206
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AS16-4656 (P)FIGURE 216 [left].-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.
[207] FIGURE 217 [right].-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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AS10-314645 (H)
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AS15-9368 (P)FIGURE 218 [left].-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.
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AS15-13345 (H)[208] FIGURE 219 [left].-A low Sun angle view of the western part of the Aristarchus Plateau shows three sharprimmed 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.
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