**GEOLOGIC MAP**

**OF THE**

**WEST SIDE**

**OF THE MOON**

By

David H. Scott, John F. McCauley,

and Mareta N. West

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--1034, 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**

Cc CRATER MATERIAL---Rim, wall, and floor deposits of craters with sharp
prominent rims, and circular to polygonal outlines; larger craters have
rough concentrically structured inner rim deposits, smooth radially
lineated outer rim deposits, and arrays of secondary craters; rays
visible on full-Moon photographs. *Interpretation:* Impact crater
materials

CEd DOME MATERIAL---Single occurrence at east edge of map (Hansteen α).
Steep-sided bulbous dome with hackly surface, very high albedo; small
linear depressions at crest; apparently superposed on adjacent mare.
*Interpretation:* Volcanic dome formed by viscous eruptive rocks; linear
crestal depressions may be vents. Similar to but somewhat more degraded
than Gruithuisen domes (Scott and Eggleton, 1973)

Ec CRATER MATERIAL---Rim, wall, and floor materials of topographically
sharp fresh-appearing craters; similar to craters of Copernican age but
slightly more subdued and rayless on full-Moon photographs.
*Interpretation:* Impact crater materials

EIm MARE MATERIAL---Forms smooth flat surfaces having low to very low
albedo and low crater density in Oceanus Procellarum, Orientale,
Grimaldi, and some craters. Spectral reflectance measurements
(Soderblom, in preparation) indicate mare ranges from red to blue in
color. Rays and Eratosthenian craters superposed, upper Imbrian and
older craters embayed. Numerous ridges, mostly trending NW and NE. Lunar
Orbiter IV photographs suggest that darker mare may have lower density
of craters than lighter mare patches (McCauley, 1973; Scott and
Eggleton, 1973). Zond 8 photographs provide excellent albedo contrast
between mare and plains material (unit Ip) throughout most of map
region. Discrimination difficult between mare and dark mantling
material. *Interpretation:* Basalt flows. Albedo variations may be
related to age color variations probably indicate changes in composition

EIph HILLY PLATEAU MATERIAL---Dome, cone, and dark mantle complexes;
positive forms include: smooth low domes; bulbous, steep, or flat-topped
domes with ramparts around summits; associated dark planar deposits that
stand above general level of Oceanus Procellarum. Includes Marius Hills,
Rümker Hills, and Aristarchus Plateau that have been mapped together for
convenience. *Interpretation:* Volcanic constructs, flows, and
pyroclastic materials (Moore, 1965; Wilhelms and McCauley, 1971; Scott
and Eggleton, 1973)

Ic2 CRATER MATERIALS---Similar to crater material of Eratosthenian age
but more subdued; superposed on all formations of Orientale Group.
*Interpretation:* Impact crater materials; some of smaller size may be
secondary craters formed by high-angle ejecta

Ip PLAINS MATERIAL---Smooth flat to undulatory terrain of intermediate
albedo; occurs mostly in topographic lows and crater floors of Imbrian
and older age but also as small pools at different elevations; locally
gradational with outer facies of Hevelius Formation but has ponded
appearance and seems to embay or bury grooves and lineaments of the
Hevelius. *Interpretation:* Multiple origins; some is ejecta from
Orientale basin and large impact craters filling low areas; includes
shock-melted rocks as well as volcanic materials. Ambiguity as to origin
precludes inclusion in Orientale Group

ORIENTALE GROUP---Includes Maunder, Montes Rook, and Hevelius Formations

Iom MAUNDER FORMATION---Occurs as smooth to rolling, intensely fractured
plains, broad linear ridges, and smooth equidimensional domes.
Restricted to central part of Orientale basin within Montes Rook ring.
Fractures, including grabens and rilles, show crude concentric and
radial patterns; these structures are covered by and predate mare
material. Scattered small smooth hills of knobby facies of Montes Rook
Formation locally appear to protrude through unit. *Interpretation:*
Probably mostly impact melt. Scattered small knobs may be fallback.
Ridges and domes are original floor material compressionally modified by
inward movement during last stages of basin formation. Fractures and
faults caused by isostatic uplift of basin floor mostly prior to mare
flooding. Some may be result of contraction during cooling

MONTES ROOK FORMATION---Divisible into a massif and a knobby facies

Iorm Massif facies ---Large rectilinear to equidimensional massifs
ranging from about 5 km to 100 km in length and several kilometers high;
forms disconnected segments that mark the second and third rings of
Orientale basin. Surfaces of individual blocks grade from smooth to
knobby where they appear to be overlain by knobby facies of the Montes
Rook Formation (unit Iork). Individual blocks separated by radial
troughs. Some blocks appear to be rotated or displaced laterally from
ring pattern. Shapes of massifs appear largely controlled by regional
structure (lunar grid). Massifs most extensively developed in southwest
quadrant of basin where ring structures are difficult to distinguish.
*Interpretation:* Structurally uplifted prebasin bedrock thickly
veneered with late arriving ejecta blocks and finer debris. Ring formed
by massif facies marks approximate extent of original crater formed by
impact; innermost ring of massifs probably represents central peak ring
also veneered by ejecta

Iork Knobby facies---Knobby, hummocky, smooth, rolling, disordered
appearing materials with irregular grooves and depressions between
individual knobs. Knobs are up to about 5 km across. In places unit is
weakly lineated in radial directions. Generally confined to region
between Montes Cordillera scarp and Montes Rook. Locally gradational
with lineated inner facies of Hevelius Formation. Large patches lie
outside Montes Cordillera scarp. Contains vague circular outlines as
much as 50 km in diameter. *Interpretation:* Uppermost part of
overturned flap of ejecta sequence at Orientale. Overlies and almost
completely buries earlier deposited inner facies of Hevelius Formation.
Knobs represent coherent material quarried from deep within transient
cavity. As these materials were last to leave the crater, they had
considerably less radial velocity (Gault and others, 1968) than earlier
ejecta units in Orientale group, thus accounting for their disordered
appearance. Thickest part of ejecta sequence; upper layers not mixed
with materials external to the transient cavity. Remnants of buried
prebasin craters visible

HEVELIUS FORMATION---Divisible into an inner, transverse, outer, and a
secondary crater facies

Iohi Inner facies---curvilinear to swirly ridges and troughs mostly
radial and sub-radial to basin; contains lobes and tongues; locally ropy
in appearance. Ridges up to tens of kilometers wide and hundreds of
kilometers long; heights may exceed 1,000 m. Innermost boundary
generally coincident with Montes Cordillera scarp where contacts are
mostly gradational with knobby facies of Montes Rook Formation (unit

Iork). On north and west sides of basin is overlapped by knobby facies;
in some places unit occurs within Montes Cordillera scarp. Vaguely
expressed concentric wavy ridges up to 100 km long present outside scarp
(concentric facies of Moore and others, 1974). Numerous primary and
secondary ghost (buried) craters present. These become more pronounced
and textures more swirly near boundary with outer facies (unit Ioho).
*Interpretation:* Continuous part of Orientale ejecta blanket emplaced
during outward flow of hot, turbulent, mobile materials. Intense
secondary cratering and gouging has also contributed to overall texture
of unit and to mixing of internal ejecta with prebasin materials outside
scarp

Ioht Transverse facies---Closely spaced arcuate ridges and troughs at
base of distal walls of prebasin craters and in elongate low areas
radial to basin; mostly between 300 and 400 km from Montes Cordillera
scarp. Best seen around east side of basin where viewing angles and sun
angles of Lunar Orbiter photographs were optimal. Originally termed
deceleration dunes (McCauley, 1968a). *Interpretation:* Ejecta of
Orientale basin accumulated against or within topographic barriers to
flow. Wavelike ridges and troughs related to yield strength and flow
characteristics of viscous material and possibly to local relief of
substrate

Ioho Outer facies---Variable characteristics; includes swirly, lineated,
hummocky, and smooth materials. Laterally continuous with inner facies
of Hevelius (unit Iohi) but difficult to define along western part of
basin where photography poor. Intergrades in places with plains material
(unit Ip) that forms ill-defined annulus around basin. Outermost
boundary with prebasin terra (unit INt) indistinct and approximately
located. Contains numerous fresh-appearing secondary craters (unit
Iohs); prebasin craters locally protrude through unit. *Interpretation:*
Thin and discontinuous part of Orientale basin's ejecta blanket. Many
secondary craters and prebasin craters not completely buried. Closely
spaced, sharp, swirly ridges occur where material flowed through
preexisting topographic gaps

Iohs Secondary crater facies---Overlapping crater chains and clusters
radial and peripheral to the basin. Individual craters as much as 30 km
across form chains several hundred kilometers long. More abundant and
larger on west side of basin than on east. Only most prominent examples,
mostly superposed on outer facies of Hevelius Formation, mapped.
Numerous elongate chains and grooves within inner facies of Hevelius
have degraded rims and outlines. *Interpretation:* Secondary impact
craters formed by ejecta from Orientale event. Chains and grooves within
inner facies of Hevelius buried by later arriving ejecta

Ic1 CRATER MATERIAL---Moderately subdued craters partly buried in places
by materials of Orientale Group. *Interpretation:* Pre-Orientale impact
material. Some may be secondary craters from Orientale event

If FRA MAURO FORMATION---Only in north-central part of map area; forms
lineated to slightly swirly material grading southwesterly into gently
hummocky to smooth slightly rolling terrain. Lineations subradial to
Imbrium basin. *Interpretation:* Ejecta from Imbrium basin mixed with
materials of substrate

INt TERRA MATERIAL (Undivided)---Gently rolling terrain with numerous
craters and indistinct depressions. Intergrades with rim materials of
older craters and, in places, with Hevelius Formation. *Interpretation:*
Interlayered breccias, shocked rocks, and glasses derived from long

period (Nectarian to Imbrian) of meteoroid bombardment of lunar crust,
including formation of Orientale basin

Nhb HERTZSPRUNG BASIN MATERIAL---Smooth rolling with faint lineations
directed toward Hertzsprung basin; only recognized south of basin where
lineations distinguish it from terra material (unit INt). Subdued;
buries or partly buries craters of Nectarian age and older including
unit Nhsc, early Imbrian and younger crater materials superposed.
Boundaries very indistinct; viewing and lighting angles of photographs
poor. *Interpretation:* Ejecta from Hertzsprung basin

Nhsc HERTZSPRUNG SECONDARY CRATER MATERIAL---Forms elongate, shallow,
highly subdued craters with long axes directed toward Hertzsprung Some
partly filled by plains material (unit Ip). *Interpretation:* Secondary
craters from Hertzsprung event

Nc CRATER MATERIAL---Subdued rim and wall deposits, terra or plains
materials mostly cover original crater floors but only mapped in larger
craters. *Interpretation:* Impact craters

NpNhf HILLY AND FURROWED MATERIAL---At east edge of map area. Gently
rolling moderately cratered terrain with numerous small smooth hills,
many slightly elongated; numerous small irregular depressions or
furrows. *Interpretation:* Origin enigmatic but has some resemblance to
knobby facies of Montes Rook Formation and might be ejecta from nearby
Humorum basin

pNc CRATER MATERIAL---Highly subdued; crater outlines generally
irregular or incomplete. Craters partly filled by younger materials;
rims merge with adjacent terrain; numerous superposed craters.
*Interpretation:* Most are impact craters

Contact

Mare ridge

Rille

Fault---Bar and ball on downthrown side

Lineament

Large trough

Mare dome

Orientale ring structure---Hachures mark steep face; barbs show steep
limb of innermost ring (fig. 1)

Older basin ring ( fig. 1 )

Dark mantling material

Buried crater outline

Buried Orientale secondary crater outline

Blue mare material (red mare unstippled)

GEOLOGIC SUMMARY

PHYSIOGRAPHIC SETTING

The map area extends from Oceanus Procellarum westward into the terrae
of the western limb of the Moon and is dominated by the Orientale basin,
a great multiringed impact structure about 1,000 km across. Elevations
increase westward across the map from about --2.5 km in Oceanus
Procellarum to more than 5 km in the highlands west of Orientale. The
terrain is exceedingly rugged outward from the Montes Cordillera around
Orientale but becomes increasingly smooth in the outer facies of the
ejecta and beyond the ejecta deposits around the basin. Eight large
older multiringed basins surround Orientale in this region of the Moon.
With the exception of Orientale, only those adjacent to Oceanus
Procellarum have their centers filled or partly filled with mare basalt.
In the discussions that follow emphasis has been placed upon those
geologic units and structures that are associated with the Orien-tale
basin. Some of Orientale\'s more noteworthy features include: (1) the
greatest number (four) of clearly distinguishable rings of any basin,
(2) the only continuously mappable ejecta blanket around a lunar basin,
(3) the greatest number of distinct basin-associated geologic units, (4)
the only basin showing impact produced basin floor materials unburied by
mare basalts, (5) the only basin showing relict crater outlines within
its major ring, (6) the last major ejecta blanket covering a large part
of the Moon, (7) the only basin having a central mascon surrounded by a
large well-defined negative gravity anomaly.

STRATIGRAPHY

PRE-ORIENTALE ROCK UNITS. The oldest rocks in the region are those
associated with pre-Nectarian craters and the probable ejecta from the
Humorum basin, shown as hilly and furrowed material. The massifs and
high scarps which form the mountainous rings around the Orientale basin
are probably in part uplifted prebasin bedrock consisting of interbedded
breccias of older ejecta blankets. Undoubtedly, however, the massifs and
scarps are overlain by ejecta from the Orientale basin just as the
circumbasin massifs of Imbrium and Serenitatis are shown by samples
collected during the Apollo 15 and 17 missions to consist of ejecta
indigenous to these basins (Wolfe and Reed, 1975). For this reason, we
have chosen to map the massifs as part of the Orientale Group and not as
prebasin bedrock as in previous mapping practice.

Along the west margin of the map, faint lineations trending northeast
are all that remain of the degraded extremities of the Hertzsprung
ejecta blanket. The ejecta blanket covers pre-Nectarian crater materials
and in turn is overlain by long stringers and lobes of the Hevelius
Formation and numerous Orientale secondary craters. The Hertzsprung
basin is believed to be younger than the Humorum basin to the east
because of the relatively sharp appearance of some of its rings and the
common occurrence though somewhat degraded form, of its secondary
craters; Humorum secondary craters have not been recognized.

The north-central part of the map area is marked by hummocky to swirly
terrain with lineations directed subradially toward the Imbrium basin
more than 1,000 km away across Oceanus Procellarum. This unit is mapped
as the Fra Mauro Formation as its general characteristics are very
similar to those of materials mapped elsewhere as the Fra Mauro at
comparable distances from Imbrium. Superposition relations between the
Hevelius and Fra Mauro Formations were not found. However, Orientale
secondary craters clearly overlie the Imbrium ejecta blanket and thus
directly confirm for the first time the previous supposition based on
morphology that the Orientale impact was the later event.

Ejecta deposits from most other major basins within and around the map
area are no longer identifiable. They probably are included in the
Imbrian and Nectarian terra

material which covers most of the area. This terra unit is inferred to
be made up of a complex multilayered sequence from both large and small
cratering events that occurred relatively early in lunar history.

ORIENTALE GROUP. Three major units associated with the Orientale basin
are defined and included in the newly-named Orientale Group of Imbrian
age (McCauley and others, in preparation). The Orientale Group is an
assemblage of all the materials cogenetic with the formation of the
basin: the Hevelius, Montes Rook, and Maunder Formations. The Orientale
basin is considered its type area.

The Hevelius Formation, the first recognized and named unit (McCauley,
1967), represents that part of the ejecta blanket which lies mostly
outside the Montes Cordillera. It is made up of four gradational facies
extending successively outward from the basin.

The inner facies is rugged and coarsely lineated with large, elongate,
irregular troughs and ridges mostly subradial to the basin center. All
but the largest preexisting craters are masked by this unit, which is
estimated to be about 3.6 km thick near the Cordillera rim (Moore and
others, 1974). Swirly textures and streamlined surface forms have led
many workers to conclude that it was deposited as a ground-hugging
surface flow.

The transverse facies consists of patches of small ridges generally
alined \[sic\] normal to the main radial lineations of the inner facies.
It occurs mostly along the distal walls of prebasin craters and in long
depressions such as that northwest of the crater Inghirami. The ridges
are probably formed as a result of a rapid decrease in velocity of the
surface flow by opposing crater walls and stresses set up by rapid
decreases in velocity between the upper and lower parts of flows within
depressions.

The outer facies is relatively thin and discontinuous over a large part
of its extent. This facies is lineated in places by narrow ridges and
grooves radial to the basin but on a much finer scale than the inner
facies. Subjacent terrain is readily discernible beneath this part of
the blanket. In places the outer facies contains large patches of smooth
plains that cover crater floors and irregular depressions. Many of the
plains appear to be laterally continuous with the outer facies; others
embay and have sharp contacts with this unit or have Orientale secondary
craters superimposed on them and thus are respectively either younger or
older than the Hevelius Formation. Those that are continuous with the
outer facies are included in the Orientale Group whereas the others
probably have a variety of origins and ages that span the impact event
and have been mapped as a separate unit, plains material. In the west
half of the map area, where photographs are particularly poor, the
boundary between the inner and outer facies of the Hevelius is locally
indeterminate and the outer limit of the formation is not easily
defined. The Hevelius is distributed irregularly and somewhat
asymmetrically around the basin and has numerous broad lobes like those
extending across Schickard and Poynting.

The secondary crater facies consists of numerous chains and clusters of
secondary craters within and beyond the mapped extent of the Hevelius
Formation. They can be recognized within a zone extending from about 600
km to 2,000 km from the basin center where they commonly occur in
clusters and linear chains. Most of the secondary craters within the
area covered by the continuous ejecta blanket arc buried to varying
degrees. Those that occur within the inner facies of the Hevelius are
highly subdued by erosion and burial by the ground surge. Secondary
craters in the outer facies, however, have relatively sharp rims and
appear to have been only thinly covered by ejecta deposits. Some
secondary craters are elongate or overlapping to form narrow sculptured
grooves; these were probably formed by low-angle trajectory material or
by blocks entrained in the ejecta cloud. Others are nearly circular in
outline, suggesting they were produced by low-velocity (less than 2.38
km/sec) fragments impacting the surface at high angles. If a lower limit
of 30° from the horizontal is assumed for the impact angle, the
equations of Giamboni (1959) yield a minimum flight time of about

10 minutes and a velocity of 1.0 km/sec for ejecta which formed the
first recognizable nearly circular craters about 600 km from the basin
center. As these secondary craters were later buried by the outrushing
ground surge, the average velocity of this debris-laden cloud must not
have exceeded 1.0 km/sec and probably was much less. The more distant
secondary craters at a range of about 2,000 km were made by ejecta
having velocities exceeding 1.5 km/sec, although ejection angles might
have varied between about 10 and 50° (Wright and others, 1963).

The Montes Rook Formation was named informally by McCauley (1968b). Its
textures, distribution, relative age, and stratigraphic position within
the Orientale Group are most important to developing the sequential
history and kinematics of basin formation. With some notable exceptions,
it is mostly confined to the region between the Montes Cordillera scarp
and Montes Rook. It has a blocky to knobby texture and on this basis is
divided into two facies: (1) The massif facies consists of large
rectilinear blocks and prominent smooth scarps described above (see
section entitled "Pre-Orientale Rock Units"); individual blocks are
measured in tens to hundreds of kilometers. (2) The knobby facies is
made up of more rounded to cone-shaped blocks ranging in size from about
10 km across to the limit of resolution (about 100 m); these are set
into a smooth gently rolling matrix. The facies is faintly lineated and,
significantly, several ghost craters are visible along its eastern edge;
these craters indicate that the original impact cavity did not extend to
the knobby facies. Northwest of the crater Schlüter and at several other
localities around the basin this facies is draped across the Montes
Cordillera scarp or outer ring of the basin. Islands of well-lineated
Hevelius Formation occur within the knobby facies and inside the scarp.
This facies is analogous to the Alpes Formation (Page, 1970) of the
Imbrium basin and has been variously interpreted as fallback, the
product of postcratering gravitational collapse, and shock fluidization
of prebasin bedrock. The knobby facies of the Montes Rook Formation is
now considered an integral part of the Orientale ejecta sequence and is
thought to be nearly unmodified by postbasin events, and, as noted
above, to lie outside a deep transient cavity that existed during basin
formation (McCauley and others, in preparation; Moore and others, 1974).
The knobby facies is interpreted as deep-seated material possibly
derived from beneath the 60-km lunar seismic discontinuity (McCauley and
others, in preparation; Toksöz and others, 1974, Moore and others,
1974). Its superposition on the Hevelius Formation, general lack of
radial lineations, and blocky forms, together with some experimental
large-scale cratering data (Roddy, 1969, 1973) suggest that it was
emplaced late in the cratering sequence and with little radial velocity
(Gault and others, 1968). It represents the uppermost part of an
overturned flap produced by impact cratering (Shoemaker, 1962).

The innermost unit of the Orientale Group, here named the Maunder
Formation after its exposures around the younger crater Maunder in the
central part of the basin, lies between the Montes Rook Formation and
mare basalt that fills the center of that basin. It has a relatively
smooth undulatory surface but with many concentric and radial fissures
which produce a wrinkled appearance. The Maunder Formation is
gradational with plains units, has a similar albedo, and resembles the
materials on the floors of the young lunar craters Tycho and
Aristarchus. It is interpreted to consist principally of impact melt
formed within the original crater floor. The pattern of the fissures
suggests uplift of the floor by isostatic adjustment resulting in normal
faulting and extension of the solidified floor material.

POST-ORIENTALE ROCK UNITS. Mare basalts and, locally, plains materials
bury or partly bury parts of the Orientale Group, and craters of late
Imbrian age and younger are superposed on its various formations. Over
most of the region, particularly the western half, Lunar Orbiter
photographs are inadequate to allow differentiation between light and
dark mare and between plains and light mare materials. However, Zond 8
photographs obtained by Harold Masursky of the U.S. Geological Survey
from

C. P. Florensky, V. I. Vernadsky Institute, Moscow, U.S.S.R., taken at
very high sun angles clearly show albedo variations associated with
these units. They also show rays and bright haloes around craters, thus
permitting the identification of Copernican age craters. Color
boundaries between red and blue mare basalts are preliminary and highly
generalized from the current work by Soderblom. Unlike other areas of
the Moon (Soderblom, 1970), there appears to be little correlation
between spectral reflectance and albedo in Oceanus Procellarum. Most of
the dark mare along the west edge of Oceanus Procellarum is red but in
some places it is relatively blue. The mare basalts within the
quadrangle range from Imbrian to Eratosthenian in age (Boyce, 1975), and
no correlation is evident between age, albedo, and color.

STRUCTURE

The ring structures of the Orientale basin are mapped along their
topographic crest lines on the accompanying structural diagram (fig. 1).
They are best seen under the low lighting of Orbiter IV
medium-resolution pictures, from which most of their characteristics and
dimensions have been described. Acquisition of Zond 8 pictures, closer
inspection of the Orbiter V data, and geologic mapping around the
entirety of the basin reveal: (1) irregularity of crests, (2) a
segmented, blocky character to the inner rings that is apparently
controlled by a preexisting fracture system, (3) numerous radial and
subradial lineaments, and (4) faults that displace or rotate individual
ring segments on the order of tens of kilometers. Only the most
pronounced of these are shown on figure 1. They appear to be analogous
to the tear faults mapped in the walls of many terrestrial craters that
developed during the cratering event and are not related to postcrater
slumping. The Montes Cordillera ring and the outer and inner Montes Rook
rings are generally indistinguishable in the southwestern part of the
basin. Here the normal concentric trends of these structures are lost
within an enormous array of structured massifs, mapped as one large
patch because of the quality of the photographs. The apparent greater
dislocation of the massifs and the lack of definition of the rings in
this region may be related to obliquity of impact, to preferential
crustal weakness along certain azimuths, or to energy asymmetry of the
initial transmitted shock wave at impact. The degree of structural
deformation observed here and the general asymmetry of the ejecta
blanket itself are consistent with a basin formed by a bolide, incident
at a moderately oblique angle from the northeast. Regardless, it is now
apparent from the above observations that the concentric asymmetry and
continuity of the Orientale rings are considerably less than previously
thought.

The Maunder Formation, characterized by extension fractures tectonically
or thermally induced, also contains numerous gently sloping ridges and
domes. Many of the ridges show axial fissures radial or concentric to
the basin center. Data from experimental explosion craters (Roddy, 1969,
1973) suggest that these ridges are compressional features formed late
in the cratering event by inward motion of material from beneath the
crater floor and inner ring.

For geologic and experimental reasons described here and elsewhere
(McCauley and others, in preparation), we consider the original rim of
the crater at Mare Orientale to lie within the circumference of the
Montes Rook. This rim approximates the hinge line around which
overturning of the flap and inversion of ejecta occur in terrestrial
impact craters and in experimental craters of all sizes. Our mapping
shows a maximum possible diameter for the original crater of
approximately 450 to 500 km. The inner Montes Rook ring probably
represents a circular central peak structure of the type described by
Milton and others (1972) and Milton and Roddy (1972). Spacing with
regard to the inferred rim crest, texture, and the highly segmented
appearance of this ring are consistent with this interpretation. The
innermost ring of Mare Orientale has a gentle inward facing scarp that
appears to be a structural terrace marking the steep part of the
original cavity into which basalt was extruded and later partly
withdrawn. This

structural terrace may represent a major strength discontinuity in the
lunar subsurface (McCauley and others, in preparation).

The Montes Cordillera ring and scarp is a seismic-wave-induced ring
fault that formed outside the original crater probably as the result of
sequential compressional and tensile stresses followed by mass movement
of material toward the basin cavity. A similar ring is observed around
experimental explosion craters (Roddy, 1969) and shows that deformation
extends well beyond the crater walls. Postcratering slumping along this
structure is considered to be a subordinate process and limited to those
areas where scarps with bright slopes appear on the photographs (less
than 20 percent of the circumference of the Montes Cordillera ring).

Outside the Montes Cordillera ring, concentric faults and grabens
persist on a diminished scale mostly visible only as lineaments within
the inner facies of the Hevelius Formation (fig. 1). Some concentric
features in the area between the craters Shaler and Baade on the south
side of the basin appear as troughs and grooves or have bright
south-facing scarps antithetic to that of the Montes Cordillera. Radial
and subradial systems are pervasive in both the inner and outer facies
of the Hevelius Formation. They consist of faults and grabens of
tectonic origin as well as grooves and ridges formed by scouring and
deposition of the ground surge and by secondary cratering. Structures
radial to the basin are more apparent than real as indicated in
figure 1. Many of the lineaments are nearly parallel and occur in two
broad zones extending across the basin. Within each zone the lineaments
trend about N.60° E. and N.50° W., and only those relatively close to
the linears extending through the basin center in these directions can
be considered radial. In the broad arcs to the north and south of the
unidirectional zones radial and subradial lineaments are present. Thus,
it seems that an ancestral structural pattern exists around Mare
Orientale where basin radial sculpture consisting of fractures and
gouges are superimposed on an older regional structural grid. In many
places, however, it is difficult to distinguish between tectonic
lineaments and those formed by ejecta gouging and scouring. Some
criteria which indicate mode of origin have been noted in the study of
Imbrium sculpture (Scott, 1972). Evidence of tectonic origin is shown by
associations of the following features: (1) variations in apparent
relative age of parts of the sculpture, (2) furrows without rims, (3)
absence of ballistic shielding---furrows cutting across crater rims,
walls, and floors, (4) absence of scalloped edges along troughs, and (5)
linear assemblages of nonradial features, especially in directions of
the two major grid systems. Ridges within Oceanus Procellarum, for
example, generally follow the northwest and northeast trends of
structures within the grid zones and are believed to represent
extrusions of basalt along fractures produced by tensile stresses (Scott
and others, 1975). Many lineaments in the ejecta may be made by material
preferentially ejected along azimuths following preexisting structural
dislocations. Elastic wave propagation is probably higher in these
directions where rock compressibility may be less; possibly this is
analogous to higher seismic velocities along rather than across planes
of stratification. Roddy (oral commun., 1975) points out that
experimental craters in coarsely crystal-line rock have exhibited
similar asymmetry in ejecta patterns and elongate rays related to joint
systems in the rocks.

TOPOGRAPHIC AND GRAVIMETRIC DATA

Laser altimeter profiles extending westward from the mare in Oceanus
Procellarum across the northern part of Mare Orientale and the Montes
Cordillera show an elevation difference of 6 km or more. The mountains
also rise about 3.5 km above the basin floor between Montes Rook and
Montes Cordillera. Supplemental profiles made by G. M. Nakata of the
U.S. Geological Survey using a stereo-analytical plotter indicate that
the mare in the center of the Orientale basin lies about 2.5 km below
the basin floor

enclosed by Montes Rook and Cordillera; thus the mare basalt within
Orientale may be about the same elevation as that along the west margin
of Oceanus Procellarum.

Outside the Cordillera ring, the ejecta blanket has large-scale
roughness where in many places slopes of 5 to 10 degrees occur between
laser points spaced approximately 30 km apart. The plains-covered floor
of the Hertzsprung basin is depressed about 3.5 km below its outer rim
and appears to be 1 to 2 km higher than the mare fill within Orientale.

Previous work (Scott, 1974) indicated that the negative gravity anomaly
associated with the Orientale basin is about --360 milligals. The
conspicuous but areally restricted mascon under the central part of the
basin has a total difference in magnitude of about 460 milligals, of
which only 100 milligals represents the uncompensated load. These values
correspond to a plug of basaltic material having about the same diameter
(300 km) as the mare-filled basin center and extending to a depth of
approximately 50 km (probably to the mantle). These data have been used
(Scott, 1974) to obtain volume estimates of the amount of material
ejected from Orientale that are in close agreement with those calculated
from basin ejecta (Moore and others, 1974).

Recent gravity studies of the Moon\'s far side (Ananda, 1975; Ferrari,
1975) show basins in this region to be associated with localized
negative anomalies. In particular, and within the map area, Hertzsprung
(lat 0°, long 130° W.) and Lorentz (lat 35°N., long 100°W.) show
anomalies of about --10 and --50 milligals, respectively. Laser
altimeter data available across Hertzsprung show that the volume
deficiency of this basin can be approximated by a disk with a diameter
of about 500 km and a thickness of 2 km. Calculations using a density
value of 3.0 g/cm3 for the displaced mass yield a theoretical gravity
value of -160 milligals for Hertzsprung. It thus appears that about 90
percent of the basin void has been compensated isostatically even though
no mare basalt is evident at the surface. The difference, then, between
farside and nearside lunar basins is that the latter are overcompensated
by basalt flows owing to their lower elevations and, consequently, more
favorable hydrostatic position. The Orientale basin has an intermediate
location on the west limb between mare and farside terra; this is
reflected by its gravity response, a broad low area encircling a central
high. Thus the position of a basin in the Moon may affect mascon
expression more than the relative age of the basin (Howard, 1970).

HISTORICAL SUMMARY

The west limb region of the Moon consists of highly cratered terra
rising 5 km or more above the mare of Oceanus Procellarum and most of
the lunar near side. Crustal material in this area of large basins and
craters has been redistributed repeatedly by giant impacts. The early
historical record of these events is apparently preserved in a complex
of interlayered breccias, which may be partly exposed in the walls of
the larger craters. Orientale, the last multiringed basin, was formed
early in lunar history during the waning stages of high impact flux by a
large body probably impacting the surface obliquely from the northeast.
Ejecta from this impact was widely scattered over the Moon and appears
to form some of the plains materials. Although the major landforms of
the Orientale basin were developed instantaneously in geologic terms,
isostatic adjustments were more protracted; the accompanying structural
dislocations including concentric and radial faults in the floor
material may have continued up to the time of mare basalt flooding after
basin formation. Subsequent impacts have produced craters generally less
than 60 km diameter.

REFERENCES

Ananda, M. P., 1975, Farside lunar gravity from a mass point model
(abs.): Proc. 5th Lunar Sci. Conf., Abs., Lunar Sci. Inst., Houston,
Tex., p. 9--11.

Boyce, J. M., 1975, Chronology of the major flow units in the western
nearside maria (abs.): Conference on Origins of Mare Basalts and their
Implications for Lunar Evolution, Lunar Sci. Inst., November, 1975,
Houston, Tex.

Ferrari A. J., 1975, Lunar gravity: The first farside map: Science, v.
188, no. 4195, p. 1297--1299.

Gault, D. E., Quaide, W. L., and Oberbeck, V. E., 1968, Impact cratering
mechanics and structures, *in* French, B. M., and Short, N. M., eds.,
Shock metamorphism of natural minerals: Mono Book Corp., Baltimore, Md.,
p. 87--99.

Giamboni, L. A., 1959, Lunar rays---Their formation and age: Astrophys.
Jour., v. 130, no. 1, p. 324--335.

Howard, K. A., 1970, Mascons, mare rock and isostacy: Nature, v. 226,
no. 5249, p. 924--925.

McCauley, J. F., 1967, Geologic map of the Hevelius region of the Moon:
U.S. Geol. Survey Misc. Geol. Inv. Map I-491.

\_\_\_\_\_\_\_\_, 1968a, Geologic results from the lunar precursor
probes: AIAA Jour., v. 6, no. 10, p. 1991--1996.

\_\_\_\_\_\_\_\_, 1968b, Preliminary photogeologic map of the Orientale
basin region, *in* G. E. Ulrich, Advanced systems traverse research
project report: U.S. Geol. Survey Interagency Rept., Astrogeology 7, pp.
32--33.

\_\_\_\_\_\_\_\_, 1973, Geologic map of the Grimaldi quadrangle of the
Moon: U.S. Geol. Survey Misc. Geol. Inv. Map I-740.

McCauley, J. F., Scott, D. H., Roddy, D. J., and Boyce, J. M., Geology
of the Orientale Basin of the Moon (in preparation).

Milton, D. J., Barlow, B. C., Brett, Robin, Brown, A. R., Glikson, A.
Y., Manwaring, E. A., Moss, F. J., Sedmik, E. C. E., Van Son, J., and
Young, G. A., 1972, Gosses Bluff impact structure, Australia: Science,
v. 175, no. 4027, p. 1199--1207.

Milton, D. J., and Roddy, D. J., 1972, Displacements within impact
craters: Internat. Geol. Cong., 24th, Montreal. 1972, proc., Sec. 15,
Planetology, p. 119--124.

Moore, H. J., 1965, Geologic map of the Aristarchus region of the Moon:
U.S. Geol. Survey Misc. Geol. Inv. Map I-446.

Moore, H. J., Hodges, C. A., and Scott, D. H., 1974, Multiringed
basins-illustrated by Orientale and associated features: 5th Lunar Sci.
Conf., Houston, Proc., Geochim. Cosmochim. Acta, Supp. 5, v.1,
p.71--100.

Page, N. J., 1970, Geologic map of the Cassini quadrangle of the Moon:
U.S. Geol. Survey Misc. Geol. Inv. Map I-666.

Roddy, D. J., 1969, Geological Survey activities, *in* Dudash, M. J.,
ed., Operation Prairie Flat preliminary report, v. 1: Defense Atomic
Support Agency, Dept. of Defense, DASA 2228-1, p. 317--333.

\_\_\_\_\_\_\_\_, 1973, Geologic studies of the Middle Gust and Mixed
Company craters, *in* Mixed Company/ Middle Gust Results mtg., 13--15
March, 1973, Proc., v. 11, Ses. 2B and 3B: General Electric
Company/TEMPO, DASIAC (Dept. of Defense Nuclear Information and Analysis
Center), p. 79--123.

Scott, D. H., 1972, Structural aspects of Imbrium sculpture, *in* Apollo
16 Preliminary Science Report, Photogeology, Pt. G: Natl. Aeronautics
and Space Admin. Spec. Pub. 315, p. 29--31 to 29--33.

Scott, D. H., 1974, The geologic significance of some lunar gravity
anomalies: 5th Lunar Sci. Conf., Houston. Proc., Geochim. Cosmochim.
Acta, Supp. 5, v. 3, p. 3025--3036.

Scott. D. H., Diaz, J. M., and Watkins, J. A., 1975, The geologic
evaluation and regional synthesis of metric and panoramic photographs:
6th Lunar Sci. Conf., Houston Proc. Geochim Cosmochim. Acta, p.
2531--2540.

Scott, D. H., and Eggleton, R. E., 1973, Geologic map of the Rumker
quadrangle of the Moon: U.S. Geol. Survey Misc. Geol. Inv. Map I-805.

Shoemaker, E. M., 1962, Interpretation of lunar craters, *in* Kopal,
Zdenek, ed., physics and astronomy of the Moon: New York, Academic
Press. p. 283--359.

Soderblom, L. A., 1970, The distribution and ages of regional
lithologies in the lunar maria: California Inst. Technology, Pasadena,
Calif., Ph.D. thesis, 139 p.

Toksöz, M. N., Dainty, A. M., Sean, C. S., and Anderson, K. R., 1974,
Structure of the Moon: Reviews of Geophysics and Space Physics, v. 12,
no. 4, p. 539--567.

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.

Wolfe, E. W., and Reed, V. S., 1975, Origin of the Taurus-Littrow
massifs (abs.): Lunar Science VI, Proc. 6th Lunar Sci. Conf., Abs.,
Houston, Lunar Sci. Inst., p. 875--877.

Wright, F. E., Wright, F. H. and Wright, Helen, 1963, The lunar surface:
Introduction to the Moon, meteorites, and comets, *in* Middlehurst, B.
M., and Kuiper, G. P., eds., The Solar System IV, The University of
Chicago Press, p. 1--56.

Mapped 1974-1976. Principle data sources; Lunar Orbiter photographs
plotted on index map, courtesy of National Aeronautics and Space
Administration, and albedo form USSR Zond 8 photographs. Adjoining
geologic maps at same scale: farside map by Stuart-Alexander (1978)
adjoins west map boundary; nearside map by Wilhelms and McCauley (1971)
adjoins and overlaps east map boundary; northside map by Lucchitta
(1978) overlaps north of 45° N.; southside map (Wilhelms, et al., 1979)
overlaps south of lat 45° S.

Geologic maps at 1:1,000,000 scale entirely or partly within area by
Wilshire (1973), McCauley (1973, 1967), Moore (1967), and Scott and
Eggleton (1973) work performed on behalf of the National Aeronautics and
Space Administration under Contract No. W-13,130.

Shaded-relief base charts, 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. East of long
80° W., Lunar Farside Chart (LMP-2). 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.