AS15-9355(P)

Figure 223

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Although this is a rather bizarre example, sharp, deep, spearhead-shaped depressions form the heads of many lunar rilles. The best-known example is the depression at the head of Rima Hadley near the Apollo 15 landing site. They probably develop by collapse into voids left by lava tubes and shallow magma chambers. This is Aratus CA in western Mare Serenitatis. The branch heading northeastward to the right edge of the stereogram, which terminates a few hundred meters beyond the picture, is the rille-a very short one. The fresh-appearing wrinkle or mare ridge that crosses a small (300 m) young crater at A (see enlargement) may postdate the spearhead depression as its extension appears to protrude along the wall at B and the floor at C. It is part of an older, broader, more subdued ridge that has been disrupted by the collapse depression. Incipient collapse along the younger ridge is visible at D. Note how the mare surface truncates a resistant layer (presumably a basalt flow) in the wall of the depression at E. This suggests either the tilting of older mare basalts eastward prior to the latest extrusion of basalt or, more likely, that the resistant layer is lenticular. -D.H.S.