Chapter 3: The Terrae

Figure 32. - Location of photographs in this chapter; numbers correspond to figure numbers. [Base map courtesy of the National Geographic Society.]
Figure 32. - Location of photographs in this chapter; numbers correspond to figure numbers. [Base map courtesy of the National Geographic Society.]
FIGURE 32. - Location of photographs in this chapter; numbers correspond to figure numbers. [Base map courtesy of the National Geographic Society.]

In the first chapter the terrae or highlands were described as the brighter, older, and generally higher standing terrain occupying most of the Moon's surface (fig. 14). Because they are older, the terrae are much more densely populated by large craters than are the maria. Even though the terrae occupy two thirds of the visible or Earth facing hemisphere (and about 85 percent of the entire surface), less was known about them than about the maria. This is so because of their greater age and apparent complexity and partly because only one of the five successful Surveyor spacecraft landed in the terrae.

Our understanding has, however, increased tremendously as a result of the Apollo missions. The last four missions have been especially rewarding in this respect. Analyses of the returned lunar samples, study of data from instruments emplaced on the lunar surface, and remote sensing instruments in the CSM have filled in many of the information gaps, but have also presented new problems.

Radiometric dates obtained on samples of terrae rocks confirm, as was believed earlier, that the terrae are older than the maria. Although the terrae are highly modified, they are composed of rock material that formed very early in the Moon's history by the process of magmatic differentiation. By this process, minerals formed within an igneous melt become segregated according to differences in their physical properties. Lighter materials rise to the top of the magma body by virtue of their lower specific gravity, and, after solidification, form low density rocks. Among the returned lunar samples thought to represent terra materials not completely altered by subsequent events, varieties of gabbroic anorthosite are the most common. This type of rock is composed largely of plagioclase with varying amounts of olivine and pyroxene. Plagioclase is a common mineral on Earth and one of rather low specific gravity.

The preponderance of anorthositic rocks in the lunar highlands is supported by data from Apollo remote sensing instruments. Some of the chemical differences between anorthositic and basaltic rocks could be determined by the X ray fluorescence and gamma ray experiments of Apollos 15 and 16. The X ray fluorescence results show a higher ratio of aluminum to silicon in the terrae than in the maria, corresponding to the known chemical difference between anorthositic and basaltic rocks. Results from the gamma ray spectrometer show that the terrae contain less iron and titanium than do the maria (Metzger et al., 1974). This also is consistent with the chemical compositions of anorthositic versus basaltic rocks.

The lower specific gravity of anorthositic rocks compared to basalts is another characteristic that was measured directly or indirectly by orbital experiments. The S band transponder experiment flown on the last five missions recorded variations in the lunar gravity field along the ground tracks. The results clearly show that the terra materials are less dense than mare materials. Indirect evidence comes from laser altimeters onboard Apollos 15, 16, and 17 that conclusively show that the terra regions are higher in average elevation than the maria. The continuous high resolution profiles of the Moon's surface provided by the electromagnetic sounder experiment on Apollo 17 substantiated the spot elevations recorded by the laser altimeter. The combined results of these three experiments indicate that most of the Moon's crust like most of the Earth's crust-is in isostatic equilibrium. In other words, areas of high elevation are underlain by low density rocks, low standing areas by high density rocks, and differences in elevation across broad areas are the result of differences in density, or specific gravity.

The ancient rock materials of the terrae have been drastically modified by various processes since their formation early in lunar history. Repetitive bombardment by impacting bodies has been the most important cause of modification. Countless impact events have resulted in the widespread redistribution of materials over the surface, the brecciation of the rocks so displaced, and the metamorphism by shock of the minerals that make up the rocks. The impact events have been so numerous and their cumulative effect has been so pervasive that few samples recognizable as original crustal material have been resumed by the Apollo missions.

Other processes that have modified the terrae are tectonism, volcanism, and mass wasting. Tectonism is visible in numerous linear structures transecting the terrae. Some have been recognized and mapped as normal faults, or as pairs of closely spaced normal faults bordering grabens. Some of the largest linear structures are on tile near side, radiating from the edge of the Imbrium basin. They are obviously related to the formation of that basin. However, over the entire Moon, the majority of linear features are oriented in northeast and northwest directions. This arrangement results in a rectilinear gridlike pattern referred to as the "lunar grid" (Strom, 1964). The origin of the lunar grid is unknown. It must have formed at an early stage because parts of it are modified and intersected by patterns of faults and gouges that radiate outward from the circular basins, themselves features of very considerable age.

Volcanism is clearly evident, for example, in the Abulfeda chain of craters extending for more than 200 km southeast from the crater Abulfeda (fig. 45). This chain is closely alined with two crater chains similar in appearance: one is near the crater Ptolemaeus and the other is near Piccolomini. Another area of possible volcanic activity is the Kant plateau, the edge of which was examined by the Apollo 16 astronauts. Both the Ptolemaeus and Kant areas are high and have abnormally high ratios of aluminum to silicon.

Mass wasting has affected the terrae by reducing differences in relief caused by cratering, tectonism, and volcanism. This form of erosion has subdued the inherent ruggedness of the terrae by moving materials from high areas to low areas. The rate of movement ranges from very slow (as by creep) to very rapid (as by avalanching). - H.M.

FIGURE 33. - This oblique view of a small area northeast of Tsiolkovsky on the Moon's far side is occupied entirely by terrae.

AS17-155-23702 (H)

FIGURE 33. - This oblique view of a small area northeast of Tsiolkovsky on the Moon's far side is occupied entirely by terrae. Like much of the far side terrae, it is unrelievedly cratered. Thousands of craters are visible, ranging in size from the limits of resolution (a few tens of meters at the lower edge of the picture) to the 75 km crater near the center. This part of the Moon is too far from any young multiringed circular basins to have been mantled by basin ejecta. In areas such as this, if anywhere, are to be found materials of the early lunar crust. - D.E.W.

FIGURE 34. - A mosaic of seven metric camera frames showing the northern part of the Crisium basin.
FIGURE 34. - A mosaic of seven metric camera frames showing the northern part of the Crisium basin. The inner lava flooded part of the basin is Mare Crisium. Basins, the principal features of the lunar terrae, are essentially large craters with more than one conspicuous concentric mountain ring. In the west half of this mosaic are two raised basin rings separated by a trough (between arrows) partly filled with mare lavas and light plains deposits. The higher and broader of the two rings forms the "shore" of Mare Crisium. In the east half of the mosaic, the rings are less obvious, partly because the Sun illumination is too high to cast prominent shadows and partly because the rings have been broken up by faulting and flooded by mare material. Craters are less numerous in this picture than in the previous one because many were destroyed or buried during the formation of the basin. Some basins, like Crisium, are deeply flooded by mare material, and others, especially on the far side, are flooded less or not at all. Therefore, the accumulation of mare material is not directly related to the formation of basins, and the terms "mare" and "basin" must always be kept distinct. - D.E.W.
FIGURE 35. - It is common for the circular form of a very old basin to survive even after all basin materials are obliterated.

AS17-1674 (M)

FIGURE 35. - It is common for the circular form of a very old basin to survive even after all basin materials are obliterated. Mare Vaporum, as shown in this south looking oblique photograph, provides a good example. Its average width is about 200 km. Its circular form marks the outer edge of the ancient, deeply buried Vaporum basin. All the terrae surrounding Mare Vaporum are blanketed by massive ejecta of the Imbrium basin, the center of which lies to the north, behind and to the right of the camera. The ejecta disappears beneath Mare Vaporum. The circularity is enhanced at the left (east) of the picture by a system of mare ridges and scarps that was localized over an old Vaporum basin ring. The cratered, linear Hyginus Rille is near the southern horizon, and the sinuous Conon Rille, to be described later in this book, is in the foreground. - D.E.W.

FIGURE 36. - The Haemus Mountains bound the southwestern edge of Mare Serenitatis and form the rim of the Serenitatis basin. They have a strongly lineated pattern that is most apparent in the lower left part of this stereoscopic view.

FIGURE 36. - The Haemus Mountains bound the southwestern edge of Mare Serenitatis and form the rim of the Serenitatis basin. They have a strongly lineated pattern that is most apparent in the lower left part of this stereoscopic view. (The width of the stereogram within this mosaic is shown by the bar across the bottom.) The trend of the linear pattern is radial to the Imbrium basin, the margin of which is about 250 km to the northwest of the edge of the picture. Carr (1966) described the mountains as composed mostly of ejecta from the Imbrium basin. The lineation may be due to shattering of the lunar crust by the Imbrium impact event, depositional fluting of the ejecta, gouges made by impacting debris from the Imbrium basin, or a combination of the three.

The prominent rilles in the upper part of the stereogram are grabens or fault troughs transecting both terra and mare surfaces. They are roughly concentric to the edge of the Serenitatis basin. The rilles become less distinct in the terrae, attesting to the easy destruction of surface features in terra material by mass wasting. Within Mare Serenitatis the rilles are partly flooded by the younger lavas that have filled the basin. A dark mantling material, named Sulpicius Gallus Formation, covers parts of the highland and mare surfaces alike. In the highlands the dark material has been removed from the tops of hills and steep slopes and reveals the underlying bright highland material. The dark mantle is conspicuous in the right center of the stereogram near the small kidney shaped crater (arrow). In this same crater and in small, young, rayed craters nearby, the Apollo 17 astronauts observed orange material. This suggests that the dark material here is similar to that sampled at the Apollo 17 landing site on the other side of the basin where orange material was found on the rim of a young crater. - B.K.L.

FIGURE 37. - An oblique view of the southeastern part of the Imbrium basin, one of the largest multiringed, circular basins on the Moon.

AS17-2433 (M)

FIGURE 37. - An oblique view of the southeastern part of the Imbrium basin, one of the largest multiringed, circular basins on the Moon. Most scientists agree that it was formed by the impact of an asteroid, comet, or other planetary body striking the lunar surface at hypersonic velocity. The Imbrium event excavated a depression nearly 1300 km in diameter in the terrae, uplifted and intensely deformed the adjacent terrae, and blanketed much of the lunar surface with debris ejected from the depression. The depressed area was subsequently flooded by lava flows to form the dark relatively smooth surface recognized as Mare Imbrium. The Montes Apenninus form the southeastern rim of the basin. They and other rugged areas of light material visible here are ancient terrae uplifted by the impact event and covered to varying thicknesses by ejecta debris. Material from the Apennine Mountains was collected by the Apollo 15 astronauts who landed near the foot of the mountains not far to the left and below (that is, to the northeast of) the area shown here. The arcuate trends parallel to the margin of the Imbrium basin are mostly faults associated with the formation of the basin. The numbers are explained in the caption for figure 38. - M.W.

FIGURE 38. - This mosaic of vertical frames covers part of the same area shown in the preceding oblique view (fig. 37), but shows the Montes Apenninus in much more detail.

AS17-1828 (M)

FIGURE 38. - This mosaic of vertical frames covers part of the same area shown in the preceding oblique view (fig. 37), but shows the Montes Apenninus in much more detail. So that the two pictures can be oriented and compared, the same two craters have been identified in each picture by numbers. The bulk of the mountain chain consists of giant blocks of lunar crust that were lifted and tilted outward by the impact that formed the Imbrium basin. These blocks have been covered by an unknown thickness of debris ejected from the basin. The hummocky deposits (H) probably were formed by the base surge a turbulent cloud of fluidized debris that moved outward along the surface from the point of impact. The hummocks resemble huge dunes. Their dimensions indicate a velocity of flow in excess of 100 km/hr and a maximum thickness of the deposits of several kilometers.

The Imbrium event is believed to have occurred 3.95 billion years ago. Later the basin was almost completely filled by successive flows of basaltic mare material. The same material also inundated parts of the outer edge of the Apennine Mountain chain, as in the lower right corner of the picture. - H.M.

FIGURE 39. - This mountain mass, called South Massif, on the southeastern rim of the Serenitatis basin towers 2000 m above the Apollo 17 landing site at the bottom left of the picture (arrow).

AS15-9297 (P)

FIGURE 39. - This mountain mass, called South Massif, on the southeastern rim of the Serenitatis basin towers 2000 m above the Apollo 17 landing site at the bottom left of the picture (arrow). The mountain is typical of the massifs forming the main rim of multiringed basins. Most lunar geologists believe that the massifs are individual fault blocks uplifted as a result of the impact event that created the basin. South Massif is composed of highly brecciated rock that was probably emplaced as ejecta from the Serenitatis basin, although similar brecciated ejecta from other more ancient and more distant basins may be present. Rocky outcrops on the top right of the mountain have shed clearly visible boulders and blocks, but most of the slopes are formed of finer mass wasted debris. Some of this debris has partially filled a small crater at the base of the mountain near the center of the picture (A). An avalanche of unconsolidated surficial material propelled by secondary impact slid off the mountainside and splayed out over the mare surface leaving a thin blanket of light colored breccia on the valley floor (B). The Apollo 17 astronauts traveled across the slide to the base of the mountain. - B.K.L.

FIGURE 40. - Across the valley of Taurus Littrow from South Massif (from fig. 39) is another big mountain known as North Massif.

AS17-136-20694 (H)

FIGURE 40. - Across the valley of Taurus Littrow from South Massif (from fig. 39) is another big mountain known as North Massif. In this scene of North Massif, an astronaut is kneeling at right. The path traced by a big boulder rolling down North Massif is indicated by arrows. This track is large enough to be visible but just barely-in some panoramic camera frames taken from orbit at an altitude of slightly more than 100 km. On both North and South Massifs boulder tracks such as this one were used by geologists as markers to find the original positions of boulders that were sampled by the astronauts. - K.A.H.

FIGURE 41. - This is a closeup view of part of the boulder track shown in figure 40.

AS17-144-22129 (H)

FIGURE 41. - This is a closeup view of part of the boulder track shown in figure 40. The track is about 10 m wide and the boulder that made the track is about 18 m in diameter (Mitchell et al., 1973). The cause of movement, other than the obvious effect of gravity, is uncertain. Various investigators have suggested that movement was initiated by seismic vibrations of internal origin, vibrations caused by repeated impact events, cyclic thermal expansion and contraction, and instability as soil accumulates above the boulder or is removed from below it. It is also possible that some tracks are formed by projectiles, presumably from impact craters, that skid or bounce along the surface before coming to rest. From detailed studies of boulder tracks, some properties related to the strength, density, and thickness of the lunar soil can be measured. - G.W.C.

FIGURE 42. - One of the high, steep peaks of the Apennine Mountains, the highest part of the Imbrium basin rim.

AS15-9804 (P)

FIGURE 42. - One of the high, steep peaks of the Apennine Mountains, the highest part of the Imbrium basin rim. Lighting is from the east (right). Lunar peaks are normally thickly mantled by their own debris; and most of this peak is mantled, but some outcrops of bedrock are also visible. These include ledges along the ridge top in the center of the picture and, probably, protrusions trending diagonally down the slopes. The fine lineations trending directly downslope are probably tricks of lighting produced by the grazing Sun illumination and not, as was believed during the Apollo 15 mission, edges of strata. Debris from the slopes has accumulated in a smooth convex upward band all along the base of the mountains but is most noticeable in the area between the two arrows. The photographed area is about 14 km wide. - D.E.W.

FIGURE 43. - This stereogram showing part of the Apennine Mountain chain is composed of two Hasselblad frames taken from the Moon's surface.

AS15-90-12187 (H) [left] AS15-90-12209 (H) [right]

FIGURE 43. - This stereogram showing part of the Apennine Mountain chain is composed of two Hasselblad frames taken from the Moon's surface. To facilitate the postmission geologic study of the landing site, the astronauts took a panoramic series (a clockwise sequence) of horizontal photographs at each major sample collecting station. Both these photographs look northeast from stations about 250 m apart along one of the EVA traverses at the Apollo 15 site. The foregrounds, which do not cover a common area, have been eliminated from the stereogram. The impression of depth in the background is greatly enhanced in comparison with the direct view of the astronauts. The mountain at the left is Mount Hadley at a distance of 20 km. The faint lineations trending downward to the left on Mount Hadley's face are the features that were believed by some investigators, at the time of the mission, to be steeply dipping strata; however, the explanation given in figure 42 now seems more likely. - L.J.K.

FIGURE 44. - A north looking oblique view of the region surrounding the Apollo 14 landing site (arrow), about 600 km south of the Imbrium basin, which is just beyond the horizon.

AS16-1420 (M)

FIGURE 44. - A north looking oblique view of the region surrounding the Apollo 14 landing site (arrow), about 600 km south of the Imbrium basin, which is just beyond the horizon. The fine, hummocky material extending through the center of the frame from the lower edge to the horizon has been mapped as ejecta from the Imbrium basin and designated the Fra Mauro Formation. The formation is most easily distinguished in the western half of Fra Mauro, the large (95 km) crater near the center. In the eastern half of the same crater the formation is either absent or is too thin to be visible. The straight rilles trending toward the lower right corner of the frame may be related to the radial stresses generated by the Imbrium event, but detailed mapping has shown that they are much younger in age. - M.W.

FIGURE 45. - These terrae of the central highlands are near the Apollo 16 landing site. The landing point is indicated by the arrow just below the lower (northern) edge of the picture.

AS16-0700 (M)

FIGURE 45. - These terrae of the central highlands are near the Apollo 16 landing site. The landing point is indicated by the arrow just below the lower (northern) edge of the picture. Here the terrae are less rugged and less densely cratered than those on the far side, shown in figure 33. The subdued appearance of these near side terrae is attributed to accumulations of successive blankets of ejecta from impact basins and possibly to the accumulation of volcanic materials in this, the topographically highest region on the Moon's near side. Descartes, for which the landing site was named, is the ancient, highly subdued crater near the center. It is 47 km in diameter and so indistinct in this view that its rim has been outlined by dashes. The unusual Abulfeda crater chain extends for a distance of 225 km across the upper part of the picture. - G.G.S.

FIGURE 46. - In contrast to figure 45, which is an oblique view looking southward, this is a vertical view arranged for stereoscopic viewing.

AS16-0976 (M) AS16-0974 (M)

FIGURE 46. - In contrast to figure 45, which is an oblique view looking southward, this is a vertical view arranged for stereoscopic viewing. North is at the top and the crater Descartes (D) is partly visible at the lower edge. The craters North Ray (N) and South Ray (S) are prominent landmarks used during the planning and conduct of the mission. The LM landed between the two. The age of South Ray is discussed in more detail in figure 105. The smooth materials filling most topographically low areas are mapped as the Cayley Formation. Samples of the Cayley excavated from the craters were collected by the astronauts. Most of the samples consisted of feldspar rich breccia. This is consistent with an early interpretation (Eggleton and Marshall, 1962) that the Cayley accumulated as ejecta from large multiringed basins, probably Imbrium (1400 km to the northwest) and Orientale (2800 km to the west). The more rugged material immediately east of the landing site is designated as the Descartes Formation. Originally interpreted as volcanic deposits, it is now thought to consist mostly of breccia, although the samples collected may not be representative of the entire formation. From the evidence presently available, its origin as basin ejecta, possibly from the Nectaris basin 450 km to the east, seems to be the most likely explanation. - G.G.S.

FIGURE 47. - The ruggedness of the grooved and furrowed terrain of the central lunar highlands is emphasized here under low Sun illumination of about 3° compared with an illumination angle of about 27° in figure 46.

FIGURE 47. - The ruggedness of the grooved and furrowed terrain of the central lunar highlands is emphasized here under low Sun illumination of about 3° compared with an illumination angle of about 27° in figure 46. In the north south direction, this mosaic covers a distance of 210 km. While these photographs were taken, the Apollo 16 crew was completing its first revolution in orbit and described the scene as follows: "In this lighting, you can see the crater Descartes, and it stands out much bigger than you would expect because of the low Sun angle. In fact, I had to look up my map to make sure that that was what I was looking at. . . . It looks very much like a big clinkery cinder field, . . . a big, rounded surface of clinkers. It is fantastic . . . boy, is that rough!" The crater Descartes itself (D) is subdued and partly covered by the rough topography. The Apollo 16 landing site (arrow) is in a gap within the grooved and furrowed unit. Contrary to premission beliefs that this unit was of volcanic origin, returned samples from the site are breccias formed by shock metamorphism of the highland rock. - F.E.- B.

FIGURE 48. - A basin forming event severely affects the terrain even beyond the outer margins of its ejecta deposits.

AS16-1411 (M)

FIGURE 48. - A basin forming event severely affects the terrain even beyond the outer margins of its ejecta deposits. This system of ridges and grooves known as Imbrium sculpture is radial to the Imbrium basin, located beyond the upper left horizon about 650 km from the center of this scene. The crater Herschel, 40 km in diameter, is at the center of the right edge. It is perched on the grooved rim of Ptolemaeus, part of whose floor and rim is visible in the lower right corner. Both secondary impact and faulting have been proposed as causes of the ridges and grooves, but in any case the sculpture must have been produced by the same event that produced the Imbrium basin. - D.E.W.

FIGURE 49. - An area centered about 900 km southeast of the Imbrium basin, illustrating again the radial fracturing and sculpturing of terra materials by the basin forming event.

AS16-0982 (M)

FIGURE 49. - An area centered about 900 km southeast of the Imbrium basin, illustrating again the radial fracturing and sculpturing of terra materials by the basin forming event. The arrow points to a 120 km long fracture that cuts the rims of the partly visible crater Albategnius in the lower left of the photograph and the crater Halley toward the upper left. It and similar trending fractures elsewhere in this picture are radial to the Imbrium basin and are related to its formation. The crater Hipparchus C (HC) is superposed on a fracture and, therefore, is younger than the Imbrium basin and the Imbrium sculpture. Light plains forming materials (LP) are younger than the Imbrium event, as indicated by the absence of fractures and the scarcity of superposed craters. Light plains deposits are a major stratigraphic unit of the terra regions and will be illustrated in more detail beginning with figure 53. - M.W.

FIGURE 50. - Between Mare Crisium and Mare Serenitatis Apollo 17 approached its eventual landing site, which is in shadow at the left edge of the photograph (white arrow).

AS17-0304 (M)

FIGURE 50. - Between Mare Crisium and Mare Serenitatis Apollo 17 approached its eventual landing site, which is in shadow at the left edge of the photograph (white arrow). The area consists of both mare and terrae. Maraldi, a crater described in figure 52, is shown by the black arrow. A markedly rectilinear pattern of major terra features and mare terra contacts is characteristic of this area. Closely spaced intersecting structures also produce a finely textured pattern of equidimensional hills. The northeast and northwest directions of the large and small structural elements are less consistent with structural trends of the nearby Crisium and Serenitatis basins than with the more distant Imbrium basin. Therefore, Imbrium ejecta or seismic energy may have produced the structures. When viewed under conditions of high angle lighting, the pattern of small hills is referred to as "corn on the cob" or as "sculptured hills." The appropriateness of these terms is demonstrated on the next figure, a stereogram enlargement of the rectangular area in the lower right corner of this picture. - D.E.W.

FIGURE 51. - The southeastern corner of figure 50 is shown here as a stereogram of two Apollo 16 pictures taken when the Sun angle was much higher.

FIGURE 51. - The southeastern corner of figure 50 is shown here as a stereogram of two Apollo 16 pictures taken when the Sun angle was much higher. The contrasting albedo (brightness) of the dark mare and bright terra is enhanced under these lighting conditions. When viewed stereoscopically, the "corn on the cob" texture of the terra is readily apparent. The rims and walls of the ancient craters Franz (F) and Lyell (L) have been severely degraded by erosion and show the same texture as the adjacent terra. These two craters contrast strongly with the much younger crater (C) whose original form has not been significantly degraded. - D.H.S.

FIGURE 52. - Maraldi, a 45 km impact crater, was shown in its regional context in figure 50. Its rectilinear shape is in striking contrast to the circular or oval shape of most lunar impact craters.

AS17-2302 (P)

FIGURE 52. - Maraldi, a 45 km impact crater, was shown in its regional context in figure 50. Its rectilinear shape is in striking contrast to the circular or oval shape of most lunar impact craters. Faulting along northwest and northeast planes, probably generated by the Imbrium event, is the cause of the unusual configuration of its walls. Debris aprons form a narrow but continuous terrace along the base of the crater wall. The high rate of mass wasting on the steepest slopes is proven by the low density of craters superposed on the crater walls in contrast to that of the much younger mare surface in the floor of Maraldi. Ultimately, as depicted near the north edge of the picture, landforms evolve toward rounded forms partly buried under their own debris aprons. - M.J.G.

FIGURE 53. - Light plains are a conspicuous feature of several lunar terra regions including the central near side highlands.

AS16-0708 (M)

FIGURE 53. - Light plains are a conspicuous feature of several lunar terra regions including the central near side highlands. In this scene essentially all flat lying areas are covered by plains deposits. No mare materials are visible. Light plains deposits are not completely planar but faintly reveal underlying relief. The subdued features that underlie the plains deposits in the floor of the large crater Albategnius (lower center) are craters and linear "Imbrium sculpture" troughs like those on the rim of Albategnius and in the adjacent highlands. The rim crest diameter of Albategnius is about 130 km. - D.E.W.

FIGURE 54. - This picture shows the subdued, 1 50 km diameter, crater Ptolemaeus. The crater is filled to about half its original depth by the Cayley Formation, a unit with a gently undulating, nearly smooth surface.

AS16-0989 (M)

FIGURE 54. - This picture shows the subdued, 1 50 km diameter, crater Ptolemaeus. The crater is filled to about half its original depth by the Cayley Formation, a unit with a gently undulating, nearly smooth surface. This unit forms similar smaller pools in numerous irregular depressions at various levels on the rim, flank, and wall of Ptolemaeus. The Cayley Formation consists of patches of light colored plains materials that fill most depressions peripheral to the Fra Mauro Formation (fig. 44) in the central earthside lunar uplands. Undulating surface features on the Cayley include very subtle circular depressions (d) 10 to 15 km in diameter that are more than an order of magnitude larger than the craters superposed on the Cayley, and irregular swells, swales, and scarps. Other surficial features are small, equidimensional, steep sided hills (h). The latter may have been formed on the surface of the Cayley by eruption of material from within the unit. In addition, more than 30 small craters on the Cayley have small central mounds (fig. 55). These mounds may represent relatively strong material that underlies a weak surficial layer of post Cayley regolith, indicating that the regolith is thicker here than on mare surfaces.

The deposition of the Cayley in pools indicates that it moved partly as a fluid. The distribution of the pools peripherally to a deposit of basin ejecta, the Fra Mauro Formation, indicates a related origin. The large, subdued crater forms suggest that the Cayley material is a draped blanket of fragmental material. Therefore, my colleague G. G. Schaber and I have suggested that the Cayley is a deposit of basin ejecta that became segregated from the ballistically transported ejecta that formed the Fra Mauro Formation around the Imbrium basin. The Cayley was transported separately as a fluidlike cloud that flowed along the ground across the whole region; portions were left behind to accumulate in local depressions - R.E.E.

FIGURE 55. - This picture shows Ptolemaeus under a Sun elevation angle of 45°.

AS16-2965 (M)

FIGURE 55. - This picture shows Ptolemaeus under a Sun elevation angle of 45°. The undulations on the Cayley Formation are so subtle that they disappear under the high Sun. The arrows point to examples of the small, relatively sharp, young craters with central mounds mentioned in figure 54. The mounds may be harder than the shallower materials and may indicate the depth of the regolith formed on the Cayley. - R.E.E.

FIGURE 56. - The very large crater or small multiringed basin Mendeleev is shown here at the same scale as the preceding two pictures of Ptolemaeus.

AS16-0875 (M)

FIGURE 56. - The very large crater or small multiringed basin Mendeleev is shown here at the same scale as the preceding two pictures of Ptolemaeus. Like Ptolemaeus, Mendeleev is largely filled by plains material. In this case, however, subsequent cratering has been much more extensive, indicating that the Mendeleev plains are older than those in Ptolemaeus. The source of these plains materials on the far side of the Moon is unknown. The linear chain of elongate craters near the left side is probably of secondary origin, formed by the impact of fragments ejected from Tsiolkovsky (850 km to the southwest). - D.E.W. and C.A.H.