G£0 LOGICAL PAPER

on:

utics find Space Administration

Stratigraphy of Part of 

the Lunar Near Side

By DON E. WILHELMS

APOLLO 15 17 ORBITAL INVESTIGATIONS

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1046-A

Prepared on behalf of the

National Aeronautics and Space Administration

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Wilhelms, Don E.

Stratigraphy of part of the lunar nearside

Apollo 15-17 orbital investigations. Geological Survey Professional Paper 1046-A

Bibliography: p. A66-A71

Supt. of Docs. No.: I 19.16:1046-A

I. Lunar stratigraphy. 2. Project Apollo. I. United States. National Aeronautics and Space Administration.

II. Title. III. Series. IV. Series: United States. Geological Survey. Professional Paper 1046-A 

QB592.W52 551.7'00999'1 80-607099

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402
CONTENTS

	Page

Abstract _______________________-____——— Al

Introduction ______________________________ 1

Previous work _________________________— 2

Scope ____________________-_____—— 2

Data sources ___-______________________ 3

Stratigraphic terminology _________________ 3

Acknowledgments _____________—___———- 6

Mare Imbrium and northern Oceanus Procellarum __———_ 6

Introduction __________________________ 6

Superposition relations __________________ 8

Mare ages on basis of small craters ___-__ ___— 14

Flooding of small craters __—————_—_——— 14

Morphology of small superposed craters _ __ ____ 14

Other properties _ ___________ __ ___ — ___ 17

Color ___________-____-_____.___ 17

Radar ________________________.___ 17

Reflectivity ____________________-___ 17

Correlations with time-stratigraphic units ____ ____ 17

Apenninus-Haemus region - ___________ _ __ ____ 19

Terra materials—Imbrium basin _______________ 19

Massif material ______________________ 22

Material of Montes Archimedes ____________ 23

Blocky ejecta ________________________ 23

Lineated and smooth ejecta ___ _______ ____ 23

Grooved material ____________________ 24

Possible impact melt _________________ 25

Slump deposits _____________________ 26

Dark materials-______________________ 26

Mare materials_____________________ 26

Mare and dark mantling material in Montes Haemus 27

Sources ___________________________ 27 

Terra east of Mare Serenitatis, west of Crisium, and north of

Tranquillitatis _________________________ 27

Pre-Imbrian materials __________________ 31

Possible Imbrium effects __________________ 31

Internal versus basin origin of structures _ ___ ____ 31

Craters ____________________________ 34

Page

Taruntius region ————— ——— __ ———— ——— ——————— - A35

Terra units __ ——————— —————— ————————— — — 35

Low albedo of some terra units —— ——— —————— ——— 38

Northern Nectaris basin rim _————— ————— ————— — 39

Theophilus______________—___————————— 39

Orientale analogs - ———————— ——— —————— ——— 43

Secondary impact craters of Imbrium _ ——— _ _ ————— _ 43

Deposits __————— ————— ———————— --————— 45

Post-Imbrium features __———————————————————— 45

Central highlands __-___——— ————————— —— ——— - 45

Grooves _____—_-————————————————————— 46

Subcircular Imbrium secondary craters ——— ————— — _ 46

Other possible secondary craters—————————— — —— - 47

Primary impact craters — ————— ———————— — ——— 47

Descartes mountains — ———— — ——————————— — 51

Other hummocky deposits and hills ————————————— 51

Plains deposits ——— —————— —————— —— ——— — — 52

Southern Oceanus Procellarum _ ————————————— ——— - 56

Fra Mauro Formation__———__ ——————— _- ——— - 56

Mare properties ______——— —————— ———— —— -_ 57

Age __________________—___—____————— 57

Color ___________—_—_____________ 60

Reflectivity _______— ——___ —— — ——— ——— 61

Thickness ____________________———_— 61

Correlations of mare properties —— ——— ———— -———— 62

Age versus thickness _____—__——— ———— - 63

Age versus color _______—___——— —— ——— G3

Color versus reflectivity_________———--——— 63

Reflectivity versus thickness ____—— ———— ———— 64

Color versus thickness ————____ ————— ————— 64

Age versus reflectivity^——————— —————— —— ___ 64

Very red terra _______———___ ————— ————— 64

Summary and conclusions ___ ————— —— ————— ————— 65

Imbrium impact —___——— ———— ————— ——————— 65

Mare materials._______——_—____———— ——— _ 66

References cited ________————__-_————— ——— _ 67

FIGURE 1.

2.

3.

4-7.

8.

9.

10.

11.

ILLUSTRATIONS

Page

Shaded relief map of part of the lunar nearside showing location of text figures - _______________________ A4 

Geologic map of northern Oceanus Procellarum, Montes Harbinger, and Aristarchus plateau ______________ 6 

Geologic map of part of southern Mare Imbrium __ _____ ___ ___________ _________ _______ _ g

Photographs of:

4. Young lava flows in Mare Imbrium _______________________________________________ 10

5. Crater Euler ____________________________________________________ -Q

6. Crater Lambert _____________________________________________________ 12

7. Craters Delisle and Diophantus________________________________________________ 13 

Stereoscopic photographs showing stratigraphic relations between older Eratosthenian mare material and secondary

craters of Timocharis and Eratosthenes ____________________________________________ 15 

Photograph of area in northern Oceanus Procellarum ____________________ ________ _____ ___ __ 16 

Color-difference photograph including region mapped in figures 2 and 3 _______ ________ _____ ______ 18 

Geologic map of terra materials in Apenninus-Haemus region _________________________________ 20

III
IV CONTENTS

FIGURE 12. Stereoscopic photographs of central part of area mapped in figure HA _______________———————————— A22

13. Photograph showing varieties of ejecta and mare features on southeast-sloping flank of Montes Apenninus —_______ 24

14. Photograph of area on flank of Orientale basin analogous to area in figure 13_________"___—————___———_ 25

15. Geologic map of dark materials in parts of Apenninus-Haemus region, Mare Serenitatis, and Mare Tranquillitatis __ 28

16. Color-difference photograph including region mapped in figure 11 _________________———————————— 30

17. Geologic map of eastern Serenitatis, western Crisium, and northern Tranquillitatis region —————————————— 32

18. Stereoscopic photographs showing east-central part of area covered by figure 17A ____—————————— ———————— 34

19. Photograph showing part of area mapped in figure 17 ____________________________________ 35

20. Photograph of area 900 km east-southeast of Orientale basin center _____________———————————————— 36

21. Geologic map of Taruntius region _____________________________________________ 37

22. Stereoscopic photographs showing southwestern corner of area in figure 21 _____________ _____ ______ 38

23. Geologic map of part of northern Nectaris rim _ _______ _____ ___________—————————————————— 40

24. Stereoscopic photographs of part of area mapped in figure 23 ___________________————————————— 41

25. Stereoscopic photographs showing features of crater Theophilus ___________————__ ——— —————— ———— 42

26. Photograph of Orientale secondary craters and basin deposits southeast of Orientale basin ——————— ————— —— 44

27. Geologic map of part of central highlands ___________________________ ____——————————— 48

28. Stereoscopic photographs of part of central highlands _________________—-_ —— ———————————— — 52

29. Photograph of Orientale analogs of many features in study area ___________———————— —————————— — 54

30. Photograph of crater Struve L ______________________________— ———————————————— — 55

31. Photograph of plains deposits in floor of crater Albategnius _______________ ———————————— __————— 55

32. Geologic map of part of southern Oceanus Procellarum _________________ ————————— —————————— 56

33. Stereoscopic photographs of central part of area covered by figure 32A ________——— —————— —————————— 58

34. Stereoscopic photographs of western part of area covered by figure 32B __________————————— —————— ———— 58

35. Stereoscopic photographs of eastern part of area covered by figure 32B _______—— ———————————————— —— 59

36. Color-difference photograph including area mapped in figure 32 _____________———————————— ———— —— 60

37. Histograms showing relations between pairs of properties of mare soil mapped in figure 32 — ————— ————— —— 62

TABLES

Page

TABLE 1. DL values in the areas mapped in figures 2 and 3 _______ ____ _ ___ __ _——— —— ——— _ —————— ———— A17

2. Crater age assignments in the northern Oceanus Procellarum-southern Mare Imbrium region __ ———— ————— __— 19

3. List of mare units mapped in figure 32 _____________—_— ——— —————————— __————————————— 63
APOLLO 15-17 ORBITAL INVESTIGATIONS

STRATIGRAPHY OF PART OF THE 

LUNAR NEAR SIDE

By DON E. WILHELMS

ABSTRACT

The geology of the part of the lunar near side west of longitude 

50°E. photographed by Apollos 15,16, and 17 has been remapped and 

reinterpreted. Emphasis is on the stratigraphy of the mare materials 

and on genetic and stratigraphic interpretations of terra units and 

landforms believed related to the Imbrium basin. The key data sets 

are stereoscopic orbital pictures taken by the three Apollo missions, 

Lunar Orbiter IV photographs of the Orientale basin, Earth-based 

telescopic color-difference images, and the Apollo 16 rock analyses.

In northern Oceanus Procellarum and southern Mare Imbrium, a 

detailed stratigraphic sequence of 20 crater and mare units has been 

determined. Working definitions of the Imbrian-Eratosthenian and 

Eratosthenian-Copernican systemic boundaries based on this sequence are proposed. The sequence is correlated with relative mare 

ages that are calibrated against absolute rock ages, and values of 

3.3±0.1 and «2.3±0.1 billion years, respectively, are estimated for 

the system boundaries.

The terrae of the north-central near side are dominated by the 

rings and continuous ejecta of the Imbrium basin. The crest of 

Montes Apenninus is part of the main rim crest of the Imbrium 

crater of excavation, modified by slumps. Blocky primary basin ejecta lofted or thrust onto the Apennine flank grades outward to 

lineated and smooth ejecta that flowed along the surface and overrode pre-basin features and Imbrium-basin secondary impact craters. Grooves in the overridden high terrain are flow lineations in the 

continuous ejecta blanket. Flowing deposits obstructed by preexisting terrain apparently piled up as small knobs and ridges. Impact melt was deposited on the Apennine flank and inside the basin 

on the Apennine bench; parts of the bench deposits are superposed on 

slump material, indicating slumping immediately after basin formation. The terra east of Serenitatis, west of Crisium., and north and 

east of Tranquillitatis consists of overlapping deposits and intersecting rings of pre-Imbrian basins modifed by superposition of Imbrium 

secondary craters, secondary or primary deposits, and northwest-oriented grooves and other landforms previously attributed to faulting.

In Maria Serenitatis and Tranquillitatis and on bordering terrae, 

the oldest dark units are mare and dark mantling materials that 

appear both red and blue on color-difference photographs. An early 

generalization that darkness and youth correlate for these materials 

is in error, and a later conclusion that lava eruptions progressed in 

time from blue (high titanium) in the east to red (low titanium) in 

the central near side to blue again in the west in Eratosthenian time 

also has many exceptions. Other studies have shown that color and 

reflectivity are both determined by composition in mature regoliths 

derived from mare lavas. Mantling materials are common on the 

terra borders of all maria studied and probably occur within maria as 

well. Many deposits considered mare because of their flatness may be 

pyroclastic accumulations. Sources can be identified for many mantling deposits but not for most maria. Adjacent mare patches occur at

several levels, and so have no common hydrostatic level. Mare or 

mantling materials have flowed from one level to another in several 

localities. Superposition of mantles on terrae gives rise to mixed 

telescopic properties.

The central highlands, northern Nectaris basin rim, and other 

outer terrae were largely shaped by Imbrium ejecta. Conspicuous 

Imbrium-radial grooves appear on the stereoscopic photographs to 

consist of coalescing elliptical secondary impact craters. Other 

grooves and ridges were formed by flow of material along the surface. 

Very few faults were found in the terrae of the entire mapped region. 

With increasing distance from Imbrium, the Imbrium secondary craters appear more circular, more widely spaced, and topographically 

sharper. Many sharp and degraded craters once considered primary 

impact craters here and elsewhere are now believed secondary to 

Imbrium. Complex and diverse landforms including mounds 

seemingly superposed on irregular craters, deposits that bury or 

partly fill craters, and various basin-radial ridges and grooves are 

explained by the nearly simultaneous impact of many closely spaced 

ejecta fragments and by interaction with primary ejecta. Many of the 

pits near the Apollo 16 site are Imbrium secondary craters superposed on Nectaris basin ejecta, but the Descartes mountains material 

at the site could have been emplaced as a flow containing much 

primary ejecta, as suggested by others. Such flows probably produced 

other thick, hummocky deposits including the Fra Mauro Formation 

and also contributed to some of the larger plains deposits in the 

transition zone between continuous Imbrium ejecta and secondary 

features. Distal plains deposits are probably composed mostly of secondary ejecta. Some small plains patches in the central highlands 

could be of Orientale origin.

Southern Oceanus Procellarum includes a complex patchwork of 

mare units and terra islands geologically like the central highlands. 

Color and reflectivity correlate closely for most units, blue mare 

units being dark and red units light. Apparently the regoliths are 

mature, and composition is the only factor controlling reflectivity for 

most units. A few anomalously bright blue units may have been 

contaminated by underlying terra materials that may include fresh 

crystalline fragments. A few anomalously dark red units may have 

been contaminated or may contain titanium-rich pyroclastic glass 

whose optical properties result from a certain degree of devitrification.

INTRODUCTION

This is the first chapter of a series describing investigations by the U.S. Geological Survey based on data 

acquired from lunar orbit, chiefly by Apollo missions 

15, 16, and 17. The stratigraphy of the near side of the 

Moon west of 50° east longitude as determined by

Al
A2 APOLLO 15-17 ORBITAL INVESTIGATIONS

photogeological techniques is the principal topic of this 

chapter. Subsequent chapters, by other authors, describe remote-sensing studies (chapter B), crater geometry (chapter C), and experimental photogrammetry 

(chapter D) and include results from earlier Apollo 

missions as well as the last three.

PREVIOUS WORK

The Apollo orbital photographs provide a close look 

at an area that has been studied geologically for many 

years starting with Gilbert (1893). Astronomers 

Baldwin (1949,1963) and Kuiper (1954,1959) also contributed many geological insights in their general 

lunar studies. Geologic mapping in the space age began 

with three near-side maps by Hackman and Mason 

(1961), followed by an intensive program of detailed 

mapping based on systematic application of stratigraphic principles developed by Shoemaker (1962) and 

Shoemaker and Hackman (1962). Maps at 1:1,000,000 

scale in the area covered by the present study were 

prepared by Marshall (1963), Eggleton (1965), Carr 

(1965, 1966), Moore (1965, 1967), Hackman (1966), 

Morris and Wilhelms (1967), Milton (1968), Howard 

and Masursky (1968), Wilhelms (1968, 1972a), Scott 

and Pohn (1972), and Elston (1972). The telescopic 

phase of this mapping was summarized by Wilhelms 

(1970), and the rest of the pre-Apollo 15 mapping was 

summarized and updated on a near-side map at 

1:5,000,000 scale by Wilhelms and McCauley (1971). 

Numerous other maps and topical studies cover parts 

of the area, including large-scale pre-mission maps 

(Eggleton and Offield, 1970; Carr and others, 1971; 

Milton and Hodges, 1972; Scott and others, 1972). 

Among the achievements of this previous work are (1) 

recognition of the temporal gap between formation of 

the Imbrium basin and Mare Imbrium, (2) correct interpretation of the impact origin of the basin, (3) correct volcanic interpretation of the mare material, and 

(4) correct general prediction of the lithology of the 

rocks collected by Apollos 11, 12, 14, 15, and 17. The 

lunar geologic framework established by these studies 

gave direction to surface exploration by Apollo and 

unmanned missions.

Despite the pre-Apollo photogeologic effort, however, 

many aspects of lunar geology were not understood. 

Predictive failures included (1) reversal by most mappers of the stratigraphic relations between lightcolored mare materials of central Mare Serenitatis and 

the dark mantling and mare materials at its margin, 

(2) misinterpretation of light-colored terra plains 

material, (3) misinterpretation of terra landforms such 

as pits, furrows, and domes, and (4) a failure to predict 

the impact nature of the rocks sampled by Apollo 4.6

that was a consequence of (2) and (3). The first error 

might have been avoided by more persistent application of the principles of superposition and more sophisticated crater counts—entirely within the discipline of 

photogeology. More timely and intensive photogeologic 

work—specifically, on the Orientale basin—might also 

have mitigated the Apollo 16 error by raising alternatives to the volcanic interpretations of the plains and 

other terra landforms. However, the origin of material 

units at the Apollo 16 site probably could not have been 

unambiguously specified by remote methods, and our 

present understanding is dependent on analyses of the 

returned rocks. This reality illustrates the need for interaction between "ground truth" and photogeology, 

which can identify and classify lunar material units, 

determine relative ages, identify problems, and propose multiple working hypotheses, but which can 

rarely prove origins (Greeley and Carr, 1976). When 

origins and absolute ages are learned by onsite exploration, photogeologists can then construct new models 

for testing.

SCOPE

This paper presents a new set of detailed and general 

interpretations for the terrae in the Apollo zone by taking into account the Apollo and Luna sample analyses 

and other information acquired or interpreted since the 

earlier synoptic mapping of the region (Wilhelms and 

McCauley, 1971). Many of the stratigraphic relations 

and landforms observed in the terrae here might still 

be interpreted logically by volcanic or other endogenic 

processes, but the Apollo 16 analyses, in particular, are 

regarded as requiring impact interpretations. No fundamentally new genetic interpretations of the maria 

are required by the sample analyses, but refinements 

in their stratigraphy that have emerged from photogeology and radiometric dating are reported. The paper 

includes more explicit and detailed explanations of 

photogeologic reasoning than do earlier descriptions of 

lunar mapping principles (Shoemaker, 1962; 

Shoemaker and Hackman, 1962; McCauley, 1967; 

Wilhelms, 1970, 1972b; Wilhelms and McCauley, 1971; 

Mutch, 1973; also see Varnes, 1974, for a penetrating 

abstract analysis of the logic of geologic maps in general).

For report purposes the overflown area west of long 

50° E. is divided into seven regions (fig. 1, heavy lines). 

Each is described by one or more geologic maps and 

annotated photographs. Topics of greatest interest 

within the regions are stressed, and the maps vary 

somewhat in methods of portrayal, symbolization, and 

subdivision of units. Because of adequate previous 

treatment, primary impact craters are not subdivided 

extensively by age except in the first region (figs. 2, 3)
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A3

or by material subunit except for the large crater 

Theophilus (figs. 23, 25). The study area includes three 

Apollo landing sites (15, 16, 17) and is near the other 

three (11, 12, 14), but the geology of the sites is not 

stressed because of extensive existing literature. The 

division into regions is based partly on natural geologic 

provinces and partly on the desire to retain the compilation scales for all maps (1:3,300,000 at the equator to 

1:3,800,000 at 30° latitude, good mapping scales for 

lunar studies in my opinion). The black and white reproduction prohibits adequate portrayal of two superposed units, so where dark mantling materials and 

terra materials are both significant, separate maps are 

drawn (figs. 11, 15).

The photographed strips (fig. 1) cover two principal 

features, the volcanic maria and the Imbrium impact 

basin and its flanks. The Apollo 15 and 17 strips of 

photographs on one hand and the Apollo 16 strip on the 

other, although oriented differently and separated 

widely in most of the area, happen to compose a radial 

sample of the Imbrium basin and its periphery. The 

Apollo 15 and 17 strips cover the basin from its buried 

center to a point about 2,000 km from the center except 

for a mare-covered gap. The Apollo 16 strip, which 

makes an obtuse angle with the radial directions, covers the basin periphery from about 1,350 km from the 

basin center in the west to about 2,350 km at the border of Mare Fecunditatis. Inner basin features are described first in this paper and outer, mostly secondaryimpact features in later sections. Mare materials are 

described in detail only in the first, second, and last of 

the seven regions. Part of Mare Tranquillitatis that 

was photographed by Apollo 15 west of long 39° E. is 

not discussed because of a photographic sun illumination too high to provide new information. Summaries 

and conclusions about the detailed regional studies are 

collected at the end of the report.

DATA SOURCES

Vertical stereoscopic metric photographs taken by 

Apollo 15, 16, and 17 mapping cameras are the chief 

but not sole data source for this work. Stereoscopy aids 

in estimating the three-dimensional form of contacts 

and volumes of deposits and in qualitatively assessing 

a scene, to the extent that stereoscopic photographs are 

superior to others when taken vertically (or subsequently rectified) at low-angle illuminations. However, the invaluable Lunar Orbiter IV photographs 

that cover most of the near side monoscopically were 

necessary supplements to the Apollo photographs, 

which cover relatively narrow strips. The Orbiter IV 

photographs provide the regional context that has 

proved vital in lunar studies because of the coarse scale

of the Moon's most significant features. Also, highresolution (100-150 m) Orbiter IV photographs were 

used to fill out small map areas along the edges of the 

Apollo strips (fig. 1) and were helpful inside the Apollo 

coverage, especially where the Apollo photographs 

were taken at sun illuminations higher than about 45°. 

Furthermore, the most important single basis for interpretations of lunar terrae has proved to be the relatively unmodified Orientale basin, photographed by 

Orbiter IV but not Apollo. An attempt was made to 

reinterpret the near-side terrae solely from relations 

exposed within them, but many ambiguities remained 

that were not resolved until comparable features of 

Orientale were examined in detail. Accordingly, this 

report includes numerous photographs of features at 

distances from Orientale that are proportional to distances of similar features from Imbrium.

Apollo panoramic photographs, which have higher 

resolution than the mapping photographs, were used to 

check certain detailed relations but are not well suited 

to areal mapping because of their narrow format.

Important data for studies of the maria are contained 

in color-difference images (Whitaker, 1972a, b, c) prepared by combining Earth-based telescopic photographs taken at two wavelengths—one at the ultraviolet end of the spectrum (0.31 to 0.40 /own with a 

maximum at about 0.37 /um) and the other at the near 

infrared end (0.73 to 0.90 /otm, with a maximum at 

about 0.82 /otm). The resulting image shows the relative 

blueness or redness of surface areas at these two 

wavelengths: relatively blue areas appear dark, and 

relatively red areas are bright. Throughout this paper 

such areas are referred to for simplicity as "blue" or 

"red" although these terms are only relative, as all 

lunar colors are reddish.

STRATIGRAPHIC TERMINOLOGY

Ages of lunar material units are currently defined as 

follows. Pre-Nectarian units are older than the Nectaris basin. The Nectarian System includes materials 

of the Nectaris basin and other, younger units that are 

older than the Imbrium basin (Stuart-Alexander and 

Wilhelms, 1975). The term pre-Imbrian is equivalent 

to pre-Nectarian plus Nectarian. The Imbrian System 

includes materials of the Imbrium basin and other, 

younger units that are older than the Eratosthenian 

System. The adjectival form "Imbrian" refers to this 

time-stratigraphic system or to the corresponding time 

period, whereas "Imbrium" refers only to the basin and 

its materials. The Imbrian-Eratosthenian boundary 

has never been formally defined (Wilhelms, 1970), nor 

has the boundary between the Eratosthenian System 

and the Copernican System, the youngest lunar time-
A4

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APOLLO 15-17 ORBITAL INVESTIGATIONS

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30) and the color-difference images (figs. 10, 16, 36). Illustration covers area between latitudes 40° N. and 25° S. and longitudes 50° E. and 70° W. Base maps ai
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STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE

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Information Center (now part of the Defense Mapping Agency).
A6 APOLLO 15-17 ORBITAL INVESTIGATIONS

stratigraphic system, usually considered to consist of 

materials of rayed craters. New working definitions of 

the Imbrian-Eratosthenian and EratosthenianCopernican boundaries are proposed in the following 

section.

ACKNOWLEDGMENTS

This study was conducted on behalf of the National 

Aeronautics and Space Administration from 1973 to 

1976. Most of the work was done as part of Experiment 

S-222, H. J. Moore, Principal Investigator, under 

NASA contract T-1167B. The regional studies that 

provided important feedback were performed under 

NASA contract W13,130. The work was facilitated by 

the excellent base maps of the LOG series prepared by

the Defense Mapping Agency, which portray the finest 

detail needed for the present purposes with good representational fidelity and locational accuracy. The manuscript benefited greatly from reviews by C. A. Hodges 

and H. J. Moore.

MARE IMBRIUM AND NORTHERN OCEANUS 

PROCELLARUM

INTRODUCTION

The northwesternmost area of the near side photographed by Apollo includes the Aristarchus plateau 

(Moore, 1967), Montes Harbinger, and the surrounding 

mare of Oceanus Procellarum (fig. 2). The unusual feaDETAILED SEQUENCE

Ca Ejecta and secondary-crater

materials of crater Aristarchus 

Csk Secondary-crater materials

of crater Kepler

Cp Materials of crater Pytheas 

Csc Secondary-crater materials

of crater Copernicus 

Edi Materials of crater Diophantus 

Em 3 Eratosthenian mare material,

youngest 

Em 2 Eratosthenian mare material,

intermediate age 

Ede Materials of crater Delisle 

Ee Materials of crater Euler 

Em i Eratosthenian mare material,

oldest

Et Materials of crater Timocharis 

Ese Secondary-crater materials

of crater Eratosthenes 

El Materials of crater Lambert 

Imi Imbrian mare material,

intermediate color 

Ik Materials of crater Krieger 

lmr-2 Imbrian mare material, red,

younger 

Iir»r i Imbrian mare material, red,

older

Idr Dark mantling material, red 

Ip Wall and secondary-crater

materials of crater Prinz 

=== Materials of Imbrium basin

25 N

SO KILOMETERS 

J

EXPLANATION

Mare and dark-mantling materials

Materials of Imbrian and younger craters 

Materials of Imbrium basin rings

UNDIVIDED UNITS

EC Materials of small Eratosthenian craters

Ic Materials of small Imbrian craters

d Dark mantling material

————— Indefinite contact

FIGURE 2.—Geologic map of part of northern Oceanus Procellarum, Montes Harbinger (right), and 

the Aristarchus plateau. Sources of data: Apollo 15 mapping-camera frames 2077-2091,
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A7

tures of this region, studied by a variety of remote sensing techniques (Zisk and others, 1977), include a 

markedly reddish mantle that may be a loosely fragmental deposit, numerous sinuous rilles, and other endogenic features. The part of Mare Imbrium photographed by Apollo (fig. 3), especially its well-developed 

volcanic flow features, has also been extensively 

studied (R. G. Strom, fig. 18 in Kuiper, 1965; Moore, 

1965; Carr, 1965; Fielder and Fielder, 1968; Whitaker, 

1972a, b; Hodges, 1973; Young and others, 1973a, b; 

Schaber, 1973; Schaber and others, 1975, 1976; Boyce 

and Dial, 1973, 1975; Boyce and others, 1975). The 

present study integrates the stratigraphic conclusions 

of these works and adds new observations of mare and 

crater units made on the excellent Apollo photographs.

The terra materials, which are parts of Imbrium basin 

rings exposed in relatively small islands, are not discussed.

The ejecta blanket of each crater is a discrete, laterally continuous layer of material that has a definite 

position in the lunar stratigraphic column. Secondary 

craters and their ejecta form deposits that are contemporaneous with each primary crater. For convenience, 

on most lunar geologic maps, the materials of a group 

of craters are together classed as a geologic map unit. 

For example, materials of all rayed craters are usually 

lumped as crater materials of the Copernican System, 

and all nonrayed post-mare crater materials are assigned to the Eratosthenian System (Shoemaker and 

Hackman, 1962; McCauley, 1967; Wilhelms, 1970). In

25

19 N

44 W

2190-2205, 2334-2350, 2473-2493, and 2741-2751; Apollo 17 mapping-camera frames 

2929-2935 (the part south of lat 25° N.).
A8

32 N

40 W

APOLLO 15-17 ORBITAL INVESTIGATIONS

35

30°

19.5 N

19°N

FIGURE 3.—Geologic map of part of southern Mare Imbrium. Explanation given with figure 2. 

Feature names can be determined from geologic unit symbols and figure 1. Sources of data: 

Apollo 15 mapping camera frames 595-602, 1002-1013, 1144-1160, 1685-1703, 1827-1851,

this report, 12 individual craters larger than 15 km are 

ranked in their proper stratigraphic place among eight 

mare or dark mantling units.

This detailed stratigraphic sequence was determined 

by a series of iterations among various types of data. 

First, the most obvious contacts were established on 

the basis of geometric superposition and transection 

relations among units visible as three-dimensional deposits or readily distinguishable by differing densities 

of small superposed craters. Such contacts amount to 

about half the total. Color-difference photographs and

data on small superposed craters were then consulted 

to locate additional boundaries. When the stereoscopic 

photographs were re-examined, subtle topographic and 

crater-density contacts not previously detected were 

commonly found at these boundaries. The complete sequence is given in the caption of figure 2.

SUPERPOSITION RELATIONS

Central Mare Imbrium contains distinct flow lobes 

that are much like those of terrestrial volcanic flows 

(R. G. Strom, fig. 18, in Kuiper, 1965; Fielder and
31 N

25 W

STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A9

20

15

B

2063-2079, 2174-2192, 2330-2337, 2460-2475, and 2734-2742; Apollo 17 mapping-camera 

frames 2115-2123, 2278-2296, 2714-2735, 2907-2932.

Fielder, 1968; Schaber, 1969, 1973; Schaber and 

others, 1975, 1976; Wilhelms, 1970; Whitaker, 1972a, 

b). The two youngest Eratosthenian mare material 

units, in particular, have many distinct flow fronts that 

are clearly superposed on lobes of the same unit or a 

more heavily cratered older unit (figs. 4-8).

Several elevated and deformed mare patches called 

"kipukas" (Nichols and others, 1974) are embayed by 

obviously younger, flat mare units. Large shieldlike 

kipukas such as that west of Euler (fig. 5) are fractured, whereas the surrounding maria are not. A patch

of old reddish Imbrian mare material (fig. 3B, map 

symbol ImrO seems to follow the raised circular structure Lambert R (Young and others, 1973b; Hodges, 

1973) and to be embayed by younger, flat, blue mare 

material (figs. 3B, 6). The older units, after covering 

the elevations, apparently subsided into the adjacent 

depressions, probably while shrinking during cooling. 

These relations and relations between ridges and sinuous rilles (Young and others, 1973a, b) indicate an extended deformational history of mare materials in the 

region.
A10 APOLLO 15-17 ORBITAL INVESTIGATIONS

>

FIGURE 4.—Young lava flows in Mare Imbrium (white arrowheads). Fine texture visible on most mare surfaces. Secondary craters of 

Copernicus (500 km in direction of black arrow) have cast conspicuous "herringbone" ejecta away from Copernicus. Rim and secondary 

craters of crater Euler partly visible along lower left edge. Apollo 15 mapping-camera frame 1157, sun illumination from right (east) 2° 

above horizontal.

The age of the dark red mantling material in the 

Aristarchus-Harbinger region is better determined by 

superposition relations than by densities of small 

superposed craters, because craters acquire indistinct 

shapes on weak fragmental mantling deposits (Lucchitta and Sanchez, 1975; Zisk and others, 1977).

Moore (1967) assigned a Copernican or Eratosthenian 

age to the mantle because, on telescopic photographs, 

rays of Aristarchus were visible on adjacent units but 

not the mantle. However, Apollo and Lunar Orbiter 

photographs show that subdued Aristarchus secondaries are superposed and that Imbrian mare material
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE All

r

FIGURE 5.—Crater Euler, 27.5 km diameter (E). A tongue of intermediate-age Eratosthenian mare material containing a sinuous rille 

(arrow) cuts across the southern ejecta and isolates a patch of ejecta. Copernicus secondaries (C) are superposed on the Euler ejecta. 

Lobate flows of youngest Eratosthenian basalt containing lava channels truncate western ejecta of Euler (arrowheads), and the ejecta 

around the entire remaining periphery also seems partly inundated by mare material. The rugged hills in the lower left quadrant are 

islands of Imbrium basin rings. North of the hills is an elevated, fractured "kipuka" of old mare material (k). Apollo 17 mapping frame 

2293; sun illumination from right (east), 3° above horizontal.

is younger than most of the red mantle (Zisk and 

others, 1977). Therefore, the mantle is Imbrian.

Ejecta or secondary craters of major impact craters 

are commonly superposed on one mare unit but partly

flooded by another (figs. 5-8). The difference between a 

slightly flooded secondary and an unmodified one 

superposed on the lavas may be subtle and detectable 

only on the best photographs, yet this distinction is
A12 APOLLO 15-17 ORBITAL INVESTIGATIONS

FIGURE 6.—Crater Lambert, 30 km diameter. Ring structure south of Lambert is Lambert R. Ejecta and secondary craters are superposed on 

mare materials in some places (A, B) but flooded by mare in others (C, D). The flooding at C was by thick flows of the intermediate-age 

Eratosthenian mare unit, whereas at D the flows (oldest Eratosthenian mare) were thin and did not obliterate the secondaries. The unit 

at D contains a sinuous rille and possibly small endogenic pits and a volcanic ridge (between letters D and E). A gashlike endogenic 

feature (arrowhead) is in the older unit to the east. Secondary craters at E seem to be outer secondaries of Eratosthenes 375 km to the 

southeast and, like Lambert secondaries, are flooded by the oldest Eratosthenian mare material. Apollo 15 mapping-camera frame 1153; 

sun illumination from right (east) 7° above horizontal.

often required to determine an age sequence. Secondary impact craters are very useful in establishing 

stratigraphic relations because of their wide extent.

The entire periphery of the crater Euler is embayed 

by mare material; its secondary craters and ejecta extend out to nearly a crater diameter but are truncated 

or filled by lava (fig. 5). South of the crater, a tongue of 

young mare material containing a sinuous rille has 

flooded the ejecta. South of the tongue, however, extensive unflooded ejecta and secondaries reappear and
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A13

FIGURE 7.—Craters Delisle (above, 25 km) and Diophantus (below, 18.5 km). The intermediate Eratosthenian mare unit containing a sinuous 

rille between the two craters partly inundates the Delisle ejecta (white arrowheads) but is overlain by Diophantus ejecta (black arrow). 

Secondaries superposed on Diophantus ejecta (C) lie on a ray radial to Copernicus and date Diophantus as older than Copernicus. Part of 

the youngest mapped mare unit lies along the right edge of the photograph (E). Apollo 15 mapping-camera frame 2074; sun illumination 

from right (east) 19° above horizontal.

show that the mare units there (oldest Eratosthenian 

and younger red Imbrian) are older than Euler.

The crater Lambert is embayed by all nearby 

Eratosthenian materials but is superposed on Imbrian 

mare in several places (fig. 6). As the oldest Eratosthenian mare embays Lambert but is overlain by Euler 

secondaries, Lambert is older than Euler.

Diophantus secondaries are superposed on Delisle 

ejecta, establishing Diophantus as the younger crater 

(fig. 7). This relation is confirmed by the fact that a
A14 APOLLO 15-17 ORBITAL INVESTIGATIONS

mare unit (intermediate Eratosthenian) flooded the 

southern and western ejecta of Delisle (as noted by 

Moore, 1965) but did not flood materials of the immediately adjacent Diophantus. Secondaries of 

Diophantus transect a sinuous rille in the part of the 

intermediate Eratosthenian mare unit that floods the 

southern Delisle ejecta. Both craters are superposed on 

the younger Imbrian red mare unit. Secondaries of 

Diophantus are also superposed on the youngest 

Eratosthenian mare unit. Thus the age sequence here, 

from oldest to youngest, is younger red Imbrian mare, 

Delisle, intermediate Eratosthenian mare, youngest 

Eratosthenian mare, Diophantus.

The crater Timocharis is also probably older than the 

old Eratosthenian mare unit that embays Lambert; 

that unit seems to truncate outer secondaries of 

Timocharis (fig. 8). Secondary craters of Eratosthenes 

also overlie the two Imbrian mare units in the 

southeast corner of the map area (fig. SB) but are truncated by the old Eratosthenian unit (fig. 8).

Ejecta of the crater Krieger overlies the younger 

Imbrian red mare and is in turn overlain by the oldest 

Eratosthenian unit and probably the intermediatecolor Imbrian unit (fig. 2JB). Therefore, Krieger is Imbrian in age but younger than the Imbrian crater Prinz 

that formed before the dark, reddish mantle.

Some small craters can also be ranked by relations to 

mare units but are not included in the detailed sequence. For example, the Eratosthenian crater at 30.5° 

N., 21° W. (Carlini B) is superposed on the younger 

Imbrian red mare unit but is embayed by the intermediate Eratosthenian unit. The entire periphery of 

the Eratosthenian crater at 21.5° N., 39.5° W. (Brayley 

C) is embayed, whereas the small adjacent crater 

(Brayley E, unmapped) is completely untouched 

(Neukum and others, 1975a). Four other mapped small 

Eratosthenian craters are superposed on all nearby 

units. Four small mapped craters are embayed by Imbrian mare materials, and so are also Imbrian.

In summary, superposition relations yield the following stratigraphic relations:

Diophantus

Youngest Eratosthenian mare material

Intermediate Eratosthenian mare material

Delisle-Euler

Oldest Eratosthenian mare material

Timocharis-Eratosthenes-Lambert

Imbrian mare material of intermediate color

Krieger

Younger Imbrian red mare material

Older Imbrian red mare material

Dark, red mantling material

Prinz

The ambiguous cases can be resolved by sizefrequency counts of small superposed craters. Neukum 

and Konig (1976) determined that Delisle is younger 

than Euler and Eratosthenes younger than Lambert. 

Other relations found by Neukum and Konig agree 

with those determined here; their complete list in the 

region is:

Aristarchus

Copernicus-Diophantus

Delisle

Euler

Timocharis-Eratosthenes

Lambert

Ambiguities in the results of Neukum and Konig are 

resolved by the superposition of Copernicus secondaries on Diophantus (fig. 7) and apparently, of 

Timocharis secondaries on those of Eratosthenes (fig. 

8). Superposition relations among secondaries add additional craters to the sequence in the Copernican System that cannot be dated by relations with mare materials (Kepler and Copernicus lie outside the mapped 

area):

Aristarchus (very fresh secondaries superposed on 

all units)

Kepler (secondaries superposed on Pytheas)

Pytheas (superposed on Copernicus secondaries)

Copernicus

Diophantus

MARE AGES ON BASIS OF SMALL CRATERS

FLOODING OF SMALL CRATERS

On good photographs many small craters (1 to 8 km) 

with subdued yet distinct rims appear flooded by mare 

material that spared the craters' interiors (fig. 9). Some 

craters are flooded nearly to the brim and look like 

deep rimless pits. According to the criteria of Trask 

(1970) the craters are Eratosthenian; craters of this 

size would be more subdued if Imbrian and would be 

sharper and brighter if Copernican. The encroachment 

by the mare on Eratosthenian craters together with an 

apparently "normal" population of superposed Copernican craters shows that the mare deposits are 

Eratosthenian.

MORPHOLOGY OF SMALL SUPERPOSED CRATERS

The method of Soderblom and Lebofsky (1972) has 

been applied to small craters here and elsewhere to 

determine the relative ages of the underlying surfaces 

(Boyce and Dial, 1973, 1975; Boyce and others, 1975; 

Boyce, 1976; Chapter B of this series). Relative ages 

are expressed as the largest diameter of craters (DL ,
FIGURE 8.—Stratigraphic relations between oldest Eratosthenian mare material and secondary crater of Timocharis (large crater, 34 km diameter) and Eratosthenes. The mare unit, with 

northeast-trending flow texture, truncates Timocharis secondaries (upper arrow). Timocharis secondaries may be superposed on Eratosthenes secondaries (lower arrow). Stereopair, parts of Apollo 15 mapping-camera frames 0600 (right) and 0602; sun illumination from right (east) 2° above horizontal at center of right frame.

01
A16 APOLLO 15-17 ORBITAL INVESTIGATIONS

3 ,."*'

,i; ««*** je? fcj ^#*^&*'..-.-•.<•."•"' '^fc£L ±. art^ *^Cstf

FIGURE 9.—Area in northern Oceanus Procellarum covered by approximately the eastern two-thirds of figure 2A. Ejecta of some of the 

largest craters are embayed by mare material (e), some are superposed on mare material (s, c) and one crater is partly embayed and 

partly superposed (x). The unit which crater x overlies is red mare material of Imbrian age; the embaying unit everywhere is blue 

Eratosthenian mare material. The craters superposed on the blue unit are probably Copernican in age (c). Apollo 15 mapping-camera 

frame 2489; sun illumination from right (east) 4° above horizontal.

meters) that are eroded to an interior slope of one degree by the net accumulated flux of small impacts. The 

units mapped here are closely consistent with ages 

based on the DL values (table 1). A few individual 

measurements within a unit disagree with the average

for the unit. Some of these measured points are so close 

to a mapped contact that the disagreement is probably 

one of exact location; Boyce and his coworkers plotted 

their data points along a standard grid, not at the craters measured. Other anomalous values seem to be
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A17

TABLE 1.—DL values of mare material units (Boyce and others, 1975) 

in the areas mapped in figures 2 and 3

[A few obviously anomalous measurements have been excluded. The uncertainty ranges 

given by Boyce and others (1975) for each measurement average plus or minus 35 and all 

but a few range between 20 and 50]

Mare material unit

Younger red Imbrian

Dark, red mantling material _

D,

._ 140 195

. _ 170-215

_ __ 210-245

255

._ 240-260

— . 270-385

360

Number of points

12

13

21

1

9

16

1

Average D,

165

200

220

Small sample

250

310

Small sample

measured on islands of old surfaces too small to map, 

on secondary impact craters to which the SoderblomLebofsky-Boyce method may not apply exactly, or on 

craters whose ejecta is flooded by a thin unit and whose 

unflooded interior slopes tell the age of an older buried 

unit (fig. 9). A few additional anomalies remain, but 

the match is so close that I am convinced of the validity 

in this region of the crater-morphology technique of 

determining relative ages.

Schaber (1973) reported DL values of the three 

Eratosthenian units as 235±20, 175±5, and 160±5 

meters (his phases I, II, and III). Schaber and others 

(1976) proposed that each of these phases represents a 

rapid and extremely voluminous eruption. The wider 

range of DL values in table 1, however, suggests that 

each unit consists of many discrete flows erupted over a 

long time.

OTHER PROPERTIES

COLOR

Color as displayed by the color-difference photographs taken by Whitaker (in Kuiper, 1965; Whitaker, 

1966, 1972a, b) clearly correlates with mare units in 

this region (fig. 10). There is a progression from relatively reddish to relatively bluish with decreasing age, 

except for the youngest Eratosthenian unit, which is 

slightly more reddish than the next oldest (Schaber 

and others, 1975). The Imbrian unit of intermediate 

color apparently fits the sequence on the basis of 

superposition relations and one DL value. Generally 

the Imbrian units are reddish and the Eratosthenian 

bluish, although small patches apparently of youngest 

Imbrian age in the Aristarchus-Harbinger area are 

bluish (Zisk and others, 1977).

Even where not correlative with age in other areas, 

color is useful in locating contacts and particularly in 

distinguishing between heavily cratered mare units 

and secondary crater fields. Except in the densest secondary fields, continuous color patterns characteristic 

of mare contacts continue across the more-ragged patterns characteristic of secondary fields.

The lava flows that flood the small Eratosthenian 

craters west of the Aristarchus plateau (fig. 9) are 

strongly blue, whereas adjacent surfaces upon which 

similar Eratosthenian craters are superposed are red; 

hence, thin flows are capable of masking underlying 

colors and imparting their own colors to the soil (which 

is the material detected by the photograph). Large 

primary and secondary craters, however, can excavate 

the underlying material and spread it as a halo of different color upon the near-surface lava or soil.

RADAR

Schaber and others (1975) have investigated the correlation between diffuse echoes of Earth-based radar 

transmissions at 3.8- and 70-cm wavelengths with 

geologic units in Mare Imbrium. They find that the 

youngest Eratosthenian flows have the weakest depolarized echoes at the 70-cm wavelength. Some of the 

intermediate Eratosthenian unit also has weak echoes, 

notably the tongue southwest of Lambert (fig. 3JB), and 

this could be part of the youngest Eratosthenian unit. 

West of 40° longitude (fig. 2A), there is a clear distinction of 3.8-cm echoes between the dark, reddish mantle 

and mare materials (Zisk and others, 1977).

REFLECTIVITY

The property of reflectivity was not used as a prime 

mapping criterion in the area. For mare units, it seems 

to correlate with color: blue units are dark, red units 

bright. Therefore the progression here is light to dark. 

The reddish mantle, however, is very dark, apparently 

because of its content of devitrified glass (Zisk and 

others, 1977). These interrelations are explored in the 

discussion of southern Oceanus Procellarum.

CORRELATIONS WITH TIME-STRATIGRAPHIC UNITS

Traditionally, rayed craters have been assigned to 

the Copernican System and unrayed craters superposed on mare materials to the Eratosthenian System. 

More exact definitions of the Copernican-Eratosthenian boundary have never been made. The relations 

younger than Eratosthenes and older than Copernicus 

established here conflict with previous assignments 

based on this scheme (table 2). The bright craters 

Timocharis and Euler are older than the rayless craters Diophantus and Delisle. The nature of the ejecta or 

of the materials impacted by the ejecta presumably accounts for the discrepancy.

In this report three mare units have been dated as 

Eratosthenian and three as Imbrian, consistent with 

traditional assignments, although previous mapping
A18 APOLLO 15-17 ORBITAL INVESTIGATIONS

FIGURE 10.—Area mapped in figures 2 and 3 (outlined) showing color differences between wavelengths of 0.31 to 0.40 /u,m on one hand and 

0.73 to 0.90 fjLm on the other. Dark is blue, light is red. Courtesy of E. A. Whitaker (Whitaker, 1966, 1972b).

has differed in detail (Wilhelms and McCauley, 1971; 

Schaber, 1973). The stratigraphic relations among 

craters and the correlations with DL values both allow 

and require us to re-examine the suitability of these 

assignments, particularly with respect to Timocharis 

and Euler, and Diophantus and Delisle.

The change that would least disrupt assignments 

over the whole Moon while removing inconsistencies 

here is to reassign Timocharis and Euler to the 

Eratosthenian System. Age assignments of mare units

in this area can then remain unchanged, whereas if 

Timocharis and Euler remained Copernican, the 

Eratosthenian maria here and probably elsewhere on 

the Moon would become either Imbrian or Copernican 

as would most craters.

If the desirability of reassigning Timocharis and 

Euler to the Eratosthenian to preserve the importance 

of that system is acknowledged, the definition of the 

Imbrian-Eratosthenian and Eratosthenian-Copernican 

boundaries can be narrowed. Informally the Imbrian-
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A19

TABLE 2.—Crater age assignments in the area (figs. 2 and 3) 

[W/M = Wilhelms and McCauley, 1971]

Crater

Aristarchus

Copernicus 

Diophantus 

Delisle

Euler _

Timocharis 

Eratosthenes

Lambert

Previous 

assignment

Copernican _

Copernican

Reference

.——Moore, 1965; W/M _ _ _—

.— _ Hackman, 1962; W/M __.

Carr. 1965: W/M

_ __Copernican __- _ _ -Shoemaker and Hackman, 

1962; W/M.

Eratosthenian

Eratosthenian

i _ -Eratosthenian

————W/M

Moore, 1965 - .

_ __ W/M 

__ Carr, 1965; W/M _ __ __.

- _ Carr, 1965; W/M _ _

__ - _ Shoemaker and Hackman, 

1962; W/M. 

_ -Carr, 1965; W/M _ - - .

Recommended 

assignment

Copernican.

Copernican. 

-Eratosthenian.

Eratosthenian.

'Moore (1965) designated the materials of Diophantus and Delisle as the Diophantus 

Formation, meaning older than some mare materials but younger than others.

Eratosthenian boundary has been regarded as the top 

of the mare material in roughly the eastern third of 

figure 3B that is overlain by secondary craters of the 

crater Eratosthenes and overlies materials of the Imbrian crater Archimedes (Wilhelms, 1970). This mare 

material, mapped here as red Imbrian of both ages 

(map symbols Imrx and Imr2 ), includes DL values 250 to 

360. Eratosthenes is overlain by mare material that 

has to be considered Eratosthenian (if not Copernican), 

which has DL values of 210 and 230 (oldest Eratosthenian unit, southeast of Lambert, fig. 6). Other large 

stretches of the same mare unit also have these DL 

values, and only three measurements are larger than 

230. Accordingly, I propose that mare units with DL 

values of 230 and smaller be considered Eratosthenian, 

units 250 and larger Imbrian, and units with intermediate values be assigned according to the weight of 

other evidence. All mare units which have been dated 

radiometrically are Imbrian in this scheme, except 

that sampled by Apollo 12 (DL 210±20; Boyce, 1976). 

Craters in the intervening range such as Lambert are 

best retained as Eratosthenian unless proved otherwise.

If the maria here are Eratosthenian, the Eratosthenian-Copernican boundary lies above the youngest 

Eratosthenian unit and has DL values smaller than 

140. No lunar maria1 are known to have smaller DL 

values. Unrayed craters younger than the youngest 

Eratosthenian unit, such as Diophantus, may be retained as Eratosthenian pending more study of superposed craters. Thus Copernicus is the oldest large 

Copernican crater in the vicinity. Assignments of 

moderately bright craters elsewhere on the Moon older 

than Copernicus, such as Langrenus and Theophilus, 

depend on determinations of DL and crater sizefrequency values (Neukum and Konig, 1976).

Approximate absolute ages have been assigned toDL 

values by Boyce (1976) by calibration with ages determined radiometrically on returned samples. Boyce estimates the ages (in billions of years) between DL 140 

and 190 to be 2.6±0.3, between 211 and 240 to be 

3.2±0.1, and between 241 and 260 to be 3.4±0.1. Thus 

the Imbrian-Eratosthenian boundary proposed here of 

between DL 230 and 250 would be about 3.3±0.1 b.y., 

and the Eratosthenian-Copernican boundary of about 

DL 140 or less would be about 2.3±0.1 b.y. or less.

APENNINUS-HAEMUS REGION

TERRA MATERIALS—IMBRIUM BASIN

The terra units of the north-central near side (fig. 11) 

are almost entirely parts of the Imbrium multi-ringed 

circular basin and its flanks. The area overflown by 

Apollos 15 and 17 includes a radial sample of the basin 

from a point on the Apennine bench (informal name by 

Hackman, 1964, 1966) within the main basin rim, over 

that rim (Montes Apenninus), and through a series of 

transitions on the Apennine flank and Montes Haemus 

to the western edge of Mare Tranquillitatis. The impact origin of the basin and the general nature of basin 

materials were established by early work (Gilbert, 

1893; Baldwin, 1949, 1963; Shoemaker and Hackman, 

1962; Hartmann and Kuiper, 1962; Hackman, 1964, 

1966). Remaining questions about emplacement of the 

materials are considered here on the basis of the 

stereoscopic photographs and comparison with the better exposed Orientale basin. Analogs pertinent to Imbrium are also displayed by the large crater Theophilus discussed under the heading "Northern Nectaris 

Basin Rim."

The current general picture of emplacement of basin 

materials is as follows. Terrestrial cratering studies 

indicate that crater rims and presumably basin rims 

are structurally uplifted, partly outthrust, and covered 

with ejecta in a stratigraphic order that is inverted 

from the original order in the target materials (Shoemaker, 1959; Roberts, 1966; Gault and others, 

1968; Moore, 1971, 1976; Gault, 1974). The ejecta 

travels outward from the basin rim in a conical 

curtain, depositing materials closest to the rim first 

and distal materials last (Shoemaker, 1962; Oberbeck, 

1975; Oberbeck and others, 1974, 1975). The close-in 

ejecta is also the last to leave the cavity, hence deposition occurs in reverse order from excavation. The 

close-in ejecta travels relatively slowly along ballistic 

trajectories (Oberbeck, 1975; Oberbeck and others, 

1974, 1975; Morrison and Oberbeck, 1975), by gliding 

on the surface (Chao, 1974, 1977), and by flowing 

(Moore and others, 1974). The outer materials impact 

after long ballistic flight with high velocities, producing secondary craters much larger than the fragments 

of impacting ejecta. Thus there is a transition outward
A20 APOLLO 15-17 ORBITAL INVESTIGATIONS

.5°N

EXPLANATION

Mare material, undivided (see fig. 15)

Materials of Imbrian and younger craters

IMBRIUM BASIN UNITS 

Massif materials — Where exposed or covered 

only by talus

Material of Montes Archimedes

Blocky ejecta — Density, size, and orientation 

of blocks shown diagrammatically

Lineated and smooth ejecta — Density of lines 

indicates degree of development of lineations

Grooved material — Spacing and shape of 

grooves indicated

Planar and ridged deposits believed to be 

impact melt — Barbed lines indicate scarps in 

this material; barbs on higher side

Slump deposits — Arrows extend from inferred 

source to present position. Queried where 

uncertain

Dashed contact divides Apenninus (west) and 

Alpes (east) types of ejecta

Topographic boundary of massifs and Montes 

Archimedes

15

13.5°N

FIGURE 11 —Geologic map of the terra materials in the Apenninus-Haemus region. The Apennine 

Bench is the terra south of Archimedes including Montes Archimedes. Photographic data 

sources: Apollo 15 mapping-camera frames 409-425, 580-597, 988-1004, 1129-1146, 1669- 

1687, 1809-1829, 2143-2170, 2287-2315, and 2696-2721; Apollo 17 mapping-camera frames
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A21

15

MARE 

13.5°NlL VAPORUM

10 N

804-813, 1229-1241, 1510-1522, 1813-1831, 21€0-2115, 2262-2279, 2697-2716, and 

2882-2909; Lunar Orbiter IV high-resolution frames 85, 90, 97, 102, and 109 supplementary 

in southeastern third of area where Apollo photographs were taken at sun illuminations 

higher than 45° above horizontal.
A22 APOLLO 15-17 ORBITAL INVESTIGATIONS

in the resulting deposits from predominantly primary 

ejecta to predominantly local material excavated by 

the secondaries. Many relations are explained by the 

fact that the inner materials override the outer materials, even though deposited earlier, because their 

momentum carries them outward behind the advancing curtain of ejecta.

The principal remaining problems may be grouped 

in two categories: (1) the origin of basin rings, particularly the question of which ring best approximates the 

rim crest and related questions about depth and 

volume of the excavation and ejecta; and (2) the exact 

nature of the trajectories (surface or ballistic) and the 

related question of the proportions of secondary and 

primary ejecta in the basin deposits at any point. Ring 

origin and rim-crest location are discussed briefly in 

the sections on "Massif Material," "Material of Montes 

Archimedes," "Blocky Ejecta," and "Slump Deposits," 

and in the later section on "Theophilus," but they are 

not major themes of this paper. The radial sample of 

Imbrium is favorable, however, for consideration of the 

emplacement processes including the primarysecondary controversy and is a recurring subject in the

paper. Because of the uncertainties, the neutral descriptive term "continuous deposits" (Oberdeck and 

others, 1974; Oberbeck, 1975) is preferable to "primary 

ejecta" even in regions like the Apennine flank where 

exposed secondary craters are scarce and primary 

ejecta must constitute a large proportion of the blocky 

ejecta and lineated deposits.

Further controversy centers on the proportions of 

solid, melted, and gaseous material in the deposits. 

This and earlier studies (Moore and others, 1974) have 

identified impact melt on basin flanks, but much ejecta 

was probably clastic; thus the terms "debris surge" 

(Morrison and Oberbeck, 1975) or "debris flows" 

(Moore and others, 1974) are appropriate. These terms 

embrace primary and secondary ejecta and mixtures of 

the two. The commonly used term "base surge" (for 

example, Lindsay, 1976) is less desirable because it 

implies gas transport, a process not yet shown to be 

important in emplacing lunar ejecta deposits.

MASSIF MATERIAL

The Apennine front is characterized by rugged massive mountains, the Moon's largest (fig. 12). They have

FIGURE 12.—Stereoscopic photographs of central part of area mapped in figure HA including steep front of Montes Apenninus massifs (ap), 

smooth material of Apennine Bench Formation (ab), part of material of Montes Archimedes (ma), head end of Rima Hadley (H), and west 

half of crater Conon (C). Apenninus type of basin ejecta, with basin-concentric structure, covers Apennine flank southeast of Apennine 

massifs. Black arrows show matching terrain of Apennine front and slump masses derived therefrom. Black cross shows intersection of a 

slump mass parallel to mountain front and a later mass elongate perpendicular to front. White arrowheads show probable Imbrium 

impact melt with fissures and flow ridges that was emplaced in gap between slumps and front. Apennine Bench Formation apparently 

subsided into basin-radial depression north of letters ab. Graben (g) apparently cuts mare as well as slump material and indicates 

basinward movement of Apennine bench long after slumping ceased. Apollo 17 mapping-camera frames 2709 (right) and 2711; sun 

illumination from right 32° above horizontal in center of right frame; 30° in left.
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A23

steep, relatively smooth slopes that are bright at high 

sun illumination. Their stratified appearance as reported by the Apollo 15 astronauts is probably a lighting artifact (Swann and others, 1972; Howard and Larsen, 1972; Wolfe and Bailey, 1972). Their massive 

structure indicates that they are parts of the rim of the 

Imbrium crater of excavation (Baldwin, 1963; Hodges 

and Wilhelms, 1976; Wilhelms and others, 1977). The 

amount of upthrust or outthrust pre-Imbrian material 

relative to basin ejecta in the massifs is uncertain 

(Carr and others, 1971). The Apollo stereo photographs 

of the tops of most massifs show textures like those of 

the adjacent ejecta (figs. 11, 12, 13), but the thickness 

of ejecta is not known; it may amount to the entire 

height of the massifs (Moore and others, 1974).

MATERIAL OF MONTES ARCHIMEDES

Montes Archimedes (fig. 12) is a chaotically structured elevated region south of the crater Archimedes. 

Bright, smooth-sloped peaks protrude above jumbled 

material, and textural elements tend to parallel the 

Apennine front. The origin of Montes Archimedes is 

tied to the question of original size of multi-ringed impact basins. For example, Head (1977) proposed that it 

is part of the rim crest of the Imbrium crater of excavation, modified by slumping. Thus Montes Apenninus 

would be external to the original crater, uplifted by 

outward-directed stresses, and the steep frontal scarp 

would be formed by faulting as Montes Archimedes 

and the adjacent shelf moved toward the basin center. 

The alternative preferred here is that the Apennines, a 

far larger structure than Montes Archimedes, approximate the main crater rim. Montes Archimedes would 

thus have been formed inside the crater either as a 

subsidiary crater rim nested inside the main excavated 

cavity (Hodges and Wilhelms, 1976; Wilhelms and 

others, 1977) or as a relatively surficial slump from the 

Apennines.

BLOCKY EJECTA

The most extensive unit is blocky ejecta from the 

Imbrium basin. It dominates the southeast Apennine 

slope (figs. 12, 13), where it is coarse, and also occurs in 

Montes Haemus. Originally it was named the hummocky member of the Fra Mauro Formation (Hackman, 1964, 1966). Later it was divided into the Alpes 

Formation (Page, 1970; Wilhelms and McCauley, 

1971) and material of Montes Apenninus (Wilhelms 

and McCauley, 1971), depending on whether its blocks 

were roughly equidimensional and widely spaced 

(Alpes) or elongate subparallel with the Apennine 

crest and closely spaced. The Alpes Formation was considered either ejecta or fractured bedrock, and the

material of Montes Apenninus as bedrock cut by 

closely spaced imbricate faults. On figure 11, the variations in orientation, spacing, and size of the blocky 

ejecta are indicated diagrammatically by the cross 

symbols. The concentric, more massive Apenninus type 

dominates in the southwest Apennines (fig. 1LA), and 

the knobby Alpes type dominates east of roughly 4° to 

5° east longitude.

The mode of emplacement of the blocky ejecta is still 

not entirely certain. The Apenninus facies apparently 

consists either of ejecta or of outthrust blocks covered 

by ejecta, for it grades outward into deposits that 

clearly have flowed (figs. 11, 13). In either case, its 

coarse structure and a lack of secondary craters 

produced by its impact indicate that it was emplaced in 

short, slow ballistic trajectories or entirely on the surface, in accord with the general picture of emplacement 

of near-rim primary deposits. Material in a comparable 

position at Orientale has textures indicative of deposition and not of structural deformation (fig. 14). The 

Alpes facies seems to be a mix of detached blocks and 

finer material and so is probably entirely depositional. 

Its more chaotic structure may indicate a longer ballistic flight and a different source than for the Apenninus 

facies. If lunar basins consist of nested craters, ejecta 

could leave the inner crater(s) at higher angles than 

from the outer and fall back upon the outer crater's 

ejecta (Oberbeck, 1975). The Alpes might represent the 

inner ejecta and the Apenninus the outer crater's 

ejecta, a relation suggested by McCauley (1977) for 

similar materials at Orientale (although he suggests 

sources closer to the basin center).

Some concentric texture in Montes Apenninus seems 

to have formed by deceleration of ground-flowing deposits (fig. 13, t) as is common at Orientale (fig. 14, t) 

(McCauley, 1968; Hodges, 1972; Moore and others, 

1974; Scott and others, 1977; see also fig. 29, p). Such 

piled-up ejecta commonly resembles sand dunes and 

has been called "deceleration dunes" (McCauley, 1968). 

Similar small, bright, blocky hills and intervening deposits in Montes Haemus (fig. 1LB) lie "downstream" 

from craters and other elevations which seem to have 

been overridden by deposits moving along the surface 

(lat 12° N., long 15° E., and lat 14.5° N., long 19° E., 

south of Auwers). However, these blocks could conceivably have been plucked from the upstream elevations.

LINEATED AND SMOOTH EJECTA

Other ejecta in the region has few or no blocks. Such 

ejecta, where lineated subradially to the basin, has 

been called the Fra Mauro Formation (Wilhelms and 

McCauley, 1971). Other material here interpreted as 

ejecta forms planar or wavy surfaces (mapped in fig. 11 

by an absence or low density of dashes).
A24 APOLLO 15-17 ORBITAL INVESTIGATIONS

FIGURE 13.—Varieties of ejecta and mare features on southeast-sloping flank of Montes Apenninus. Blocks alined upper right to lower left such 

as those between mountain front (ap) and crater Conon (C) are part of the Apenninus facies. More widely spaced knobs (al) are part of the 

Alpes facies (Alpes Formation); here they are alined radially away from basin. At bottom of photographs, knobs seem to be covered by a 

smooth facies (s). Grooves (g) probably were formed by surface flow, and transverse ridges (t) probably from deceleration of flow. Mantle at 

(m) contains narrow fissures that suggest contraction of cooling material; this may be impact melt, as may other smooth ejecta (s). Narrow 

transverse fissures (f) in other ejecta could also be contraction cracks. Dark mantling material (d), D-caldera (arrowhead), and subsidence 

features (arrow) like those in Apennine Bench Formation (fig. 12) are post-Imbrium, mare-related features. Same scale as figure 12. Parts 

of Apollo 17 mapping-camera frames 1821 (right) and 1823; sun illumination from right (east) 13° above horizontal in center of right 

frame, 10° in left.

The lineated ejecta shows clear evidence of flow generally away from the basin (shown by orientation of 

dashes on fig. 11). Some lineations curve around obstacles. Most lineations in this area are narrow, but some 

are distinct grooves (fig. 13, g) as at Orientale (fig. 14, 

g) (Moore and others, 1974, figs. 5 and 7). The lineated 

and smooth ejecta appear to grade from blocky or other 

coarsely structured ejecta at both basins. The gradation could reflect a finer grain size or greater melt content in the outer ejecta (Moore and others, 1974) or a 

greater lateral velocity component during emplacement.

GROOVED MATERIAL

Much of the high terrain beyond about 150 km from 

the Apennine crest is grooved and streamlined.

Grooves, on the average about 2 km apart, are radial to 

the basin and are straight or gently curved. Mountains 

and slopes facing the basin are the most commonly 

grooved features, but some slopes facing away from the 

basin are also grooved. Reflectivity is mostly high, although some occurrences are covered by dark material 

(mapped in fig. 15). Ejecta is banked up against both 

sides of some of the grooved hills.

Evidence in the area is insufficient to preclude either 

an erosional or a depositional origin of the grooves, but 

similar occurrences on pre-Orientale eminences are 

continuations of flow features within the adjacent 

ejecta deposits (fig. 14) and purely erosional grooves 

this close to the basin seem rare at Orientale.

Most eminences in the Apennine-Haemus region lie 

along continuations of rings of the pre-Imbrian 

Serenitatis basin (Wilhelms and McCauley, 1971) and
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A25

FIGUEE 14.—Area on flank of Orientale basin at same position relative to basin rim, similar 

azimuth (east-southeast), similar scale, and similar geology as the Imbrium area in figure 13. 

Crater Eichstadt (E) 50 km diameter. Topographic basin rim of Orientale is Montes Cordillera (co). Concentrically oriented ejecta near basin rim and Eichstadt grades outward into 

smoother, radially lineated ejecta (s) with occasional deep flow grooves (g). Transverse ridges 

of ejecta (t) are piled up where ejecta flow encountered obstacles. Concentric structure of 

inner ejecta may have a similar deceleration origin or may result from low original ejection 

velocities and trajectories. Narrow transverse fissures (f) similar to those of the Imbrium 

area are also present, but their significance is uncertain; they may indicate shrinkage of hot 

impact-melted material in ejecta. Part of Lunar Orbiter IV frame 173-H; sun illumination 

from right (east) 15° above horizontal.

presumably are massifs of that basin, but some are 

Imbrium basin secondaries emplaced only 

momentarily before being overridden by a debris surge 

(Oberbeck, 1975; Wilhelms, 1976). Some occurrences 

are not strongly grooved but consist of single or alined 

knobs ("volcanic domes," Morris and Wilhelms, 1967). 

These knobby features may also have been overridden 

and are included in the grooved unit (fig. 11). The 

greater contrast in albedo and texture between ejecta 

deposits and the eminences relative to the Orientale 

analogs may be a result of erosion of the Imbrium 

ejecta from steep slopes.

POSSIBLE IMPACT MELT

Three types of terrain in the area may be formed by 

material that was melted or partly melted by energy of

tne impact that formed the Imbrium basin. The first is 

the lineated and smooth ejecta discussed above. 

Second, cracks in some wavy surfaces suggest origin as 

shrinkage of molten material (fig. 13, m). Material in 

comparable positions in the Orientale ejecta ("fissured 

facies") is interpreted as impact melt by Moore and 

others (1974).

The third and largest expanse of possible impact 

melt is the planar or wavy material inside the Apennine scarp and around Montes Archimedes (fig. 12). 

This terrain was named the Apennine Bench Formation by Hackman (1964, 1966) and interpreted by him 

and E. M. Shoemaker as impact melt (oral commun., 

1963). Subsequently a volcanic origin was proposed 

(Wilhelms, 1970). Fluid emplacement seems indicated 

by the smooth surface, irregular fractures roughly radial and concentric to the basin that resemble shrink-
A26 APOLLO 15-17 ORBITAL INVESTIGATIONS

age cracks, and gentle, smooth-walled depressions that 

suggest subsidence of viscous fluid into underlying 

cavities (fig. 12, above symbol ab). Volcanism cannot be 

excluded, but an impact origin is more consistent with 

current understanding of Orientale (Scott and others, 

1977) and the lunar terrae in general. Fractured melt 

overlies slumps from the Apennine front (fig. 12), in 

accord with inferences by Dence (1971) that melt in 

terrestrial craters is emplaced after slumping.

SLUMP DEPOSITS

Much evidence suggests that the hilly material immediately northwest of the Apennine front slumped 

from the mountains. The mounds and ridges nearest 

the front roughly parallel the Apennine crest, are most 

massive opposite the largest concavities, and in general can mentally be fit back into the front in jigsaw 

puzzle fashion (figs. 11, 12, arrows). Their finer 

morphology—mostly irregular imbricate structure and 

a few massif-like hills—also matches that in the 

Apennines. Lineations radial to the front are probably 

scour or flow grooves along the direction of movement.

Superposition relations show that the slumps formed 

episodically. Superposition of the impact melt shows 

that major slumping occurred within a short time after 

the basin formed. Near the head of Rima Hadley, a 

massive, linear slump ridge parallel to the front is 

transected by a later hummocky tongue of slump material (Carr and others, 1971) perpendicular to the front 

(fig. 12, black cross). The topography of these intersecting slumps matches "missing" parts of the Apennine 

crest. The later, radial flow apparently was a fragmental mass that travelled farther than the earlier more 

coherent slump. Long-continued movement toward the 

basin center—though probably regional subsidence 

rather than true slumping—is indicated by grabens 

parallel to the Apennine front that cut the mare material as well as the slumps (fig. 12, g).

DARK MATERIALS

The regions of Mare Serenitatis mapped here (fig. 15) 

and to the east (fig. 17) contain typical and longstudied examples of the two major morphologic types of 

lunar dark material, mare material and dark mantling 

material. The returned samples are generally assumed 

to show that mare materials are basaltic lava (overlain 

by regolith derived from the lava by impacts) and that 

the dark mantling materials consist of glassy droplets 

of the same approximate composition as the lava 

emplaced in "fire fountains" (Heiken and others, 1974). 

The darkest of both types occur in the border zone of 

Mare Serenitatis and Mare Tranquillitatis including

the highland rim. The dark border was once thought 

younger than the brighter center (Carr, 1966; 

Wilhelms and McCauley, 1971). The dark mantling 

materials are level (marelike) in places, and this was 

thought to indicate that they overlay the mare. The 

dark mare materials were believed younger than the 

lighter ones on the basis of sparse crater counts and 

analogy with the better understood relations in the 

northern Procellarum-southern Imbrium region discussed above, where the younger mare units are indeed 

the darker. The Apollo photographs show, however, 

that the lighter mare materials of central Serenitatis 

abut and embay the border mare materials and hence 

are younger (Howard and others, 1973). The dark 

mantling materials are also clearly embayed by mare 

materials in most places; the level parts of the mantle 

may overlie old mare material, or they may be thick 

accumulations of dark mantling materials themselves, 

as will be discussed.

MARE MATERIALS

Mare units are mapped (fig. 15) in the eastern twothirds of the area of figure 11 much as in Mare Imbrium and northern Oceanus Procellarum. Most of central Mare Serenitatis is composed of red mare material 

in which 18 DL values ranging from 225 to 320 were 

measured (Boyce and others, 1975). This wide range 

presumably reflects a complex stratigraphy, but in the 

absence of strong color boundaries (fig. 16), only two 

red units (both Imbrian, map symbols Imr, and Imr2 ) 

could be tentatively distinguished in this study.

The marginal zone of Mare Serenitatis appears more 

diverse than the interior. The easternmost unit of the 

margin in this area is the oldest (older Imbrian blue 

mare material) and is a continuation of the materials 

that fill northern Mare Tranquillitatis (basalt of 

Plinius area of Howard and others, 1973; McCord and 

others, 1972; Thompson and others, 1973; Johnson and 

others, 1975). The unit slopes sharply from 

Tranquillitatis toward Serenitatis (arrows) and is 

faulted by grabens. This slope presumably results from 

subsidence of central Mare Serenitatis. Locally faulted 

blue mare materials west of the unit appear to be 

younger Imbrian.

The western margin of Serenitatis includes a unit of 

Eratosthenian blue mare material that was detected 

on the color difference photographs and indicated by 

three low DL values (caption, fig. 15); it was then 

mapped in detail according to embayments of small 

Eratosthenian craters. Another marginal unit that 

could be Eratosthenian (map symbol Elm) is intermediate in color, recalling a unit similar in age and 

color in northern Oceanus Procellarum.
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A27

Mare Vaporum seems to contain the unit of 

Eratosthenian or Imbrian age and intermediate color 

and a similarly colored but clearly Eratosthenian unit 

that embays deposits of the crater Manilius.

Thus the sequence here contains a red-to-blue progression like that in the northern Procellarum-southern Imbrium region, but some young units are not the 

bluest and extensive blue units occur in the old part of 

the sequence. As will be discussed in the section on 

southern Oceanus Procellarum, color and reflectivity 

correlate in mature mare regoliths, and both depend on 

composition of the underlying basalt. This explains 

why the generalization that young lavas are dark could 

not be correctly extrapolated to the present region— 

young lavas are dark only if they happen to be blue 

(titanium rich). Blue old lavas are also dark.

The presence of the old blue materials here and not 

in the northern Procellarum-southern Imbrium region 

has been taken as an indication that erupted lunar 

mare lavas progressed in time from blue in the east to 

red in the center to blue again in the west (western 

Oceanus Procellarum contains many young blue lavas) 

(Soderblom and Lebofsky, 1972; Soderblom and Boyce, 

1976). However, there are numerous exceptions to this 

generalization here and elsewhere including to the 

east in Sinus Amoris, where very old red materials are 

exposed (fig. 17).

MARE AND DARK MANTLING MATERIAL IN 

MONTES HAEMUS

Dark materials were deposited on most of Montes 

Haemus at levels considerably higher than Maria 

Serenitatis and Tranquillitatis (fig. 13). The terra 

along the border of Mare Serenitatis is thickly mantled 

by a deposit called the Sulpicius Gallus Formation 

(Carr, 1966) that grades outward to a thinner blanket 

(fig. 15). Small bright peaks protrude through all but 

the thickest parts of these deposits. The mantle was 

probably deposited even on high ground, for some 

material on steep slopes is streaked with dark as well 

as bright material (the latter presumably derived from 

the underlying bedrock). The thick deposits are distinctly reddish (for example, Lucchitta and Schmitt, 

1974). Unit Idr north of Menelaus is an arched, faulted 

deposit (dark member of the Tacquet Formation; Carr, 

1966) that was included in the blue mare unit called 

basalt of the Plinius area (Howard and others, 1973), 

but is more like the Sulpicius Gallus Formation because it surrounds some bright terra peaks and is reddish (Johnson and others, 1975).

Farther inland are dark "lakes" of Imbrian dark materials arranged in two color belts parallel to Montes 

Haemus: the belt closer to Mare Serenitatis is red or 

intermediate in color, the outer belt blue. Most patches

in both belts appear to be as old as the older blue and 

red mare units along the Serenitatis margin, but the 

stratigraphic relation between red and blue "lakes" is 

uncertain. In addition, a few small patches of intermediate or uncertain color and age are mapped as 

mare, undivided (map unit m), and an elevated, blue, 

young, probably Eratosthenian patch forms a swell 

surmounted by the "D-caldera" (fig. 13) (Whitaker, 

1972d; El-Baz, 1973).

The distinction between mantling and mare materials is unclear in this region. The "lake" materials 

would normally be mapped as mare materials because 

of their overall level surfaces, but they thinly cover 

Imbrium basin materials; in the best low-sun stereoscopic pictures, linear radial structures of Imbrium 

show through. Most patches have sloping contacts that 

also reflect the subjacent topography. Thin, probably 

gradational deposits mantle the adjacent terra. Some 

of these mantles connect "lakes" and seem to have 

flowed from one lake to another (arrows in fig. 15). 

Other dark mantles (mapped as unit d or db) are sufficiently like the adjacent terra, only slightly more 

bluish, as to suggest that their original color was 

weakened by mixture with terra material. Thus all of 

Montes Haemus may have been mantled early in the 

Imbrian epoch of basaltic volcanism by pyroclastic 

deposits—"mare materials" being those which accumulated in sufficient thickness or on suitable flat 

substrate to appear flat.

SOURCES

The region has an unusually large number of irregular craters, rilles, and cones which might have been 

sources for the dark materials. Aratus CA is an irregular crater in the mare (Greeley, 1973). A dark cone is 

perched high on the terra (fig. 15, h, lat 14° N., long 

15.3° E.). The most common possible sources, especially 

numerous in the Sulpicius Gallus Formation, are irregular craters that appear too deep on the stereo 

photographs to be secondary impact craters. The Sulpicius Gallus rilles are likely additional sources for 

much of that formation (Carr, 1966). Some volcanic 

depressions could have formed by passive collapse 

without eruption.

TERRA EAST OF MARE SERENITATIS,

WEST OF CRISIUM, AND 

NORTH OF TRANQUILLITATIS

The origin of the complex terra described here is less 

clear than that of the Imbrium-dominated ApennineHaemus region. Imbrium and several pre-Imbrian 

basins have undoubtedly shaped this region, but their 

relative contributions are uncertain (Wilhelms and 

McCauley, 1971; Wilhelms, 1972a, c, 1973; Scott and
A28 APOLLO 15-17 ORBITAL INVESTIGATIONS

3°E 23° E

15 N
EXPLANATION

Elm

Imbj

Idr

Volcanic hills (h) or depressions

Eratosthenian mare material, blue except intermediate

color south of Manilius - Three DL values 160-215,

avg. 185 

Eratosthenian or Imbrian mare material, intermediate or

unknown color - Three DL values, 200-245, avg. 230 

Imbrian mare material, red, younger — Twelve DL values

225-270, avg. 250; high albedo 

Imbrian mare material, blue, younger - Five DL values

235-265, avg. 260 

Imbrian mare material, red, older - Six DL values 285-320,

avg. 295 

Imbrian mare material, blue, older - No DL values in this

area; faulted; dark 

Dark mantling material, red

db

Dark mantling material, color and age unknown-Line with 

crosses separates thicker (northeast) and thinner mantles 

Dark mantling material, blue, age unknown

Mare material, mostly blue, age unknown Materials of Imbrian and younger craters 

Materials of Imbrium basin, undivided (see fig. 11)

—— Scarp within geological unit; barb on higher side

Dark-haloed crater - Shows that dark material underlies 

crater ejecta

— Scarp and possible flow direction— — Indefinite contact a

FIGURE 15.—Geologic map of dark materials in parts of the Apenninus-Haemus region, Mare 

Serenitatis, and Mare Tranquillitatis. Based on Apollo 15 mapping-camera frames 398-416, 570-588, 977-995, 1119-1137, 1659-1678, 1800-1819, 2029-2048, 2143-2162, 2287-2305, 

2426-2445, 2694-2712; Apollo 17 mapping-camera frames 796-813, 1221-1241, 1502-1522, 

1805-1825, 2091-2108, 2252-2271, 2688-2708, 2882-2901; and color-difference photograph 

reproduced in figure 16.

I

to

CD
A30 APOLLO 15-17 ORBITAL INVESTIGATIONS

FIGURE 16.—Color-difference photograph of region mapped in figure 11 (outlined). Wavelength pairs 0.31-0.40 /jum and 0.73-0.90 fjum. Dark

is blue, light is red. Courtesy of E. A. Whitaker (Whitaker, 1966, 1972b).

Pohn, 1972; Scott and others, 1972; Head, 1974a). The 

dominant northwest-southeast topographic "grain" of 

the terrane (figs. 17, 18, 19) is radial both to Imbrium 

(1,200 km to the northwest) and, in places, to the 

nearby Serenitatis or Crisium basins. This grain and a 

weaker, roughly northeast set of lineations follow the 

most common "lunar grid" directions (Strom, 1964; 

Scott and others, 1975) and have been ascribed to impact rejuvenation of endogenic structures (Scott and 

Pohn, 1972; Scott and others, 1972; Head, 1974a). After 

considering the pre-Imbrium basins, I conclude that 

Imbrium ejecta has played a major role in shaping the 

terrane and that endogenic interpretations are unnecessary. This conclusion relies less on direct observations here, which remain ambiguous, than on observations of the informative Orientale analog and of 

Imbrium features described elsewhere in this report.

The dark nonterra materials are not discussed in detail because they are similar to those described above 

or have been treated extensively in the literature. 

Novel features in the older part of the sequence include 

a heavily cratered red unit (fig. 17, symbol Imr0) in

Sinus Amoris and the dark, blue, probably pyroclastic 

mantling deposit of the Apollo 17 site (Scott and Pohn, 

1972; Scott and others, 1972; Hinners, 1973; Lucchitta, 

1973; Pieters and others, 1973; Thompson and others, 

1973; Muehlberger and others, 1973; Adams and 

others, 1974; Head, 1974b; Lucchitta and Sanchez, 

1975; Wolfe and others, 1975). In addition, reddish 

dark mantling materials were found in this study on 

the Crisium basin rim just east of the mapped area at 

the latitude of Macrobius. A red Eratosthenian mare 

unit apparently is also present (fig. 17, symbol Em). 

Steplike distribution of patches of mare material connected by probable flows starting at elevations substantially above that of the predominant mare level 

was noted in the "lakes" peripheral to Crisium 

(Wilhelms, 1973) and at either side of Sinus Amoris 

(fig. 17, arrows; fig. 19, m). A line of small dark volcanic cones also occurs at a high elevation in the crater 

Tisserand (fig. 19). The Apollo 17 mission returned excellent photographs of other small volcanic features 

(Scott, 1973a; Bryan and Adams, 1973; Young and 

others, 1973b) and of mare ridges and scarps (Hodges,
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A31

1973; Howard and Muehlberger, 1973; Scott, 1973b; 

Young and others, 1973b; Muehlberger, 1974; Lucchitta, 1976). However, Mare Tranquillitatis south of 

Dawes, a diverse area that includes a large sinuous 

rille, disc-shaped filled craters or domes, flow lobes, 

large mare ridges, orthogonal faults in different units, 

and degraded terra islands, was not well photographed.

PRE-IMBRIAN MATERIALS

The pre-Imbrium basins most likely to contribute to 

the topography and lithology of the region are 

Serenitatis, Crisium, and Tranquillitatis (StuartAlexander and Howard, 1970; Wilhelms and 

McCauley, 1971; Wilhelms, 1972a, 1973; Short and 

Forman, 1972; Scott and Pohn, 1972; Head, 1974a). 

The Tranquillitatis basin is very obscure but may have 

contributed to the gross form of the terra. The massifs 

around the Apollo 17 landing site (figs. 17A, 18) are 

part of the most conspicuous ring on the southeast side 

of the Serenitatis basin, which may be double (Scott, 

1972c, 1974; Reed and Wolfe, 1975; Wolfe and Reed, 

1976). Groups of irregular, overlapping craters north of 

the area mapped here are alined radially to the southern Serenitatis basin and are probably its secondary 

impact craters (Wilhelms, 1976). Crisium basin massifs and radial lineations (of possible non-Crisium origin) occur near the eastern map boundary (figs. 17B, 

19), and hummocky terrain west of Proclus (fig. 19) 

that resembles the Alpes Formation of Imbrium may 

be ejecta of Crisium. Otherwise, few features attributable to Crisium have been identified in spite of the 

proximity of the area to this large and relatively 

youthful-appearing basin. Possible reasons are that 

Crisium ejecta was ejected preferentially in other directions or that it is covered here by Serenitatis ejecta. 

This younger age of Serenitatis, though contrary to 

intuition because of its degraded appearance, is supported by superposition relations outside the mapped 

area. Its "old" appearance probably results from 

modification by Imbrium ejecta.

POSSIBLE IMBRIUM EFFECTS

Five general types of small-scale topographic elements oriented northwest-southeast are superposed on 

probable pre-Imbrian terrain: small, irregular pits (p), 

lineations (1), terra mantling material (t), pits intermixed with, or on the Imbrium side of, lineations (pi), 

and lineations and a thin mantle (It) (figs. 17, 18, 19). 

These features are best interpreted by comparison with 

the more continuous exterior features of the Orientale 

basin. Orientale is about three-fourths the size of Imbrium, so a comparison region 900 km from the center

of Orientale was selected (fig. 20). Small pitlike secondary craters (p) and lineations (1, pi) score the preOrientale terrane. Lineations on the southeast side of 

secondary pits (pi) apparently are partly the coalescent 

ejecta of the pits and partly flow features in continuity 

with thick masses of continuous ejecta farther west toward the basin (figs. 26, 29). Isolated lineations (1) far 

from the basin are likely to be secondary ejecta and 

gouges, but origin as flow grooves of continuous ejecta 

is also possible. If the Orientale lineations were more 

degraded or poorly photographed, they might be interpreted as faults. Thus regional context is essential in 

interpretations of landforms. By analogy with the 

Orientale scene, the pits east of Mare Serenitatis are 

interpreted as Imbrium secondary craters, and the 

lineations as textures of either primary or secondary 

ejecta.

The mantles on the pre-Imbrian terrane (units t and 

It) are also probably of Imbrium origin, as are mantling 

and plains deposits that are sufficiently thick to 

obscure the pre-Imbrian topography and that appear 

clearly to be of Imbrian age (units It, Itl, and Ip). 

Orientale is surrounded at a similar distance by mantles and plains in comparable amounts that are gradational with secondary crater ejecta and with the 

continuous ejecta on the basin flanks (fig. 20, same 

symbols). The covered terrane consists either of prebasin crater rims or of secondary craters of Orientale 

itself (fig. 20, c; compare figs. 26 and 29, c). Continuous 

and secondary basin ejecta may appear quite similar, 

and the former may flow outward past secondary craters (figs. 20,26, 29). The Imbrium origin of the features 

east of Mare Serenitatis presumably would be more 

apparent if the zone between there and the 

Apenninus-Haemus region were still exposed like its 

Orientale counterpart (fig. 20).

INTERNAL VERSUS BASIN ORIGIN OF STRUCTURES

Not only fine lineations but also major terra blocks 

and mare-terra "shorelines" formed by steep scarps are 

oriented northwest-southeast (figs. 17, 18, 19). Endogenic interpretations were based on the assumption 

that these gross structures were too large and too 

"squared off" to have been produced by ejecta. 

Moreover, this trend is parallel to the Cauchy rille and 

scarp south of the area, which cut the mare so are presumably endogenic though perhaps impact initiated 

(Wilhelms, 1972a). Yet structures of considerable 

magnitude appear to have been formed around Orientale by accumulations of primary and secondary ejecta 

(fig. 20, X). Hence the large structures of the terra discussed here could be tongues of ejecta from Imbrium 

either alone or superposed on radial ejecta of 

Serenitatis.
A32

27° N

23 E

APOLLO 15-17 ORBITAL INVESTIGATIONS

30°

35°

37 E

26.5 N

FIGURE 17.—Geologic map of the eastern Serenitatis, western Crisium, and northern 

Tranquillitatis region. Based mostly on Apollo 15 mapping-camera frames 383-400,
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A33

26.5 N

37° E

^\\SINUS

\ YAMORIS

15

11

B

2278-2287, and Apollo 17 mapping-camera frames 289-314, 432-453, 1484-1506, 

2671-2692; Lunar Orbiter TV frame H-78 was supplementary.

EXPLANATION

IMBRIUM-RELATED 

UNITS

THICK UNITS OF IMBRI

AN AGE 

Light plains material

Terra-mantling material

Lineated terra-mantling 

material

INFERRED IMBRIUM FEATURES SUPERPOSED 

ON PRE-IMBRIAN 

TERRANE

Lineated

Pitted

Terra mantle

Lineated with terra mantle

Pitted and lineated

"Sculptured hills" (possibly 

Serenitatis ejecta)

Closed or partly closed rim 

crest of secondary crater 

(possibly of Serenitatis)

OTHER UNITS 

(see fig. 15 for other mare units)

Very smooth plains material

— Probably emplaced or 

smoothed by Proclus 

impact

Imbrian mare material, 

intermediate color

Dark mantling material, blue

— Only thickest deposits 

are mapped

Dark mantling material, 

blue, with marelike (flat) 

topography

Imbrian mare material, red, 

very old — Could be terra 

plains material in part

Mare dome material

Materials of Imbrian and 

younger craters

Hilly terrain with no evident

Imbrium influence 

Scarp and possible flow

direction

Apollo 17 landing site
A34 APOLLO 15-17 ORBITAL INVESTIGATIONS

FIGURE 18.—Stereoscopic photographs showing east-central (right-central) part of area covered by figure 17A. Arrowhead indicates Apollo 17 

landing site; d, dark mantling material. Labelled craters are Vitruvius (V), Maraldi (Ma), and Littrow (L). Serenitatis massifs surround 

Apollo 17 site; smaller hills in remainder of picture have been termed "sculptured hills." Rugged, ragged elevation between Vitruvius 

and arrow with shaft at top of picture ("Vitruvius front," Head, 1974a) may be a Serenitatis basin ring. Typical occurrences of mapped 

units (fig. 17) are shown by geologic symbols. Hilly, pitted, and lineated terrain (pi) on rim of Littrow and adjacent terrain; thick lineated 

mantle (Itl) grades to thinner mantle with lineations on rim of crater Maraldi (It). Structure (white line) transverse to Imbrium radials 

helps give region an orthogonal patern. Parts of Apollo 17 mapping-camera frames 444 (right) and 446; sun elevation 18° in center of 

right frame, 14 ° in left. North at top.

Coarse and fine lineations not radial to Imbrium, 

Serenitatis, or Crisium also bound many of the small 

knoblike mounds called "sculptured hills" or "corn-onthe-cob" in Apollo mission terminology (fig. 18; Lunar 

Sample Preliminary Examination Team, 1973). Major 

scarps of the roughly northeast-southwest trend seem 

too irregular to be endogenic and are probably parts of 

pre-Imbrian concentric basin structure like the 

"Vitruvius front" (fig. 18; Head, 1974a). The small hills 

could be Imbrium or Serenitatis ejecta blocked by obstacles, as is common around Orientale (fig. 29). An 

origin as an inner knobby facies of Serenitatis ejecta 

comparable to the Alpes Formation of Imbrium is also 

consistent with the morphology and distribution of the 

small hills (Head, 1974a).

CRATERS

Stereoscopic study and comparison with the periphery of the Orientale basin have drastically revised previous interpretations of the larger, degraded craters in 

this area. Most were previously believed to be primary 

impact craters of pre-Imbrian age (Wilhelms and 

McCauley, 1971; Scott and Pohn, 1972), and their subdued topography and irregular outlines were ascribed 

to burial and deformation by the nearby pre-Imbrian 

basins. Origin by secondary impact of Imbrium and 

perhaps Serenitatis basin ejecta now seems more 

likely. This conclusion is far from obvious and must be 

supported by the Orientale analog, where nearly identical craters of almost certain secondary impact origin 

occur (figs. 20, 26, and 29; discussed further in later 

sections). Morphologies of the Orientale secondaries 

differ greatly because of different degrees of modification by lineated and planar primary and secondary 

ejecta, but the craters are contemporaneous (Wilhelms, 

1976). This wide morphological range around a given 

basin shows that secondaries of different basins may 

also resemble one another.

The recognition of craters as secondary of course 

reduces the number formerly identified as primary and 

leads to revised estimates of impact flux (Oberbeck and 

Morrison, 1976; Wilhelms, 1976). Among the craters 

covered by figures 18 and 19, the only visible craters of 

any size likely to be primary impact craters older than 

Imbrium but younger than the local pre-Imbrian 

basins are Maraldi, which is deep, terraced, and rug-
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A35

FIGURE 19.—Labelled craters are Maraldi (Ma, compare fig. 18), Romer (R), Macrobius (M), Tisserand (T), Proclus (P), Franz (F), and Lyell 

(L). Large maria are Mare Tranquillitatis (MT) and Sinus Amoris (SA); cluster of mare domes is near scale bar; mare patches at high 

elevations, m; secondary impact crater of Imbrium or Serenitatis basin, c. Other letter symbols refer to typical occurrences of geologic 

units or features (compare figs. 17, 18, 20). Large scarps (X) could be Imbrium basin ejecta (compare fig. 20, X). Uncontrolled mosaic of 

Apollo 17 mapping-camera frames 295 (right) and 302; sun illumination from right (east), varies from 20° above horizontal at right edge 

to 6° at left.

ged, and perhaps Tisserand, whose depth is intermediate between depths typical of primary and secondary craters.

TARUNTIUS REGION

The stereoscopic Apollo mapping photographs and 

the re interpretations of terra units forced by the Apollo 

16 results are the impetus for reinterpretations in the 

Taruntius region east of Mare Tranquillitatis previously mapped by the author (Wilhelms, 1972a; 

Wilhelms and McCauley, 1971). Many of the morphological forms and albedo variations in the region were 

previously ascribed to volcanic rocks of different compositions and ages. Because of the high sun illumination of the Apollo photographs, reinterpretations are 

not attempted west of long 39°E. and are partly dependent in the rest of the region on the observations made 

of the better photographed terra north of Mare 

Tranquillitatis discussed above.

TERRA UNITS

On an earlier large-scale map (Wilhelms, 1972a) 

eight geologic units were mapped to express the diverse topography and albedo of the noncrater part of 

the terra. Internal origin was the favored interpretation for four units and was considered possible for the
A36 APOLLO 15-17 ORBITAL INVESTIGATIONS

FIGURE 20.—Area 900 km east-southeast of Orientale basin center. Labelled craters are Vieta (V), Piazzi C 

(P), and Lacroix (L); the latter two are also on figure 26. Geologic symbols 1, p, t, It, pi, Ip, It, and Itl have 

same meaning as in Imbrium-influenced area west of Mare Serenitatis (figs. 17, 18, 19). The pitted and 

lineated terrain (pi) north of Piazzi C is part of the continuous Orientale ejecta, whereas the other, similar 

occurrences and the pitted (p) and lineated (1) terrain are cut off from the continuous ejecta and could be 

secondary in origin. Similar uncertainties between continuous ejecta or secondary origin pertain to the 

thin terra mantling deposits (t, It) and the other deposits (Ip, It, Itl), but all are clearly of Orientale origin. 

Unit c, Orientale secondary craters partly covered by above units. Unit m, mare material. High scarp at 

(X) apparently is formed entirely of ejecta. Part of Lunar Orbiter IV frame 160-H, sun illumination from 

right (east) 17° above horizontal.

others. In the remapping (fig. 21), six units are employed that reflect new evaluations of the landforms.

Lineations radial to the Imbrium basin pervade the 

center of the area. Dense lineations are observed, even
39° E 40°

STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE

45°

A37

50° E

2"S

80 KILOMETERS 

I

IMBRIUM-RELATED UNITS 

THICK UNITS OF IMBRIAN AGE

Light plains material 

Terra-mantling material 

Lineated terra-mantling material

INFERRED IMBRIUM FEATURES SUPERPOSED ON PRE-IMBRIAN TERRANE

Lineated

Pitted

Terra mantle

Lineated with terra mantle

Pitted and lineated

EXPLANATION

I ^"""' I Rim crest of secondary crater of Imbrium basin

OTHER UNITS 

I Imr2 Imbrian mare material, red, younger

I Imri I Imbrian mare material, red, older 

Blue flat dark materials 

Blue dark mantling material

(pl)

Materials of Taruntius; barbed line marks rim 

crest

Hilly terrain with no evident Imbrium influence

Flow direction of blue materials on short steep 

slopes (short arrows)

Flow direction of blue materials on slopes without scarps (long arrows)

Buried contact (unit pl buried)

FIGURE 21.—Geologic map of Taruntius region. Based on Apollo 15 mapping-camera frames 1777-1788, 2010-2018, 

2406-2413, and 2675-2682. Contacts of the Taruntius materials are taken from Wilhelms (1972a); the Apollo 

photographs are less satisfactory for mapping these subtle features than the Lunar Orbiter I low-sun photographs 

used for that mapping.
A38 APOLLO 15-17 ORBITAL INVESTIGATIONS

on high-sun photographs (fig. 22), in mantling deposits 

(unit Itl) and in hilly terrain southeast of numerous 

irregular pits (unit pi). The radial lineations resemble 

those north of Mare Tranquillitatis and elsewhere at 

similar distances from Imbrium, which is centered 

1,950 km to the northwest. Hence it is inferred that the 

mantling deposits are secondary or primary Imbrium 

basin ejecta and that the lineated and pitted deposits 

are Imbrium secondaries and their ejecta which are 

superposed on pre-Imbrian materials (probably of the 

Nectaris and Crisium basins). The patch of lineated 

material that seems to embay the crater Taruntius M 

could be a tongue of Imbrium deposits.

Volcanic or tectonic interpretations are also unnecessary for the irregular craters. Secchi and Taruntius 

L, for example, previously considered endogenic and 

pre-Imbrian because of their shape and degradation 

(Wilhelms, 1972a), in fact resemble other large Imbrium secondary craters at comparable distances from 

the basin.

LOW ALBEDO OF SOME TERRA UNITS

In the Taruntius region and adjacent areas, the 

dichotomy between mare and terra that is distinct on

most of the Moon is blurred. Previous mapping vacillated between mare and terra-mantling designations 

for units having properties intermediate between typical wavy, textured, red, high-albedo terra material and 

flat, smooth, blue, dark mare material (compare the 

maps by Wilhelms, 1972a, and Wilhelms and 

McCauley, 1971). The proposed relation to Imbrium of 

the lineations and pits in the units with intermediate 

properties shows them to be of terra affinity. Thus the 

darkening is probably due to mantling by materials of 

mare affinity. The stereoscopic photographs (fig. 22) 

support the telescopically observed relation (Wilhelms, 

1972a) that the flattest areas are the darkest and the 

rugged areas bright. This relation is consistent with 

superposition of a mantle upon the whole terra followed by shedding from slopes, as observed more 

clearly in Montes Haemus.

This hypothesis is supported by stereoscopic observations of dark materials that are blue on the colordifference photographs (fig. 16). Blue spots and streaks 

appear superposed on an otherwise red background, 

which includes both the main mare material and the 

dark and light terra materials. The blue materials appear to have flowed from Mare Tranquillitatis to Mare 

Fecunditatis across the intervening isthmus of terra

FIGURE 22.—Stereoscopic photographs showing southwestern corner of area of figure 21; S, crater Secchi. Arrows representing inferred flow directions of blue materials same as on figure 21. Parts of Apollo 15 mapping-camera frames 2409 

(right) and 2411 (partial); sun illumination from right (east) 72° above horizontal in center right frame (that is, 18° from 

0° phase angle).
STRATIGRAPHY OF PART OF THE LUNAR NFAR SIDE A39

(arrows, figs. 21, 22). Tranquillitatis is obviously 

higher than Fecunditatis; Defense Mapping Agency 

topographic orthophoto map LTO 61C4 shows a difference of about 200 m. The descent of the blue material 

occurs in a series of steps and cascades. In places it 

forms smooth surfaces that probably represent deep 

pools, while elsewhere it only thinly mantles the terra 

material, darkening it but leaving the various textures 

including rims of superposed craters still visible.

NORTHERN NECTARIS BASIN RIM

The overall terra topography north of Mare Nectaris 

is that of the pre-Imbrian Nectaris basin rim, but origins of the smaller features including diverse craters, 

Hrregulaf pits, and hills (figs. 23, 24) are less obvious. 

Most were previously believed to be volcanic (Elston, 

1972; Wilhelms and McCauley, 1971) but are here interpreted as products of the impact of Imbrium basin 

ejecta. Non-Imbrium features include parts of the Nectaris basin and relatively few large primary impact 

craters. Stereoscopic photographs of the large (100 km) 

crater Theophilus (fig. 25) provide a guide to the Imbrium interpretations, as do further references to the 

Orientale analog (fig. 26). Mare and dark mantling 

materials are inadequately photographed, so are not 

discussed.

THEOPHILUS

Lunar impact craters and basins constitute a continuum of impact features that increase in complexity 

with increasing size (Gilbert, 1893; Baldwin, 1949, 

1963; Stuart-Alexander and Howard, 1970; Hartmann 

and Wood, 1971; Hartmann, 1972; Howard, 1974; 

Hodges and Wilhelms, 1976). Therefore the generally 

accepted interpretations of crater features can be 

applied, with caution, to analogous but more complex 

and puzzling features of basins. The high, rugged central peak of Theophilus (fig. 25, A) probably formed by 

instantaneous rebound after impact in response to excavation of the crater, a mechanism advocated by 

many workers (for example, Baldwin, 1963; Milton and 

Roddy, 1972). The inner rings of basins may form in a 

similar fashion, enhanced in layered target materials 

(Hodges and Wilhelms, 1976; Wilhelms and others, 

1977), although the origin of basin rings is problematical. Rolling plains (B) and hummocky material 

(C) on the crater floor are believed to consist mostly of 

impact melt because of their general resemblance to 

floor materials of the younger crater Tycho that preserve textures indicative of melt origin. The presence 

of abundant impact melt on floors of large craters supports interpretations of hummocky and planar material in Orientale as melt (Moore and others, 1974;

Head, 1974c; McCauley, 1977; Scott and others, 1977) 

and by extension, a similar interpretation of the Apennine Bench Formation advanced earlier in this paper. 

The wall terraces of large craters (D) indicate enlargement of the crater rim immediately after cratering. Hummocky parts of the Theophilus crater wall (E) 

are readily explained as slump features and are probably analogous to chaotic slumps identified along the 

Apennine front.

The Theophilus crater rim also resembles the Imbrium basin rim. Like the Apennine massifs, the immediate rim crest (fig. 25, F) is sharply uplifted, appears smooth at fine scale, and has a high albedo. The 

inner part of the rim flank is a massive, hummocky 

raised ring that terminates at an abrupt escarpment 

(G) at about 25 km or one-half a crater radius from the 

rim. Montes Apenninus has a similar distinct outer 

dropoff, although the raised montes is proportionally 

narrower—125 km wide as compared with a distance 

of 600 km from the basin center.

At the base of the escarpment (G) are pools of material apparently emplaced in a fluidlike state (H, I). The 

contacts of some pools are sharp, whereas others are 

gradational as if a widespread sheet of fluid covered the 

hummocky (presumably mostly clastic) ejecta (J) and 

pooled in depressions. These relations accord with 

studies of fresher lunar craters which indicate the 

plains-forming material is impact melt that flowed into 

place after deposition of the clastic ejecta (Howard and 

Wilshire, 1975). Cracked material interpreted as impact melt was noted on the Orientale (Moore and 

others, 1974) and Apennine flanks, but the abundance 

of these pools at Theophilus and other craters suggests 

that more may be present around the much larger 

basins than can be identified by cracks or other textures (Moore and others, 1974).

Outside the massive rim flank and the pools of impact melt are continuous deposits, secondary impact 

craters, and discontinuous deposits. Clearly these are 

formed by ejecta from the crater, but as noted their 

exact means of emplacement is controversial. The 

morphology of the ridged ejecta closest to the crater (K) 

could result from flow of primary ejecta along the surface. Close to it, however, the ridges form a herringbone pattern (L), demonstrated by laboratory experiments to result from the intersection of ejecta 

i blankets of secondary craters that formed close together and nearly simultaneously (Oberbeck and 

Morrison, 1974). Closely spaced secondary craters with 

well-developed herringbone texture are evident farther 

out (M). Crater interference also produced blocklike 

rim intersections in places (N). Planar deposits that 

are gradational with the herringbone and that therefore probably consist of additional secondary crater
A40 APOLLO 15-17 ORBITAL INVESTIGATIONS

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FIGURE 24.—Stereopairs of area outlined in figure 23. The large craters in the left two frames are Isidorus (left) and Capella (with central peak); part of mare-filled crater Gutenberg at 

right-hand edge (letter U in its mare fill). Letters designating features are the same as in figures 23 and 25; a through g have analogs in figure 26. Apollo 16 mapping-camera frames 422, 

424, 426 (partial), right to left. Sun illumination from right (east) 37° above horizon in center.

I

H
to

FIGURE 25.—Stereopairs showing features of crater Theophilus, To right (east) of Theophilus is crater Ma'dler, 28 km diameter. Apollo 16 mapping-camera frames 427 (partial), 429,

and 431 (partial), right to left. Sun illumination from right (east 1 30 above horizon.

A. Central peak

B. Plains on crater floor

C. Hummocky material on crater floor

D. Wall terraces

E. Hummocky wall material

F. Raised crater rim crest

G. Massive hummocky crater rim one-half crater-radius wide

H. Pool of plains material (probably impact melt) at bottom of slope of (G)

I. Another pool of probable impact melt, less cratered than (H)

J. Smooth ejecta, probably a coating of impact melt like (H) on clastic ejecta

K. Radially ridged ejecta

L. Herringbone pattern of secondary craters

M. Group of sharper secondaries with herringbone

N. Closely clustered secondaries with rugged rim intersections

P. Thin mantle of material on mare surface

Q. Material similar to P but thick and bright enough to obscure nature of underly­ 

	ing unit
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A43

ejecta are also present in this outer regime (P, Q). 

Hence there is clearly a general transition from predominantly primary ejecta deposits on the inner rim 

flank (G) to predominantly secondary deposits at the 

edge of the continuous Theophilus deposits (L-Q), but 

the exact point of transition is not known.

ORIENTALE ANALOGS

Many Orientale features in a comparison area 1,200 

km from the basin center (fig. 26), proportional to the 

1,800 km of the discussion area from the larger Imbrium, resemble outer Theophilus features and landforms superposed on the Nectaris rim. Just as sharply 

defined satellitic craters appear at about one crater diameter (100 km) from the rim crest of Theophilus (fig. 

25, M), concentrations of conspicuous craters appear at 

about one basin diameter (930 km) from the topographic basin of Orientale (Montes Cordillera). Some 

Orientale satellitic craters are readily identified as 

secondary by their moderately shallow depths and irregular outlines (fig. 26, a), like typical Theophilus 

secondaries. Others are morphologically diverse and 

have been interpreted as primary impact craters of different ages (for example, Karlstrom, 1974). The diversity can be explained, however, by mutual interaction 

(Oberbeck and Morrison, 1974) and differential burial 

of secondary impact craters that formed nearly simultaneously (Wilhelms, 1976). For example, rims have 

interacted to produce linear septa between the craters 

(fig. 26, b). Similar interactions occur at Theophilus 

but are harder to detect because of the smaller scale 

(fig. 25, N). At Orientale the typical craters are converted into flat-floored craters through burial by deposits from the basin and from other secondaries (fig. 26, 

c); craters that are located in depressions or are otherwise susceptible to burial are more deeply filled than 

others. Coalescing craters form linear valleys (fig. 26, 

e) or small, diverse clusters (f). Very sharp craters that 

resemble primary impact craters could also be secondary to Orientale (fig. 26, d), perhaps formed by fragments that were ejected at high angles and impacted 

late, as proposed for craters at Tycho (Shoemaker and 

others, 1968). A random distribution of such highangle impacts may explain the difference in crater 

density of two impact-melt pools at Theophilus (fig. 25, 

H, I).

Orientale and Theophilus deposits are also similar. 

Ejecta ridges splay outward from the secondary craters 

of both, although the ridges are more linear at Orientale than the V-shaped herringbone of Theophilus. 

Smooth deposits at the distal ends of the secondaries 

are also probably secondary crater ejecta, local material excavated by secondary impact of basin ejecta and 

redistributed in debris clouds (Oberbeck and others, 

1974, 1975). Previously the extensive outer plains 

around Orientale (for example, fig. 26, g) and Imbrium 

were interpreted as volcanic, but the smaller analogous plains at Theophilus (fig. 25, P, Q) are unlikely to 

be coincidental mantles of post-Theophilus volcanic 

materials.

SECONDARY IMPACT CRATERS OF IMBRIUM

Central to the Imbrium study in this area is a group 

of diverse craters mostly between 10 and 18 km in diameter (figs. 23, 24). Their rim crests are subcircular or 

irregular in plan and range from subdued to sharp in 

profile. Some appear so sharp that the pre-Apollo 16 

investigators thought that they were probably not Imbrium basin secondaries, although this possibility was 

recognized (Elston, 1972). Eggleton's (in Ulrich and 

others, 1980) interpretation that the diverse craters 

are Imbrium secondaries is supported by their similar 

sizes and shapes, their compound elliptical elements, 

and Imbrium-radial orientation of the compound craters, crater clusters, and accompanying lineations.

Although the overall appearance of the craters is 

similar enough to suggest a genetic relation, individual craters vary considerably in detailed morphology, 

and so the relation would not be suspected without the 

Theophilus and Orientale analogs. Some have typical 

secondary-crater morphology (figs. 23 and 24, a) as at 

Orientale (fig. 26, a), but morphologic variability of 

others led to interpretations as endogenic or primaryimpact craters of different ages. Especially misleading 

was the interaction between crater rims (b) and the 

filling by deposits (c). Even sharp, deep, unfilled craters 

(d) once considered young primaries are now thought 

secondary because of their compound nature and associated Imbrium-radial lineations, although this origin 

is not certain either here or at Orientale (fig. 26, d). 

Linear valleys that resemble grabens can be formed by 

overlapping secondary craters (e).

Hills once considered volcanic can also be explained 

by secondary-impact mechanisms. The large compound 

crater (b) on the east wall of Capella (northwest of the 

post-Imbrium crater "R") has a rugged moundlike floor 

that may be formed by interaction of simultaneous craters, or perhaps by landslide of material from the 

Capella rim flank. Clustered craters or irregular pits of 

widely diverse morphology (f) are also probably secondary to Imbrium. These and the associated mounds 

and deposits were once interpreted as endogenic because of their morphologic complexity (Elston, 1972; 

Wilhelms and McCauley, 1971), but similar, better observed features at Orientale (fig. 26, b, f) were produced
A44 APOLLO 15-17 ORBITAL INVESTIGATIONS

by rim interactions. Intercrater septa are radial to Imbrium on the northwest flanks of Isidorus and Gutenberg (fig. 24, f). During nearly simultaneous impact of 

closely spaced, small projectiles, ejecta of the youngest

crater would have buried older craters and collected in 

depressions, accounting for the seeming inconsistency 

that secondary craters can be flooded by their own deposits.
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A45

DEPOSITS

Deposits of Imbrian age occur on much of the northern Nectaris rim. The northern part of the mapped 

area, consisting of plateaus, low hills, and depressions, 

is heavily to lightly mantled (fig. 23, h). Plains deposits 

appear transitional with the mantles (figs. 23, 24). A 

few have sharp contacts and are flat and smooth (fig. 

24, g, at top center). In contrast, little or no mantle is 

observed on rugged terrain including the steepest massifs, a few crater rims, the vicinity of the craters 

Isidorus and Capella, and the terrain east of about 

40.5°E. longitude (the farthest from Imbrium). The deposits are probably local materials that have been excavated and ejected from secondary impact craters as 

in the outer regime of Theophilus (fig. 25, P, Q) and 

Orientale (fig. 26, g). Abundant primary ejecta is unlikely to have reached this far from Imbrium by ground 

flow, because the Nectaris rim would have intercepted 

it. Therefore the deposits are probably ejecta from the 

secondaries or, perhaps, primary ejecta that arrived in 

comminuted form along ballistic trajectories (Chao and 

others, 1975). Imbrium deposits are treated more fully 

in the discussion of the region to the west.

A peculiar knobby or hummocky terrain (fig. 23, i, j) 

is also mantled in places (i) though it appears relatively fresh in a trough cut in the Nectaris basin rim 

(j). This knobby unit resembles the Alpes Formation of 

Imbrium and may be the equivalent Nectaris basin 

ejecta or Imbrium secondary ejecta blocked by the 

trough as observed at Orientale.

POST-IMBRIUM FEATURES

Post-Imbrium primary impact craters in the area, 

besides Theophilus, provide some information useful in 

interpreting crater landforms. The small primary impact crater Capella A (fig. 24, R) has a compound outline apparently due to collapse of part of its rim. The 

primary origin (and Copernican age) of Capella A is 

demonstrated by the line of small sharp secondaries to 

the southwest. Another presumed primary, Capella J

(S), was influenced by the pre-existing terrain; the wall 

of Capella J that formed against the large horseshoeshaped crater is higher than the opposite west wall. 

When degraded, Capella J will probably look like two 

mounds—a high one on the horseshoe crater rim and a 

low one opposite. Thus both Capella A and Capella J 

mimic forms of secondaries.

The peculiar knobby or hummocky fill of the crater 

(T) next to Capella J may be ejecta from the neighboring Imbrium secondaries or from Capella J. Alternatively the raised, structured floor may result from crater floor rebound. Rebounded crater floors are common 

near mare boundaries (Pike, 1971; Schultz, 1976). Cylinderlike uplift may explain the unusual interior of 

the nearby crater Gaudibert (Brennan, 1975). Uplift 

along the mare border is also indicated by a small fault 

between the mare (U) and a higher shelf along the 

terra (V).

CENTRAL HIGHLANDS

The central location and good telescopic coverage of 

the tract described here have exposed it to scrutiny 

since the early days of lunar geologic research (Gilbert, 

1893). The western two-thirds, closest to the Imbrium 

basin, is characterized by "Imbrium sculpture"— 

grooves and ridges radial to the basin that have been 

variously attributed to an "outrush of matter" from 

Imbrium (Gilbert, 1893; also Baldwin, 1949,1963) or to 

Imbriurn-related faulting or volcanism along faults (Shoemaker, 1963, p. 349; Hartmann, 1963, 1964; 

Wilhelms, 1970, p. F15; Scott, 1972a). Extensive plains 

deposits and various small hills or "domes" have been 

interpreted either as related or as not directly related 

to Imbrium. Part of the Fra Mauro Formation is in the 

western part of the area, and in the east is the Apollo 

16 landing site, which was selected in the hope of sampling terra volcanic materials of the plains and of a 

peculiar hilly and furrowed terrane called the Descartes mountains (site selection history by Hinners, 

1972; Muehlberger and others, 1972). The discovery 

that these units are composed of impact breccias (Lunar Sample Preliminary Examination Team, 1972)

FIGURE 26.—Orientale secondary craters and basin deposits southeast of Orientale basin. North arrow (lower right) is on 

dark mare, and light plains fill of crater Schickard, about 190 km diameter. Left-hand arrows point to center of 

Orientale basin, 1,200 km from Schickard, a direction also indicated by obvious linear pattern of the terrain (ejecta 

deposits), a, typical well-formed secondary craters; partly filled by materials of the projectile or ejecta that flowed 

from nearer the basin, b, pairs of typical secondaries whose rim intersections are straight septa. Linear ejecta of the 

secondaries (a, b) has covered other secondaries "downstream" from the basin, c, degraded, irregular secondary 

craters filled by mare materials and by plains deposits composed of secondary or primary Orientale ejecta. d, four 

sharp, deep, conical craters that resemble primary impact craters but could be secondaries, e, linear valleys composed 

of overlapping secondaries, f, cluster of diverse small craters like a-d; all are probably secondaries, g, plains probably 

composed of ejecta of secondaries (a, f). Probable pre-Orientale primary impact craters are not numerous; besides 

Schickard they include Inghirami A (I), Lacroix (L), Piazzi C (P), and Wargentin (W). Central part of Lunar Orbiter 

IV frame 167-H; sun illumination from east (right) 17° above horizontal.
A46 APOLLO 15-17 ORBITAL INVESTIGATIONS

fundamentally altered the course of geologic investigations of the lunar terrae (Howard and others, 1974; 

Ulrich and others, 1980).

Like the terra east of Serenitatis, this tract lies at 

distances from Imbrium intermediate between the 

Apennine-Haemus region, dominated by Imbrium continuous ejecta, and the northern Nectaris basin rim 

and Taruntius regions, in which Imbrium secondary 

features are conspicuous. The morphology here, including a progressive change with increasing distance from 

Imbrium (west to east), is fully consistent with an Imbrium origin for most features, although details of formative processes remain to be clarified—in particular 

the origin of the Descartes terrane and the relative 

importance of primary and secondary (locally derived) 

Imbrium basin ejecta. The role of Orientale basin ejecta in the area is also under debate.

The descriptions and discussions of Imbrium-related 

features are keyed to the geologic sketch map (fig. 27) 

and the sterescopic photographs (fig. 28) by means of 84 

numbered localities ("loc."). The discussions progress 

by topic; the locality numbers progress roughly west to 

east. Features probably younger than Imbrium are lettered. Some of the lettered features and some of the 

pre-Imbrian terrain in the east are discussed only in 

the captions. Again, the Orientale basin (figs. 29, 30) 

provides essential support for the interpretations; an 

attempt to decipher the area by detailed scrutiny of its 

features without reference to Orientale proved unproductive.

GROOVES

The stereoscopic photographs of the area (fig. 28) and 

Lunar Orbiter photographs of the Orientale periphery 

(figs. 20, 26, 29) contain abundant evidence that the 

original ejecta explanation of the Imbrium-radial 

grooves (Gilbert, 1893) is more nearly correct than the 

later endogenic explanations. Most of the larger 

grooves consist of overlapping or coalescent elliptical 

or irregular craters and their ejecta that were undoubtedly produced by impact of ejecta from ballistic 

trajectories (fig. 27, heavy lines; figs. 285, 28C, especially Iocs. 5, 31; figs. 26 and 29, e, h; fig. 31; Head, 

1976). Very few faults were identified even tentatively 

in the entire mapped area (fig. 28, Iocs. B, S).

Other grooves are not craterlike or pitlike. At Orientale, many grooves and ridges were produced by flow 

along the surface of an apparently massive deposit (fig. 

29, j, k, m, p, q; Moore and others, 1974). Some long and 

narrow grooves apparently formed where the deposit 

was free to flow rapidly (j) and grade to smaller grooves 

or to dunelike deposits transverse to the direction of 

flow where it was obstructed (k, p). The flow grooves 

are commonly deflected by subjacent topography (q).

Thus these grooves are part of the continuous deposit 

and are not secondary impact craters, although they 

may contain ejecta of secondary craters that has been 

caught up in the debris flow.

The distinction between ground-flow and ballistic 

grooves is harder to make in the Imbrium area than at 

Orientale because Imbrium is more highly degraded. 

Moreover, continuous-ejecta flow grooves and secondary impact crater chains commonly are juxtaposed by 

flow of the continuous deposit past or over the secondary features (fig. 29, r). Short, closely spaced, pit-like 

grooves radial to Imbrium that resemble secondaries 

pervade the area between about 10° and 18° east longitude (fig. 28Z), Iocs. 64, 65), including the "furrowed 

unit" (Milton and Hodges, 1972) or "Smoky Mountain" 

facies of Descartes terrane (loc. 64) north of the Apollo 

16 site. Some lineated high terrain is so far from the 

Imbrium basin that the grooves are presumed secondary (fig. 28Z>, loc. 70) as at Orientale (fig. 29, h). In most 

cases, Imbrium grooves are similar on diverse terrain 

(fig. 28C, Iocs. 34, 35, 37), and so their origin cannot be 

as readily established on the basis of interactions with 

pre-basin terrain as they can at Orientale. Hence, the 

distribution of flow grooves in the central highlands is 

uncertain.

SUBCIRCULAR IMBRIUM SECONDARY CRATERS

In the east and south parts of the central highlands, 

subcircular craters (fig. 28C, Iocs. 40, 43; fig. 28Z), loc. 

77) are more numerous than grooves. The craters 

resemble the Imbrium secondaries on the northern 

Nectaris basin rim and southern highlands (Scott, 

1972b) but are somewhat more irregular, more highly 

modified by plains and mantling deposits, and more 

complexly intermingled with pits, hills, deposits, and 

other features once interpreted as volcanic (Howard 

and Masursky, 1968; Wilhelms and McCauley, 1971). 

As noted above, individual secondary craters in a cluster can vary widely in morphology (figs. 26 and 29, a-f) 

(Wilhelms, 1976). Younger craters overlap slightly 

older ones (fig. 285, loc. 23). Ejecta cones interfere to 

produce ridges (Oberbeck and Morrison, 1974), and 

ejecta of the whole group is cast away from the basin, 

mantling downstream craters (figs. 26 and 29, 

southwest side of a and b; fig. 28C, Iocs. 40-43; fig. 

28Z), Iocs. 49, 73, 76). Both secondary and primary deposits collect in depressions (fig. 29, c, g, i), and if the 

depression is or contains a secondary crater, the secondary can be filled and thus look much older than its 

immediate neighbor (fig. 29, c; fig. 28, Iocs. 6, 8,11,18, 

20, 26, 28, 39, 40-43, 46, 48?, 49, 50?, 58, 76). Latearriving deposits that do not collect in depressions can 

also modify secondaries (fig. 29, r, s; fig. 28A, Iocs. 1, 3).
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A47

Small pits that may be late Imbrium secondaries appear to modify some larger secondaries (Iocs. 4, 6, 16?, 

18, 49, 74), but few such pits are observed at Orientale. 

Degradation on slopes has contributed to the diversity 

among secondaries (Iocs. 17, 19, 42?, 77).

Intersection of adjacent coeval secondary crater rims 

can produce unusual landforms. A large hill at loc. 43 

(fig. 28C) is typical of highland "domes" interpreted as 

volcanic (Wilhelms and McCauley, 1971). The craters 

were considered Imbrian or pre-Imbrian in age and 

volcanic in origin, and the hills were considered part of 

the igneous complex, inasmuch as no impact process 

could be thought up for them. However, an Orientale 

analog (fig. 30) which also has a laboratory analog 

(Oberbeck and Morrison, 1974, fig. 18) shows that such 

features do form by secondary impact (Wilhelms, 

1976). Interactions of intersecting craters can also produce central peaks (Oberbeck, 1973). The deep 

central-peak crater Zollner (fig. 28D, loc 71) resembles 

a primary impact crater but may have been produced 

by interactions of secondary ejecta (Eggleton, in Ulrich 

and others, 1980), as were other complex details of the 

Zollner chain (loc. 72, 73, 74).

Internal origin was once regarded as the obvious explanation of the puzzling Miiller complex of craters (fig. 

285, loc. 23) (Howard and Masursky, 1968; Wilhelms 

and McCauley, 1971). The three largest Miiller craters 

have diameters and depths typical of Imbrium secondaries and are at the proximal end of a pair of Imbriumradial grooves. The straight chain trending N. 65° W., 

however, makes such an obtuse angle with the Imbrium radial direction as to suggest a different origin. 

The chain has earmarks of a secondary chain including 

parallel lineations, overlap of craters from sharp to 

subdued, and accompanying smooth deposits. A source 

to the east-southeast is indicated, but no source of an 

appropriate age (late Nectarian or early Imbrian judging from morphology) lies in that direction. The chain 

is overlapped by or coeval with Imbrium-radial 

grooves, so an Imbrium-secondary origin is possible in 

spite of the alinement. However, internal origin is supported by alinement with the similar Abulfeda chain 

outside the area, which also lacks an obvious primary 

impact source.

OTHER POSSIBLE SECONDARY CRATERS

Many deep, topographically sharp, high-rimmed 

bright craters are of primary impact origin (for example figs. 27, 28 Iocs. A, C, R, and possibly L), but others 

may be secondary to the Orientale basin. This origin is 

suggested by the orientation of the clusters (F, N) and 

of the herringbone ejecta (N). Size, shape, freshness, 

and superposition on Imbrium features are also consistent with Orientale origin although they do not exclude Imbrium origin (E, G, H, J, numerous craters a 

few kilometers across in the area of fig. 27A). The 

Orientale basin is centered 2,550 km west of the mapped area, comparable to the distance from Imbrium of 

similar though more subdued outer secondaries of Imbrium (fig. 24, a, b, d). Catena Davy (K) may be of 

Orientale origin (Oberbeck and others, 1975), although 

another possible source in the right direction and distance is the Eratosthenian crater Bullialdus. The 

larger compound crater Davy G (L) at the end of the 

chain, however, is too large to be of Bullialdus origin. 

Alternative origins are secondaries of Orientale and a 

multiple primary impact caused by tidal breakup 

mechanisms suggested by Sekiguchi (1970) and Aggarwal and Oberbeck (1974).

Secondary craters of the Imbrian crater Arzachel 

have played a troublesome role in the region. Howard 

and Masursky (1968), Carr (1969), and Wilhelms and 

McCauley (1971) believed that the herringbone ridge 

in the floor of the crater Alphonsus (M) was endogenic, 

but the Apollo photographs clearly show the herringbone and other features typical of a chain of secondary craters. Small, sharp craters superposed on 

other features are also possible Arzachel secondaries. 

Those in the Miiller complex (fig. 28B, Iocs. 23, 24) 

were interpreted as volcanic (Howard and Masursky, 

1968; Wilhelms and McCauley, 1971), and additional 

sharp secondary craters may complicate the stratigraphic relations and morphology in other crater 

groups (fig. 28£, Iocs. 20, 21).

Several degraded craters are believed likely Nectaris 

basin secondaries (figs. 27, 28, Iocs. 14?, 15, 48, 50) on 

the basis of membership in a group of subequal size 

(mostly outside the mapped area; see Lunar Orbiter IV 

photograph H-101) lying about one basin diameter 

from the Nectaris basin rim crest. However, the Nectaris origin of any single crater is difficult to establish 

in this Imbrium-affected region.

PRIMARY IMPACT CRATERS

When the secondary craters from all sources are subtracted, surprisingly few craters remain that might be 

of primary impact origin. Probable primaries younger 

than Imbrium and larger than 8 km are shown with 

the diagonal line pattern (fig. 27). The Orientale and 

Imbrium secondaries reduce the numbers of Imbrian 

and Eratosthenian primary craters once mapped here 

(Howard and Masursky, 1968). Twelve recognized 

probably primaries of Nectarian age have the diameters 21, 24, 30, 31, 32, 33, 36, 38, 47, 100, 115, and 132 

km; no Nectarian primary craters smaller than 21 km 

were recognized. If they exist they are obscured by the
A48 APOLLO 15-17 ORBITAL INVESTIGATIONS

0°

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A

HEOPHILUS

VXX//

B
FIGURE 27.—Geologic map of the part of the central highlands covered by vertical 

photographs taken by the Apollo 16 mapping camera. Geologic symbols as in figure 

23. Numbers and capital letters refer to geologic units or features cross-referenced to 

figure 28 except 14, 52-57 and 78-84, only on figure 27; 22 and 53 omitted. Numbers 

are concerned with the Imbrium basin; letters, post-Imbrium features. Principal 

sources of data: Apollo 16 mapping-camera frames 152-173, 430-449, 966-997, 

1256-1288, 1650-1682, 1945-1977, 2171-2202, 2798-2819, and 2944-2997. Apollo 

16 panoramic-camera frames 4553-4593, 4614-4676, 5371-5408 and Lunar Orbiter 

IV high-resolution frames 84, 89, 96, 101, 108 and 113 supplemented the Apollo 

mapping-camera coverage.

The following numbered and lettered localities also appear on figure 28 except as 

noted.

On figures 28A,B:

1. Thick-appearing hummocky deposits with no visible Imbrium secondary craters.

2. Smooth-appearing hill that may have been overridden by deposits.

3. Deposits and small secondary craters on elevated knobby terrain.

4. (Two localities—large regions west, northwest, and northeast of Ptolemaeus) 

Mantling deposits with grooves visible on terrain of intermediate elevation, 

plains in the depressions. Much of area also finely pitted by more equidimensional craters.

5. (Six localities) TheJiighest terrain, strongly grooved, has few visible deposits.

6. Ragged, grooved, and mantled north-facing depressions that probably are Imbrium secondaries.

7. Islands in mare are grooved pre-Imbrian terrain like that of "mainland."

8. Extensive deposits in lows between secondary grooves.

9. Inner rim material of Alphonsus consists of small hills and Imbrium secondaries, alined radially to Imbrium but less strongly than in Iocs. 5 and 6. 

On figure 285:

10. Southeast rim of Ptolemaeus has a fine knobby texture less strongly indicative 

of Imbrium effects than other nearby terrain.

11. (Two localities) Imbrium secondary craters buried by plains in the floors of 

Albategnius (fig. 31) and Ptolemaeus.

12. Northwest-facing scarp in plains (fig. 31).

13. Sharp grooves on rugged terrain. Plains deposits have flowed to lowest accessible places.

14. Alphonsus B (not on photograph).

15. Elongate plains-filled crater degraded like loc. 6 but radial to Nectaris.

16. Numerous subcircular pits on wall of crater Klein apparently younger than the 

grooves of loc. 13.

17. Imbrium features on walls of Albategnius and Klein blurred by downslope 

movement.

18. Circular crater (Albategnius G) and irregular, smaller crater adjacent on 

southeast are probable Imbrium secondaries.

19. Rim of Ptolemaeus seems relatively unaffected by Imbrium here; Imbrium 

features possibly degraded by downslope movement.

20. Large group of subcircular craters, mostly Imbrium secondaries including Albategnius B, the large elongate crater filled and embayed by plains. The 

sharper craters that extend to crater Hipparchus J (loc. 21) are radial to 

Arzachel and may be its secondaries.

21. Deep crater Hipparchus J of uncertain origin, 

mary or an Orientale secondary, older than the 

20 group (22 omitted).

23. The Miiller complex.

24. Small, equal-sized craters that continue from hig

tion suggesting Arzachel as source.

25. Mantle superposed on Miiller complex.

26. (Three localities) Level plains materials that ov

Arzachel secondaries(?) of the Miiller complex 

ondary that otherwise resembles loc. 18.

27. Wall and rim of Hipparchus has texture of smo

28. Low interior of Hipparchus including some larg

rims are degraded into rounded knobs and pit

deposits.

29. Large, high, level area on which impact featur

nearby Imbrium grooves. 

On figures 285, C:

30. Sharply grooved north-facing slope of about 6°.

31. East rim of Albategnius scored by a large groov

daries (see also fig. 31). 

On figure 28C:

32. Fairly level shelf between north-facing (loc. 30

which Imbrium grooves are less conspicuous t

33. Mounds in crater Hind consisting of Imbrium d

from the walls by impact of Imbrium ejecta.

34. Imbrium grooves and deposits on lowland.

35. Similar grooves on Hipparchus rim.

36. Sharp furrowlike craters transverse to the Imbr

37. Strongly grooved terrain of variable slope.

38. (Two localities) Finely grooved texture represen

secondary ejecta flow (compare fig. 29, m).

39. Deeply filled secondary craters encompassing sm

of ejecta (compare fig. 26, a-c; fig. 30).

40. Ritchey group of secondaries.

41. (Two localities) Thin, finely lineated mantle on 

42. Terrain that seems smoothed either by slope mo

43. Mound and other materials seemingly superpos

pare fig. 30).

44. Hills that are likely composed of ejecta which has 

45. Similar to loc. 44; higher proportion of plains.

46. Andel M (Imbrium secondary?), filled by thick h

On figures 28C, D:

47. Knobby deposits resting against wall of crater 

Nectarian primary) that resemble, in degraded 

Orientale (compare fig. 29, p).

48. Complexity of the large smoothed-off crater sugg

ondary origin. 

On figure 28D:

49. (Two localities) Degraded probable Imbrium se

plains.

50. Plains fill a very degraded central-peak(?) crat

impact or by a coalescence of Imbrium or Nec

51. Thin mantle over hummocks.
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STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A51

swarm of Imbrium secondaries, but regardless, the 

total number of Nectarian primaries here and 

elsewhere is smaller than commonly assumed. The 

same is true of pre-Nectarian primaries (such as 

Ptolemaeus and Hipparchus); many of the smaller craters once considered pre-Nectarian may be secondaries 

of Nectaris and Imbrium (fig. 28C, D, Iocs. 46, 48, 50, 

71).

DESCARTES MOUNTAINS

The origin of the complex of hills, furrows, and craters known as the Descartes mountains (Milton and 

•Hodges, 1972) has long been one of the puzzles of the 

area. They had been interpreted as hummocky ejecta of 

the Imbrium basin (Eggleton and Marshall, 1962), but 

subsequently, endogenic interpretations replaced this 

impact interpretation (Milton, 1968; Wilhelms and 

McCauley, 1971; Milton and Hodges, 1972; Head and 

Goetz, 1972) and led to selection of the base of the 

mountains as the Apollo 16 landing site (Hinners, 

1972).

The returned samples are impact breccias (Lunar 

Sample Preliminary Examination Team, 1972), and 

the regional pattern discussed above shows that the 

landforms must have some relation to Imbrium, as 

originally believed. The reasons for the jumbled orientation of the furrows and pits and their intimate mixture and mutual superposition with hills in the "hilly 

unit" (Milton and Hodges, 1972) or Stone Mountain 

facies of the Descartes terrane (fig. 28D, loc. 66) are 

still problematical. Explanations include deceleration 

and jostling by movement of ejecta along the ground, as 

seen at Orientale (fig. 29, p; Hodges, 1972; Head, 1972; 

Hodges and others, 1973; Hodges and Muehlberger, in 

Ulrich and others, 1980), and Imbrium secondary craters superposed on Nectaris ejecta (Wilhelms, 1972d). 

Hodges and others (1973) proposed that a tongue of 

Imbrium ejecta moved down a trough that affords a 

relatively clear path from the Imbrium basin. Part of 

this deposit has flowed into the crater Descartes (fig. 

28D, loc. 67). This interpretation is consistent with the 

observations of a complex of blocks, striations, mantles, plains, and scoured-looking hills (Iocs. 54-58) tentatively ascribed here to ground flow in the trough. 

Moreover, ridges transverse to the Imbrium radial direction (fig. 28D, Iocs. 61, 66, 76) could be "scum" built 

up by deceleration of deposits flowing along the ground 

from Imbrium. A similar feature in another area (fig. 

28C, loc. 36) lies transverse to the Imbrium radial direction and might have formed by abrupt slowing of a 

ground-flowing deposit (loc. 34). Transverse features 

are common at Orientale (fig. 29, p), though some may 

not originate by deceleration (fig. 29, upper s). Pits in

the proposed Descartes flow (loc. 67) do not contain 

plains deposits like those of the adjacent secondary 

fields (Iocs. 64, 65)—plains that are likely to be secondary ejecta, as will be discussed. The weight of evidence, 

therefore, supports the decelerated-flow origin for the 

Descartes mountains. However, this conclusion is tempered by the general observation that closely packed 

secondary impacts can produce a great variety of features and by the gradation of the Descartes 

morphology with a very likely secondary field (fig. 2SD, 

loc. 64) which continues on high slopes (loc. 69).

OTHER HUMMOCKY DEPOSITS AND HILLS

Other thick-appearing deposits have stratigraphic or 

morphologic relations suggestive of massive flow along 

the surface from points nearer the basin. A thick deposit in Andel M (loc. 46) looks like a tongue of ejecta that 

demolished and flowed over the north rim of that crater 

and banked up against the south wall (Moore and 

others, 1974). Similar relations were observed in the 

crater Julius Caesar north of the mapped area (Morris, 

1964; Morris and Wilhelms, 1967; see also fig. 29, p). 

Knobby deposits in Andel (loc. 47) and Hind (loc. 33) 

might also result from blocked surface flow. Furthermore, surface flow probably emplaced the large, apparently thick hummocky deposit in the northwest corner 

of the area (fig. 27A, loc. 1). This deposit has a rolling 

surface and includes small rather domelike hummocks. 

It resembles both the outermost terra of Montes 

Haemus described above, which lies at the same distance (1,350 km) from the center of the Imbrium basin, 

and the type area of the Era Mauro Formation (see 

section on "Southern Oceanus Procellarum").

In addition to the "hummocks" in the deposits, small 

rounded hills occur in several places. Knobs that are 

surrounded by plains deposits have been given both 

"hot" and "cold" interpretations. The former include 

"volcano or surface expressions of intrusions" (Howard 

and Masursky, 1968; Wilhelms and McCauley, 1971) 

and eruptions from hot plains material (Eggleton and 

Schaber, 1972). "Cold" mechanisms include remnants 

of buried terrain (Howard and Masursky, 1968; 

Wilhelms and McCauley, 1971) and fragmental debris 

(Eggleton and Schaber, 1972), presumably entrained 

in the impact ejecta flow.

Igneous or quasi-igneous origin is not necessary to 

explain the hills. Many hills (fig. 28C, D, Iocs. 39, 44, 

45, 59, 60) occur near grooves and apparently scoured 

surfaces (fig. 27, Iocs. 54-57), suggesting override by a 

ground-hugging flow. They might be transported 

blocks, but in the Orientale analog small hills are composed of ejecta that accumulated against obstacles (fig. 

29, p) or in secondary craters (fig. 29, a, c; fig. 30).
A52 APOLLO 15-17 ORBITAL INVESTIGATIONS

B

FIGURE 28.—Stereoscopic Apollo 16 mapping-camera photographs of part of the central highlands; sun illumination from right (east). A, 

frames 1676, 1678, 1680 right to left; sun elevation 15° in center. B, frames 1667, 1669, 1671 right to left; sun elevation 24° in center. C,

PLAINS DEPOSITS

In much of the area, rugged terrain is grooved or 

pitted by secondaries, and adjacent depressions are 

filled by smooth deposits (fig. 28, Iocs. 3-8, 13). Some 

deposits have nearly planar surfaces and appear at 

least moderately thick (Iocs. 11, 13, 15, 26, 39, 58, 61, 

63); more commonly they have wavy surfaces but are 

thick enough to conceal Imbrium grooves and other 

structures (Iocs. 8, 28, 40, 43, 45, 48, 49, 51-54, 60, 75, 

76, 80) or do not conceal underlying features but only 

"blur" them (25, 62, 68, 81). The planar and wavy deposits seem intergradational.

The prevalence of plains and the necessity to interpret the impact breccias sampled by Apollo 16 in typical plains deposits have led to considerable discussion 

of the origin of the plains. Imbrian-age plains material 

was first identified as an outer facies of Imbrium ejecta 

(Eggleton and Marshall, 1962; Eggleton, 1964, 1965) 

and later separated from the ejecta unit as the Cayley 

Formation to avoid premature interpretations (Wilhelms, 1965, 1970). Volcanic interpretations supported before the Apollo 16 mission by relations in 

Alphonsus (fig. 27, Q), where light plains and volcanic 

dark mantling materials seem intergradational (Carr, 

1969; McCauley, 1969). Howard and Masursky (1968)
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A53

D

frames 1269, 1271, 1273 right to left; sun elevation 21° in center. D, frames 0437, 0439, 0441 right to left, sun elevation 18° in center. See 

figure 27 caption for explanation of symbols.

and mappers of other areas (summarized by Wilhelms, 

1970) recognized the gradation of the plains with 

mantling material on adjacent higher ground and interpreted the different topographic expressions as the 

result of ponding and differential compaction of a deposit that fell upon the entire area. Ash-flow tuff was 

the favored material because of its light color and 

propensity to differential compaction, though basin 

ejecta was not excluded (Howard and Masursky, 1968). 

Plains in the large craters Ptolemaeus, Albategnius, 

and Hipparchus transect the Imbrium grooves in the 

crater walls, a stratigraphic relation suggesting that 

the plains deposits are younger than the Imbrium

basin (summarized by Wilhelms, 1970). Probable Imbrium secondaries also formed inside the craters and 

were subsequently buried by the surface deposits (fig. 

28, Iocs, 11, 28; fig. 31); Hodges and Muehlberger (in 

Ulrich and others, 1980) estimated a thickness of 200 

m or less for the deposits. These superposition relations 

seemed more easily explained by volcanic than by 

basin-ejecta origins. Nevertheless, impact origins now 

seem required for most plains, as originally suggested 

(Gilbert, 1893; Eggleton and Marshall, 1962), although 

volcanism is still favored by some workers for lightly 

cratered deposits such as those in Albategnius 

(Neukum and others, 1975b).
A54 APOLLO 15-17 ORBITAL INVESTIGATIONS

It has been suggested that the stratigraphic hiatus 

implied by the transection of Imbrium sculpture and 

supported by counts of superposed craters on the plains 

and adjacent highlands (Soderblom and Boyce, 1972) 

and young ages of some plains (Neukum and others, 

1975b; Boyce and others, 1974, 1975) was caused by 

deposition of Orientale ejecta on Imbrium ejecta (Chao 

and others, 1975; Hodges and Muehlberger, in Ulrich

and others, 1980). According to this hypothesis, Orientale deposits have "reset" the ages based on smallcrater morphology and density. Some support for the 

idea was found in this study. Plains embay two craters 

that seem too deep and sharp to be Imbrium secondaries that close to the basin and thus could be postImbrium primary impact craters or Orientale secondaries (fig. 2&B, Iocs. 21, N). Other possible Orientale 

secondaries were mentioned above including a peculiar 

pair, embayed by plains (H) and alined along an Imbrium groove, that is sharper than most other Imbrium 

secondaries. A post-Imbrium age of some plains is also 

indicated by several level pools which are cratered by 

only very sharp, presumably young craters and which 

apparently embay Arzachel secondaries (fig. 2&B, loc. 

26). Such plains could be emplaced by the Orientale or 

Arzachel secondaries. They could also be primary 

Orientale ejecta, because fine-grained ejecta might accumulate without significant cratering and re-ejection 

of local material (Morgan and others, 1974; Chao and

•^FIGURE 29.—Orientale analogs of many features in the study area.

Part of Lunar Orbiter IV frame H-172. Sun illumination from

right 16° above horizon. Overlaps with figure 26; letters a-g and w

identify the same or similar features as on that figure. Letters 1, o,

t, u, and v omitted.

a. Subcircular secondary craters

b. Intersecting secondaries

c. Subcircular secondaries flooded by smooth ejecta. These craters could be contemporaneous with a and b craters but were 

subjected to burial because of their location in depressions

d. Apparent post-Orientale primary impact crater, but could be 

Orientale secondary

e. Groovelike chains of overlapping secondaries

f. Clusters of small intersecting secondaries

g. Crater Phocylides filled with smooth deposits that are probably mostly secondary ejecta

h. High crater rim scored by secondary grooves

i. Inghirami A, a pre-Orientale primary crater that has survived 

with relatively little damage except floor flooding by sharpbordered smooth ejecta

j. (near upper left edge) Grooves formed within continuous, 

mostly primary ejecta by rapid flow along the surface

k. Simultaneous basin-radial and transverse grooves and ridges

formed by slowly flowing ejecta; obviously transitional with j 

m. Outer, finely lineated primary ejecta

n. Smoother continuation of m

p. Transverse ridges and furrows formed by rapid deceleration of 

ejecta as it encountered crater floor after flowing rapidly 

over wall

q. Additional flow lineations deformed by underlying terrain

r. Pits, probably Orientale secondaries, that have been overridden by ground-flowing ejecta

s. (near lower left edge) Lobes of ejecta that have encroached on 

the secondary craters. Upper occurrence has transverse 

lineations apparently formed by burial of ridges 

w. Crater Wargentin (85 km diameter) filled with smooth secondary or primary ejecta to higher level than ejecta closer to 

basin
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A55

FIGURE 30.—Crater Struve L, 900 km northeast 

of Orientale basin (direction of arrow). The 

complex feature resembles loc. 43 (fig. 28) and 

is clearly of secondary impact origin. The hill 

that seems superposed on the crater rim was 

probably formed by interference of closely 

spaced impacts. Hummocky material in crater 

floor could be part of the crater-forming projectile or other ejecta that flowed into the crater from Orientale or from other secondaries. 

Part of Lunar Orbiter IV frame 174-H; sun 

illumination from right (east) 15° above horiFIGURE 31.—Plains deposits in floor of crater Albategnius appear 

to have buried Imbrium secondary craters (11). Unburied secondaries are on rim (loc. 31). A ridge (arrow; loc. 12) may indicate 

marelike deformation or flow of plains material. Part of Apollo 16 

mapping frame 449. Sun illumination from right (east) 5° above 

horizontal.

others, 1975; Hodges and Muehlberger, in Ulrich and 

others, 1980). Finally, seismic shaking produced by the 

Orientale impact might have smoothed the plains 

(Schultz and Gault, 1975).

Nevertheless, the concentration around Imbrium indicates that most plains deposits here must be of Imbrium origin (compare fig. 29, c, g, i, n, w). The stratigraphic gap can be explained as follows even if the 

entire stratigraphic section and not only the subsurface layers are of Imbrium origin. Except for the deposits in Albategnius (loc. 12, a marelike ridge possibly 

indicating deformation of a coherent plate) and similar 

deposits (loc. 26), the gap is not as great as suggested 

by the frequency counts, because most of the excess 

craters counted by Soderblom and Boyce (1972) on the 

highlands are Imbrium secondaries. The counted and 

buried (fig. 31) secondaries need be only slighty older 

than the plains to produce the observed age relations, 

as at Orientale (fig. 29, g-h); only a slight delay of 

ponding of Imbrium ejecta after formation of the 

sculpture is required (E. M. Shoemaker, oral commun., 

1963). The late-arriving deposit may have been a gasbearing or otherwise fluidized fraction of Imbrium 

primary ejecta segregated from the clastic, hummocky 

ejecta (Eggleton and Schaber, 1972) or a debris surge 

containing more secondary ejecta than primary ejecta 

(Oberbeck, 1975; Morrison and Oberbeck, 1975; Oberbeck and others, 1975).

Both primary and secondary ejecta probably compose 

plains material. Some of the Orientale smooth materials are outward continuations of the textured continuous deposits that presumably are heavily primary 

(fig. 29, n; Eggleton and Schaber, 1972; Hodges and 

others, 1973; Moore and others, 1974). The voluminous 

plains in Ptolemaeus, Albategnius, and Hipparchus 

and in the trough northwest of the Apollo 16 site, relative to adjacent secondary-pitted highlands, favors 

predominantly primary origin of these Imbrium plains 

deposits (Hodges and Muehlberger, in Ulrich and 

others, 1980). Farther from the basin, plains deposits 

are less extensive relative to the density of nearby secondaries and probably consist predominantly of secondary ejecta (fig. 28, loc. 65). At Orientale, plains are 

distal to secondary impact craters (figs. 26, 29, g; fig. 

30) and are undoubtedly ejecta of those craters. Both 

the continuous ejecta and the secondary ejecta plains 

or Orientale fill secondary craters (fig. 29, c, s) and 

large depressions (fig. 29, g, w). Thus most plains are 

related to basins, but sources cannot always be determined because of the similar appearance of plains of 

primary and secondary-impact origin. Plains around 

both Orientale and Imbrium are concentrated where 

the primary regime gives way to the secondary impact 

regime.
A56 APOLLO 15-17 ORBITAL INVESTIGATIONS

,46 W 45' 30 W

14 S

80 KILOMETERS

EXPLANATION

Mare units — Indicated by 3-component symbols, 

some with underlines; first component is a 

number indicating color: 1, reddest, 5, bluest; 

second component is a letter indicating albedo: 

b, bright, c, intermediate, d, dark; third 

component is a number indicating age: 1, oldest, 

4, youngest. Single underline, probably thin; 

double underline, very thin

Dark mantling material

Possible endogenic feature - h, hill or cone; c, 

crater or other depression; s, sinuous rille head

Very red terra

Materials of Imbrian and younger craters

Plains material

Fra Mauro Formation

Probable Imbrium-basin secondary crater

Pre-Imbrian crater and terra materials pitted by 

Imbrium basin secondary craters or smoothed by 

mass wasting

—— --- Contact - Dashed where approximately located or 

inferred on the basis of a single property

-"•a~--u— Inferred depositional mare contact along scarp - 

Ticks on high side

SOUTHERN OCEANUS PROCELLARUM

Both the terra and mare geology in this region are 

well shown by the Apollo 16 strip of photographs (figs. 

33-35). The stratigraphically complex maria are 

mapped in the entire region that was photographed 

west of long 12° W. (fig. 32), and the relations among 

age, color, overall reflectivity, and thickness were 

studied in detail. The crater Letronne, an Imbrian-age 

crater 125 km in diameter, is the major structure in the 

west half of the mapped area (figs. 32A, 33; Marshall, 

1961, 1963). Thin mare units (and mare ridges not 

mapped) partly trace its submare continuation, and its 

central peak is partly exposed. In the center of the area 

is Montes Riphaeus (figs. 32B, 34; Eggleton, 1965),

which consists of several arcs that must be crater rim 

crests. Extrapolation of the largest arc gives a diameter of about 200 km. Mare Cognitum (the Ranger 7 

impact site; Heacock and others, 1965) must be mostly 

a fill of this crater or basin. The terrae mostly continue 

the geological style of the central highlands described 

above, and only the Fra Mauro Formation and certain 

exceptionally red terrae will be discussed in this section.

FRA MAURO FORMATION

In the eastern part of the region (figs. 32, 35) is the 

peninsula that includes the craters Fra Mauro, Bonpland, Parry, and Guericke and the hummocky Fra 

Mauro Formation. North of the area are the type area
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A57

30 W 12 W

FIGURE 32.—Geologic map of part of southern Oceanus Procellarum. Based on Apollo 16 

mapping-camera frames 1680-1689, 1975-1984, 2201-2216, 2816-2842, and 2972-2998.

of this unit (Eggleton, 1964; Wilhelms, 1970) and the 

Apollo 14 landing site (Eggleton and Offield, 1970; Sutton and others, 1972; Swann and others, 1972, 1977). 

The Fra Mauro here and the hummocky deposit in the 

northwest part of figure 28A (loc. 1) are similar and 

presumably are parts of the same stratigraphic unit.

Although the Imbrium basin origin of the Fra Mauro 

Formation is clear, the exact manner of emplacement 

and content of primary and local material are controversial. Early workers (Eggleton, 1964; Wilhelms, 

1970) assumed that it is primary ejecta because it 

resembles continuous, radially ridged deposits of craters that appeared to consist of debris that flowed outward along the surface (for example, fig. 25, K). This 

conclusion has been disputed by Oberbeck and his 

coworkers (for example, Oberbeck and others, 1974; 

Morrison and Oberbeck; 1975; also Head and Hawke, 

1975), who believe that local material excavated and 

redistributed in the ejecta of secondary impact craters 

should dominate the deposit. These workers believe 

that the ridges characteristic of continuous ejecta deposits of basins and large craters are formed by the 

interference of secondary crater ejecta.

Relations in the present area and to the east suggest 

that the original explanation is more likely than

ballistic emplacement. Ridges and grooves in the Fra 

Mauro are much less distinct than those in the central 

highlands, and no distinct secondary craters are visible 

either in the formation or nearer the basin. The deposits are more like the massive continuous deposits of 

Orientale closer to the basin (fig. 14), yet they are now 

adjacent to grooves, ridges, and craters of secondary 

origin (fig. 35). Thus the deposits probably flowed into 

the region from a point considerably farther north and 

therefore contain more primary ejecta than they would 

if formed by impact of ejecta where they now rest. They 

are likely to contain more primary material than the 

adjacent plains, which probably contain appreciable 

ejecta from the secondary craters on the pre-Imbrian 

crater rims.

MARE PROPERTIES

AGE

Four ages of maria (4, youngest; 1, oldest) were 

distinguished. The dating technique of Boyce and 

others (1974) based on the crater degradation model 

furnished ages of some of the largest units, but for the 

many small patches of mare material in this complex 

region, ages were extrapolated from well-dated units
A58 APOLLO 15-17 ORBITAL INVESTIGATIONS

FIGURE 33.—Stereoscopic photographs of central part of area covered by figure 32 (left). Rugged terra at left (west) is east rim of pre-mare 

crater Letronne. Post-mare crater below scale bar is Herigonius Abundant kipukas and shelves continue the terra features, probably 

parts of Letronne and an old basin (Wilhelms and McCauley, 1971), beneath thin mare layers (t—compare double underlines in fig. 

32A). Apollo 16 mapping-camera frames 2837 (right) and 2839; sun illumination from right (east) 8° above horizontal in right frame 

and 6° in left.

FIGURE 34.—Stereoscopic photographs of western part of area covered by figure 32B. Large, rugged terra feature is Montes Riphaeus. 

Shieldlike feature is Darney Chi (D). Same scale as figure 33. Parts of Apollo 16 mapping-camera frames 2829 (right) and 2831; sun 

illumination from right (east) 17° above horizontal in right frame and 15° in left.
FIGURE 35.—Stereoscopic photographs of eastern part of area covered by figure 32S. The terra is informally called the Fra Mauro peninsula. Pre-lmbrian craters are Fra Mauro (F), Bonpland 

(B), Parry (P), and Guericke (G). Elongation of rims of flooded irregular craters between Parry and Guericke suggests Imbrium secondary origin. Fra Mauro Formaton fills much of crater 

Fra Mauro and buries an Imbrium secondary at (f). Possible volcanic features shown on geologic map (fig. 32B) are indicated by arrowheads. The volcanic features in the center frame 

could be the sources of the dark smooth materials that surround them; the dark materials thinly mantle the light cratered plains (Cayley Formation), which were once interpreted as older 

materials of the same type as the dark mantling materials. Imbrian mare-filled primary impact crater Parry A (A) is 14 km in diameter. Apollo 16 mapping-camera frames 2203, 2205, 

2207 (right to left); sun illumination from right (east) 13° above horizontal in center.

01 

CD
AGO APOLLO 15-17 ORBITAL INVESTIGATIONS

elsewhere by means of estimated crater densities. 

Comparison with units mapped in northern Oceanus 

Procellarum and Mare Imbrium shows that age unit 4 

is Eratosthenian (DL 230 or smaller) and units 2 and 1 

are Imbrian (DL 265 or larger). For age unit 3, of the 

eight most reliable DL measurements, two are 265, 

clearly Imbrian, and the other six are between 230 and 

245, in the transition range considered to be either 

Eratosthenian or Imbrian depending on the weight of 

other evidence. Parts of age unit 3 apparently embay 

small Eratosthenian craters as do the northern 

Eratosthenian units, whereas other parts are older 

than secondary craters of the crater Bullialdus centered south of the map area, whose Eratosthenian age 

is shown by embayment by Eratosthenian mare materials and superposition on Imbrian mare materials. 

Thus age unit 3 probably includes both Eratosthenian 

and Imbrian materials.

COLOR

Five color divisions were recognized on the colordifference images supplied by E. A. Whitaker (fig. 36; 

Whitaker, 1966, 1972d). From relatively red to relatively blue, they are numbered from 1 to 5 in the 

geologic unit symbols.

These colors are primarily a mapping tool, but compositional inferences can be drawn from them if certain

assumptions are made. The pair of wavelengths 

(0.31-0.40 ^m and 0.73-0.90 ^m) used to construct the 

images is sufficiently close to the pair (0.40 and 0.56 

/u,m) used by the remote-sensing group at the Massachusetts Institute of Technology to estimate 

titanium content of mature soils derived from lunar 

mare rocks (McCord and others, 1972, 1976; McCord 

and Adams, 1972; Charette and others, 1974; Pieters 

and McCord, 1975, 1976). The M.I.T. group relates 

titanium content to the slope of the curve drawn between those two points on their continuous spectra. 

Therefore the color-difference photographs are probably reasonable data from which to estimate titanium 

content of mare soils (Whitaker, 1972b; Zisk and 

others, 1977) provided the soils are mature and do not 

contain much pyroclastic glass or other variables.

The M.I.T. workers assume that all mare soils are 

mature—that is, they contain more than 70 percent of 

the glass-bonded impact-produced aggregates known 

as agglutinates, whose production is a function of time 

(for example, Charette and Adams, 1975; Pieters and 

McCord, 1976; McCord and others, 1976). As discussed 

below, this assumption is substantiated by the present 

study. Therefore, for soils derived from mare lavas, 

relatively red values (1, 2), which appear bright on the 

color-difference photographs, generally represent low 

titanium and blue (dark) values (4, 5) represent high 

titanium.

FIGURE 36.—Color-difference photograph of area mapped in figure 32 (outlined). 

Wavelength pairs are 0.31-0.40 /mi and 0.73-0.90 /u,m. Dark is blue, bright is 

red. Courtesy of E. A. Whitaker (Whitaker, 1972 b, d).
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A61

"Blue" color does not always signify high titanium, 

however. Immature soils are blue in the wavelength 

pair 0.56 and 0.95 ^m (McCord and others, 1976). Such 

soils are found on the walls of fresh craters and on 

other steep slopes where they have not had a chance to 

become mature because of continual reexposure of 

fresh material by down-slope movement. Many of the 

fresh craters in the present area appear blue on the 

color-difference images (fig. 36). Therefore some bluing 

effect due to freshness and not composition may be 

visible on the photographs and must be considered in 

drawing interpretations.

Soils with a high content of pyroclastic glass may 

show a different relation between color and composition than do mature mare soils in which most glass is 

agglutinitic. For the pyroclastic glasses, devitrification 

is more significant in determining color than is composition (Pieters and others, 1973; Adams and others, 

1974; Zisk and others, 1977). For example, the very red 

and very dark color of the Aristarchus plateau may 

result from a mixture of one part fresh (orange) glass 

and two parts devitrified (black) glass of identical 

(high) titanium content (Zisk and others, 1977), 

whereas the blue color of the Apollo 17 mantle may 

result from a higher ratio of devitrified glass (Adams 

and others, 1974). The effect of pyroclastic glass on the 

soil colors in the present area cannot normally be distinguished from that of the lava compositions, but 

certain anomalies in the relation between color and 

albedo to be discussed do suggest the presence of pyroclastic glass in the soils of some units.

A few complete spectra are available from the southern part of the area (Johnson and others, 1973; Pieters 

and others, 1975). They confirm that unit Ib2 centered 

at lat 13° S., long 36° W., has low titanium content. The 

spectral region which shows as high in titanium covers 

several of the units mapped here but probably corresponds most closely with the blue units 4d4 and 4d3.

REFLECTIVITY

Three steps of reflectivity have been recognized qualitatively on high-illumination telescopic photographs 

and to some extent can be seen on the Apollo photographs (figs. 33-35). Almost all maria are darker than 

the terrae. A few spots in the terrae are darker than 

the maria (fig. 32, map symbol d; fig. 35). Most such 

spots are topographically uneven, suggesting that the 

terra is overlain by only a thin dark mantling deposit, 

probably pyroclastic. Many such deposits occur on or 

around probable volcanic craters, hills, or sinuous rille 

heads (fig. 35, arrowheads). All dark mantles so 

mapped that are large enough to be detected on the

color-difference images are blue, more like those at the 

Apollo 17 site than at Aristarchus.

Reflectivity of mare soils without pyroclastic material is generally agreed to be mostly a function of the 

iron and titanium content of the soil, although the 

exact nature of the dependence is uncertain (Gold and 

others, 1977). The M.I.T. group believes that the abundance of agglutinates made opaque and dark by 

titanium and iron is the controlling factor (Adams and 

McCord, 1973) whereas others ascribe much of the 

darkening to an increase in iron and titanium as coatings of other small soil grains as well (Wells and 

Hapke, 1977; Gold and others, 1977). In either case, 

soils darken proportionally to time of exposure as the 

agglutinates or coated grains (probably both) increase 

in abundance at the expense of the brighter crystalline 

rock materials, which have the same bulk composition 

as the lavas from which they are derived. Once maturity is reached, probably no further darkening occurs, 

and reflectivity of mature soil is a function of titanium 

and iron content alone. Steep slopes retain their 

brightness, regardless of the albedo of the source material, because exposure of fresh materials by downslope 

movement (Shoemaker and Hackman, 1962) is faster 

than accumulation of agglutinates or other dark soil 

grains.

THICKNESS

Thickness is a fourth variable which must be taken 

into account, because reflectivities and colors of thin 

units seemingly might be altered by contamination 

with underlying materials. Lateral mixing appears to 

be ineffective in changing color because many color 

boundaries are very sharp (Kuiper, 1965), but vertical 

mixing by impacts is evident in rays and could occur 

elsewhere. Thickness of the mare units can be estimated from their topography. Thin units are to be expected on shelves bordering terra, on rises that evidently are buried terra features, and in tracts where 

numerous small terra islands protrude through the 

mare surface (for example, fig. 33, t). Map symbols of 

units believed to be thin are underlined, twice if believed to be very thin (fig. 32).

Elevation is also a factor in estimating age. Old units 

commonly are elevated above young units on a shelf or 

a kipuka (Nichols and others, 1974). The young units 

have flowed into depressions that either remained after 

the old units were thinly draped on rolling topography 

or that were created by post-depositional subsidence or 

sagging of the old units, events that are indicated by 

fracturing of mare material on many of the kipukas. 

Scarps judged to be depositional are distinguished 

where possible from fault scarps (fig. 32). Other scarps 

not mapped here border the special mare structures
A62 APOLLO 15-17 ORBITAL INVESTIGATIONS

known as mare ridges and arches, whose origin is still 

uncertain (Strom, 1971; Colton and others, 1972; 

Young and others, 1973a; Hodges, 1973; Lucchitta, 

1976).

CORRELATIONS OF MARE PROPERTIES

In order to determine their degree of interdependence, the four variables of age, color, reflectivity, and 

thickness are plotted against each other in six histograms (fig. 37). The histograms were constructed from 

a table of units (table 3) that in turn was extracted 

from the map (fig. 32). The vertical scale of the histograms is an arbitrary one that considers both the areal 

extent and the number of patches of a unit. Each occurrence of a unit smaller than 1 square degree (900 

km2 ) was given a value of 1. Each occurrence between 1 

and 6 square degrees was given a value of 2, and each 

larger occurrence was given a value 3. Thus, separate 

occurrences have more weight relative to their area 

than do large occurrences. Patches that were considered isolated parts of another unit were not counted 

separately. This procedure was followed because separate eruptions were considered more significant to the 

volcanic history than voluminous eruptions.

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FIGURE 37.—Relations between pairs of mare soils in mapped area (fig. 32; table 3). Ordinate is a measure of areal abundance of mare 

units in arbitrary units explained in text. A, Age versus thickness. B, Age versus color. C, Color versus reflectivity. D, Reflectivity 

versus thickness. E, Color versus thickness. F, Age versus reflectivity.
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE 

TABLE 3.—List of mare units mapped in figure 32

[First symbol, a number, refers to color: 1, reddest; 5, bluest. Second symbol, a letter, refers 

to reflectivity: b, bright; c, intermediate; d, dark. Third symbol, a number, refers to age: 1, 

oldest; 4 youngest. Single underline, probably thin; double underline, very thin. Italic 

numbers after dash refer to relative abundance (see text)]

A63

Color

1

(Very

red)

2

(Red)

3

(Intermediate

color)

4

(Blue)

5

(Very

blue)

Age Bright

Thick Thin Very Thin

4

3 Ib3 7 lb3-2

2 lb2-4 lb2-4 lb2-3

1 Ibl 2 __. __. Ibl 7

4 2b4-7 _ ... ......

3 2b3-70 2b3-4

2 2b2-5 2b2-2 __ ..

1

4 ._ . __

3 ...... ______ 3b3-7

2 ___ 3b2 7 3b2-3

1 ______ 3bl-7 ~I7__

4

3 ...... ______ __ __

2 ___ ______ 4b2-3

1 _ ___ ... _ 4bl-4

4 _ ___ ______ ______

3 ______

2 .. __ ______ ______

1

Reflectivity

Intermediate

Thick Thin Very Thin

lc3-7 ... _ ___

___ ______ lc2-7

2c3-5 __ __ 2c3-2

2c2-2 2c2-2

3c4-2 3c4-2

___ ___ 3cl-7

4c4 1 \c\ 5

4c3-7 4c3-2 4c3-3

______ 4c2-2 ~._

5c3-7 ___ 5c3-7

______ 5c2-2 5c2-7

._... "ScT 7

Dark

Thick Thin Very Thin

Id4~7 ______ __— -

2d4-2 __— . — —

-.— . 2d2-2

3d4-2 ___

3d3-5 3d3-7 — —

4d3-72 4d3-5 4d3-2

______ 4d2-7 4d2-7

______ ______ 4dl-7

5d4-6 5d4-7 5d4-7

5d3-3 5d3-7

______ 5dl-3 5dT-7

AGE VERSUS THICKNESS

Apparent thickness increases with decreasing age 

(fig. 37'A ), as expected from the exposure of old units on 

shelves and kipukas and the fill of the deeper basin 

centers and other depressions by the younger units. 

However, the old units also undoubtedly underlie the 

younger units in the deeper depressions, and so volumetrically, units of ages 1 and 2 might be at least as 

abundant as units of age 3. The dropoff in abundance in 

age 4, however, is clearly real, for there is nothing to 

bury the young units in this area.

AGE VERSUS COLOR

There is some change of color with age (fig. 375). The 

oldest group (sparsely exposed) has more blue than red. 

Reds gain the ascendancy in age unit 2; each of the red 

categories is about equal to the intermediate color and 

to the two blues combined. In age unit 3, which has the 

most exposed materials, the increase is in the three 

middle colors; in total amount the extremes are as 

abundant as in age unit 2, although the proportions of 

red and blue are reversed. The balance in the three 

middle colors is also shifted to the blue. Age unit 4 is 

dominated by blue. This progression from dominance

by blue to red and back to blue is the same as the 

general pattern for the Moon as a whole (Soderblom 

and Lebovsky, 1972; Soderblom and Boyce, 1976), although it is neither pronounced here nor universally 

true for the rest of the Moon.

The fact that all colors are represented in all ages 

indicates that there is no strong purely age-related 

control such as impact cratering upon color. The 

changes with age must be a property of the erupted 

magmas.

COLOR VERSUS REFLECTIVITY

There is a good correlation between color and reflectivity (fig. 37C). Red mare units are bright; blue units 

are dark; intermediate color correlates with intermediate reflectivity. This correspondence accords with 

the inference discussed above that composition determines both color and reflectivity in mature, nonpyroclastic soils.

The correlation is even stronger than indicated in 

the histogram if certain exceptions are evaluated. The 

bright units of colors 3 (intermediate) and 4 (blue) are 

all thin (table 3). This may result from contamination 

by underlying bright terra materials or by fresh, immature materials that are blue. The single very red
A64 APOLLO 15-17 ORBITAL INVESTIGATIONS

unit that is dark is a pronounced anomaly, especially 

considering that the few reflectivities observed as intermediate for this color are questionable because of 

the small size of the units or position near rayed craters. The anomaly (unit Id4), lat 13.5° S., long 28°-30° 

W., is very smooth and lies near several endogenic features. It could be pyroclastic as, proposed for the Aristarchus plateau material that is also dark and red 

(Zisk and others, 1977). The dark units of color 2 (red) 

may also be pyroclastic (unit 2d4 in Letronne) or may 

overlie dark mare units that could contaminate them 

(unit 2d2 lat 10° S., long 37° W.). Abundant iron is 

another possible cause of exceptionally dark glasses 

(Johnson and others, 1973).

REFLECTIVITY VERSUS THICKNESS

A change of reflectivities by contamination is suggested by a plot of reflectivity versus thickness (fig. 

37D). Thin and very thin units are more abundant relative to thick units in the bright class than in the other 

two reflectivity classes. This suggests a brightening by 

mixing with underlying terra materials.

Although red color and high reflectivity usually correlate, there are some bright units that are blue. As 

noted above, these blue units are all thin, and so their 

anomalously high reflectivity could be the result of 

contamination. Any brightening of the red materials 

that might occur by mixing could not be detected in the 

data, because soils derived from red lavas are naturally 

bright.

COLOR VERSUS THICKNESS

A histogram of color versus thickness (fig. 37E) tests 

whether colors might be changed by contamination as 

reflectivities seem to be. The thickness referred to is 

that of the total estimated mare section down to the 

terra basement; another mare unit could intervene between the terra and the surface mare unit.

Blue is the dominant color among the thin and very 

thin units, so the red color of terra units does not seem 

to affect the mare colors. In the Barney Chi region, 

some of the bluest material is superposed on some of 

the reddest terra material. Elsewhere, however, rays 

change the color of mare units (fig. 10), and so the 

absence of change noted here is probably due to the 

lack of sufficiently large impacts.

Thin and very thin units together exceed thick units 

for the very red and very blue color classes and are 

almost equal to the thick units for blue unit 4. For the 

very red units, this correlation is probably a result of 

the geologic circumstance that the old units remain 

exposed only where they were emplaced on terra

shelves and kipukas. Thin blue mare units in all age 

classes occur near terrae, however, as do the dark blue 

mantling deposits (which differ little in observed properties and possibly lithology from many of the materials mapped as blue mare materials). A possible 

genetic connection between blueness (high titanium 

content) and thinness, shallowness of source, or proximity to terra material needs to be further explored.

AGE VERSUS REFLECTIVITY

Low reflectivity had been thought generally diagnostic of young materials, as it indeed seems to be in Mare 

Imbrium (described above; Wilhelms, 1970), but many 

dark units are now known to be old because iron and 

titanium content and not age determine reflectivity of 

mature soils, as noted above. If soil maturity had not 

been reached in the age of exposure of the mare units 

in the present area, reflectivity should vary with age 

for a given color.

However, reflectivities show much the same trend 

with age (fig. 37F) that colors do (fig. 37B). This is to be 

expected in view of the strong correlation between reflectivity and color if soils are mature. There seems to 

be no other effect of age on reflectivity in the data. 

Moreover, no anomalously high reflectivities were discovered such as might be produced by other factors 

such as high feldspar content of the lavas. Therefore 

even the regoliths on the Eratosthenian units here are 

probably mature.

VERY RED TERRA

Almost all lunar terrae are redder on the colordifference photographs (fig. 36) than the maria, but 

some are exceptionally red. Very red areas here are 

part of southern Montes Riphaeus, a few terra islands 

east of that area (fig. 34), and most of the terra material around lat 12° S., long 26° E., including the 

shieldlike feature Barney Chi (fig. 34; Wood and Head, 

1975). These red areas are also very bright. Pieters and 

others (1975) showed that a feature near the mapped 

area similar to Barney Chi is compositionally distinct 

from other terrae (and that it is not red if its whole 

spectrum is considered). Malin (1974) suggested that 

the red areas may be pre-mare basalt relatively 

radioactive and high in potassium, rare earths, and 

phosphorus (KREEP).

If the Barney Chi plains are composed of old lavas, 

they would be the only nonmare volcanic lavas yet 

identified on photographs. They probably are of Imbrian age, as they seem not to be affected by Imbrium 

secondaries. The rugged highlands of the southern
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A65

Montes Riphaeus and the other red islands might have 

been mantled by fire-fountain equivalents of the lavas. 

Alternatively, both impact and volcanic rocks in the 

region may have formed from chemically anomalous 

materials with red color.

SUMMARY AND CONCLUSIONS

The Apollo 15, 16, and 17 mapping cameras photographed representative parts of the two most conspicuous features of the near side of the Moon, the Imbrium 

multi-ringed circular basin and the maria. Previous 

studies have shown that the basin is of impact origin 

and that the maria are composed of basaltic lavas. 

Limits set by the Apollo 16 samples and comparisons 

with the Orientale basin have led to the interpretations that most terra landforms in this region were 

produced by the Imbrium impact. Many of these landforms were previously ascribed to volcanism. The most 

significant new results for the maria concern their 

stratigraphic sequence. Working definitions are proposed for the Imbrian-Eratosthenian and 

Eratosthenian-Copernican system boundaries, and 

their absolute ages are estimated at 3.3±0.1 and 

2.3±0.1 billion years or less, respectively.

IMBRIUM IMPACT

The Imbrium impact contributed more to the Moon's 

present appearance than any other single event. It 

uplifted Montes Apenninus, the Moon's largest 

mountain structure; emplaced masses of ejecta on the 

Apenninus flank and beyond; mantled, grooved, and 

cratered outlying highlands; and provided a large 

basin that was subsequently filled with lava.

Montes Apenninus was formed as part of the rim of 

the Imbrium basin—the edge of the excavation. Material ejected or thrust from the basin in low trajectories, 

perhaps without leaving the surface, was deposited 

close to the rim and formed imbricate blocks. Other 

more chaotic blocky ejecta may have been transported 

more violently. Some blocky ejecta of both types grades 

outward to lineated or smooth material that flowed 

along the surface, mantled elevated terrain, filled depressions, and banked up against obstacles. The deposits commonly mantle or fill secondary impact craters of 

Imbrium. These relations are consistent with models of 

ejection processes whereby ejecta travels outward in a 

conical curtain whose lower ejecta fragments are deposited first and nearest the basin rim and whose upper 

fragments, traveling much faster, impact later and 

farther from the basin. The outer, high-velocity ejecta 

produces craters larger than the impacting masses,

and the inner, slow ejecta accumulates as thick, continuous deposits that move outward behind the advancing ejecta curtain. Like all high crater walls, the 

Apennine front was modified by large slumps soon 

after ejecta deposition and before impact melt inside 

the basin solidified. Impact melt was also deposited on 

the mountain flank as it is on crater flanks.

The rest of the terra in the study area was also 

shaped by Imbrium ejecta, with an outward increase in 

the proportion of secondary to primary deposits and 

landforms as expected from the higher impact velocities of the outer ejecta. The complex topography of the 

interbasin terra east of Mare Serenitatis can be explained, without recourse to endogenic mechanisms, by 

the interaction of secondary or primary Imbrium ejecta 

with massifs and deposits of the Serenitatis and 

Crisium basins, whose structure in turn was probably 

inherited partly from older basins. Similarly, 

landforms and stratigraphic relations in the peninsula 

east of Mare Tranquillitatis, on the northern Nectaris 

basin rim, and in the central highlands were produced 

by impact of Imbrium ejecta upon Nectaris-basin deposits and secondary craters and widely spaced Nectarian 

and pre-Nectarian primary craters. Closely spaced secondary impacts, followed in favorable localities by a 

debris flow of mixed secondary and primary ejecta, 

produced most of the Imbrium features in the outer 

terra. The deposits are grooved, hummocky, rolling, or 

planar depending on the amount of material and the 

topography of the terrain over which they moved and 

on which they came to rest. Some lobes consisting 

largely of primary ejecta are believed to have flowed 

large distances from the basin. Certain large basinradial ridges that were once considered tectonic are 

now believed to be composed of ejecta. Craters on the 

ridges and other places protected from the later deposits are sharper than simultaneously formed craters in 

depressions subject to burial. Intercrater septa, long 

ridges, grooves, peaks, "domes," and thin and thick deposits were produced by interference of adjacent secondaries and account for numerous landforms once 

considered volcanic. Groovelike chains of elliptical secondaries formed closest to the Imbrium basin, whereas 

sharp and degraded subcircular craters up to 30 km in 

diameter once considered primaries of various ages 

characterize the more distal terrain.

Non-Imbrium origin is possible for some terra features (besides primary craters) in the study area. Some 

plains seem to embay scattered Orientale secondaries 

or post-Imbrium primary craters in the central highlands and could have been emplaced by post-Imbrium 

impacts. A volcanic origin is not excluded for some 

plains materials. Certain exceptionally red terra materials of diverse morphology may also be volcanic.
A66 APOLLO 15-17 ORBITAL INVESTIGATIONS

MARE MATERIALS

In northern Oceanus Procellarum and southern 

Mare Imbrium, the stratigraphic sequence of mare materials and crater materials (from old to young) is: Imbrian System—(1) Imbrium basin, (2) crater Prinz, (3) 

reddish dark mantle of the Aristarchus region, (4) reddish mare material (two units), (5) crater Krieger, (6) 

mare material of intermediate color; Eratosthenian 

System—(7) crater Lambert, (8) crater Eratosthenes, 

(9) crater Timocharis, (10) bluish mare material, (11) 

crater Euler, (12) crater Delisle, (13) bluish mare 

material (two units), (14) crater Diophantus; Copernican System—(15) crater Copernicus, (16) crater 

Phytheas, (17) crater Kepler, and (18) crater Aristarchus.

The colors of the mare units (as seen on certain 

color-difference images) have been shown by other 

workers to result from the titanium content of the regolith derived from the lava. Blue colors indicate high 

titanium, red colors indicate low titanium. Thus the 

above sequence progresses from low to high titanium. 

Blue units older than this sequence occur along the 

margins of southern Serenitatis and northern 

Tranquillitatis, and young blue units are common in 

western Oceanus Procellarum. This has led to the suggestion that lunar lavas follow a general east-west 

progression in time from high titanium to low and back 

to high. A rough blue-red-blue progression applies to 

the detailed stratigraphy in southern Oceanus Procellarum, where thick red units of late Imbrian age fill the 

deepest depressions as in the northern basins. 

However, there are exceptions in all areas. For example, red mare as old or older than the blue occurs in 

Sinus Amoris, Mare Imbrium, and Oceanus Procellarum, and Eratosthenian red or intermediate-color 

materials occur in Mare Serenitatis, Mare Vaporum, 

and southern Oceanus Procellarum.

The proposal was made early that lunar surface materials darken with time, for crater rays fade with time 

and steep slopes, where relatively fresh rock presumably is exposed, are bright. This proposal plus the observed progression from bright to dark of lavas in 

regions like northern Procellarum and southern Imbrium led to the suggestion that the youngest mare 

lavas and dark mantling materials were always the 

darkest. The dark, blue lavas and the mantles along 

the Serenitatis-Tranquillitatis border were accordingly considered young until the Apollo photographs 

showed otherwise. Now it is believed that the darkening is caused by the production of dark glassy particles 

and coatings on other particles in the regolith at the 

expense of brighter crystalline rock fragments and that 

this darkening progresses only to a point of equilibrium. In mature regoliths, reflectivity is a function of

titanium and iron content. This relation seems 

confirmed by comparison of properties in southern 

Oceanus Procellarum, where reflectivity correlates 

with color for most units, indicating that soils are mature and that composition is normally the only determinant of reflectivities of mare regoliths. Departures 

from this relation may be due to pyroclastic (lavadroplet) particles or to contamination by materials of 

underlying units by cratering. Other workers have 

shown that anomalously low reflectivities of red mantling units, in particular, may indicate high contents of 

titanium- or iron-rich glasses in certain states of devitrification. Anomalously high reflectivities of some 

blue units (all thin) may be due to impact contamination from underlying, perhaps crystalline materials.

There is evidence that the early stages of mare formation included eruption of much pyroclastic material, 

some of which reached high elevations. Thick deposits 

of red and blue dark mantling material occur high on 

Montes Haemus along the Serenitatis border, thin blue 

mantles are common in all areas, a new red deposit was 

found on the Crisium basin rim, and the terra margins 

of all maria are darkened patchily in a way that suggests dark mantles were once abundant and have 

become inconspicuous only because of mixing with underlying terra materials by impact and downslope 

movement. Perhaps many small maria are merely accumulations in depressions or on flat surfaces of "firefountained" lava droplets.

Almost all conspicuous patches of dark mantling 

material surround or adjoin irregular craters, rilles, 

sinuous rille "cobra heads," cones, crater chains, or 

other features of undoubted internal origin. These features presumably were sources for the pyroclastic materials. Some of them occur at high elevations on the 

terra. Visible sources of lavas are much rarer. "Dcaldera" in Montes Apenninus surmounts one of the 

highest known occurrences of mare material, a domeshaped swell of young blue material. Other sources are 

inferred from the high level of certain pools of mare 

material, from which a steplike cascading to lower 

levels has occurred. Thus there is no common hydrostatic level of mare material.

REFERENCES CITED

Adams, J. B., and McCord, T. B., 1973, Vitrification darkening in the 

lunar highlands and identification of Descartes material at the 

Apollo 16 site: Proc., Lunar Sci, Conf., 4th, Geochim. et Cosmochim. Acta, supp. 4, v. 1, p. 163-177.

Adams, J. B., Pieters, Carle, and McCord, T. B., 1974, Orange glass: 

Evidence for regional deposits of pyroclastic origin on the Moon: 

Proc., Lunar Sci. Conf., 5th, Geochim. et Cosmochim. Acta, supp. 

5, v. 1, p. 171-186.
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A67

Aggarwal, H. R., and Oberbeck, V. R., 1974, Roche limit of a solid

body: Astrophys. Jour., v. 191, p. 577-588. 

Baldwin, R. B., 1949, The face of the Moon: Chicago, 111., Univ.

Chicago Press, 239 p.

———— 1963, The measure of the Moon: Chicago, 111., Univ. Chicago 

Press, 488 p.

Boyce, J. M., 1976, Age of flow units in the lunar nearside maria 

based on Lunar Orbiter IV photographs: Proc., Lunar Sci. Conf., 

7th, Geochim. et Cosmochim. Acta, supp. 7, p. 2717-2728.

Boyce, J. M., and Dial, A. L., Jr., 1973, Relative ages of some nearside mare units based on Apollo 17 metric photographs: Apollo 

17 Prelim. Sci. Rept., NASA SP-330, p. 29-26 to 29-28.

———— 1975, Relative ages of flow units in Mare Imbrium and Sinus 

Iridum: Proc., Lunar Sci. Conf., 6th, Geochim. et Cosmochim. 

Acta, supp. 6, v. 3, p. 2585-2595.

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lunar near side light plains and maria: Proc., Lunar Sci. Conf., 

5th, Geochim. et Cosmochim. Acta, supp. 5, v. 1, p. 11-23.

———— 1975, A summary of relative ages of lunar nearside and farside plains: Astrogeology 66, U.S. Geol. Survey interagency report (open file), 26 p.

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features of Mare Serenitatis: Apollo 17 Prelim. Sci. Rept., NASA 

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of the Moon: U.S. Geol. Survey Misc. Geol. Inv. Map 1-462, scale 

1:1,000,000.

———— 1966, Geologic map of the Mare Serentitatis region of the 

Moon: U.S. Geol. Survey Misc. Geol. Inv. Map 1-489, scale 

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1969, Geologic map of the Alphonsus region of the Moon: U.S.

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scales 1:250,000 and 1:50,000.

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———— 1977, The Ries crater of southern Germany, a model for large 

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1605-1613.

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———— 1965, Geologic map of the Riphaeus Mountains region of the 

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———— 1964, Stratigraphy and structure of the Montes Apenninus 

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———— 1964, Radial structures surrounding lunar basins, II: Orientale and other systems; Conclusions: Arizona Univ. Lunar and

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Hartmann, W. K., and Kuiper, G. P., 1962, Concentric structures 

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A68 APOLLO 15-17 ORBITAL INVESTIGATIONS

Hartmann, W. K., and Wood, C. A., 1971, Moon: Origin and evolution of multi-ring basins: The Moon, v. 3, no. 1, p. 4-78.

Heacock, R. L., Kuiper, G. P., Shoemaker, E. M., Urey, H. C., and 

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analyses and interpretations: Pasadena, Calif., Jet Propulsion 

Lab. Tech. Report 32-700, 154 p.

Head, J. W., 1972, Small-scale analogs of the Cayley Formation and 

Descartes mountains in impact-associated deposits: Apollo 16 

Prelim. Sci. Report, NASA SP-315, p. 29-16 to 29-20.

———— 1974a, Morphology and structure of the Taurus-Littrow highlands (Apollo 17): Evidence for their origin and evolution: The 

Moon, v. 9, p. 355-395.

———— 1974b, Lunar dark-mantle deposits: Possible clues to the distribution of early mare deposits: Proc., Lunar Sci. Conf., 5th, 

Geochim. et Cosmochim. Acta, supp. 5, v. 1, p. 207-222.

———— 1974c, Orientale multi-ringed basin interior and implications 

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p. 327-356.

———— 1976, Evidence for sedimentary origin of Imbrium sculpture 

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1977, Origin of rings in multi-ringed basin: Evidence from

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Head, J. W., and Hawke, B. R., 1975, Geology of the Apollo 14 region

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Proc., Lunar Sci. Conf, 6th., Geochim. et Cosmochim. Acta,

supp. 6, v. 3, p. 2483-2501. 

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———— 1973, Apollo 17 site selection: Apollo 17 Prelim. Sci. Report,

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p. 29-20 to 29-24.

———— 1973, Mare ridges and lava lakes: Apollo 17 Prelim. Sci. 

Rept., NASA SP-330, p. 31-12 to 31-21.

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Cosmochim. Acta, supp. 4, v. 1, p. 1-25.

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1976, p. 612-613.

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———— 1975, Geologic map of the crater Copernicus: U.S. Geol. Survey Misc. Geol. Inv. Map 1-840, scale 1:250,000.

Howard, K. A., Carr, M. H., and Muehlberger, W. R., 1973, Basalt 

stratigraphy of southern Mare Serenitatis: Apollo 17 Prelim. 

Sci. Rept., NASA SP-330, p. 29-1 to 29-12.

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25-58 to 25-62.

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the Taurus-Littrow region: Apollo 17 Prelim. Sci. Rept., NASA

SP-330, p. 31-22 to 31-25. 

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Space Physics, v. 12, no. 3, p. 309-327. 

Howard, K. A., and Wilshire, H. G., 1975, Flows of impact melt at

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Taurus-Littrow region of the Moon: Proc., Lunar Sci. Conf., 4th, 

Geochim. et Cosmochim. Acta, supp. 4, v. 1, p. 149-162.

———1976, Mare ridges and related highland scarps—Result of vertical tectonism?: Proc., Lunar Sci. Conf., 7th, Geochim. et Cosmochim. Acta, supp. 7, p. 2761-2782.

Lucchitta, B. K., and Sanchez, A. G., 1975, Crater studies in the 

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Lunar Sample Preliminary Examination Team, 1972, Preliminary 

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NASA SP-315, p. 7-1 to 7-58.

————1973, Preliminary examination of lunar samples: Apollo 17 

Prelim. Sci. Rept., NASA SP-330, p. 7-1 to 7-46.

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by systematic geologic mapping, in Runcorn, S. K., ed., Mantles 

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————1968, Geologic results from the lunar precursor probes: Am. 

Inst. Aeronautics Astronautics Jour., v. 6, no. 10, p. 1991-1996.

————1969, Geologic map of the Alphonsus GA region of the Moon: 

U.S. Geol. Survey Misc. Geol. Inv. Map 1-586, scale 1:1,000,000.

————1977, Orientale and Caloris: Physics Earth and Planetary

Interiors, v. 15, p. 220-250. 

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Icarus, v. 29, p. 1-34.
STRATIGRAPHY OF PART OF THE LUNAR NEAR SIDE A69

Malin, M. C., 1974, Lunar red spots: Possible pre-mare materials: 

Earth and Planetary Sci. Letters, v. 21, p. 331-341.

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———1963, Geologic map and sections of the Letronne region of the

Moon: U.S. Geol. Survey Misc. Geol. Inv. Map 1-385, scale

1:1,000,000. 

Milton, D. J., 1968, Geologic map of the Theophilus quadrangle of the

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1:1,000,000.

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Geol. Survey Misc. Geol. Inv. Map 1-748, scales 1:250,000 and

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———1967, Geologic map of the Seleucus quadrangle of the Moon: 

U.S. Geol. Survey Misc. Geol. Inv. Map 1-527, scale 1:1,000,000.

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and Anders, Edward, 1974, Lunar basins: Tentative characterizations of projectiles, from meteoritic elements in Apollo 17 

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Caesar quadrangle of the Moon: U.S. Geol. Survey Misc. Geol. 

Inv. Map 1-510, scale 1:1,000,000.

Morrison, R. H., and Oberbeck, V. R., 1975, Geomorphology of crater 

and basin deposits—Emplacement of the Fra Mauro Formation: 

Proc. Lunar Sci. Conf., 6th, Geochim. et Cosmochim. Acta, supp. 

6, v. 3, p. 2503-2530.

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Geochim. et Cosmochim. Acta, supp. 5, v. 1, p. 101-110.

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investigation of the Apollo 16 landing site: Apollo 16 Prelim. Sci. 

Rept., NASA SP-315, p. 6-1 to 6-81.

Muehlberger, W. R., and 19 coauthors, 1973, Preliminary geologic 

investigation of the Apollo 17 landing site: Apollo 17 Prelim. Sci. 

Rept., NASA SP-330, p. 6-1 to 6-91.

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Princeton, N. J., Princeton Univ. Press, 391 p. (2d ed.).

Neukum, Gerhard, and Konig, Beate, 1976, Dating of individual 

lunar craters: Proc., Lunar Sci. Conf., 7th, Geochim. et Cosmochim. Acta, supp. 7, p. 2867-2881.

Neukum, Gerhard, Konig, Beate, and Arkani-Hamed, J., 1975a, A 

study of lunar impact crater size-distributions: The Moon, v. 12, 

p. 201-229.

Neukum, Gerhard, Konig, Beate, Fechtig, H., and Storzer, D., 1975b,

Cratering in the earth-moon system: Consequences for age 

determination by crater counting: Proc. Lunar Sci. Conf., 6th, 

Geochim. et Cosmochim. Acta, supp. 6, v. 3, p. 2597-2620.

Nichols, D. J., Young, R. A., and Brennan, W. J., 1974, Lunar 

kipukas as evidence for an extended tectonic and volcanic history of the maria [abs.]: Lunar Sci. Conf., 5th, Houston, Texas, 

Lunar Sci. Inst., p. 550-552.

Oberbeck, V. R., 1973, Simultaneous impact and lunar craters: The 

Moon, v. 6, p. 83-92.

———1975, The role of ballistic erosion and sedimentation in lunar

stratigraphy: Rev. Geophys. and Space Physics, v. 13, no. 2, p.

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Oberbeck, V. R., Horz, F., Morrison, R. H., Quaide, W. L., and Gault,

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Oberbeck, V. R., and Morrison, R. H., 1974, Laboratory simulation of

the herringbone pattern associated with lunar secondary crater

chains: The Moon, v. 9, p. 415-455.

————1976, Candidate areas for in situ ancient lunar materials: 

Proc. Lunar Sci. Conf., 7th, Geochim. et Cosmochim. Acta, supp. 

7, p. 2983-3005.

Oberbeck, V. R., Morrison, R. H., Horz, F., Quaide, W. L., and Gault, 

D. E., 1974, Smooth plains and continuous deposits of craters 

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Acta, supp. 5, v. 1, p. 111-136.

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Moon: U.S. Geol. Survey Misc. Geol. Inv. Map 1-666, scale 

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Pieters, Carle, Head, J. W., McCord, T. B., Adams, J. B., and Zisk, S. 

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Definition using remote sensing and lunar sample information: 

Proc., Lunar Sci. Conf., 6th, Geochim. et Cosmochim. Acta, supp.

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Sci. Inst. contribution 234, p. 125-129.

————1976, Characterization of lunar mare basalt types: 1. A remote 

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Proc., Lunar Sci. Conf., 7th, Geochim. et Cosmochim. Acta, supp.

7. v. 3, p. 2677-2690; also frontispiece of v. 1. 

Pieters, Carle, McCord, T. B., Zisk, S. H., and Adams, J. B., 1973,

Lunar black spots and the nature of the Apollo 17 landing area:

Jour. Geophys. Research, v. 78, no. 26, p. 5867-5875. 

Pike, R. J., 1971, Genetic implications of the shapes of martian and

lunar craters: Icarus, v. 15, no. 3, p. 384-395. 

Reed, V. S., and Wolfe, E. W., 1975, Origins of the Taurus-Littrow

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the Moon: U.S. Geol. Survey Misc. Geol. Inv. Map 1-602.

———— 1973, Lava flows in Mare Imbrium: Geologic evaluation from 

Apollo orbital photography: Proc., Lunar Sci. Conf., 4th, 

Geochim. et Cosmochim. Acta, supp. 4, v. 1, p. 73-92.

Schaber, G. G., Boyce, J. M., and Moore, H. J., 1976, The scarcity of 

mappable flow lobes on the lunar maria: Unique morphology of 

the Imbrium flows: Proc., Lunar Sci. Conf., 7th, Geochim. et 

Cosmochim. Acta, supp. 7, p. 2783-2800.

Schaber, G. G., Thompson, T. W., and Zisk, S. H., 1975, Lava flows in 

Mare Imbrium: An evaluation of anomalously low earth-based 

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Schultz, P. H., 1976, Floor-fractured lunar craters: The Moon, v. 15, 

p. 241-273.
A70 APOLLO 15-17 ORBITAL INVESTIGATIONS

Schultz, P. H., and Gault, D. E., 1975, Seismic effects from major 

basin formations on the Moon and Mercury: The Moon, v. 12, p. 

159-177.

Scott, D. H., 1972a, Structural aspects of Imbrium sculpture: Apollo

16 Prelim. Sci. Rept., NASA SP-315, p. 29-31 to 29-33.

———— 1972b, Geologic map of the Maurolycus quadrangle of the 

Moon: U.S. Geol. Survey Misc. Geol. Inv. Map 1-695, scale 

1:1,000,000.

———— 1972c, Geologic map of the Eudoxus quadrangle of the Moon: 

U.S. Geol. Survey Misc. Geol. Inv. Map 1-705, scale 1:1,000,000.

———— 1973a, Mare Serenitatis cinder cones and terrestrial analogs: 

Apollo 17 Prelim. Sci. Rept., NASA SP-330, p. 30-7 to 30-8.

———— 1973b, Small structures of the Taurus-Littrow region: Apollo

17 Prelim. Sci. Rept., NASA SP-330, p. 31-25 to 31-28.

1974, The geologic significance of some lunar gravity

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