclastic facies of mare material. The
dark material in Waterman (lat 26° S., long 128° E.) could also be
volcanic or could be impact melt ejected from Tsiolkovsky.

BRIGHT SWIRLS

Swarms of light-colored sinuous markings (stipple pattern on geologic
map), which resemble the Reiner gamma feature on the western near side
(McCauley, 1967), occupy an irregular area in and around Mare Marginis
(El-Baz, 1972b). They seem to have no intrinsic relief and are visible
on mare material, several terra units, and both the floors and rims of
craters. They are bright like crater rays, but no certain source craters
have been discovered. No plausible theory of origin is readily
suggested. They may be caused by chemical alteration of the surface
materials by gases released from the lunar interior (El-Baz, 1972b), or
they may be ray materials, perhaps from Giordano Bruno or a smaller
unmapped impact crater on the north rim of Goddard (lat 17° N., long 90°
E.), deposited in a sinuous pattern by unknown processes.

GEOLOGIC HISTORY

After a period of crustal formation about which little is known, a rain
of large and small impacts created a dense array of overlapping craters
and multi-ring basins whose remnants are now exposed in a
topographically high, "primitive" terra. These include very old craters
and the basins Tsiolkovsky-Stark. Al- Khwarizmi-King, Lomonosov-Fleming,
and probably unidentified others.\* Six other basins formed during this
period are large enough or young enough to retain rugged

rims and traces of their surrounding ejecta. The formation of basins and
craters are the only recorded events of pre-Nectarian time.

The Nectarian Period was initiated by a large impact west of the mapped
area that created the Nectaris basin, blanketed the pre-Nectarian
crater-and-basin terrain with lineated and probably nonlineated ejecta,
and created clusters of secondary craters. A similar, but probably
later, impact created the Humboldtianum basin north of the mapped
region, and smaller impacts formed three other basins and numerous
craters.

Like the Nectarian Period, the succeeding Imbrian Period began with a
large basin-forming impact outside the mapped region. Traces of this
Imbrium impact may be present here as plains and terra-mantling
materials and perhaps as crater clusters and furrowed and pitted
terrain. Alternatively some of these materials may have been produced by
volcanism or structural dislocation during this or later times. Another
massive impact, on the western limb of the Moon, created the Orientale
basin, whose ejecta may also have influenced this region.

Sometime during the Imbrian Period, perhaps before the formation of the
Orientale basin, basaltic volcanism created dark mare plains in some of
the pre-Nectarian and Nectarian basins and craters. This early volcanism
was followed in later Imbrian time by more widespread basaltic volcanism
in the same and other basins and craters, including some craters of
Imbrian age. Before and during this late Imbrian mare volcanism, impacts
decreased drastically in number and continued at a much lower rate
during the Eratosthenian and Copernican Periods up to the present.

REFERENCES

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\_\_\_\_\_\_1972b, The Alhazen to Abul Wafa swirl belt: An extensive
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\_\_\_\_\_\_1973, Al-Khwarizmi: A new-found basin on the lunar far side:
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14 lunar orbital photography: Proc. 3d Lunar Sci. Conf., Geochim. et
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\_\_\_\_\_\_1973b, Geologic map of the Petavius quadrangle of the Moon:
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Wollenhaupt, W. R., 1972, Analysis and interpretation of lunar
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Arnold, J. R., 1973, Lunar surface radioactivity: Preliminary results of
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1972, S-band transponder experiment, *in* Apollo 15 preliminary science
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Geology mapped 1971-72. Data sources: Lunar Orbiter and Apollo
photographs, plotted on coverage map, courtesy of National Aeronautics
and Space Administration; X-ray fluorescence data courtesy of I. Adler,
Goddard Spaceflight Center, NASA; gamma-ray data courtesy of A.E.
Metzger, and laser altimeter data courtesy of W.L. Sjogren, Jet
Propulsion Laboratory, California Institute of Technology. Brightness
data from Apollo 8 and Apollo 17 Hasselblad frames taken during
trans-Earth coast.

Adjoining geologic maps at same scale: near-side map by Wilhelms and
McCauley (1971) overlaps between lat 32° N., and 32° S. west of long 70°
E., and between lat 32° and 48° N., and S., west of long 58° E.; central
far-side map by Stuart-Alexander (1977) adjoins east map boundary;
north-side map by Lucchitta (1977) overlaps north of 45° N.; south-side
map overlaps south of lat 45° S.

Geologic maps at the 1:1,000,000 scale entirely or partly within area by
Stuart-Alexander (1971), Casella and Binder (1972), Hodges (1973a, b),
Grolier (1974), and Olson and Wilhelms (1974).

Work performed on behalf of the National Aeronautics and Space
Administration under contract Nos. W-13,130 (mapping; Wilhelms) and NGR
09-015-206 and T-1167B (integration of remote sensing data; El-Baz).

Shaded relief base charts; 2d edition, October, 1970, prepared by
Defense mapping Agency, Aerospace Center (formerly Aeronautical Chart
and Information Center, U.S. Air Force), St. Louis, Mo. 63118. West of
long 100° E., Lunar Earthside Chart (LMP-1); east of long 80° E, Lunar
Farside Chart (LMP-2). Dots refer to center of named feature. Some
feature names shown on these charts have been deleted here. Names shown
are selected from the approved International Astronomical Union (IAU)
list of 1970, with the addition of Al-Khwarizmi (lat 7° N., long 107°
E.) and Necho (lat 5° S., long 123° E.), names approved by the IAU in
1973

[^1]: \* Note added in proof: Mountains in the southeast corner of the
    map area (mapped as units pNbm and pNbr and as province bo) may be
    parts of a ring of a very old basin not identified during the
    preparation of this map. This is the South Pole---Aitken basin,
    about 2200 km in diameter, the largest lunar impact basin) so far
    discovered (Howard and others, 1974; Stuart-AIexander, 1977).