AS16-4530(P)

Figure 136

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Linear features of external and internal origin are contrasted in this area of southernmost Mare Tranquillitatis. The northtrending line of overlapping, very irregular craters along the east edge of the picture is clearly a chain of secondary impact craters. Its trend is radial to Theophilus, a large crater of early Copernican age that lies about 105 km south of this area. The flaring shapes of some of the craters and their state of preservation also suggest that Theophilus is the primary crater. The narrow, straight rifle or graben that extends westward across the picture is clearly of internal origin. It formed when tensional forces ruptured the crust, causing the floor of the rifle to subside along faults. As is discussed later, straight rifles are commonly the sites of volcanic cones or of blankets of volcanic ejecta; however, there are no signs of volcanism here that can be related to this rifle.   -M.J.G.