Chapter 6: Rimae
![Figure 184. - Location of photographs of sinuous rilles; numbers correspond to figure numbers. [Base map courtesy of the National Geographic Society.]](./images/p178.jpg)
Figure 184. - Location of photographs of sinuous rilles; numbers correspond to figure numbers. [Base map courtesy of the National Geographic Society.]
Sinuous Rimae
The sinuous valleys on the Moon (called sinuous rimae or rilles) have been the focus of much debate. The most extreme view is that they were cut by flowing water. However, study of the details of their shape and the nearly complete lack of water in returned lunar samples make this hypothesis very unlikely. Two other possible hypotheses are that they were formed by faulting and subsidence of the lunar crust and that they are lava channels or collapsed lava tubes. The wide variety in shapes suggests that both of the latter processes have been involved. Some sinuous rilles seem to represent almost pure fault troughs while others represent valleys that were formed by flowing lava and modified by filling of slump materials from the walls.
Data obtained by the Apollo 15 astronauts when they landed beside Hadley Rille indicate that layered basaltic lava flows are exposed in the walls of the valley. These rocks may be the only ones sampled during Apollo that had not been moved one or more times by impact; that is, they were "bedrock" samples. Hadley Rille starts in a volcanic crater on the flank of the Apennine Mountains and flows into the Imbrium basin. The floor of the channel is very irregular; possible eruptions of lava along the channel course may have added to the material flowing from the source crater. Also, the meander pattern of the rille, with points projecting into the channel on one side and rounded on the opposite shores, indicates a great deal of modification by flowing lava of what may originally have been a fault trough.
In some sinuous channels a small valley has formed within a larger valley, indicating at least two episodes of valley formation. Other meandering channels do not start or stop in a crater-they are formed on lava plains that are so flat it is difficult to tell the direction of flow. Still other channels have formed on mountainsides covered by hummocky ejecta thrown outward from major impact basins; these rilles may have been formed by some process other than lava flows.
In summary, there appear to be several different types of sinuous valleys on the Moon. One current theory is that most of them are channels formed by basaltic lavas. The valleys may have originated as fault troughs that were later modified and obscured by lava flows, impact ejecta, or landslide material from the walls. A few of the sinuous valleys (those that have formed on the hummocky mountainous terrain) still are difficult to explain by the processes suggested so far. - H.M.
AS15-2611 (M)
FIGURE 185. - This south looking oblique view, centered near 25.5° N, 50.5° W, depicts a prominent "cobra head" rille, Vallis Schroteri (Schroter's Valley). In the foreground is the Aristarchus plateau, and in the background the smooth surface of Oceanus Procellarum. The two large craters in the middleground are Aristarchus (38 km diameter) on the left, and Herodotus (30 km diameter) on the right. At first look the sinuous, flat floored Schroter's Valley and the tightly meandering channel within bear a striking resemblance to river valleys on Earth, and some viewers of the Moon have thought that sinuous rilles were formed by flowing water. In detail, however, sinuous rilles differ from river valleys in many respects; for instance, Schroter's Valley becomes smaller toward its downstream end. They are much like an entirely different terrestrial feature-lava channels. (See fig. 189.) Most geologists think that Schroter's Valley is a channel through which lava flowed from the circular crater at the "head" of the rille to the lower elevation of Oceanus Procellarum, a distance of approximately 175 km. Figures 186 to 188 are enlargements of the four sided areas outlined in this photograph. - M.C.M.
AS15-0341 (P)
FIGURE 186. - Schroter's Valley in the Aristarchus plateau is one of the largest lunar sinuous rilles (width in picture is about 5 km). The valley consists of an arcuate rille (1) that contains a meandering sinuous rille (2). The valley here traverses what appears to be a lava plain embaying low hills in the southern part of the picture. The blocky outcrop ledges, probably lava layers, near the rim (3) and the blocks at the inside base of the slopes (4) are of interest as is the downslope movement of material in the walls, which results in partial burial of the inside rille (5) and shows that the valley is laterally enlarged by mass wasting processes.
The sinuous shape, uniform width, presence of low levees, irregular depressions at the head of this and other rilles (outside the picture), and uniform cratering of floor and surrounding terrain suggest that the feature originated as lava flow channels, or collapsed lava tubes (Greeley, 1971). The rilles in the picture probably formed by the draining of a large lava flow channel and a smaller channel in a somewhat later flow that was confined within the boundaries of the larger channel. Incision by thermal erosion of lava streams with turbulent flow is an alternative explanation for the formation of sinuous rilles such as Schroter's Valley (Hulme, 1973).
Other conspicuous features in the pictures are secondary crater clusters from the young crater Aristarchus (6). The secondary clusters cross the rille at (7), and show that the crater Aristarchus is younger than the rille. - B.K.L.
FIGURE 187. - A view near the mouth of Schroter's Valley. Clearly shown are the old broad valley stage of formation of the valley-largely controlled by faulting-and the fresher, younger, meandering inner valley that probably represents a basaltic lava channel. Rocks returned by Apollo 15 from the banks of a similar channel are layered flows of vesicular (full of gas bubble holes) basalts. The valley drops 1600 m along its length a slope similar to terrestrial lava channels. Secondary craters from the bright fresh impact crater to the east show that the impact crater is younger than the valley. The valley like all terrestrial and lunar lava channels-gels narrower and shallower downstream, possibly reflecting the cooling of the lava and its loss of mobility as it gets farther from its source. - H.M.
AS15-0332 (P)
FIGURE 188. - The pile of volcanic rocks that surround the crater at the head of Schroter's Valley has been called the Cobra Head. The crater is 40 km in diameter, the pile of volcanic rocks is 100 km in diameter. The abundant secondary craters on the volcano and the channel show that the crater Aristarchus, from which they were thrown, is younger than Schroter's Valley. The straight line segments of the valley that turn at sharp angles show that the basic form of the valley has been made by faulting, or breaks, in the lunar crust. The sinuous, meandering valley indicates that the basic straight valley shape has been modified heavily by later lava flows coursing down the valley. Samples returned by Apollo 15 from the side of a similar valley, Rima Hadley, strongly indicate that the rocks in the valley walls are basaltic lava flows. The rocks are layered because of the flow and have many holes (vesicles) formed by gas escaping from the rocks when they were still molten. - H.M.
AS15-9309 (P)
FIGURE 190. - Pictured here is a lunar sinuous rille located in the eastern part of Mare Serenitatis. It is interpreted to be a lava channel because it resembles terrestrial lava channels like the one in figure 189. The rille starts in a crater (presumably the volcanic source) and follows a sinuous course downhill. The channel edges are marked by low ridges resembling those that form on actively flowing lava channels on Earth; the terrestrial ridges are formed when the outer edges of the molten lava chill and make natural levees. The lower reaches of the channel may be a lava tube that formed when the upper surface of the flowing lava chilled and froze while the hotter lava in the interior continued to flow. On the other hand, this ridge at the end of the channel may be a fracture along which lava has been extruded. Similar features are called "squeeze ups" in terrestrial lava flows. A possible small lateral lava flow is visible near the end of the feature. - H.M.
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FIGURE 189. - This volcanic crater and lava channel are near Craters of the Moon National Monument in southeastern Idaho. The resemblance to lunar "cobra head" rilles (like the one in fig. 188) is immediately apparent. The crater lies in the gently sloping Snake River Plain, a broad expanse of volcanic rock with craters and linear and sinuous features thought to have formed in the same manner as their lunar counterparts. Apollo astronauts studied this area in preparation for lunar missions. The crater pictured is approximately 100 m in diameter, and the associated channel is over 5 km long. - M.C.M.
AS15-2606 (M)
FIGURE 191. - The sinuous rilles in this photograph, east of the Aristarchus Plateau, are a particularly interesting complex of these unusual features for which no entirely convincing terrestrial analogs have yet been recognized. The rilles are at least partly controlled by fracture and most originate in craters. Rille A, beginning on the flank of the crater Prinz, appears to have had a distinct two cycle history, producing crater in crater and rille in rille structures. Rille B crosses a ridge of highland material without deviation or deformation, suggesting that the feature was superposed-that is, let down-from an earlier higher mare surface; the ridge appears to have been eroded. A shallow narrow rille (not visible in this photograph) occurs within the broader valley of rille C and is traceable across the elongate collapsed depression that bisects the main rille. Craters at the heads of rilles probably represent source vents for fluids that either eroded the rilles or formed lava tubes that drained, contributing to the volume of mare lava in Oceanus Procellarum.
Ejecta from the young crater Aristarchus forms lightcolored streaks or "rays" across the dark mare surface; the high albedo of the rays may be due in large part to disruption of the surface by secondary craters. - C.A.H.
AS15-2083 (M)
FIGURE 192. - A low Sun angle, larger scale view of part of the area shown in figure 191. Rima Prinz I (1) graphically displays many of the features considered to be indicative of lunar basaltic lava channels. The rille starts in a crater on the side of the ancient crater Prinz (just off the photograph) and descends about 300 m, becoming narrower and shallower downslope. It is a "two story" channel with a broader older channel and crater inside of which is a younger, more sinuous, channel with its source vent. Samples returned by Apollo 15 from the very similar looking Rima Hadley were from a vesicular (full of holes formed by gas bubbles) flow of layered basalt. The next channel to the west (2) also gets narrower and shallower downslope. It is the best example of distributaries-that is, a branching network of smaller channels at the downstream end of a larger channel.
Krieger (3) is a "Gambart type" crater inferred to be volcanic in origin. Its flat floor, irregular shape, and highly irregular external deposits resemble the crater Gambart south of Copernicus, which was studied in 1967 by Apollo 17 astronaut H. H. Schmitt. The deposits from Krieger lie on the surface of the mare basalts, indicating that the crater is quite young. Its youthfulness is confirmed by the freshness of the crater floor deposits and the characteristic shape of these deposits. A nice example of a sinuous rille, interpreted as a lava channel (4), runs out of the crater onto the mare surface. This lava surface is marked by wrinkle ridges (5)- complex mare ridges, generally asymmetric, with a braided ridge along one edge. These ridges are interpreted to be faults or breaks in the mare lava flows along which a later generation of molten lava has been both intruded, raising the already cooled mare lava flows, and extruded onto the mare surface.
A small (8 km diameter), young impact crater (6) is excavated into the mare material. The continuous ejecta blanket formed by the base surge-turbulently flowing ejecta riding on the surface-is particularly well shown. It forms a typical dune pattern (7); the crest to crest distance (200 m) is an index of the velocity of flow of the base surge. Similar dune lava features have been seen forming around terrestrial experimental craters. - H.M.
AS15-0321 (P)
FIGURE 193. - A larger scale view of part of the area of figure 192 shows the bend in Rima Prinz II. It and Rima Prinz I form subparallel linear depressions that originate in small craters on the north flank of the crater Prinz. Both make right angle bends about 1/4 to 1/3 of the way from their source. The maximum width of the part of Rima Prinz II shown in this photograph is about 1.5 km. It is at least 100 m deep in the dark mare materials, and shallower in the rugged and older circumbasin (terra) materials. The rille is normal to the terra ridge where it cuts across the ridge, and is younger than the youngest materials it incises.
The processes by which lunar rilles form are open to controversy. Their sinuosities lack the characteristics of meanders in most terrestrial streams. The rims along the north portion of Rima Prinz II have both a rectilinear zigzag pattern and subdued arcuate sinuosity. In general, a concave reentrant is opposite a protruding wall. The rille floor displays the same crater density and the same crater size distribution as the mare materials adjacent to the rim. The morphological evidence, therefore, suggests that tensional stresses in the lunar crust probably caused the rille floor to subside between the steep normal faults that form the rille walls. The zigzag pattern of the walls probably is caused by irregular faulting along conjugate joints and fractures in the lunar grid. - M.J.G.
AS15-0320 (P)
FIGURE 194. - The sinuous rilles here are part of a network controlled to some degree by fractures. In an area view (fig. 192) it may be seen that the east west segment at A parallels prominent linear trends of several rilles northeast of the Aristarchus Plateau. Fracturing alone, however, cannot explain the origin of the rilles. As shown by the larger rille (B), material has clearly been removed from the walls so that, despite their parallelism, they cannot be fitted together. In addition, a terrace occurs at C, suggesting two cycles of rille formation. The larger rille is nearly obliterated at its juncture with rille A, suggesting that formation of A may have been slightly later, and that some sort of fluid erosion and overbank flooding may have been involved. - C.A.H.
Sinuous Rimae
AS15-88-11982 (H)
FIGURE 195. - Relief is exaggerated in this near terminator (low Sun) photograph of the northwest margin of the Aristarchus Plateau. A broad graben (approximately 15 km wide) separates the rugged linear mountain chain from the sloping plateau surface. The crenulate profiles of mare wrinkle ridges (some as high as 200 m) are visible on the horizon; one mare ridge bisects the graben. Sinuous rilles are prominent in the graben; several originate on the plateau. The surface of the mare in the graben appears to be higher on the west side of the mare ridge, suggesting faulting associated with ridge formation. The rille A crossing the ridge is nearly 100 km long and has interlocking meanders, which preclude its formation as merely a crack in the mare surface; erosion by downstream transport of a fluid seems necessary to explain such sinuosity. Origin of the diamond shaped Aristarchus plateau itself is enigmatic, although its relatively straight edges suggest fault control. - C.A.H.
AS15-0349 (P)
FIGURE 196. - Sinuous rille (A) is unusually shallow for its width and has probably been mantled or filled since its formation. The entire area shows evidence of an older mantled and subdued topography on which secondary craters (B) from the 40 km primary crater Aristarchus (to the southeast) have been superposed. Among the mantled features are a straight rille (C), craters (D), and numerous polygonal depressions (E), which may be subsidence features, possibly caused by withdrawal of magma at depth, or differential compaction of lava over small highland blocks. The Aristarchus plateau is to the southeast. - C.A.H.
AS15-93-12725 (H)
FIGURE 197. - A number of mare related features are present in this oblique view looking north across a mare surface between Mare Imbrium and Oceanus Procellarum. A broad mare arch is visible in the foreground. As it is followed toward the northwest the arch gives way to a series of small ridges, which then converge on an unusual sinuous chain of craters, elongate ridges, and elongate depressions. This sinuous chain leads into a large arcuate elongate depression at the boundary between the highlands and the mare. Similarity of parts of this structure to parts of some sinuous rilles, for example, Rima Hadley, suggests that its strange morphology may be a poorly understood variation of a lava channel, possibly a partially collapsed lava tube. - J.W.H.
AS17-3128 (P)
FIGURE 198. - The branching sinuous rille in this photograph is part of a continuous single rille extending more than 300 km across the mare in northeast Oceanus Procellarum. The width of the rille is essentially constant (approximately 250 m) throughout its length. The secondary branches, all of which rejoin the main "stream," are shallower than the larger channel. Slope is probably northeast toward the center of Mare Imbrium. Like some other lunar rilles, this one crosses several mare ridges with no apparent deviation or deformation; unlike many rilles, particularly those near the Aristarchus plateau, there is no associated crater at either end. The origin of lunar sinuous rilles remains controversial. Among the alternatives proposed are lava channels and lava tubes, but fracture control is decidedly apparent in some places. Some sort of fluid erosion, however, seems necessary to account for the configurations of many rilles with exactly parallel walls from which material has been removed; lava may be the most plausible agent for erosion inasmuch as no evidence of water exists in the lunar samples. The diversity among rilles suggests that several genetic hypotheses may be required to explain all of them. - C.A.H.
AS15-2076 (M)
FIGURE 199. - Delisle and Diophantus are two relatively young craters, 27 km and 19 km in diameter, located north of the area shown in figure 198, in the western reaches of Mare Imbrium. Both are younger than the mare materials in which they are excavated, and also younger than the narrow sinuous rille between them. The rille is sculptured by lineaments radial to the rim crests of Diophantus or Delisle; the lineaments presumably were produced by ejecta from one or both craters.
Both craters exhibit characteristics of young lunar impact craters that have undergone relatively little degradation. The rim crest is sharp, slump terraces inside the walls are distinct, and the density of younger superposed craters is low: only one very young rayed crater, 3 km in diameter, is superposed on Diophantus ejecta.
Five major morphologic facies of crater ejecta can be distinguished in concentric zones away from the crater rim crests: (1) closest to the rim crest, a zone of pitted terrain, dotted with tiny circular craters and large blocks; (2) a zone of rolling troughs and ridges with a smooth surface, grading outward into the third type; (3) elongate or chevronlike secondary impact craters, the rim crests of which form ridges radial to the rim crest of the primary crater; (4) a reticulate network of discontinuous ejecta, forming ridges radial to the rim crest of the primary crater; and (5) isolated secondary impact craters that dot the surface of preexisting mare materials.
The scarp bounded mountains west and southwest of Delisle are massifs of light toned circumbasin materials that are older than the mare and crater materials around them. - M.J.G.
AS15-1702 (M)
FIGURE 200. - Low Sun angle photograph of mare surface taken by the metric camera. Two large impact craters, Diophantus and Euler, occur in the upper left and lower right comers of the frame; the terminator lies at the left margin of the picture. Many secondary crater chains and smaller primary craters pepper the mare surface, and lava flow fronts, mare ridges, and rilles are common. The rilles exhibit a marked branching or dendritic network pattern. The lava channels become narrower and shallower downstream, to the northeast; terrestrial lava channels also commonly become shallower downstream. The lobes of the lava flows also point northeastward. Apparently the material filling the basin moved from southwest to northeast in this region. The lava flows and rilles obviously antedate the impact craters that pockmark the surface. The outlined area shows the location of figure 201. - H.M.
AS17-23718 (H)
FIGURE 201. - This low Sun angle, high resolution picture shows in greater detail the branching network of lava channels displayed in figure 200. The long shadows along the channel network are formed by its natural levees of chilled lava. The channels branch downslope, as is customary in lava distributary systems on Earth. Small impact craters that postdate the lava flows riddle the channel margins and bottoms and saturate the lava surfaces. Many form lines of secondary craters. Tectonic displacements (faults) offset the lava surfaces and cut across the lava channels; they are, therefore, considered to be younger than the lava channels. - H.M.
FIGURE 202. - This Apollo IS photograph depicts the sinuous Hadley Rille, the Apennine Mountains trending from lower left to upper right, and the smooth surface of Palus Putredinus (Marsh of Decay) in the upper left quarter. The Apollo 15 landing site at 26.4° N, 3.7° E (arrow) was selected because of the variety of important lunar surface features concentrated in the small area. The Apennine Mountains, with almost 5 km of relief in the area pictured, are a part of a ring of mountains that surrounds the Imbrium basin and in which very old rocks are thought to be exposed. The smooth lava flows of Palus Putredinus formed later, and Hadley Rille, sharply etched in the mare surface, is thought to be one of the youngest rilles on the Moon. The V shaped rille originates in a cleft at the base of the mountains in the south and gradually becomes shallower and less distinct to the north and west. Layered rocks crop out in its walls at several places. Near the landing site, the width of the rille is 1.5 km and its depth is more than 300 m. - M.C.M.
AS15-85-11451 (H)
FIGURE 203. - Astronaut James Irwin and the Apollo 15 rover are perched here above the rim of Hadley Rille. This scene looks northwestward down the rille from the flank of St. George crater (the largest crater in fig. 204). The astronauts discovered that layered basalts crop out in the upper walls of the rille. Talus blocks line most of the walls and the floor of the rille. The rille apparently was once narrower and deeper but has widened by backwasting of the rims. - K.A.H.
AS15-11720 (H)
FIGURE 204. - Here is how Hadley Rille first appeared to the Apollo 15 astronauts in the LM. They landed in the right foreground. The large subdued crater in the foreground, St. George, is on a mountain to which astronauts drove. Hadley Rille is mainly in the mare basalts of Palus Putredinus. The rille cuts against older mountains in the foreground and at the upper right. Along the top (north) of this photo the rille is discontinuous and is similar in many respects to a partly collapsed lava tube, although it is much bigger than any terrestrial counterpart. The rille is 1.5 km wide and over 300 m deep. It is thought to be a giant conduit that carried lava from an eruptive vent far south of this scene. Topographic information obtained from the Apollo 15 photographs supports this possibility; however, many puzzles about the rille remain. - K.A.H.
AS15-9432 (P)
FIGURE 205. - This closeup of Hadley Rille was taken by the Apollo 15 panoramic camera. This part of the rille appears along the left side (south) of figure 204 under different lighting conditions. Rocky outcrops along the top of the rille walls appear to be cut by fractures, called joints. The blocks that have rolled to the bottom of the rille are huge, up to tens of meters across. - K.A.H.
AS15-9816 (P)
FIGURE 206. - This part of Hadley Rille is far south of the landing site. Lines of outcrops of mare basalt in the upper rille walls suggest thick layering in the basalt. Notice how the edges of the rille stay parallel to each other. One origin suggested for the rille is that it is a graben or fault valley. In its present form, however, the rille could not have formed as a fracture because the sides will not fit back together. - K.A.H.
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FIGURE 207. - The large (approximately 100 km) crater Posidonius is filled with mare lava to a level higher than the surrounding surface of Mare Serenitatis. The most interesting and perplexing feature of this crater is Rima Posidonius II-the highly sinuous rille that follows a devious course from the north rim of the crater at upper right (outside the photograph) through the breach in the crater rim at center. The rille is topographically controlled in part, hugging the juncture between lava and crater material. Erosion by some sort of fluid may have formed the rille; material appears to have been removed from it. An alternative explanation might be that the feature represents a drained and collapsed lava tube. The fluid involved probably was emitted from the craterlike depression at the head of the rille in the crater's north wall. If the rille is assumed to be contemporaneous with the lava filling, a lava of low viscosity would seem to be required to explain the channel's high sinuosity. - C.A.H.
Straight Rimae
![Figure 208. Location of photographs of straight rilles; numbers correspond to figure numbers. [Base map courtesy of the National Geographic Society]](./images/p198.jpg)
Figure 208. Location of photographs of straight rilles; numbers correspond to figure numbers. [Base map courtesy of the National Geographic Society]
In many places the lunar surface is broken and a portion is downdropped, forming trenchlike features known as straight rimae or rilles. Some of these rilles are large enough (tens of kilometers across) to be visible on Earth based photographs; others are so small (a few meters across) that they are barely visible on the highest resolution orbital pictures.
Some of these trenches cut across the surrounding plains, uplands, and craters and may record preferred directions of breakage of the lunar crust caused by internal stresses (the so called lunar grid). Others ring crater floors and may be related to uplift of the floor caused by crustal readjustment after impact. A few contain low rimmed dark halo craters that are interpreted to be volcanic vents. Many trenches are curvilinear; some appear to be transitional between straight rilles and sinuous rilles. - H.M.
AS8-13-2225 (H)
FIGURE 209. - This oblique view looks westward over the crater Goclenius (large crater in the foreground) at the western edge of Mare Fecunditatus. The area is typical of the edge of shallow mare basins in that ejecta deposits and interiors of older craters along the margins are partially flooded by mare material. The pitted portions of the intercrater areas in the background are crater deposits that have not been flooded by the smoother mare material. Both the craters and maria are cut by linear rilles (Rime Goclenius I and II); rilles are also seen along other shallow mare basin margins. In this picture, several linear rilles, each about 1 km wide, can be seen crossing from right to left across the mare into the floor of Goclenius. The rilles are thought to be similar to fault bounded troughs on Earth (grabens) and may have originated when the central portion of the mare basin settled. - J.W.H.
AS15-93-12641 (H)
FIGURE 210. - This picture of the interior of the crater Humboldt (approximately 200 km in diameter) was taken looking southward. The terraced crater wall lies in the background, and the central peaks are visible in the lower right hand portion of the picture. The crater floor is typical of those that exhibit both radial and concentric cracks, or fissures. The fissures and cracks appear to be related to the uplift of the crater floor subsequent to the formation of the crater. The deformation may be related to upwelling of portions of the crust in an attempt to reach isostatic equilibrium, or it may be coincident with intrusion of lavas below the crater floor. Small patches of dark mare material can be seen along the edge of the crater floor in the lower left and middle right hand portions of the picture. - J.W.H.
AS16-2478 (M)
FIGURE 211. - This oblique metric photo shows part of the lunar highlands where the ancient crust is saturated with large craters. Portrayed here are the crater Alphonsus (middle ground) and the ancient crater Ptolemaeus (foreground). The floor of Alphonsus is broken by faults that form a polygon roughly parallel to the walls. Dark halo craters lie along these faults. The rims of the dark halo craters fill in the fault troughs. This relationship indicates that the craters must have been formed by material ejected from the central vents rather than by collapse of material into the cracks. However, unlike impact craters with their hummocky ejecta and lines of secondary craters, the smooth rimmed deposits have been interpreted as finegrained volcanic ejecta.
Lunar transient events have been observed many times in the crater Alphonsus. Red glows have been documented and spectra have been recorded by Kozyrev (1971) that apparently confirm the existence of gaseous emissions. These events are thought to be related to orbital parameters; when gravitational stresses are high, the crust shifts and gas escapes from the interior at regular intervals. If this is true, a low level of activity still continues to affect the lunar crust and interior.
General and detailed contour maps have been made (Wu et al., 1972) of Alphonsus using metric and panoramic photography obtained by Apollo 16. (See figs. 212 to 215. Fig. 212 is outlined in this figure, and fig. 213 is outlined in fig. 212.) - H.M.
FIGURE 212. - This map, compiled of the floor of Alphonsus at a scale of 1:300000 shows an array of faults, volcanic centers, and impact craters. The smooth flanks and youthful appearance of the central peak are apparently the result of the downslope movement of fragmental material that has filled in any impact craters as fast as they formed. - H.M.
FIGURE 213. - This map, compiled at a scale of 1:40 000, shows about 10 times the detail of the floor and crater shapes seen in figure 212 (see outlined area). The locations of profiles of two dark halo volcanic craters (A A' and B B') are shown on the map. - H.M.
FIGURE 214. - These sketches of the crater profiles of figure 213 contrast markedly with profiles of young impact craters. - H.M.
FIGURE 215. - A sketch geologic map of the area of figure 213 was done by Richard Hereford of the U.S. Geological Survey. It shows the distribution of the geologic units by type and age. The ancient crater walls, central peak and ridge, and varied floor deposits are displayed. - H.M.
AS16-4656 (P)
FIGURE 216. - This oblique view of part of the flat floor of the ancient crater Alphonsus shows faults that break the floor along straight line fractures. Spaced at irregular intervals along some of the rilles are dark halo craters that have broad low rims. The first detailed pictures of these craters were taken by Ranger 9 in 1964. This photograph confirms the detail in the Ranger picture and confirms the hypothesis that the dark halo craters are volcanic in origin. Their constructional rims indicate that they cannot have formed by collapse of material running back into the fault zone. The smooth rim deposits indicate that fine grained material was ejected uniformly from the volcanic vent. These deposits differ markedly from the ejecta patterns around bright, young impact craters. - H.M.
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FIGURE 217. - Rima Ariadaeus is a fine example of a straight rille. Ariadaeus Rille is over 300 km in length; a portion of the central section of the rille about 120 km in length is pictured here. A linear section of the crust is dropped down along parallel faults or breaks in the crust to form a graben or fault trough. The ridges crossing the trough and the surrounding plains units have been offset by the trough, proving that they are older than the faults. Some craters are cut off by the faults and are, therefore, older. Other craters lie on the wall of the trough and are younger than the faulting. The faulting must be relatively young because so few craters appear to be younger than the faults, and because the edges of the trough appear to be crisp and little affected by slumping and other mass wasting.
There is a gradation between straight rilles, gently curving rilles, and sinuous rilles modified by volcanic flows. This example shows no trace of associated volcanism; it is, therefore, considered to be the end member of the sequence, where only pure faulting is involved. - H.M.
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FIGURE 218. - Rima Fresnel I (1) approximately parallels the Apennine Mountain front (2) along the eastern margin of the Imbrium basin. The rille and its several branches at the northern end fall into the class of linear rilles that are usually attributed to a ten signal structural regime causing the formation of grabens. Faults in the highlands (3), parallel to some of the rilles, support the contention of structural origin for the rilles. Some of the curvilinear segments of the rilles, however, suggest that they may have served locally as lava channels.
The rille traverses a plain that was called the Apennine Bench (4) and interpreted to be formed of older mare lavas (Hackman, 1966). The inside of the rille is flooded by younger mare lavas (Carr, Howard, and El Baz, 1971) (5), which also embay its northern truncated terminus (6). Some younger mare lavas may have poured over the bench material and buried a crater (7). Old lava benches on the inside margin of basins are a common feature on the Moon. They are usually crossed by old rilles and embayed and partially buried by younger lavas. - B.K.L.
AS15-13345 (H)
FIGURE 219. - A low Sun angle view of the western part of the Aristarchus Plateau shows three sharp rimmed linear depressions that occur in the higher unit and terminate at the edge of the plateau. Their floors are filled with mare material that is similar to the surrounding materials of Oceanus Procellarum (lower right corner). The chain of craters in the mare of Oceanus Procellarum is believed to be secondaries from the crater Aristarchus (outside of the view). As described by Alfred M. Worden, the Apollo 15 CMP, the mare materials in this region display a brownish tint, as compared to the metal gray materials of Mare Imbrium farther east. - F.E.- B.
AS17-2313 (P)
FIGURE 220. - Some of the freshest and youngest fractures yet found are in the Littrow area. The sharpest gash in this view runs along the right side of one of the much older Littrow rilles. The surface material in this area appears to be soft; blocks are very rare and most craters appear subdued. Therefore, it seems likely that the fresh fractures here, which are smaller than those in the preceding figures, will not survive long before they are obliterated by mass wasting. - K.A.H.
AS17-2313 (P)
FIGURE 221. - This detailed view of a mare surface near the eastern edge of Mare Serenitatis, just west of the Apollo 17 landing site, shows the numerous small grooves. They are unusual features that have not been observed in such large numbers elsewhere. They are here developed in a thick accumulation of regolith overlying an ancient mare basalt. At first glance, resembling chains of secondary impact craters, they are more logically interpreted as structural features. A likely explanation is that they are the result of drainage of unconsolidated regolith into openings caused by fissuring in the consolidated bedrock. At some localities, drainage did not occur at a uniform rate along the fissure but was concentrated at certain points, resulting in a series of unequally spaced, pitlike depressions (arrows) along the groove. - B.K.L.