**GEOLOGIC MAP**

**OF THE**

**BACH (H-15)**

**QUADRANGLE**

**OF MERCURY**

By

Robert G. Strom, Michael C. Malin,

and Martha A. Leake

1990

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--2015, as part of the Atlas of Mercury, 1:5,000,000 Geologic Series.
Hardcopy is available for sale from U.S. Geological Survey, Information
Services, Box 25286, Federal Center, Denver, CO 80225)

Please direct questions or comments about the digital version to:

Richard Kozak

U.S. Geological Survey

2255 N. Gemini Drive

Flagstaff, AZ 86001

e-mail: rkozak@flagmail.wr.usgs.gov

**DESCRIPTION OF MAP UNITS**

PLAINS MATERIALS

pvs **Very smooth plains material**---Most common east of long 70° in
depressions within smooth plains material; occurs elsewhere in craters
of c2 to c5 age. Distinguished by extremely smooth surface and near
absence of superposed craters. Nearer terminator, relief is more
detectable and surface is slightly hummocky. *Interpretation:* Impact
melt in youngest craters or volcanic material in older depressions such
as crater floors

ps **Smooth plains material**---Commonly in c2 to c5 craters and
widespread to local depressions, particularly east of long 90°.
Distinguished by smooth to moderately rough surfaces (appearance rougher
near terminator) and few superposed craters (mostly c4 and c5 and their
secondaries). Associated with scarps and ridges. *Interpretation:*
Impact melt in younger craters and volcanic material in older craters
and in extensive intercrater areas

psi **Intermediate plains material**---Forms large intercrater areas
east of long 85° and floors some craters of c1 to c3 age; superposed by
some c2 craters and larger c3 to c5 craters. Intermediate in texture and
crater density between smooth plains and intercrater plains units.
Moderately rough texture pockmarked by many small craters and
secondaries. May be interbedded with ejecta of craters and basins of c2
to c4 age. *Interpretation:* Volcanic materials in c1 craters and
intercrater plains. May be volcanic or impact melt within younger
craters

pi **Intercrater plains material**---Forms large tracts exterior to
craters, embaying rims of c1 and c2 craters. Surfaces are heavily
cratered and textured by knobs, ridges, and pits; superposed craters of
many sizes, some aligned in clusters or chains. Age relations to c1
craters commonly ambiguous. *Interpretation:* Volcanic. Interbedded with
ballistically emplaced ejecta of c1 to c3 craters and basins; overlain
by ejecta from craters of all ages

CRATER AND BASIN MATERIALS

\[These materials are divided into three associated facies: crater
materials, mapped to the break in slope of the exterior rim and
including central peak and peak ring materials (in large craters and
basins); floor materials; and a radial facies. Secondary crater chains
are mapped as a distinct unit. Crater morphologic age is divided into
five classes similar to those used in lunar mapping. Classification
criteria are also similar, allowing for gravitational differences
between the Moon and Mercury, and are virtually the same as those of
McCauley and others (1981). Craters smaller than l5 km in diameter not
mapped. *Interpretation of craters:* Formed by impact, surrounded by
ballistically emplaced ejecta\]

c5 **Material of very fresh craters**---Craters characterized by sharp
rims, crisp interior forms (for large craters), rays, and very few
superposed craters. Unit includes crater floor material except where
mapped separately on basis of floor-wall contact and floor morphology

cp5 **Central peak material of c5 craters**---Forms isolated or grouped
hills and arcuate ridges that, in larger craters, may be aligned along
lineament directions. *Interpretation:* Highly shocked and deformed
country rock elevated during impact

cf5 **Crater floor material of c5 craters**---Hummocky to smooth; very
few superposed craters. *Interpretation:* Impact melt and fallback
ejecta

cr5 **Radial facies of c5 craters**---Smooth to hummocky deposits radial
to primary crater. *Interpretation:* Material ejected from c5 craters
during formation; debris followed ballistic trajectories to impact
around craters, fragmenting local bedrock and regolith

cs5 **Secondary crater chains radial to c5 craters**---Continuous to
discontinuous chains of small craters radial to a primary crater at lat
67°S., long 56°, lying outside its radial facies. *Interpretation:*
Craters formed from material ejected from c5 craters, as described for
radial facies

c4 **Material of fresh but slightly modified craters**---Distinct but
visibly modified rims and interior features; well-preserved radial
facies around larger craters

cp4 **Central peak material of c4 craters**---Like central peak material
of c5 craters but modified by superposed small impacts and depositional
degradation

cf4 **Crater floor material of c4 craters**---Some more hummocky than
very smooth or smooth plains materials. *Interpretation:* Same as crater
floor material of c5 craters

cr4 **Radial facies of c4 craters**---Continuous ejecta blanket beyond
exterior rim's break in slope; discontinuous ejecta facies at greater
distances. Degraded by smaller impacts (c5 craters and their
secondaries). *Interpretation:* As for radial facies of c5 craters but
surrounds c4 craters

cs4 **Secondary crater chain radial to a c4 crater**---Only one mapped
(lat 71°S., long 176°), well beyond radial facies of crater Marti.
*Interpretation:* As for secondary crater chains radial to c5 craters
but older

c3 **Material of modified craters**---Relatively low, semirounded but
continuous rims and subdued interior landforms

cp3 **Central peak material of c3 craters**---Like central peak material
of c5 craters but modified by subsequent impacts, ejecta, and seismic
shaking. Partly buried by plains units

cf3 **Crater floor material of c3 craters**---Hummocky

cr3 **Radial facies of Bernini and Bach basins**---Continuous ejecta
blanket radially striated by crater chains and ridges. Overlies
intercrater plains unit and is locally embayed by smooth plains material

cs3 **Secondary crater chains radial to c3 basins and large
craters**---Like secondary crater chains radial to c5 craters but some
lie on top of radial facies of c3 craters; degraded by subsequent
impacts

c2 **Material of strongly modified craters**---Low rims and shallow
interior relief. Interior forms scarce; secondary crater fields absent

cp2 **Central peak material of c2 craters**---Rare. Interpretation: As
for central peak material of c5 craters but severely degraded

c1 **Material of extremely subdued craters**---Low, partial rims. Some
craters barely distinguishable from surrounding plains materials.
Interior features rare. Secondary crater fields absent

cp1 **Central peak material of c1 basins and craters**---Severely
subdued remnants of hills and hummocks in vaguely arcuate to circular
arrays. Clearly identified only in Cervantes basin, but materials there
can also be interpreted as rim remnants of a nested crater degraded by
formation of plains materials. Unit may occur also as extremely

degraded mound in crater at lat 68° S., long 68°. *Interpretation:* As
for central peak material of c5 craters but severely degraded

csu **Material of secondary crater chains, undivided**---In troughlike,
elongated depressions or connected lines of closely spaced small
craters. Relation to larger craters cannot be recognized; age
indeterminant. *Interpretation:* Secondary craters of a primary located
past terminator

> **Contact**---Dashed where approximately located; dotted where buried;
> queried where doubtful
>
> **Arcuate scarp**---Line at base of scarp; barb points downslope;
> dotted where buried. Abrupt boundary separating surfaces of different
> elevation. Interpreted to be scarp of probable thrust fault
>
> **Ridge**---Interpreted as formed by faulting or extrusion. Crest
> commonly flat. Symbol on crest
>
> **Lineament**---Linear segment of crater rims, ridges, or scarps of
> low relief. Interpreted to be joints in newly cooled, stressed rock
> later faulted during periods of stress
>
> **Crater rim crest**---Distinct, generally continuous; dashed where
> approximately located; queried where doubtful
>
> **Crater rim crest, subdued**---Buried by younger material or degraded
> by age
>
> **Area of ray material**---Bright, high-albedo patches or streaks
> roughly radial to fresh c5 craters, noted more commonly at higher sun
> angles. Interpreted to be freshly excavated ejecta from either primary
> or secondary craters

**INTRODUCTION**

The Bach region encompasses the south polar part of Mercury poleward of
lat 65° S. About half of the region was beyond the terminator during the
three Mariner 10 encounters and hence not visible. The entire mapped
area was covered by near-vertical photography from the second encounter,
and the eastern part, from long 15° to about 110°, was covered by
oblique photography from the first encounter. No third-encounter images
were acquired. The entire visible area may be viewed stereoscopically by
combining images from the first and second encounters taken at different
viewing angles or by combining second-encounter images of the same area
taken at different viewing angles. These combinations provided excellent
qualitative control of topographic relief and a good quantitative
photogrammetric base. However, sun-elevation angles of the images are
limited to less than 25°, and image resolutions are no higher than about
0.5 km per picture element. Therefore, the south polar geologic map
reflects mostly large-scale processes and topographic information,
whereas other mercurian quadrangle maps benefit from greater albedo
discrimination and, in some cases, higher resolution.

The imaged part of the Bach region covers about 1,570,000 km2. Its
surface consists of craters of a wide variety of sizes and morphologies,
as well as plains units, scarps, and ridges. It includes three
double-ring basins that range from 140 to 200 km in diameter: Bach
(after which the region is named), Cervantes, and Bernini. Another large
crater, Pushkin, is 240 km in diameter and occurs at the map boundary at
lat 65° S., long 25°. Both Bach and Bernini display extensive fields of
secondary craters. An unusual area between lat 69° and 80° S. and long
30° and 60° consists of young, relatively smooth plains marked by many
flat-topped ridges unlike any seen in other areas of Mercury. Scarps
similar to Discovery Rupes (in the Discovery quadrangle adjacent to the
north) are relatively common throughout the Bach region. The most common
terrain units in the region are the plains units, which display a wide
range of small-crater densities.

**STRATIGRAPHY**

CRATER AND BASIN MATERIALS

Superposition relations among craters and basins, and their ejecta,
provide the best means of establishing the relative time-stratigraphic
order of crater and basin materials. Relative to the Moon, stratigraphic
relations among mercurian craters are more clearly discerned because
Mercury has a lower density of large craters (Malin, 1976a), and its
enhanced gravitational acceleration has restricted the distribution of
ejecta (Gault and others, 1975). These attributes of the mercurian
crater population allow stratigraphic sequences to be constructed over
large regions.

The degree of crater degradation is determined by qualitative assessment
of their landforms such as rim crests, interior wall terraces and
slumps, central peaks, continuous ejecta deposits, and secondary crater
fields (see Malin and Dzurisin, 1977; McCauley and others, 1981). To the
extent that degradational changes are systematic with increasing age,
they can be used to correlate local and regional stratigraphic sequences
over the map region. On the basis of this morphologic evaluation, five
crater ages are defined and used to make stratigraphic assignments.
However, the low sun angle at which images in the region were acquired
may make craters appear younger than in other parts of Mercury where
images were taken at higher sun angles.

The attributes of the five morphologic classes, as illustrated by
McCauley and others (1981) and Leake (1982), are given in the
description of map units. However, these attributes are valid only for a
given size crater and in a relative sense. A representation (fig. 1) of
the classification scheme has two important constraints. (1) A given set
of criteria, as in a fresh secondary-crater field, denotes an older age
for larger craters than for smaller ones, because the landforms of
larger

craters take longer to degrade than analogous features of smaller
craters. (2) The lines of "constant morphology" converge slightly at
larger crater diameters. Thus less morphologic distinction occurs among
large craters than among small ones.

Of the region's three double-ring basins, Bach (200 km in diameter) and
Bernini (140 km in diameter) are moderately fresh (of c3 age) and have
well-defined secondary-crater fields, whereas Cervantes (200 km in
diameter) is degraded (c1). The inner rings of the three basins are
about half the diameter of the outer rings. Bach's inner ring, the most
complete, is open only to the southeast; it consist of an almost
continuous series of sharp-crested hills. The area within it and part of
the area between it and the outer ring are filled with smooth plains
material. The inner rings of Cervantes and Bernini consist of
discontinuous, low, rounded hills, Bernini has a small central peak.

As first noted by Gault and others (1975), the continuous ejecta
blankets and secondary crater fields surrounding mercurian craters are
smaller than their lunar counterparts, and the boundary between the two
features is much less distinct. As a consequence, continuous and
discontinuous ejecta are mapped together in the Bach region as "radial
facies." With this exception, the morphological elements of mercurian
craters are virtually identical with those on the Moon. Therefore, all
of the craters within the Bach region are probably the result of impact
by meteorites, small planetesimals, and possibly comets.

PLAINS MATERIALS

About 60 percent of the map area consists of tracts of planar surfaces
having a variety of small-scale textures. These tracts range in size
from a few square kilometers within craters to areas larger than 10,000
km2 that surround and separate large craters: the so called "intercrater
plains" (Trask and Guest, 1975; Malin, 1976b). The origin of the plains
material is uncertain. Strom and others (1975), Malin (1976b), Trask and
Strom (1976), Strom (1977,1979), and Leake (1982) presented arguments in
favor of volcanism, whereas Wilhelms (1976) and Oberbeck and others
(1977) argued for an impact-related origin through processes similar to
those responsible for the lunar Cayley Plains (fluidized ejecta sheets
or ballistically deposited secondary-crater ejecta).

In general, our intercrater plains and intermediate plains units
correspond to "intercrater plains," and our smooth plains and very
smooth plains units correspond to deposits previously mapped as "smooth
plains" (Trask and Guest, 1975). Plains formation occurred throughout
the period when visible craters were formed and most likely throughout
the period of intense impact cratering (Malin, 1976b; Strom, 1977). The
time scale for production and retention of plains units is crudely
similar to that for the production and retention of craters.

The oldest and most extensive plains material in the Bach region, the
intercrater plains material (unit pi), is characterized by a gently
rolling surface and a high density of superposed craters less than 15 km
in diameter. Most of these small craters occur in strings or clusters
and are irregularly shaped; they appear to be secondaries from craters
of c2 through c5 age. Therefore, the intercrater plains unit is thought
to be older than most c2 craters. Its relation to c1 craters is not
clear. The highly degraded nature of c1 craters makes it impossible to
determine whether the craters predate, postdate, or are contemporaneous
with the intercrater plains unit. However, the presence of shallow
depressions, which may be ancient craters, within this plains material
suggests that the unit flooded a preexisting population of craters and
therefore was emplaced sometime during the period of late heavy crater
bombardment. The two proposed origins for this plains unit, as volcanic
or basin-ejecta material, cannot be unambiguously resolved by geologic
relations in the Bach region. However, a volcanic origin is favored
because of (1) the widespread distribution of the plains material
throughout the imaged regions of Mercury, (2) the

apparent lack of source basins large enough to supply such great amounts
of impact melt, and (3) the restricted ballistic range of ejecta on
Mercury.

The intermediate plains material (unit psi) is concentrated mostly in
the northeastern part of the Bach region. It is similar in morphology to
intercrater plains material but has a lower density of small craters. On
the basis of the reasoning applied to the intercrater plains material,
the intermediate plains unit is also tentatively ascribed a volcanic
origin.

Materials of the smooth plains (unit ps) and very smooth plains (unit
pvs) are also concentrated mainly in the eastern part of the map area.
The smooth plains unit has a lower density of small craters than does
intermediate plains material and a somewhat hummocky surface with
scattered small hills and knobs. The hummocks within fresh c5 craters
may be mantled floor materials or incipient peak rings (see, for
example, crater Callicrates at lat 66°S., long 32°; FDS 27402). The very
smooth plains unit has virtually no visible small craters and displays
smoother planar surfaces than those of the smooth plains unit. It occurs
in the lowest areas within smooth plains material (including areas
within buried crater depressions) and commonly within older craters. The
areas of greatest concentration of smooth and very smooth plains
materials also contain the most ridges, which suggests that ridges and
the younger plains units are genetically related. Very smooth plains
material for instance, commonly lies at the base of ridges or scarps. It
occurs as small patches within the smooth plains unit that fills the
crater Pushkin. Smooth plains material embays the ejecta blanket of a c3
crater on Pushkin's rim at lat 66°S, long 28° (FDS 27402) and fills the
interior and part of the outer-ring area of Bach. The distribution of
these two youngest plains units may indicate that the smooth plains
material as mapped is nothing more than a thin, discontinuous layer of
very smooth plains material that mantles the older units. In this
respect, it is similar to the lunar Cayley Formation, which is probably
basin ejecta. However, unlike plains material of the lunar uplands, no
source basin is evident for the mercurian smooth and very smooth plains
units within the imaged part of the Bach region. Although such a source
basin may lie within the part not imaged, intervening areas do not
contain smooth or very smooth plains materials. For these reasons we
tentatively ascribe a volcanic origin to most of the smooth and very
smooth plains material. The ridges appear to be of volcano-tectonic
origin; the fracturing may have provided the means by which lavas
reached the surface to form these younger plains units. Some very smooth
and smooth plains materials that form the floors of c5 and c4 craters
may be impact melt.

**STRUCTURE**

The map region displays a wide variety of structural features, including
lineaments associated with ridges, scarps, and polygonal crater walls.
Joint-controlled mass movements are most likely responsible for the
polygonal crater-wall segments; segments as long as 100 km suggest that
these fractures extend deep into the lithosphere. The most conspicuous
trends of these lineaments are east-west, N.50°W., and N. 40° E. (fig.
2). More trends are north-south, N.20°E., and N.70°E.

Large ridges and scarps are the most prominent structural features in
the low-sun-angle Mariner 10 pictures of the Bach region. They are most
numerous between long 0° and 90°, where they have no preferred
orientation.

Ridges may have been formed by several processes, including tectonism
and extrusion, or they may be buried crater-rim segments. Several large
ridges may represent uplift of plains materials by normal faulting.
Other ridges are arcuate to circular, which suggests that they are
segments of old, subdued crater and basin rims. Near Boccaccio (centered
at lat 81°S., long 30°), ridges are domical in cross section and have
smooth tops with small irregular or rimless craters along their crests;
they appear to overlap both a c3 and a c1 crater (FDS l66751). In turn,

these ridges are superposed by c3 craters and c4 ejecta. The ridges may
be volcano-tectonic features, composed of extrusives along fissures.
However, they are mapped only as ridges because we cannot determine if
they are volcanic material that should be mapped as a separate unit or
uplifted intercrater plains. These same structures may have been the
source of older plains units.

Lobate scarps are the most common structural landforms in the Bach
region. Almost all have convex slope profiles, rounded crests, and
steep, sharply defined lobes. Three types are seen in the map region:
(1) very small (\<50 km long, \~100 m high), irregular scarps that
commonly enclose topographically depressed areas; they are restricted to
the intermediate and smooth plains units in the eastern part of the map
region; (2) small (\~100 km long, \~100 m high), arcuate or sinuous
scarps, also confined primarily to the intermediate and smooth plains
units in the eastern part of the map region; and (3) large (\>100 km
long, \~1 km high), broadly arcuate but locally irregular or sinuous
scarps whose faces are somewhat steeper. Several of these scarps (lat
83°S., long 80°) deform craters and offset preexisting features
vertically (FDS 166751). The morphology and structural relations of the
scarps suggest that most result from thrust or reverse faults. However,
an extrusive origin has been suggested by Dzurisin (1978) for a scarp
more than 200 km long that extends from about lat 70° S. to the map
border between long 45° and 52°; he based this interpretation on albedo
differences between the two sides of the scarp and on partial burial of
craters transected by it.

Age relations among structural features are not readily apparent. In the
Bach region, the youngest craters cut by a scarp are of c4 age; the
oldest crater to superpose a scarp is a c3. These relations suggest that
scarp formation occurred in c3 to c4 time. Very smooth plains material
flanks some scarps and ridges and, if the material is ponded extrusives
or mass-wasted products, may postdate the structures. Scarps and ridges
are abundant in intercrater, intermediate, and smooth plains units, but
they are not embayed by intermediate and intercrater plains materials.
These relations suggest that the structures began to form after
emplacement of these two oldest plains units. Some of the oldest craters
and basins, such as Cervantes, have polygonal shapes at least as marked
as more recent craters, suggesting that some structural lineaments are
older than c1 craters.

**GEOLOGIC HISTORY**

Murray and others (1975) proposed that Mercury's history could be
divided into five periods: (1) accretion and differentiation, (2)
"terminal heavy bombardment," (3) formation of the Caloris basin
(centered off map sheet at lat 30°N., long 195°; U.S. Geological Survey,
1979), (4) filling of the large basins by "smooth plains," and (5) a
period of light impact cratering. Although these divisions have
withstood well the assessments of subsequent investigators, they do not
define a stratigraphy. Because the geologic map of the Bach region
constitutes a synthesis of observation with interpretation, we shall
explore several aspects of the region's geologic development.

The history of the region begins prior to the formation of any presently
visible surface, when Mercury's internal evolution played a key role in
determining subsequent landform development. Because it is the planet
nearest the Sun, Mercury represents one extreme in possible
cosmochemical models of planet formation. Even before the Mariner 10
mission, Mercury's high density and photometric properties suggested a
large core, presumably iron, and a lithosphere of silicate materials.
Evidence for an intrinsic dipolar magnetic field (Ness and others, 1974)
reinforces interpretations favoring a large core. This core, which
formed partly as a result of radiogenic heating, produced additional
heating, leading to global expansion and the formation of extensional
fractures in the lithosphere (Solomon, 1976, 1977). These fractures may
have provided egress for the eruption of the oldest plains material
during the period of heavy bombardment. Also about

this time other structural lineaments developed, possibly as a result of
stresses induced by tidal spin-down from a more rapid rotation rate
(Burns, 1976; Melosh, 1977; Melosh and Dzurisin, 1978). The major
east-west lineament trend in this polar region (noted in previous
section) conforms to a prediction of Melosh (1977) for the orientation
of normal faults. However, no unambiguous evidence for tensional faults
occurs in the Bach quadrangle.

A population of large, very indistinct, degraded craters, first noted in
stereoscopic images by Malin (1976b), occurs within the oldest
(intercrater) plains material and is thought by most workers to be
coeval with or older than that material. The intercrater unit,
presumably volcanic extrusions through tensional fractures, is the most
voluminous plains material in the map region. Many large c1 and c2
craters have shallow interiors but moderately well preserved rim
features, suggesting that at least some of these craters have undergone
topographic adjustment due to isostatic phenomena (Schaber and others,
1977). This adjustment may have been facilitated by a high-temperature
mantle that was conducive to "crustal plasticity" (Malin 1976b; Malin
and Dzurisin, 1977). The lesser amount of intermediate plains material
indicates decreasing plains formation, some localized within older
basins.

Scarps such as Vostok Rupes (in the Discovery quadrangle adjacent to the
north) are apparently the expression of thrust faults; they suggest that
planetary contraction may have stressed the lithosphere (Strom and
others, 1975) at about the time that c3 craters and smooth plains
material were formed. Following core formation, lithospheric cooling and
consequent contraction may have closed the conduits, restricting
formation of plains material (Solomon, 1977). By c4 time, such formation
was greatly reduced.

Theoretical studies by Melosh (1977), based on observations recorded by
Dzurisin (1978), suggested that tidal spin-down combined with core or
lithospheric contraction could explain many of the tectonic features of
Mercury. The scarps occurring in the polar regions do appear to be the
result of thrust faulting, which substantiates the suggestion that
contraction occurred concurrently with spin-down. Linear structures
(other than some ridges) are thus interpreted to form as a result of
these two active processes. Fracture and lineament patterns around the
Caloris basin (Strom and others, 1975) suggested to Pechmann and Melosh
(1979) that Mercury's despinning period began before global contraction
started and ended during the contraction's early phases.

Plains formation and cratering continued at reduced rates during the
early phases of planetary cooling and contraction. c3 craters are
distinguishable by partial retention of secondary craters and by locally
prominent morphologic features (McCauley and others, 1981). These
characteristics suggest a decreasing rate of resurfacing and of crater
modification (Malin and Dzurisin, 1977). The smaller extent of the
smooth and very smooth plains units, compared with that of older plains
materials, suggests considerable heterogeneity of mercurian crustal
materials. Subcrustal zones of tension may have allowed molten materials
to reach the surface through fractures beneath craters, even during the
period of global contraction (Solomon, 1977). Ridges of domical cross
section cut some c4 craters and, at places, flank areas of young, very
smooth plains material. Thus, possible volcanic extrusions associated
with tectonic activity may have continued into the period of formation
of c4 craters and the oldest very smooth plains material.

The period of tectonic adjustment of the mercurian lithosphere lasted at
least through the time of formation of smooth plains material; c4
craters that formed during this period are cut by scarps and are
superposed on them. Some very smooth plains material, most of which
postdates c4 craters, appears to postdate the scarps that it commonly
embays. Superposition relations of scarps in other regions

of Mercury indicate that tectonic activity may have continued into c5
time (Leake, 1982).

However, the time of formation of c5 craters and very smooth plains
material has, for the most part, been tectonically quiescent. During
this period, with the exception of a scattering of extremely fresh
craters and some minor mass wasting (Malin and Dzurisin, 1977), almost
no geologic activity has occurred near the mercurian south pole. The
youngest smooth plains and the very smooth plains materials that occur
within c5 craters may be impact melts.

**REFERENCES CITED**

Burns, J.A., 1976, Consequences of the tidal slowing of Mercury: Icarus,
v. 28, no. 4, p 453--458.

Dzurisin, Daniel, 1978, The tectonic and volcanic history of Mercury as
inferred from studies of scarps, ridges, troughs, and other lineaments
Journal of Geophysical Research, v. 83, no. B10, p. 4883--4906.

Gault, D.E., Guest, J.E., Murray, J.B., Dzurisin, Daniel, and Malin,
M.C., 1975, Some comparisons of impact craters on Mercury and the Moon:
Journal of Geophysical Research, v. 80, no. 17, p. 2444--2460.

International Astronomical Union, 1977, Working Group for Planetary
System Nomenclature, *in* 16th General Assembly, Grenoble, 1976,
Proceedings: International Astronomical Union Transactions, v. 16B, p.
330--333, 351--355.

Leake, M.A., 1982, The intercrater plains of Mercury and the Moon: Their
nature, origin, and role in terrestrial planet evolution \[Ph. D.
dissertation, University of Arizona, Tucson\], *in* Advances in
Planetary Geology---1982: National Aeronautics and Space Administration
Technical Memorandum 84894, p. 3--534.

Malin, M.C.,1976a, Comparison of large crater and multiring basin
populations on Mars, Mercury and the Moon: Lunar and Planetary Science
Conference, 7th, Houston, 1976, Proceedings, p. 3589--3602.

\_\_\_\_\_\_1976b, Observations of intercrater plains on Mercury:
Geophysical Research Letters, v. 3, no. 10, p. 581--584.

Malin, M.C., and Dzurisin, Daniel, 1977, Landform degradation on
Mercury, the Moon, and Mars: Evidence from crater depth/diameter
relationships: Journal of Geophysical Research, v. 82, no. 2, p.
376--388.

McCauley, J.F., Guest, J.E., Schaber, G.G., Trask, N.J., and Greeley,
Ronald, 1981, Stratigraphy of the Caloris Basin, Mercury: Icarus, v. 47,
no. 2, p. 184--202.

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