**GEOLOGIC MAP**

**OF THE**

**NORTH SIDE**

**OF THE MOON**

By

Baerbel K. Lucchitta

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--1062, 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**

**BASIN MATERIALS**

Ia ALPES FORMATION---Angular or rounded, closely spaced hills and
irregular hummocks

If FRA MAURO FORMATION---Sinuous, curvilinear, and straight ridges
draping over underlying landscape. Surface texture locally hummocky

IpNbm MATERIAL OF IMBRIUM BASIN MASSIFS---(Imbrian, Nectarian, and pre-
Nectarian) Bright, high, steep-sided mountain blocks and ridges

Ipnl IMBRIUM LINEATED BASIN MATERIAL---(Imbrian, Nectarian, and pre-
Nectarian) Linear subparallel arrays of scalloped troughs and ridges
radial to Imbrium basin

NpNbm MATERIAL OF HUMBOLDTIANUM BASIN MASSIFS---(Nectarian and pre-
Nectarian) High, rugged mountain blocks and chains forming basin rings

NpNbp MATERIAL OF RUGGED BASIN TERRAIN---(Nectarian and pre-Nectarian)
Irregular mountain blocks and rugged hills, forming inner rings of
Birkhoff and Schwarzschild basins, and Bel\'kovich central peaks

Nbl LINEATED BASIN MATERIAL---Irregular hilly material and broadly
undulating ridges and troughs radial to Humboldtianum and Birkhoff
basins

Nb UNDIVIDED BASIN MATERIAL---Craterlike rims and terraced walls of
Birkhoff and Schwarzschild basins

INbl UNDIVIDED LINEATED BASIN MATERIAL---(Imbrian and Nectarian)
Intersecting ridges, troughs, and vaguely lineated material radial to
Imbrium and Humboldtianum basins

**OTHER TERRA MATERIALS**

Ip UNDIVIDED PLAINS MATERIAL---Light, smooth, flat surface in low areas.
Higher density of craters than mare

Ip2 YOUNGER PLAINS MATERIAL---Light, smooth, flat surface Lower density
of craters than Ipl. Contacts mostly sharp

Ip1 OLDER PLAINS MATERIAL---Light, fairly smooth, flat to locally
undulatory surface. Crater density like that on Fra Mauro Formation.
Contacts locally diffuse

It TERRA MATERIAL---Light, slightly hummocky surface. Smooth and level
on medium-resolution pictures

INt TERRA MATERIAL---(Imbrian and Nectarian) On high-resolution
pictures, hummocky plain with some subdued hills (kilometers in size),
mostly in low areas. On medium-resolution pictures light, fairly smooth
and level plains. Surface more undulating and contacts more diffuse than
those of plains units. Mixture of hilly material and plains material in
areas too small to map separately. Includes unit INp of some adjacent
maps

IpNt TERRA MATERIAL---(Imbrian, Nectarian, and pre-Nectarian) on
high-resolution pictures, smooth to undulating, pitted, and grooved
surface on topographically high areas. On medium-resolution pictures,
resembles unit Nt

Nt TERRA MATERIAL---Irregular hills and depressions, a few kilometers to
tens of kilometers in size. On medium-resolution pictures, somewhat
subdued highlands. On high and low areas

pNt TERRA MATERIAL---Rugged mountains, irregular depressions, and
clusters of old craters and crater rim remnants

NpNt TERRA MATERIAL---(Nectarian and pre-Nectarian) Irregular hills,
ridges depressions, furrows, and crater remnants

**MARE AND DARK MANTLE MATERIALS**

Em YOUNGER MARE MATERIAL---Dark, smooth, flat surface of bluish color.
Lower albedo and lower density of craters than Imbrian mare

Im OLDER MARE MATERIAL---Dark, smooth, flat surface in Mare Frigoris,
Oceanus Procellarum and Imbrium and Humboldtianum basins. Mostly on
lunar near side

EId DARK MATERIAL---(Eratosthenian or Imbrian) Dark, smooth, or hummocky
material draping over underlying landscape. Locally associated with
rimless depressions or grooves

**CRATER MATERIALS**

Cc MATERIAL OF RAYED CRATERS---Sharp topographic detail at all
photographic scales, sharp continuous rimcrest. On high-resolution
pictures, deep rugged interior, terraced walls, radially ribbed rim
materials, and secondary crater clusters and chains. Rays visible on
near side images. Small craters are sharp-rimmed with deep bowls

Ccc MATERIAL OF CRATER CHAIN---Linear chain of sharp-rimmed, overlapping
irregular craters radial to Philolaus

Ec MATERIAL OF FRESH CRATERS---Fairly sharp topographic detail, sharp
continuous rimcrest. On high-resolution pictures, deep, somewhat subdued
interior, terraced walls, radially ribbed rim materials, and secondary
crater clusters and chains. Small craters have deep bowls

Ecc MATERIAL OF CRATER CHAIN---Linear crater chain radial to Pythagoras

Ic2 MATERIAL OF MODERATELY FRESH CRATERS---Subdued but distinct
topographic detail, fairly continuous rimcrest. On high-resolution
pictures and around large craters (Plato, Plaskett), terraced walls,
radially ribbed rim facies, and secondary craters. On medium-resolution
pictures, outer rim facies indistinct or smooth. Small craters have deep
bowls, rim more subdued than those in unit Ec

Icc2 MATERIAL OF CRATER CLUSTERS AND CHAINS---Round to irregular
overlapping craters and chains, somewhat subdued rims and fairly deep
interiors

Ic1 MATERIAL OF MODERATELY SUBDUED CRATERS---Subdued topographic detail,
fairly continuous rimcrest. Terraced walls, radial rim facies and
secondary craters on very large craters (Iridum, Compton); on other
craters outer rim facies indistinct or smooth. Small craters have
shallow bowls with subdued rims, irregular shadows

Icc1 MATERIAL OF CRATER CLUSTERS AND CHAINS---Round to irregular
overlapping craters, subdued rims, shallow interiors, irregular shadows

Nc MATERIAL OF SUBDUED CRATERS---Continuous to interrupted subdued
rimcrest. Only large craters show terrace remnants; outer rim facies
only on d\'Alembert. Small craters have smooth walls, mostly flat floors

Ncc MATERIAL OF CRATER CLUSTERS AND CHAINS---Round to irregular,
overlapping craters, rounded rims; irregular, shallow interiors or flat
floors

pNc MATERIAL OF HIGHLY SUBDUED CRATERS---Discontinuous, subdued rim-
crests and rounded, curved or straight rim remnants

IpNcl MATERIAL OF LINEATED CRATERS---(Imbrian, Nectarian, and
pre-Nectarian) Linear subparallel arrays of troughs and ridges radial to
Imbrium basin; on crater rims

Approximate contact

Crest of buried crater rim

Crest of basin ring structure (generalized). Dashed where poorly defined
or inferred

Fractured crater floor, occurs in craters of Nectarian, Imbrian, and
Copernican age.

INTRODUCTION

Geologic mapping of the north side of the Moon follows the practices
established by Shoemaker and Hackman (1962) and Wilhelms (1970). Most
units and map conventions conform to those of the near- side geologic
map by Wilhelms and McCauley (1971), except for a larger size cutoff (20
km) for craters and patches of other units. A significant departure is
division of pre-Imbrian materials into a Nectarian System and
pre-Nectarian materials. The base of the Nectarian System is defined as
the base of the Janssen Formation, the lineated ejecta blanket in the
Nectaris basin (Stuart-Alexander and Wilhelms, 1975). On the present
map, units are assigned to the Nectarian by comparison of crater
morphologies. The base of the informally designated upper Imbrian series
is defined by materials of the Orientale basin.

Identification of geologic units is based mostly on Orbiter photographs,
supplemented in places by Zond 7 and Mariner 10 images. The coverage for
the front side is excellent (Orbiter IV high-resolution pictures) and
identification of units is made fairly confidently. The coverage of most
of the far side is less good (Orbiter IV and V medium-resolution
photographs, locally with highly oblique viewing angles). Thus
identification of geologic units on the far side is tentative, and
location of contacts approximate. The low sun illumination angle on all
pictures near the pole also complicates recognition of units.

The mapping of the lower latitudes of the near side in this map area
closely follows that of the detailed maps at a scale of 1:1,000,000
listed in the credit note. This map is slightly modified in view of the
different scale and scope of the present rendition.

GENERAL FEATURES

The north side of the Moon reflects the Moon's general dichotomy: the
near side is imprinted by the Imbrium impact and flooded by mare
material; the far side is characterized by cratered highlands mantled to
various degrees. A third major element, on the east limb, is the
Humboldtianum basin and its ejecta blanket. This general setting is
displayed on the province map (Fig. 1).

The Imbrium basin units include basin massifs that partially define
Imbrium basin rings, and the Alpes and Fra Mauro Formations. In
addition, Imbrian lineated material trends northeast, radial to the
basin. Thick plains material fills many craters in a belt beyond the Fra
Mauro Formation, and smooth terra material covers highland surfaces
beyond it. Mare Frigoris on the east and Oceanus Procellarum on the west
occupy a broad depression across the near side in the map area.

On the far side, craters and basins dominate the surface: the old basin
here named Coulomb-Sarton and the two small younger basins, Birkhoff and
Schwarzschild, are recognized The intervening areas are mantled by hilly
terra units whose roughness increases with age; the oldest and roughest
unit consists mostly of ancient crater rims.

Radial ejecta deposits and secondary craters of the Humboltianum basin
are pronounced and extensive. The basin rings encircle the basin and the
superimposed crater Bel'kovich.

Among post-Imbrium craters the early Imbrian craters Iridum and Compton
are most prominent; their ejecta blankets obscure sizable areas on the
near and far sides. Several large Eratosthenian and Copernican craters
on the near side give rise to a dense secondary crater population and to
the bright appearance of the north pole area on full Moon photographs.

GEOLOGIC UNITS

BASIN MATERIALS

Multiringed basins produced by impact have long been recognized on the
Moon (Hartmann and Kuiper 1962; Baldwin, 1963; Stuart-Alexander and
Howard, 1970; Wilhelms, 1970; Hartmann and Wood, 1971). Younger basins
typically consist of one or

more mountainous rings with rugged intervening terrain and a radial
ejecta blanket beyond the outermost ring. Older basins display traces of
ring structures composed of mountains or uplifted crater rim segments.
Mantled terra material near the older basins attests to the presence of
degraded ejecta blankets.

Basin ring structures are assigned a pre-basin and basin age on the
assumption that pre-basin rocks are structurally displaced to form the
basin rings (Wilhelms and McCauley, 1971). Recent investigators,
however, concur that the outer basin rings may be predominantly composed
of ejecta (Head, 1974; Howard and others, 1974; Moore and others, 1974;
Reed and Wolfe, 1975; Scott and others, 1977). The ejecta are assigned a
basin age because included pre-basin rocks are thought to have been
thoroughly modified by the impact. Not included here in the definition
of basins are craters that display both ring structures and central
peaks.

Imbrium basin materials

The northern part of the Imbrium basin extends into the map area. The
inner and second basin rings follow the massifs, which form "islands" in
Mare Imbrium and blocky mountains near Vallis Alpes. The second ring
becomes indistinguishable underneath the ejecta blankets of craters
Plato and Iridum. The outer ring, represented by Montes Apenninus
farther south, diverges outward in the map area and can be traced
through mountain massifs near crater Aristoteles, through "islands" in
Mare Frigoris, and possibly through the north "shore" of the mare. The
basin rings are inferred to result from structural adjustment upon
impact, either as frozen tsunamilike waves (Baldwin, 1972, 1974) or by
rebound and collapse (Head, 1974; Howard and others, 1974).

The Alpes Formation occurs on both sides of the outer Imbrium basin
ring. It was previously interpreted as Imbrium basin ejecta and
structurally modified older rock (Page, 1970; M'Gonigle and Schleicher,
1972; Lucchitta, 1972). The present study suggests different origins in
different places: near the basin ring, where its texture is coarse, the
Alpes may be fractured fallback ejecta, as suggested for similar
material near the main ring of the Orientale basin (McCauley, 1968;
Head, 1974) or a fallback ejecta facies of deep-seated origin (Scott and
others, 1977); farther outward from the Imbrium basin rings, where its
texture is fine, the Alpes may be a hummocky facies of Imbrium ejecta
(Scott, 1972; Lucchitta, 1972). Some of the hummocky appearance may
result from dense cratering by small secondary projectiles.

The Fra Mauro Formation is a thick deposit of Imbrium ejecta (Eggleton,
1964; Wilhelms, 1970) that buries cratered highlands north of Mare
Frigoris and extends to about 700 km from the inner basin ring with
tongues over 1000 km. These distances are comparable to those of ejecta
deposits on the south side of the basin (Wilhelms and McCauley, 1971).
The Fra Mauro also occurs west of the Imbrium basin, where it is locally
deposited against the far walls of craters, but it is not present
northeast and east of the basin, where lineated material and the Alpes
Formation take its place. The Fra Mauro may have been deposited from a
dense, rapidly moving cloud of debris, in the manner envisioned for the
Orientale ejecta (McCauley and Masursky, 1968; McCauley, 1968; Masursky,
1968), or as collision products from primary and secondary ejecta debris
(Oberbeck and others, 1974, 1975; Morrison and Oberbeck, 1975). Analysis
of samples returned from the Fra Mauro Formation south of the Imbrium
basin suggest that the formation consists of breccia in various stages
of thermal metamorphism (Sutton and others, 1972; Wilshire and Jackson,
1972; Warner, 1972; Chao and others, 1972).

Lineated basin material occurs mainly northeast of the Imbrium basin but
is also present to the northwest and west. The grooves forming the
lineations traverse highland terrain and gouge old crater rims; they are
similar to grooves of "Imbrium sculpture" southeast of the basin
(Wilhelms and McCauley, 1971), and the unit in the map area was likewise
interpreted to consist of radially faulted bedrock (Lucchitta, 1972).
Although some faulting may have taken place, the lineated material
extending to the northeast is now thought to be

linear chains of secondary craters produced by concentrated projectiles
traveling in low trajectories (Moore and others, 1974; Wilhelms, 1976).
There is less primary ejecta and more locally excavated material than in
the Fra Mauro Formation, and the lineated material obscures the
underlying landscape much less than does the Fra Mauro. The asymmetrical
distribution of the lineated material (mainly northeast of the basin)
and the Fra Mauro (north and northwest) may result from an oblique
approach of the basin-forming bolide.

Older plains material and the terra material, even though probably
associated with the Imbrium event, are discussed below under "terra
material".

Humboldtianum basin materials

The Humboldtianum basin near the east limb of the Moon is believed to be
of middle Nectarian age, as Nectarian craters are both superposed on it
and partly obliterated by it, and because its well-developed rings and
ejecta blanket appear morphologically sharper than those of the Nectaris
basin. Its interior is partly flooded by mare material. The two basin
rings form massifs. The large crater Bel\'kovich is superimposed on the
inner basin ring. This ring, which resembles the rim of Bel\'kovich, may
nearly delineate the original impact cavity of the basin. The outer ring
is similar to though more degraded than the Cordilleran scarp around the
Orientale basin and likewise probably originated by faulting (McCauley,
1968; Howard and others, 1974; Head, 1974). On the east side, the outer
ring forms a scarp that is partly buried by ejecta from crater Compton.
The outer ring is elliptical and surrounds both the inner basin and
Bel\'kovich. These relations and the similar degradation of their rims
suggest that Humboldtianum and Bel\'kovich were emplaced nearly
simultaneously. Even though Bel\'kovich caused the northern deflection
of the Humboldtianum ring structure, it is mapped as a crater because it
has central peaks and not peak rings.

Between the Humboldtianum basin rings, hummocky material grades into
lineated material, and there are mountains (unit NpNbr) along grooves
radial to both the inner ring of Humboldtianum and Bel\'kovich. These
grooves and massifs may have been produced by secondary impacts from
Humboldtianum and Bel\'kovich. Lineated material interpreted to be
ejecta locally occurs inside the outer basin ring on the south side; the
occurrence confirms that the outer ring was formed outside the original
impact cavity. Humboldtianum ejecta includes sinuous, hummocky, and
radially grooved deposits; they are not mapped separately like the
Imbrium ejecta, mostly because of the poor picture quality around part
of the basin. The sinuous, grooved facies is predominant south of the
basin, whereas a hummocky and hilly facies extends to the northwest,
where it partly buries older craters near the blanket margin. West of
the basin a grid is formed where Humboldtianum basin radial ejecta is
transected by Imbrium basin linear chains (unit INbl).

Birkhoff and Schwarzschild basin materials

The small basins Birkhoff and Schwarzschild on the far side are
transitional to craters: they have well-defined continuous terraced rims
resembling those of impact craters, but have inner rings of mountains
rather than central peaks (unit NpNbr), and thus are similar to basins
Their rims are assigned a basin age because they are thought to consist
predominantly of ejected debris. Both basins are considered of Nectarian
age on the basis of morphologic sharpness and the Nectarian and younger
age of superimposed craters; both basins are older than the
Humboldtianum basin. Birkhoff, on the western far side, is heavily
cratered, suggesting an old age;however, traces of a radially grooved
ejecta blanket on its southeast side and radially arranged secondary
chains and clusters suggest that it is young. The crater Stebbins, which
is superimposed on the basin rim, is filled with plains material that
has a wrinkle ridge along the projected trend of the former basin rim,
indicating the presence of a former structural zone of weakness
(Hartmann and Wood, 1971). Schwarzschild, on the eastern far side,
appears to be partially buried by

Humboldtianum ejecta. Mantled hilly highlands (unit Nt) near these
basins are probably a degraded or smooth facies of basin ejecta.

Other basin materials

An old basin centered near the west limb on the far side (about lat 51°
N long 122° W) (Hartmann and Wood, 1971), here named Coulomb-Sarton
basin for two unrelated superimposed craters, is composed of alternating
concentric rings of highly cratered terrain and smoother terrain mantled
by plains or other terra material. The basin is ancient, and only its
outline remains expressed topographically; basin-related deposits are
degraded beyond recognition.

OTHER TERRA MATERIALS

Light, smooth, and relatively densely cratered plains materials cover
large areas on the near side but are much less abundant on the far side,
where they mostly occupy crater interiors (undivided plains unit).

Older plains material (Ipl) deeply fills craters at the distal end of
the Fra Mauro Formation. Crater density and the shadow measurement
technique show that the unit may be contemporaneous with the Fra Mauro
Formation (Boyce and others, 1974). The older plains unit is slightly
more hummocky than the younger plains unit (Ip2), locally drapes over
the pre-existing landscape, and has gradational contacts with the Fra
Mauro Formation. These facts suggest that this plains material is a
distal and possibly thinner facies of Imbrium ejecta, possibly mixed
with local material derived from secondary craters (Morrison and
Oberbeck, 1975). A similar transitional relation is seen at the distal
end of the Orientale ejecta blanket, where a finely textured facies is
almost indistinguishable from plains materials (Hodges and others, 1973;
Moore and others, 1974).

The smoother and less densely cratered younger plains unit can be
identified confidently only in a few places, mostly north of Mare
Frigoris; where mapped elsewhere its identification is tentative. The
younger plains unit north of Mare Frigoris is considered of
Orientale-basin age (Boyce and others 1974), on the basis of crater
densities and the shadow measurement technique (Soderblom and Lebofsky,
1970). If this interpretation is valid, this younger plains material
represents a foreign element in an area dominated by Imbrium basin
deposits. This young plains unit could be a tongue of thick Orientale
basin ejecta; perhaps similar thin unrecognized ejecta covers the entire
northern near side without obscuring the underlying older units. Or,
perhaps the main plains area is composed of thin Orientale ejecta upon
old mare material, a relation that would explain both the smoothness and
the high albedo of the area. Later impact into such a mantled mare
should eject dark mare material and form dark-halo craters; their
absence could be explained by the obscuring effect of the extensive
young ray systems of the area. In summary, the thick plains units on the
northern near side appear to be primary and secondary ejecta from the
Imbrium basin and probably contain a substantial contribution from local
highland materials, redistributed by secondary impact (Oberbeck and
others, 1975). The plains may possibly be underlain by ejecta from
Humboldtianum and overlain by a thin veneer of ejecta from Orientale.

Most of the hummocky Imbrian terra material probably is similar in
origin to the older plains material but represents a facies transitional
between plains material and the Alpes Formation.

Older, relatively smooth mantling material (INt) occurs mostly inside
Nectarian and older craters and basins on the far side of the Moon. It
looks like plains material on medium-resolution pictures but is
distinctly more hilly than plains material on high-resolution frames. It
consists of older hilly material and intervening Imbrian plains on a
scale too fine to map separately or of an older plains unit degraded to
a more hummocky surface by cratering and mass wasting.

A mantle (IpNt) with diffuse contacts locally smooths and levels
highland areas north of and beyond the belt of older plains material. It
differs from the Fra Mauro Formation in not

displaying sinuous ridges or completely burying craters. It probably
represents a transitional facies at the distal end of the Imbrium ejecta
blanket, where Imbrium ejecta mixed with local materials thinly covers
the pre-existing highlands. North of the Humboldtianum basin, the unit
may include much Humboldtianum ejecta.

A hummocky terra unit (Nt) fills craters and mantles highlands. Its
roughness suggests that it is older than more planar surfaces, and its
smoothing of highland terrain indicates that it buries the older
landscape and probably consists of ejecta blankets from numerous older
craters and basins. In the Humboldtianum basin some of this material may
be remnants of impact melt, but even there it more likely is ejecta from
Bel\'kovich.

Nectarian and pre-Nectarian terra material covers large areas on the far
side of the Moon. It has a rough surface composed of hilly material that
appears to mantle the landscape, interspersed with rugged terrain that
is mostly composed of remnants of ancient craters. The hilly morphology
is thought to be the result of primary roughness of old overlapping
ejecta blankets, partially degraded by later impacts and concomitant
secondary gouging, structural adjustments, seismic shaking, and mass
wasting.

Pre-Nectarian terra material consists almost entirely of old craters,
crater segments, or crater clusters. Because of the absence of obvious
mantling deposits, the material is envisioned to consist mostly of
ancient crater debris.

MARE AND DARK MANTLE MATERIALS

In the map area mare material is restricted mostly to the near side,
where it forms Mare Imbrium, Mare Frigoris, Oceanus Procellarum, Mare
Humboldtianum, and patches near the Humboldtianum basin. On the far side
only one small patch was observed, in crater Campbell. The albedo of
Mare Frigoris is relatively high for maria (Pohn and Wildey, 1970) and
appears not to be intrinsic to the mare surface but caused by superposed
ray material from the young craters farther north. The color of Mare
Frigoris is reddish, much like the surrounding highlands, but bluish
mare surfaces having lower albedo than Mare Frigoris elsewhere (Pohn and
Wildey, 1970) occur in Sinus Roris and the northwest margin of Oceanus
Procellarum (Whitaker, 1972).

The dark bluish mare unit at the northwest margin of Oceanus Procellarum
is old (Boyce, 1976) and here assigned an Imbrian age. By contrast, dark
bluish mare material in Sinus Roris floods secondary craters of the
Eratosthenian crater Pythagoras and coincides approximately with Boyce's
(1976) two youngest mare units. It is assigned an Eratosthenian age on
this map.

All mare materials are assumed to be basalts in accordance with the
findings from the Apollo missions. The bluish maria probably are basalts
with a higher titanium content than the reddish maria (McCord and Adams,
1972).

Dark mantling material covers Imbrium basin ejecta in the crater J.
Herschel, and ejecta and mare materials along the southeastern margin of
Mare Frigoris. It also occurs on the floor of the crater Compton. In all
places the material is associated with rimless craters and fissures or
with grabens. It is best interpreted as a pyroclastic deposit (McGetchin
and Head, 1973; Lucchitta, 1973), possibly caused by fire fountains
(Heiken and others, 1974). Its age is considered Imbrian on the basis of
evidence elsewhere (Howard and others, 1973; Lucchitta and Schmitt,
1974); however, the freshness of associated depressions suggests that it
may be younger.

CRATER MATERIALS

A sequence of large Eratosthenian and Copernican impact craters extends
across the western high lands of the northern near side. Their age
decreases from west to east; Pythagoras is oldest, Anaxagoras is
youngest. Pythagoras has faint rays, and some of its secondary craters
are flooded by mare material. Anaxagoras is believed to be as young as
Aristarchus on the basis of its morphologic sharpness. Anaxagoras and
the 35-km crater

Thales on the eastern north side are the sources of the extensive ray
system that brightens the northern highlands. The crater Hayn on the
east limb has sent rays southward across Mare Humboldtianum, and some of
these link with others and contribute to some of the very long rays on
the eastern near side of the Moon. Secondary craters are well developed
around all of these young craters, but most are too small to map. Chains
of secondaries from Philolaus extending to the northwest are parallel to
each other and to the direction radial to Imbrium and possibly reflect a
pre-existing Imbrium-radial fracture system that influenced the
direction of ejection. On the far side of the Moon, more Copernican
craters may be present than have been identified because the lack of
photographs displaying albedo variations prevents the detection of rays.

Two prominent late Imbrian craters are Plato on the Imbrium basin margin
and Plaskett on the far side near the north pole. Plato formed early in
the epoch, as its secondary craters are flooded by Imbrian mare
material. It differs from other craters in having an unusually reddish
ejecta blanket (Whitaker, 1972). Plaskett's secondary craters trend
across the pole towards the near side and contribute to the dense swarm
of secondary craters in that area. Near the west limb many small
relatively sharp Imbrian craters and clusters may be secondaries of the
Orientale basin. At this distance from the basin the individual craters
were probably excavated by projectiles with high trajectories that
produce circular craters similar to primary ones.

Iridum and Compton are large and conspicuous early Imbrian craters. Both
predate mare emplacement. Secondary craters from Iridum can be
recognized on the map by their orientation radial Iridum and their
smaller size than Imbrium secondary crater clusters. Secondaries from
Iridum are mapped as far away as the crater Meton, where they overlie
older plains material. Compton is distinguished from other craters by an
inner ring and a central peak, thus being transitional between craters
and basins (Hartmann and Wood, 1971; Howard and others, 1974). The floor
of Compton has the grooved and cracked appearance of the floor of the
Orientale basin and may consist of shock-melted material (Saunders,
1968; McCauley, 1968; Howard and others, 1974). The central peak may
have formed by rebound (Wilshire and Howard, 1968; Howard and others,
1972), and the ring by faulting and slumping of the crater walls, which
were displaced inward and upward along curved fault planes (Howard and
others, 1974).

Crater clusters of early Imbrian age and clusters of craters larger than
the secondary craters from Iridum are especially numerous northeast of
the Imbrium basin near the margin of the Imbrium ejecta blanket; some of
these form linear chains approximately radial to the Imbrium basin.
These clusters and chains are interpreted to be secondary craters from
Imbrium. Many of the linear chains are straight and narrow and could be
grabens; however, traces of raised rims and scalloped walls suggest an
origin as chains of overlapping secondary craters. The Linear chains
typically are more degraded than the clusters, possibly owing to the
successive impact erosion of earlier arrivals by later ones or to the
low trajectory of the impacting body. Many of the early Imbrian crater
clusters on the far side of the Moon may be secondary to the Imbrium
basin, but the association is not certain.

Nectarian craters are equivalent to those previously mapped by Wilhelms
and McCauley (1971) as late pre-Imbrian and include relatively
fresh-looking middle pre-Imbrian craters. Nectarian craters are
identified by morphologic comparison with craters superposed on the
Nectaris ejecta blanket. On the northern front side, a relative dearth
of Nectarian craters can be attributed to the proximity to the Imbrium
basin: the impact buried and destroyed many older craters. These craters
now morphologically appear and are mapped as pre-Nectarian even though
their real age may be Nectarian. On the far side large Nectarian craters
show traces of radial ejecta (d'Alembert) or smooth highland mantles
nearby which may be their degraded ejecta blankets. Large Nectarian
craters (d'Alembert, Rowland, Sommerfield) appear more degraded than
smaller ones of the same age. This degradation may have occurred early
in the history of the crater, and, as for basins, may result from
destructive

rebound and isostatic adjustment; alternatively, it may be a later
effect due to the erosion caused by the many superimposed craters.

On the far side of the Moon, where photographs are poor, early Imbrian
and late Nectarian craters are difficult to distinguish. Both may show
moderately dissected rims, terrace remnants on the walls, and central
peaks; age assignment is therefore tentative, and in fact, the unusually
large number of early Imbrian craters mapped suggests that many late
Nectarian ones may be included.

Fields of subdued but nearly uniformly degraded clusters of Nectarian
craters of similar size surround the Humboldtianum basin near the margin
of the ejecta blanket and are inferred to be secondary craters of
Humboldtianum. Remnants of probable Nectarian clusters of secondaries
also exist in the area of Imbrium lineated material north of eastern
Mare Frigoris, where the crater rims are pervasively cut by "Imbrium
sculpture." Fields of crater clusters southeast of Birkhoff on the west
limb may be secondary craters of Birkhoff, but the craters appear
somewhat larger than expected from this small basin; it cannot be ruled
out entirely that some of these craters and some of the clusters of
subdued craters northeast of the Imbrium ejecta blanket and northwest of
Humboldtianum are early arrivals from Imbrium rather than secondary
craters of Birkhoff and Humboldtianum. Other Nectarian crater clusters
occur throughout the north side and where not near and radial to known
basins are inferred to be secondary crater clusters from unknown
sources.

Pre-Nectarian craters include all old degraded craters previously mapped
as early pre-Imbrian, and most of those mapped as middle pre-Imbrian
(Wilhelms and McCauley, 1971). Many of them consist of rim remnants.
They locally occur in overlapping clusters, resulting in a rough
crater-rim topography. These craters and their ejecta occupy the oldest
known surfaces on the Moon.

Lineated crater material occurs mostly northeast, north, and west of the
Imbrium basin. It is the crater equivalent of the terra unit "basin
lineated material" and is inferred to have originated similarly through
gouging of older materials by dense secondary bombardment from Imbrium.
The lineated basin material north of the basin differs from that in
other areas, forming massive linear mountain chains radial to the
Imbrium basin. The mountains may be remnants of ancient crater rims that
were faulted, or they may be the rim remnants of large and irregular
secondary craters from Imbrium. Also attributed to secondary impact may
be a northeast-trending valley system east of crater Epigenes that
resembles the Vallis Bouvard structure of the Orientale Basin.

GEOLOGIC HISTORY

The first surface record in lunar history is that of the formation of
numerous craters and basins by impacts, which must have followed early
differentiation and formation of a feldspathic crust (Toksöz, 1974). The
pre-Nectarian cratering and basin formation may have preceded or been
part of the terminal cratering event around 3.9 b.y. ago detected by
Tera, Papanastassiou, and Wasserburg (1974a, b), whereas the Nectarian
Period may have coincided with most of this cratering.

On the north side of the Moon, basins and craters were formed during
pre-Nectarian time, but most of them were destroyed by subsequent
impacts. Only one ancient basin remains preserved as topographically
high ring structures. Similarly, most ejecta from pre-Nectarian craters
was destroyed, but ejecta from those emplaced during late pre-Nectarian
time may have contributed to the present-day terra mantles.

The Nectaris basin impact farther south initiated the Nectarian Period,
and relatively soon afterward the Birkhoff and Schwarzschild basins were
excavated. Later, almost simultaneous impacts created the Humboldtianum
basin and the crater Bel\'kovich and produced ring structures around
both impact sites. Ejecta formed a blanket of lineated and hummocky
debris and clusters of secondary craters. Deposits from other basins and
craters mantled highland surfaces and filled craters during the
Nectarian Period .

The Imbrium impact initiated the Imbrian Period and profoundly altered
the topography of the northern front side. It broke the surface near the
impact site into fault-block massifs and threw the terrain farther from
the impact into nearly concentric highs and lows, the latter now being
occupied Sinus Roris and part of Mare Frigoris. Simultaneously, vast
amounts of ejecta were thrown around the basin and partly buried the
older landscape, forming the Alpes and Fra Mauro Formations and much
plains material. Secondary low-angle projectiles gouged older crater
rims and created the lineated terrain to the northeast. Secondary
projectiles were also thrown to great distances on the far side of the
Moon, forming crater chains and clusters. Older craters near the Imbrium
basin were severely modified by structural disturbance, secondary
impact, and burial.

The Orientale basin impact created secondary craters mostly near the
west limb and possibly spread a thin sheet of ejecta across most of the
north side. Locally, tongues of ejecta may have been thick enough to
bury low areas and to form the younger plains unit. Alternatively, mare
basalt may have been erupted in northeastern Mare Frigoris before the
Orientale impact and been buried by thin, light-colored Orientale ejecta
to form the younger plains units. The main eruptions of mare basalt,
however, postdate the Orientale impact, and the basalts flooded
depressions, including the Imbrium basin, Mare Frigoris, Oceanus
Procellarum, and part of the Humboldtianum basin. Lava fountaining may
have spread dark mantles over highlands and mare during this time. A
younger basalt, possibly with a higher titanium content than the earlier
north-side lavas, later buried flows in part of Sinus Roris .

Cratering had decreased markedly by the beginning of the Imbrian Period
around 3.9 b.y. ago (Tera and others, 1974a,b). Subsequently, only a few
large craters were emplaced on the north side the Moon. Among these are
the large craters Compton, Iridum, and Plato, which impacted before mare
flooding, and a series of large craters on the northern nearside
highlands, which impacted after most flooding, during the Eratosthenian
and Copernican Periods. Of these Pythagoras is the oldest, Anaxagoras
the youngest. The most recent event is the emplacement of ray material
over the northern highlands, originating from craters Anaxagoras and
Thales.

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Mapped 1971-1974. Data sources; Lunar Orbiter photographs (1967)
predominantly; some Zond 7 (1969) and Mariner 10 (1973) frames plotted
on index map. Photographs courtesy of National Aeronautics and Space
Administration. Adjoining geologic maps at same scale: Nearside map by
Wilhelms and McCauley (1971) overlaps between long 50° W to 40° E and
lat 48° N to 64° N; and between long 62° W to 58° E and lat 45° N to 48°
N. Eastside, westside and central farside maps overlap between 45° N and
50° N; eastside map by Wilhelms and El-Baz (1976), between long 50° E
and 40° E, farside map by Stuart-Alexander (1978)) between long 140° E
and 140° W, and westside map by Scott, McCauley, and West (1977) between
140° W and 50° W.

Geologic maps at 1:1,000,000 scale entirely within northside map by
Ulrich (1969), M'Gonigle and Schleicher (1972), Lucchitta (1972); and
partly within, maps by Scott and Eggleton (1973), Schaber (1969), Page
(1970), Scott (1972), and Grolier (1974).

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

Shaded-relief base chart, North Polar Region of LMP-3, 2d ed., October,
1970, prepared by ACIC (Aeronautical Chart and Information Center), U.S.
Air Force. Overlaps 5° with Lunar Earthside Chart LMP-1 and Lunar
Farside Chart LMP-2. Horizontal positions of shaded relief features
based on ACIC Positional Reference System, 1969. Feature names from the
International Astronomical Union catalogue amended 1970.