**GEOLOGIC MAP**

**OF THE**

**CENTRAL FAR SIDE**

**OF THE MOON**

By

Desiree E. Steuart-Alexander

1978

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--1047, 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**

**CRATER MATERIALS**

(Only craters 20 km or more in diameter are mapped)

Cc MATERIAL OF VERY SHARP-RIMMED RAYED CRATERS

Ec MATERIAL OF SHARP-RIMMED CRATERS

Ic2 MATERIAL OF SINGLE CRATERS---Orientale basin age or younger

Ic1 MATERIAL OF SINGLE CRATERS---Older than Orientale basin and younger
than Imbrium basin

Ic MATERIAL OF IMBRIAN CRATERS, UNDIVIDED

Ico MATERIAL OF ELONGATE CLUSTERS OF CRATERS APPROXIMATELY RADIAL TO
ORIENTALE BASIN---*Interpretation:* secondary craters of the Orientale
basin

Icc MATERIAL OF ELONGATE AND IRREGULAR CLUSTERS OF CRATERS---Some not
obviously related to any particular Imbrian basin or crater, others
secondary craters of adjacent Imbrian crater to which they are
approximately radial

Ifc FURROWED, RAISED CRATER FLOOR OR FILL MATERIAL---Most are convex
upward. Texture equally sharp in Nectarian and Imbrian craters.
*Interpretation:* postimpact filling, probably lava

Nc MATERIAL OF SUBDUED CRATERS---Older than Imbrium basin and younger
than Nectaris basin

Ncc MATERIAL OF ELONGATE CLUSTERS OF SUBDUED CRATERS---Craters
approximately the same size. *Interpretation:* secondary craters of the
Nectarian basin to which they are adjacent and (or) approximately radial

pNc MATERIAL OF SUBDUED TO VERY SUBDUED CRATERS---Older than Nectaris
basin

**BASIN MATERIALS**

> (Multiringed circular structures 300 km or more in diameter as
> measured across most prominent ring)

Nb MATERIAL OF RAISED RIMS AND SLUMPED WALLS OF BASINS---Primarily the
outermost ring. *Interpretation:* disrupted bedrock largely covered by
ejecta

Nbl LINEATED MATERIAL SURROUNDING BASINS---Linear elements approximately
radial to basin; locally only weakly developed. *Interpretation:* basin
ejecta deposited ballistically and as massive flows

NpNbm MATERIAL OF NECTARIAN BASIN MASSIFS---Massive mountain blocks,
mainly form part of outermost basin rings; inner ring in Korolev.
*Interpretation:* uplifted and structurally complex blocks of prebasin
bedrock; may be covered by basin ejecta

NpNbr MATERIAL OF RUGGED MOUNTAINS AND MOUNTAIN SEGMENTS OF NECTARIAN
BASINS---Smaller than massifs (unit NpNbm); mainly form

inner rings. *Interpretation:* uplifted and complexly faulted prebasin
bedrock; may be covered by basin ejecta

pNb PRE-NECTARIAN BASIN MATERIAL, UNDIVIDED---Subdued, eroded mountain
rings and arcuate segments of rings

pNbm MATERIAL OF PRE-NECTARIAN BASIN MASSIFS---Relatively massive single
mountain blocks or part of continuous ring. *Interpretation:* same as
unit NpNbm

pNbr RUGGED MATERIALS OF PRE-NECTARIAN BASINS---Discontinuous blocky
mountains forming arcuate ring segments; smaller than massifs (unit
pNbm). *Interpretation:* same as unit NpNbr

**OTHER MATERIALS**

Ip MARE MATERIALS OF DARK PLAINS---*Interpretation:* basaltic lavas, by
analogy with returned Apollo samples. STIPPLED PATTERN: light streaks
and swirls in Mare Ingenii. *Interpretation:* Surficial markings of
uncertain origin, probably not related to underlying mare rocks

INp MATERIAL OF GROOVES AND MOUNDS---Covers craters and other terrae of
pre-Nectarian through Imbrian age. Craters have mainly radial grooves on
rim and walls; some mounds Level terra has mounds and grooves.
Particularly well developed around Mare Ingenii and crater Van de
Graaff. *Interpretation:* origin uncertain; general area antipodal to
Imbrium basin; therefore could be depositional site of Imbrium ejecta
that traveled around the Moon, or mass-wasting caused by Imbrium seismic
shaking; alternatively, may be some unidentified local phenomenon unique
to this area

Np SMOOTH LIGHT PLAINS---Generally higher density of craters than on
maria. *Interpretation:* may be related to formation of an Imbrian basin

INp LIGHT PLAINS---Higher density of craters than unit Ip.
*Interpretation:* may be related to various Imbrian and/or Nectarian
basins

Np HIGHLY CRATERED LIGHT PLAINS---*Interpretation:* may be related to
Nectarian basins

It RELATIVELY FRESH-APPEARING, IRREGULAR TERRA---Low relief; low density
of superposed craters. *Interpretation:* probably a complex mixture of
local erosional debris and crater and basin ejecta

Nt ROLLING TERRA---Moderately high density of craters, particularly
craters of diameter less than 20 km. *Interpretation:* same as unit It

NpNt IRREGULAR TERRA---Covers large areas and has high density of
craters larger than 20 km wide. *Interpretation:* same as unit It

pNt CRATERED TERRA---High density of arcuate low hills or crater
segments, and pre-Nectarian craters. *Interpretation:* same as unit It
except contains erosional remnants of pre-Nectarian craters

Contact

Generalized crest of basin ring structure

Slope---Drawn at base of steep slope; barb points down slope

Covered crater rim crest

**INTRODUCTION**

This map is one of a series of geologic maps that will cover the entire
Moon at a scale of 1:5,000,000[^1] The geology of the central far side
is compiled largely from NASA Lunar Orbiter and Apollo photographs and
Soviet Zond photographs; the discussion and interpretation of geologic
units incorporate the geochemical and geophysical data obtained from
orbiting spacecraft. Geologic units are based on the stratigraphic
framework established by Shoemaker and Hackman (1962) as subsequently
revised (summarized by McCauley, 1967; Wilhelms, 1970; Mutch, 1972;
Stuart-Alexander and Wilhelms, 1975). Description of map units is brief
because they largely follow units described on the nearside 1:5,000,000
map of Wilhelms and McCauley (1971). The reader unfamiliar with lunar
maps, mapping principles, and terminology is referred to that map. Only
new units and features or units whose origins are still much in doubt
are discussed here.

**GEOLOGIC SUMMARY**

**GENERAL FEATURES**

Broad geologic patterns, formed primarily by the arrangement and
relative abundance of units common to all provinces, divide the central
lunar far side into four general provinces. The most diversified
province fills the southern third of the mapped area and is coincident
with a nearly obliterated pre-Nectarian basin, here named the South
Pole--Aitken basin for two landmarks along its rim. Within this basin is
the largest far-side concentration of maria and light plains and also
the unique occurrence of a strangely grooved material (unit Ig). Ringing
the basin and locally covering it is a province consisting of a wide
band of pre-Nectarian cratered terra (unit pNt). Two provinces
constitute the northern half of the map area: a zone of nondescript
irregular terra (unit NpNt) occupies most of it, and a smaller,
irregular patch of pre-Nectarian cratered terra (unit pNt) occurs along
the north-east to north-central border of the area. Key features within
all provinces are the multiringed circular basins and some areally
restricted terra and plains units. An unrelenting rain of bolides
produced the shotgun pattern of craters randomly superposed over the
entire map area.

**PRE-NECTARIAN MATERIALS**

The oldest discernible structures are subdued mountain remnants of the
South Pole--Aitken basin. The northernmost chain of mountains (lat 16°
S., long 160--170° W.) was first recognized on Apollo 8 photographs but
was not thought to be associated with any basin (Wilhelms and others,
1969). Extensive detailed mapping now shows that this chain is the
largest of several linear mountains scattered around the periphery of a
giant depression. A line drawn through these mountains describes a crude
circle centered at about lat 50° S., long 180° W. near the crater Von
Kármán. K. A. Howard and H. G Wilshire (written commun., 1976) have
identified several isolated mountains that continue the skeletal
outlines of this great basin, including mountains at lats 80° S. and 85°
S. that locate a possible southern boundary. Also, Wilhelms and El-Baz
(1977) have mapped some isolated mountains at lat 38° S. long 130° E.
that probably mark the western limits of the basin. In order to include
these mountains in the circumference of the basin, I have drawn on this
map a generalized basin ring north of the western mountain segments near
the crater O'Day. Projecting the ring in a circle southward would
include the mountains near the South Pole and give the basin a diameter
of about 2,000 km, making it about 700 km larger than the Imbrium basin
(Markov, 1962; Hartmann and Kuiper, 1962; Stuart-Alexander and Howard,
1970; Hartmann and Wood, 1971).

A topographic low in the area of the South Pole--Aitken basin was first
identified from Zond photographic altimetry (Rodionov and others, 1971)
and confirmed by Apollo laser altimetry. The photographic measurements
were made about 10° east of the center of the basin as mapped here and
intersected a segment of about 60° of longitude (1,800 km). Apollos 15
and 17 crossed the northern segment of the basin and furnished detailed
topographic profiles along a narrow strip (Wollenhaupt and Sjogren,
1972a; Wollenhaupt and others, 1973). The terrain within the northern
segment is highly irregular (fig. 1), both because of the high density
of superposed craters and because of the uneven mountain segments that
form the residual rings of this large basin. The photographic data are
more generalized than the laser data and show the average basin depth to
be 5 to 6 km below the central far-side terra and more than 7 km below
the southern near-side terra along long 10°E. The most recent
compilations of laser altimetry also show that the lowest areas within
the basin are as deep as any on the near side of the Moon and more than
6 km below the adjacent highlands (Kinsler and others, 1975; Bills and
Ferrari, 1975).

The second oldest feature mapped is a smaller basin in the northern
irregular terra province. The basin is centered at lat 19° N., long 175
E., near the crater Buys-Ballot and small patches of maria. It is here
named the Freundlich-Sharonov basin for two conspicuous superposed
craters. The western mountains of this basin were first reported by
Campbell, O'Leary, and Sagan (1969) and Baldwin (1969). The eastern
perimeter is identified by well-developed, arcuate scarps that face
basinward. As mapped here, the basin is 600 km in diameter, although
Baldwin (1969) considered these outermost mountains to be inner rings.
His postulated outer two rings at 1,000 km and 1,600 km are not
confirmed; in fact, the laser altimetry from Apollo 16 (Wollenhaupt and
Sjogren, 1972b) shows topographic highs in areas that should be
intermountain lows if any outer rings had existed.

The remaining pre-Nectarian materials comprise two additional basins,
many craters, and heavily cratered terra. One basin, Apollo, lies
entirely within the South Pole--Aitken basin. The other, here named the
Keeler--Heaviside basin for the two craters that almost fill it, lies
immediately north of the South Pole--Aitken basin and within the
southern province of old terra (unit pNt). This province surrounds and
locally penetrates the South Pole--Aitken basin, supporting the great
antiquity of that basin. The northern province of old terra (unit pNt)
is much more irregular and does not seem to be related to any structure.
The terra consists of rolling, rough terrain because much of it is
arcuate hillocks that are probably remnants of old battered craters.

The irregular terra (unit NpNt) is considered to contain both
pre-Nectarian and Nectarian materials. Fewer pre-Nectarian features lie
within its limits than within the older terra, the unit is probably a
mixture of ejecta from Nectarian and pre-Nectarian basins and craters.
It undoubtedly contains admixtures of even younger ejecta, but these do
not show. The unit forms monotonous, uneven, rolling ground. The
physical nondistinctiveness of this terra is paralleled by the
geochemical monotony of the terrain as discussed below.

**NECTARIAN SYSTEM**

In general, Nectarian units correspond in kind to those of the
pre-Nectarian and therefore are interpreted as products of similar
processes. The main differences occur in the random distribution of a
major Nectarian units and in the smaller average size of Nectarian
basins relative to pre-Nectarian ones. Also, certain physical
differences are apparent because of the comparative youthfulness of
Nectarian features. Not only are details such as crater rim-crests a
little sharper, but the texture of parts of ejecta blankets around
Nectarian basins is sufficiently distinct to permit mapping them as a
lineated unit (Nbl), separate from the basin rims (units Nb and NpNbr).

Within the Nectarian System are the oldest discernible units among the
light plains materials that cover approximately 5 percent of the lunar
surface. Nectarian highly cratered light plains (unit Np) are scattered
across the map area as fillers in a few basins and large craters. More
abundant are the light plains mapped here as consisting of both Imbrian
and

Nectarian materials (unit INp), but most abundant are the Imbrian plains
which form 4 percent of the lunar surface (Howard and others, 1974, p.
319). Eratosthenian and Copernican plains exist, but none are identified
at the scale of this map. The comparative scarcity of Nectarian plains
may not be due to lack of original plains materials but rather to their
destruction by impact and perhaps other processes that tend to roughen
smooth surfaces.

Leading contending theories of the origin of light plains seem to be
those that relate the plains to basin formation. By one theory they form
from the lateral transport of fluidized ejecta (Eggleton and Schaber,
1972); by another they form from ballistic transport of ejecta (for
example, Moore and others, 1974; Chao, 1974); and by another they are
primarily erosional in origin, consisting of mass wasted material and of
local material that has been reworked and redeposited by secondary
craters of distant craters or basins (Oberbeck and others, 1974, 1975).
An earlier concept was that the light, smooth plains were predominantly
very fluid lavas or pyroclastic rocks (for example, Milton and Hodges,
1972; Trask and McCauley, 1972), but rocks from the plains sampled by
the Apollo astronauts were predominantly impact-brecciated
plagioclase-rich rocks (Apollo 16 Preliminary Examination Team, 1973).

**IMBRIAN SYSTEM**

Contrasts between Imbrian and older features indicate that the Imbrian
Period was a time of transition in the history of the Moon. One of the
changes occurred in the influx of large bolides, which is reflected in
the relative numbers of large craters in each lunar system. There are
only three Imbrian basins on the entire Moon, none of which is in this
map area where the largest Imbrian crater is only about 145 km in
diameter. Here there are 14 Imbrian craters 100 km or more across, 38
Nectarian craters, and 47 pre-Nectarian. Whether or not impacts of this
size reached a steady state (Baldwin, 1969; Howard and others, 1974),
younger craters and basins have obliterated older ones, so that 47 is a
minimum count for the pre-Nectarian.

Despite the discrepancy in numbers of large impact features, the maximum
time available for the pre-Nectarian and Nectarian combined (Tera and
others, 1974; Stuart-Alexander and Wilhelms, 1975) about equal to the
minimum length of the Imbrian Period, that is, roughly 600 m.y.,
assuming that the top of the Imbrian lies within the timespan of the
deposition of the Apollo 15 mare basalts (Carr and El-Baz, 1971). The
absolute age for those basalts is 3.3 ± 0.1 g.y (Papanastassiou and
Wasserburg, 1973), leaving three-quarter of all lunar history with no
basins.

Another change within the Imbrian Period is a reduction in the flux rate
for smaller craters. This change is less dramatic than differences in
numbers of basins but is important and has been well documented (for
example, Soderblom and Lebofsky, 1972; Boyce and others, 1974).

Another transition in lunar evolution is recorded by the plains units,
which form an important part of the Imbrian record. The oldest mare
basalts (dark plains, unit Im) that have been recognized belong to the
Imbrian System (Stuart-Alexander and Howard, 1970); they may be younger
than the Orientale basin (Wilhelms, 1970; Wilhelms and McCauley, 1971).
Maria on the central far side are harder to date stratigraphically than
those on the near side because they form in isolated patches that are
not in contact with easily distinguishable Orientale or Imbrium basin
materials. The far-side mare appear to be lighter (higher albedo) than
most near-side maria, possibly the result of lightening with age
(Stuart-Alexander and Howard, 1970) or alternatively perhaps slight
differences in chemical composition.

Imbrian smooth light plains (unit Ip) are scattered across the area but
appear to be somewhat more abundant in the east half to two-thirds. As
this is the region closest to Orientale, a control by that basin is
possible, as noted by Howard, Wilhelms, and Scott (1974) and Scott and
others (1977). The Concentration here is greatest in and around the
basins Korolev and Apollo, which, while closest to Orientale, are not
directly in line with

the major extent of the Orientale ejecta blanket as mapped by Scott and
others (1977). There is a slight concentration of light plains in the
northeast, where most of the identified Orientale secondaries (unit Ico)
occur, but there is no preferred juxtaposition or other apparent
relation between the secondaries and the light plains. Therefore, this
mapping does not resolve the origin of light plains.

Origin of the unit of grooves and mounds (Ig) is uncertain. No other
lunar terrain is exactly like it. Its features are sharper than those of
\"hilly and pitted terrain\" (Wilhelms and McCauley, 1971; Wilhelms and
El-Baz, 1977) and its positive elements less regular than those of
"hilly and furrowed terrain" (Wilhelms and McCauley, 1971). It is more
strongly and sharply developed on crater rims and walls than are the
other two units. Distribution of the grooves and mounds is limited to an
area within the South Pole--Aitken basin from lat 20° S. to 47° S., long
147° E. to 178° W. The largest exposure of the unit covers about 23,000
km2. The juxtaposition of all exposures suggests a common origin but not
necessarily one related to the huge containing basin.

Three proposed explanations for the origin of the grooves and mounds are
extrusive volcanism, impact of ejecta from the Imbrium basin, and
mass-wasting caused by a shock wave from the Imbrium basin impact.
Before Apollo 16 landed at Descartes, some sort of highland volcanism
seemed a reasonable explanation for units such as the hilly and furrowed
material on the Moon\'s near side, but only impact-brecciated plutonic
rocks were collected there, not volcanic rocks. This does not
categorically exclude a volcanic interpretation for terra materials, but
it does make them less probable for terrain of similar morphology.

Both hypotheses that stress a relation of the grooved unit to the
Imbrium basin do so because of the unit's antipodal position to Imbrium.
Similar associations occur elsewhere: an area of furrowed and pitted
terrain near Mare Marginis is antipodal to the Orientale basin (Moore
and others, 1974; Wilhelms and El-Baz, 1977), and an area of hilly and
pitted terrain of Mercury is antipodal to the Caloris basin (Murray and
others, 1974, p. 176). Schultz and Gault (1975) have proposed that these
features may have formed by extensive mass-wasting caused by seismic
waves associated with the impact that produced the basin. P waves and
surface waves would eventually converge at the antipodal point, the P
waves before arrival of ejecta and the surface waves simultaneously with
the ejecta. Reflected tensile waves would converge beneath the antipodal
points, adding to disruption of the surface. Moore, Hodges, and Scott
(1974, p. 91), however, ascribe the terrain features primarily to
convergence of ejecta at the antipodal point. The strongest evidence
supporting an impact-related hypothesis comes from the apparent
contemporaneity of all parts of the unit; the grooves and mounds seem to
be of comparable freshness everywhere yet are superimposed on craters
that range in age from pre-Nectarian to uppermost Nectarian (perhaps to
lower-most Imbrian).

The physical appearance of the unit suggests that ejecta deposition
dominated and that seismic shaking played a subsidiary role. Mounds and
grooves out on the level terra are not related to slopes in arrangement
or shape as would be expected if they formed by mass wasting.
Additionally, in some places material seems to have piled up on top of
crater rim crests and some grooves are normal to crater rim crests,
suggesting ejecta deposition. However, the down-slope movement of
abundant material inside crater walls could have been produced by
shaking from ejecta impacting the surface or from seismic waves.

**ERATOSTHENIAN AND COPERNICAN SYSTEMS**

In the map area, the Eratosthenian and Copernican Systems are only
represented by craters 75 km wide or smaller; no other materials were
identified. Craters of both systems are randomly distributed.
Eratosthenian craters are more abundant than Copernican, corroborating
the decline in flux rate proposed by Soderblom and Lebofsky (1972).

**GEOCHEMISTRY AND GEOPHYSICS**

Significant data have been collected from orbit by Apollos 15, 16, and
17, Zond 6, and lunar Orbiter 5. Results most relevant to the central
far side were obtained from laser altimetry, photogrammetric whole-disk
altitude measurements, gamma-ray spectroscopy, subsatellite
magnetometry, and gravity calculations based on spacecraft
perturbations. X-ray fluorescence coverage of the central far side was
insufficient for analysis because the data are limited to narrow bands
in the westernmost 12° of longitude. Results of other experiments
expected to produce useful data should be released in the near future.
These include data from infrared thermal emissions, alpha-particle
spectrometry, and radar sounding.

**ALTIMETRY**

Altitude measurements computed by various methods have been compiled by
Bills and Ferrari (1975) into a generalized topographic map for a wide
belt around the Moon encompassing lat 45° N. to 45° S. Detailed
topographic profiles in an east-west direction were provided by the
reduction of laser altimetry data recorded by Apollos 15, 16, and 17
(Wollenhaupt and Sjogren, 1972a and b; Wollenhaupt and others, 1973),
and in a polar cross section from the photogrammetric altimetry of Zond
6 (Rodionov and others, 1971). Kinsler, Merrill, and Srnka (1975) have
prepared a map at a resolution of 1 km of the orbital tracks along which
laser altimetry was collected by Apollos 15. 16, and 17.

Comparison of results from the different methods of altimetry are in
reasonable agreement once the photogrammetric data have been converted
from a geometrically spherical Moon to a Moon spherical about the center
of mass. As discussed earlier, both the laser altimeter and
photogrammetric limb measurements confirm a major deep depression in the
southern hemisphere of the far side (the South Pole--Aitken basin,
fig. 1) and show that this depression is as deep as any basin on the
near side. Additionally, the laser altimeter indicates that there is a
major topographic high north of the basin and east of long 180°. Along
the Apollo 16 ground track (fig. 2), the high extends from about 180° to
135° W.; along the Apollo 15 (fig. 1) and 17 ground tracks (Kinsler and
others, 1975) it extends from about 155° W. to 125° W. These data
suggest that there may be a broad arch, trending north- westward, that
averages more than 3 km above a spherical Moon. Unfortunately the
photogrammetric data are so poor or nonexistent in a 25° strip (Rodionov
and others, 1971) that includes the Apollo 16 ground track that neither
the topographic high nor the continuation or termination of the light
terrain northward can be confirmed. From the photogrammetric data alone,
however, it seems that the terrain north of about lat 25° N. does not
rise significantly above the spherical Moon radius (Rodionov and others,
1971). It is significant that the southern near-side highlands, and area
not sampled by the Apollo laser altimeter, do appear as a major
topographic high in the photogrammetric data .

If the photogrammetric data are correct, as they seem to be, then there
are no significant differences in altitudes between the near and far
sides, only between basins and nonbasin terra. Altitude differences of 7
km or more exist between the basin deeps and adjacent terra masses.
Major near-side and far-side basins seem to be equally deep, and major
terra masses rise equally far above the mean radius of the Moon on both
hemispheres. This equality suggests that the crust on the southern
near-side highlands is as thick as that postulated for the far side by
Kaula, Schubert, Lingenfelter, Sjogren, and Wollenhaupt (1973).

**GAMMA-RAY SPECTROMETRY**

A plot of total lunar radioactivity measured by orbiting gamma-ray
spectrometers is included here for the central far side (fig. 3).
Selected areas of the whole plot, which covers about 20 percent of the
lunar surface (Metzger and others, 1973a, b), have been used to
calibrate the orbital values with those at some of the landing sites by
Trombka, Arnold, Reedy, and Peterson (1973). They found that the orbital
values tend to be higher than the

ground-based measurements but that the trends are in good agreement. The
orbital data are averaged over 2° squares, 60 km on a side, and
generally show that maria are more radioactive than terrae. Data
reduction is still being refined, but it is apparent that there are
variations within the terrae, within one mare, and from one mare to
another; the western maria on the near side are the most radioactive.
Radioactivity on the central far side seems low (fig. 3), except for an
anomaly that extends from long 160° E. to 166° W. and includes the Van
de Graaff region.

The source of the anomaly is not apparent. It might be caused by the
four patches of maria that are spread out under the ground track, but
these make up less than 20 percent of the area. Most of the maria lie
south of the ground track and should not strongly affect the data.
Alternatively, the radioactivity may be related to the grooved terrain
(unit Ig), although this terrain does not appear to extend under the
easternmost 10° of the anomaly. If this unit were formed by ejecta from
the Imbrium basin, it might correlate with the high radioactive values
measured on the surface at the Apollo 14 site (Eldridge and others,
1972) and recorded in that general area from orbit by Apollo 16. The
terra rocks there have been mapped as Imbrium basin ejecta (Eggleton,
1964, 1965; Wilhelms and McCauley, 1971).

Further analysis of the gamma-ray data has resulted in preliminary
values for concentrations of iron, magnesium, and titanium as well as
separate values for potassium and thorium plus uranium, rather than
total radioactivity. These concentrations have been published (Metzger
and others, 1974) using generally fairly large resolution cells (fig. 4)
for the preliminary analysis. The main far-side highland areas---B, C,
D, and E of fig. 4---all have a relatively consistent iron content that
is a little lower than the value of 7.2 weight percent given for the
northern highlands immediately west of the central far side. The
far-side highlands are consistent with the southern highlands that are
an extension of area E. Area A, coincident with the Mendeleev basin, has
a lower iron content than the rest of the central far side, but the iron
percent is comparable to, or higher than, that in the highlands
extending from the Hertzsprung basin eastward across Orientale ejecta
and Orientale's northern rim (Metzger and others, 1974). The Van de
Graaff area has the highest iron content recorded on the far side. This
is higher than the average terra value but lower than most maria and
could be explained by its mare patches. Magnesium, unlike iron, does not
seem to exhibit any pattern in the central far side, except that the
values in the northern strip are a little lower than those in the
southern strip; Van de Graaff falls in the middle. This contrasts with
the general correlation of higher magnesium values over most maria
relative to most terrae, but the correlation is not so marked or
consistent as that of iron (Metzger and others, 1974, p. 1074, and table
3 and 4). As titanium values have been published for the southern ground
tracks only, generalizations cannot be made. In the Van de Graaff area,
however, the titanium values do seem to be too low for a typical mare.
Finally, most values for potassium and thorium plus uranium, and the
ratios between these elements, are consistent with the near-side terrae
values. The values in the van de Graaff cell, however, appear similar to
the near-side eastern maria, particularly Mare Serenitatis. The
resolution cells for the eastern maria include areas of terra but not as
much as in the Van de Graaff cell.

In summary, the gamma-ray data for the central far side show that all
areas except Van de Graaff have chemical compositions comparable to
other terrae areas. However, the resolution cells for most of the
geochemical results are too large to pick up local variations in
composition. The Van de Graaff cell is unique in its chemistry: iron,
thorium plus uranium, and potassium contents are higher than typical
terra and lower than most maria; magnesium and particularly titanium are
too low for mare. Probably the chemical values reflect a mixture of
materials that include terra, the grooved material (unit Ig), and some
mare.

**MAGNETIC DATA**

Approximately 20 percent of the Moon\'s magnetic field has been mapped
from orbital magnetometers by measuring the vector magnetic fields
(Sharp and others, 1973; Russell and others, 1973, 1974, 1975). Most of
the vector magnetic coverage is duplicated and also

extended by electron scattering measurements (Howe and others, 1974;
McCoy and others, 1975, Lin and others, 1975; 1976; Anderson and others,
1976; R. P. Lin, unpub. data, 1976). Complementary work on a much
coarser scale (10° x l0° resolution cells) has been compiled from
measurements of solar wind compressions that are interpreted as being
caused by the lunar magnetic field (Russell and Lichtenstein, 1975). A
general conclusion based all these data sets is that the Moon is
extensively magnetized in both the terrae and maria, but that magnetic
anomalies are much more numerous in the terrae than in the maria. The
strongest fields detected to this time occur on the far side (figs. 5
and 6). The main anomalies detected by the vector magnetic fields are
also detected by the other two methods.

Four main magnetic anomalies are evident in figures 5 and 6 (centered at
about 22° S. and 170° E., 20° S. and 170° W., 0° and 165° E., and 5° N.
and 175° W.) and are apparent in Lin's electron scattering maps (Lin and
others, 1975; R. P. Lin, unpub. data, 1976); Lin and others show a fifth
anomaly (2° N. and 155° W.). The two southern anomalies are the
strongest and could be considered as one large anomalous zone from the
limb compression data and the electron scattering data. The western-most
of these anomalies is near the crater Van de Graaff and is the largest
anomaly so far detected on the Moon. The two anomalies appear distinct
and separate on the vector field measurements (Sharp and others, 1973;
Russell and others, 1974; figs. 5 and 6), and Russell and Lichtenstein
(1975) also interpret their data as indicating two separate anomalies.
The fine-scale plot of the Van de Graaff anomaly shows a highly
structured field (Sharp and others, 1973, fig. 10) whose origin is still
being debated (Sharp and others, 1973; Strangway and others, 1973;
Stuart-Alexander, 1975).

Origin of all lunar magnetic anomalies is still in doubt. It is
difficult to ascribe them to the surface geologic units because, at
least in this map area, the units under each anomaly are diverse in
origin and age: a pre-Nectarian crater, pre-Nectarian through Nectarian
terra, a Nectarian basin, Imbrian grooved material, and ejecta blankets
of early Imbrian craters. The only consistent relation discernible to me
is that each anomaly is close to the major structural rim of an old
basin. The southern anomalies are actually just within the proposed
outermost rim of the South Pole--Aitken basin, although the western-most
anomaly near Van de Graaff is also close to the outermost rim of the
pre-Nectarian Keeler--Heaviside basin. The northern anomalies lie, from
west to east, near the projection of the northern rim of
Keeler-Heaviside basin, outside the southern rim of the pre-Nectarian
Freundlich--Sharonov basin, and straddling the rim crest of the
Nectarian Korolev basin. With the exception of Korolev, the youngest
(Nectarian) basins in the mapped area---Mendeleev and Moscoviense---do
not appear to have magnetic anomalies. Apollo, another of the old
basins, was too far from the ground track to register, although Russell
and Lichtenstein (1975) believe that the easternmost of their major
anomalies is associated with that basin. Note that these anomalies, with
the possible exception of South Pole--Aitken basin, occur along only
small segments of the basin rims.

**GRAVITY DATA**

Computations of lunar gravity fields have been based on direct tracking
from Earth of an orbiting spacecraft. Results for most of the near side
were based on Lunar Orbiter data; computations for the far side were
much more difficult. Two workers have recently made new attempts to
combine the data from both Lunar Orbiter and Apollo spacecraft
perturbations over the far side (Ferrari, 1975; Ananda, 1975; Ananda and
Ferrari, 1975). There still appear to be problems in reducing the data.
For instance, there is a periodicity in Ferrari\'s map that might be
suspect; the positive anomalies appear in north-south bands, nicely
balanced by parallel bands of negative anomalies. Ananda's data, on the
other hand, show a more irregular distribution of positive and negative
anomalies that is more comparable to the near-side reduction (see for
example, Sjogren, 1974). The problem with the reduction is that the
anomalies do not correlate directly with physical features on the ground
but are commonly offset; the direction of offset may be consistent.

The map that combines the two reductions (Ananda and Ferrari, 1975, pl.
13) has made "Qualitative adjustments. . . to eliminate solution errors
and to locate major anomalies over corresponding topographic features."
Unfortunately, this map may have introduced other errors. It shows the
Moscoviense and Mendeleev basins to be gravity lows of equal value,
forcing some of Ananda's data to the west and some to the east, whereas
Ananda's original map shows a positive anomaly centered slightly west of
Moscoviense and a negative one equally offset to the west of Mendeleev.
Because Moscoviense contains a significant quantity of mare rocks, the
negative anomaly over it should be less than over Mendeleev in fact, the
Moscoviense anomaly could well be positive. Additionally, the negative
anomalies immediately east of Moscoviense and Mendeleev on Ananda\'s map
coincide quite well with the Freundlich--Sharonov basin as mapped here.
The revised Ananda-Ferrari map shows this area as strong gravity high,
which is inconsistent with a "dry" basin, even a very old basin.
Finally, the Ananda map generally shows the interbasin areas,
particularly the old pre-Nectarian terra, as gravity highs. This
consistent picture may be more nearly correct than the subsequent
attempt to fit the data to certain basins, thereby missing others.

**RADON-222 AND POLONIUM-210**

Distribution of radon-222 and her delayed daughter polonium-210 was
measured by Apollos 15 and 16 over approximately 15 percent of the Moon
(Bjorkholm and others, 1973a, b). Because radon-222 is a decay product
of uranium, the distribution of all three elements might be expected to
be grossly parallel. Bjorkholm, Golub, and Gorenstein (1973a) maintain
that this is so, but as they have published only one orbital path for
radon-222 (1973a, fig. 1), their conclusion is not entirely supportable.
They have published a map of the distribution of polonium-210 (1973b) on
a 10° grid wherein the values vary from one place to another, but not in
a consistent pattern. Apparently no particular signature differentiates
mare from terra, whereas most of the other geophysical-geochemical
measurements do. There is no readily discernible far-side/near-side
distribution pattern and no apparent anomalies or the central far side.

**INFRARED DATA**

Apollo 17 flew an infrared scanning radiometer to measure thermal
emission during lunar orbit. Preliminary analysis of the data (Low and
Mendell, 1973) indicates that the Eratosthenian crater Birkland on the
rim of Van de Graaff (lat 30° S., long 175° E.) has a slower cooling
rate than the surrounding terrain. A second, smaller anomaly is adjacent
to the Birkland ejecta blanket within Van de Graaff (Low and Mendell,
1973). Both of these anomalies are consistent with the findings on the
near-side by Shorthill and Saari (1969) that show younger craters to
have slower cooling rates than older craters.

A preliminary map of the infrared data that has been made of a 15° band
for long 168° to 145° W. (Mendell, 1974) does not go far enough south to
include Birkland crater. It does, however, include part of the double
crater Van de Graaff where there are four bright, young, probably
Copernican craters too small to be mapped here. These show up as
anomalies on Mendell's map (1974). Four other small anomalies are
apparent on his map (17° S. and 170° W., 19° S. and 163° W., 12° S. and
152° W., and 13° S. and 150° W.), and these too are over, or close to,
bright sharp craters of Copernican age too small to be mapped here. From
these results, it is clear that the infrared scanning technique and
resultant maps are useful mapping tools for identifying young, fresh
features and would be particularly helpful in areas where photography is
poor.

**CONCLUSIONS**

For the central far side of the Moon, the pre-Nectarian and Nectarian
were the most significant geologic periods, whereas on the near side,
the Imbrian System dominates the geology. The decline in impact flux
between the Nectarian and Imbrian Periods is clearly evident on the far
side in the relative paucity of Imbrian and younger craters and by the

reduction in diameter of the largest craters in the three youngest
systems. All of the central far-side basins are Nectarian or older, as
are most of the other mapped materials. The basins are the structures
that control the emplacement of most maria and apparently control the
surface geochemical and geophysical properties of most areas. In
particular, the giant depression of the southern hemisphere, here named
the South Pole--Aitken basin, exerts a tremendous control over much of
the geology of the central far side. It is the largest lunar basin whose
structure is discernible by more than one parameter. It is one of the
lowest areas on the Moon. Most of the far-side maria lie within its
boundaries. Finally, the preponderance of all geologic, geochemical, and
geophysical anomalies on the central far side lie within its boundaries;
how much of this is coincidental (because of the huge size of the basin)
is still uncertain.

Two extensive units may not be controlled by Nectarian or older basins.
One is the light plains material (unit Ip), which is widespread over the
area primarily as a filler in craters and other low areas. There is some
indication of a concentration in the eastern half of the area,
suggesting a possible relation to an Imbrian basin, possibly Orientale.
The other is the enigmatic unit of grooves and mounds (Ig) that seems to
be related to the formation of the Imbrium basin.

In general. the geochemical results are consistent with the typical
near-side signatures that distinguish terrae from maria. Possible
exceptions are the analyses over the part of the South Pole--Aitken
basin that includes the crater Van de Graaff. There is a radioactive
high in this region that could be related to maria under and adjacent to
groundtracks of the orbiting spacecraft. Iron content is consistent with
the percentage of maria present, whereas both magnesium and titanium
seem low for maria. This supports the suggestion that part of the
radioactive anomalies may be produced by some other material under the
groundtrack, possibly the grooves and mounds (unit Ig).

The magnetic data appear very significant but are inconclusive. The
highest anomaly so far detected on the Moon lies within the South
Pole--Aitken basin, near the crater Van de Graaff, and another
significant anomaly lies farther east in the basin. The geologic
correlation with the three other significant magnetic anomalies on the
far side may be their location near the outer ring of a major basin.

**REFERENCES**

Ananda, Mohan, 1975, Farside lunar gravity from a mass point model,
Proc. 6th Lunar Sci. Conf.: Geochim. et Cosmochim. Acta, suppl. 6, v. 3,
p. 2785*--*2796.

Ananda, M. P., and Ferrari, A. J., 1975, Lunar farside gravity, Proc 6th
Lunar Sci Conf.: Geochim. et Cosmochim. Acta, suppl. 6, v. 3,
Frontispiece, pl. 13.

Anderson, K. A., Lin, R. P., McCoy, J. E., McGuire, R. E., Russell, C.
T., and Coleman, P. J., 1976, The large magnetized region associated
with Rima Sirsalis, *in* Abstracts, 7th Lunar Sci. Conf.: Houston. Lunar
Sci. Inst., p. 16*--*18.

Apollo 16 Preliminary Examination Team, 1973, The Apollo 16 Lunar
Samples---petrographic and chemical description: Science, v. 179, p.
23*--*34.

Baldwin, R. B., 1969, Ancient giant craters and the age of the lunar
surface: Astronom. Jour., v. 74, no. 4, p. 570--571.

Bills, B. G., and Ferrari, A. J., 1975, Lunar topography, Proc. 6th
Lunar Sci. Conf.: Geochim. et Cosmochim. Acta, suppl. 6, v. 3,
Frontispiece, pl. 2.

Bjorkholm, P. J., Golub, Leon, and Gorenstein, Paul, 1973a, Distribution
of Rn-222 and Po-210 on the lunar surface as observed by the alpha
particle spectrometer, Proc. 4th Lunar Sci. Conf.: Geochim. et
Cosmochim. Acta, suppl. 4, v 3, p. 2793--2802.

\_\_\_\_\_1973b, Distributions of Polonium-2l0, Proc. 4th Lunar Sci.
Conf.: Geochim. et Cosmochim. Acta, suppl 4, v. 1, Frontispiece, pl. 5.

Boyce, J. M., Dial, A. L., and Soderblom, L. A., 1974, Ages of the lunar
nearside light plains and maria, Proc. 5th Lunar Sci Conf.: Geochim. et
Cosmochim. Acta, Suppl. 5, v. 1, p. 11--23.

Campbell, M. J., O'Leary, B. T., and Sagan, Carl, 1969, Moon---Two new
mascon basins: Science, v. 164, p. 1273--1275.

Carr, M. H., and El-Baz, Farouk, 1971, Geologic map of the
Apennine-Hadley region of the Moon---Apollo 15 pre-mission map: U. S.
Geol. Survey Misc. Inv. Map I-723 (sheet 1).

Chao, E. C. T., 1974, Impact cratering models and their application to
lunar studies---a geologist's view, Proc. 5th Lunar Sci Conf: Geochim.
et Cosmochim. Acta, suppl 5, v 1, p 35--52.

Eggleton, R. E., 1964, Preliminary geology of the Riphaeus quadrangle of
the Moon and definition of the Fra Mauro Formation, *in* Astrogeologic
Studies Ann. Prog. Rept., August 1962--July 1963, pt. A: U.S. Geol.
Survey open-file report, p. 46--63.

\_\_\_\_\_1965, Geologic map of the Riphaeus Mountains region of the
Moon: U.S. Geol. Survey Misc. Geol. Inv. Map I-458.

Eggleton, R. E., and Schaber, G. G., 1972, Cayley Formation interpreted
as basin ejecta, *in* Apollo 16 preliminary science report: NASA SP-315,
p. 29--7 to 29--16.

Eldridge, J. E., O'Kelley, G. D., and Northcutt, K. J., 1972, Abundances
of primordial and cosmogenic radionuclides in Apollo 14 rocks and fines,
Proc. 3rd Lunar Sci. Conf.: Geochim. et Cosmochim. Acta. suppl. 3, v. 2,
p. 1651--1658.

Ferrari, A. J., 1975, Lunar gravity---the first farside map: Science, v.
188, p. 1297--1300.

Hartmann, W. K., and Kuiper, G. P., 1962, Concentric structures
surrounding lunar basins: Arizona Univ. Lunar and Planetary Lab.
Commun., v. 1, no. 12, p. 51--67.

Hartmann, W. K., and Wood, C. A., 1971, Moon---origin and evolution of
multi-ring basins: The Moon, v. 3. p. 3--78.

Howard. K. A., Wilhelms, D. E., and Scott, D. H., 1974, Lunar basin
formation and highlands stratigraphy: Reviews Geophys. and Space
Physics, v. 12. no. 3, p. 309--327.

Howe, H. C., Lin, R. P., McGuire, R. E., and Anderson, K. A., 1974,
Energetic electron scattering from the lunar remanent magnetic field:
Geophys. Research Letters, v. 1, no. 3, p. 101--104.

Kaula, W. M., Schubert, G., Lingenfelter, R. E., Sjogren, W. L., and
Wollenhaupt, W. R., 1973, Lunar, topography from Apollo 15 and 16 laser
altimetry, Proc 4th Lunar Sci. Conf.: Geochim. et Cosmochim., Acta,
suppl. 4, v. 3, p. 2811--2819.

Kinsler, D. C., Merrill, R. B., and Srnka, L. J., 1975, Apollo laser
altimetry, Proc. 6th Lunar Sci. Conf.: Geochim. et Cosmochim. Acta,
suppl. 6, v. 3, Frontispiece, pl. 1.

Lin R. P., Anderson, K. A., McGuire, R. E., and McCoy, J. E., 1976, Fine
scale lunar surface magnetic fields detected by the electron reflection
method, *in* Abstracts, 7th Lunar Sci. Conf.: Houston, Lunar Sci. Inst.,
p. 492--494.

Lin, R.P., McGuire, R.E., Howe, H.C., and Anderson, K.A., 1975, Mapping
of lunar surface remanent magnetic fields by electron scattering, Proc.
6th Lunar Sci. Conf.: Geochim. et Cosmochim. Acta, suppl. 6. v. 3, p
2971--2973, and Frontispiece, pl. 3 and 4.

Low, F. J., and Mendell, W. W., 1973, Infrared scanning radiometer, *in*
Apollo 17 preliminary science report: NASA SP-330, p. 24--1 to 24--6.

Markov, A. V., 1962, Description of the lunar surface, *in* A. V.
Markov, ed., The Moon: Univ. Chicago Press, p. 75--99.

McCauley, J. F., 1967, The nature of the lunar surface as determined by
systematic geologic mapping, *in* Runcorn, S. K., ed., Mantles of the
Earth and terrestrial planets: New York and London, John Wiley & Sons,
p. 431--460.

McCoy, J. E., Anderson, K. A., Lin, R. P., Howe, H. C., and McGuire, R.
E., 1975, Lunar remanent magnetic field mapping from orbital
observations of mirrored electrons: The Moon, v. 14, p. 35--47.

Mendell, W. W., 1974, Apollo 17 infrared scanning radiometer lunar
farside radiance map, Proc. 5th Lunar Sci. Conf.: Geochim. et Cosmochim.
Acta, suppl. 5, v. 1, Frontispiece, pl. l0.

Metzger, A. E., Trombka, J. I., Peterson, L. E., Reedy, R. C., and
Arnold, J. R., 1973a, Lunar surface radioactivity---Preliminary results
of the Apollo I5 and 16 gamma-ray spectrometer experiments: Science, v.
179, p. 800--803.

\_\_\_\_1973b, Net lunar radioactivity, Proc. 4th Lunar Sci. Conf.:
Geochim. et Cosmochim. Acta, suppl. 4, v. 1, Frontispiece, pl. 2.

Metzger, A. E., Trombka, J. I., Reedy, R. C., and Arnold, J. R., 1974,
Element concentrations from lunar orbital gamma-ray measurements, Proc.
5th Lunar Sci. Conf.: Geochim. et Cosmochim. Acta, suppl. 5, v. 2, p.
1067--1078, and v. 1, Frontispiece, pl. 2.

Milton, D. J., and Hodges, C. A., 1972, Geologic maps of the Descartes
region of the Moon---Apollo 16 pre-mission maps: U.S. Geol. Survey Misc.
Inv. Map I-748.

Moore, H. J., Hodges, C. A., and Scott, D. H., 1974, Multiringed
basins---illustrated by Orientale and associated features, Proc. 5th
Lunar Sci. Conf.: Geochim. et Cosmochim. Acta, suppl. 5 v. l, p.
71--100.

Murray, B. C., Danielson, G. E., Davies, M. E., Gault, D. E., Hapke,
Bruce, O'Leary, Brian, Strom R. G., Suomi, Verner, and Trask, Newell,
1974, Mercury's surface---preliminary description and interpretation
from Mariner 10 pictures: Science, v. 185, p. 169--179.

Mutch, T. A., 1972, Geology of the Moon, a stratigraphic view:
Princeton, N. J., Princeton Univ. Press, 2d ed., 391 p.

Oberbeck, V. R., Hörz, F., Morrison, R. H., Quaide, W. L., and Gault, D.
E., 1975, On the origin of the lunar smooth-plains: The Moon, v. 12, p.
19--54.

Oberbeck, V. R., Morrison, R. H., Hörz, F., Quaide, W. L., and Gault, D.
E., 1974, Smooth plains and continuous deposits of craters and basins,
Proc. 5th Lunar Sci. Conf.: Geochim. et Cosmochim. Acta, suppl. 5, v. 1,
p. 111--136.

Papanastassiou, D. A., and Wasserburg, G. J., 1973, Rb-Sr ages and
initial strontium in basalts from Apollo 15: Earth and Planetary Sci.
Letters, v. 17, p. 324--337.

Rodionov, B. N., Isavnina, I. V., Avdeev, Yu. F., Blagov, V. D., and
others, 1971, New data on the Moon's figure and relief based on results
from the reduction of Zond-6 photographs: Cosmic Research, v. 9, no. 3,
p. 410--417.

Russell, C. T., Coleman, P. J., Jr., Lichenstein, B. R., Schubert, G.,
and Sharp, L. R., l973, subsatellite measurements of the lunar magnetic
field, Proc. 4th Lunar Sci. Conf.: Geochim. et Cosmochim. Acta, suppl.
4, v. 3, p. 2833--2845.

Russell, C. T., Coleman, P. J., Jr., and Schubert, G. , 1974, Magnetic
fields, Proc. 5th Lunar Sci. Conf.: Geochim. et Cosmochim. Acta, suppl.
5, v. 1, Frontispiece, pl. 3--8.

\_\_\_\_1975, Magnetic fields, Proc. 6th Lunar Sci. Conf.: Geochim. et
Cosmochim. Acta, suppl. 6, v. 3, Frontispiece, pl. 5--12.

Russell, C. T., and Lichenstein, B. R., 1975, on the source of lunar
limb compressions: Jour. Geophys. Research, v. 80, p. 4700--4711.

Schultz, P. H., and Gault, D. E., 1975, Seismic effects from major basin
formations on the Moon and Mercury: The Moon, v. 12, p. 159--177.

Scott, D. H., McCauley, J. F. and West, M. N., 1977, Geologic map of the
west side of the Moon: U. S. Geol. Survey Misc. Geol. Inv. Map I-1034.

Sharp, L. R., Coleman, P. J., Jr., Lichenstein. B. R., Russell, C. T.,
and Schubert, G., 1973, Orbital mapping of the lunar magnetic field: The
Moon, v. 7, no 3/4, p. 322--341.

Shoemaker, E. M., and Hackman, R. J., 1962, Stratigraphic basis for a
lunar time scale, *in* Kopal, Zdenek, and Mikhailov, Z. K., eds., The
Moon---Internat. Astron. Union Symposium 14, Leningrad 1960: London,
Academic Press, p. 289--300.

Shorthill, R. W., and Saari, J. M., 1969, Infrared observations on the
eclipsed Moon: Seattle, Wash., Boeing Sci. Research Labs. Doc.
D1-82-0778, 73 p.

Sjogren, W. L., 1974, Lunar gravity at 100 km altitude, Proc. 5th Lunar
Sci. Conf.: Geochim. et Cosmochim. Acta, suppl. 5, v. 1, Frontispiece,
plate 1.

Soderblom, L. A., and Lebofsky, L. A., 1972, Technique for rapid
determination of relative ages of lunar areas from orbital photography:
Jour. Geophys. Research, v. 77, p. 279--296.

Strangway, D. W., Gose, W. A., Pearce, G. W., and McConnell, R. K.,
1973, Lunar magnetic anomalies and the Cayley Formation: Nature, v. 246,
p. 112--115.

Stuart-Alexander, D. E., 1975, Lunar magnetic anomalies and the Cayley
Formation: Nature, v. 253, p. 658.

Stuart-Alexander, D. E., and Howard, K. A., 1970, Lunar maria and
circular basins---a review: Icarus, v. 12, p. 440--456.

Stuart-Alexander, D. E., and Wilhelms, D. E., 1975, The Nectarian
System, a new lunar time-stratigraphic unit: U.S. Geol. Survey Jour.
Research, v. 3, no. 1, p. 53--58.

Tera, Fouad, Papanastassiou, D. A., and Wasserburg, G. J., 1974,
Isototopic evidence for a terminal lunar cataclysm: Earth and Planetary
Sci. Letters, v. 22, p. 121.

Trask, N. J., and McCauley, J. F., 1972, Differentiation and volcanism
in the lunar highlands---photogeologic evidence and Apollo 16
implications: Earth and Planetary Sci. Letters, v. 14, p. 201--206.

Trombka, J. I., Arnold, J. R., Reedy, R. C., and Peterson, L. E., 1973,
Some correlations between measurements by the Apollo gamma-ray
spectrometer and other lunar observations, Proc. 4th Lunar Sci. Conf.:
Geochim. et Cosmochim. Acta, suppl. 4, v. 3, p. 2847--2853.

Wilhelms, D. E., 1970, Summary of lunar stratigraphy---telescopic
observations: U.S. Geol. Survey Prof. Paper 599-F, 47 p.

Wilhelms, D. E., and El-Baz, Farouk, 1977, Geologic map of the east side
of the Moon: U.S. Geol. Survey Misc. Geol. Inv. Map I-948.

Wilhelms, D. E., and McCauley, J. F., 1971, Geologic map of the near
side of the Moon: U.S. Geol. Survey Misc. Geol. Inv. Map I-703.

Wilhelms, D. E., Stuart-Alexander, D. E., and Howard, K. A., 1969,
Preliminary interpretations of lunar geology, *in* Analysis of Apollo 8
photography and visual observations: NASA SP-201, p. 16--21.

Wollenhaupt, W. R., and Sjogren, W. L., 1972a, Comments on the figure of
the Moon based on preliminary results from laser altimetry: The Moon, v.
4. no. 3/4. p. 337--347.

\_\_\_\_\_1972b, Apollo 16 laser altimeter, *in* Apollo 16 preliminary
science report, NASA SP-315, p. 30--1 to 30--5.

Wollenhaupt, W. R., Sjogren, W. L., Lingrenfelter, R. E., Schubert, G,
and Kaula, W. M., 1973, Apollo 17 laser altimeter, *in* Apollo 17
preliminary science report: NASA SP-330, p. 33--41 to 33--44.

Primary data source, NASA Lunar Orbiter moderate-resolution photographs
(shown on index map). High resolution frames lie within
moderate-resolution frames and were supplementary data source; they were
main data for frame 30. Zond 8 (USSR) and Apollo 15, 16, and 17
photographs supplementary.

Work performed on behalf of the National Aeronautics and Space
Administration under Contract No. W-13,130.

Part of shaded-relief base chart Lunar Farside Chart (LMP-2), 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. The lunar surface features shown on this chart
were interpreted from the photographic records of Lunar Orbiter
Missions, I, II, III, IV, and V. Ray patterns and albedo differences are
incomplete, owing to limitations of the source photographs. Horizontal
positions of the features based on ACIC Positional Reference System,
1969. Feature names were taken from the International Astronomical Union
list of 1970.

[^1]: The series is an outgrowth of the original 1:1,000,000 quadrangle
    maps constructed for the nearside in support of the unmanned and
    Apollo spaceflight programs (Wilhelms, 1970).