Vol 50, No 5, May 2013 - BAA Lunar Section

west of Riccioli G (Fig. 3) shows theprominent ridge mentioned above,marking the southern edge. However,there is no sign of a correspondingnorthern edge to the ‘valley’.However, a plot taken at the midpointof the Miyamori feature,southwards from the crater LohrmannN, does suggest the profile of a valley(Fig. 4). It is not completelyconvincing, but it does leave open thepossibility that the ‘Miyamori Valley’is indeed a graben-like feature,perhaps heavily modified by ejectafrom the Orientale basin.Fig. 3 (top right). N-S elevationplot west of Riccioli G.Fig. 4 (right). Elevation plot southfrom Lohrmann N.Fig. 3.Fig. 4.‘Larrieu’s Dam’There is no such ambiguity over‘Larrieu’s Dam’, properly known asPolybius K – it is certainly not a dam!However, it is an oddity – a peculiar polygonal crater with astraight, sharply defined, dam-like wall abutting what appearsto be an older, much-degraded enclosure to the north-west(Fig. 5). The appearance of a dam is indeed very striking whenthe lighting conditions are just right and Polybius K is filledwith shadow. It looks very much like the sort of straightboundary between contiguous craters that is produced by thesimultaneous impact of two bodies. However, it isimmediately obvious that Polybius K is much younger than thedegraded enclosure on the other side of the ‘dam’, so there canbe no question of simultaneous impact. It is also much deeper,as the elevation profile in Fig. 6 (measured from Polybius J toPolybius K) clearly shows. Nigel Longshaw has argued thatthere is evidence for a small-scale ‘grid’ pattern in this area,possibly attributable to fault lines, and that the deformation ofPolybius K might be the result of that. Certainly there are otherexamples of straight ridges nearby, such as that separating theadjacent craters Polybius F andPolybius C, but it is arguable whetherthese constitute a grid system and thequestion remains open.Fig. 5. Polybius region from Lunar Orbiter IV,Frame 084.Fig. 6. NW-SE plot from Polybius J to Polybius K.One thing is certain: both the‘Miyamori Valley’ and ‘Larrieu’sDam’ are worthy of furtherinvestigation, not only at thetelescope but also using the othertools we now have at our disposal.Bill LeatherbarrowDirector, BAA Lunar SectionBAA Lunar Section Circular Vol. 50 No. 5 May 2013 3