AS15-2295(M)

Figure 65

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This photomosaic of an area of relatively young mare lavas in southwestern Mare Imbrium shows a complex of overlapping lava flows. The complex has been traced to its apparent source northeast of the mountain mass Mons Euler (formerly called "Euler ß") where the approximate location of a fissure has been deduced by detailed geologic mapping  (Schaber, 1973). Individual flows are recognizable in this low-Sun (about 4°) picture as elongate lobes bounded by steep scarps. They are shown on the accompany-
ing  sketch map . Many contain one or more small rilles that are interpreted as flow channels. As individual flows are traced southward toward their source, they narrow and converge or terminate in the vicinity of the postulated fissure. South of the fissure distinct flow scarps are absent. A row of dark volcanic cinder cones along the southeast side of Mons Euler is alined with the postulated fissure, further strengthening the idea that this is an area of eruption. It is likely that the fissure is covered by its own lavas. The succession of geologic events in this area is easily decipherable. Secondary impact craters (as at S) from the large crater Copernicus overlie the lavas; hence the lavas are pre-Copernican, or Eratosthenian, in age. In turn the lavas have inundated part of the ejecta from the crater Euler; therefore, Euler is also pre-Copernican. Before Apollo pictures became available, it had been mapped as a Copernican crater. At the present time, only the lavas can be assigned an absolute age. About 2.5 billion years old (Schaber,  1973), they are older than all but a very few rock outcroppings in the entire United States. On the other hand, they are much younger than most, if not all, the samples collected on the Moon during the six lunar landings. Solid line shows position of lower left corner of figure 66.   -G.G.S.