On the present map, units are assigned to the Nectarian by comparison of crater morphologies. The base of the informally designated upper Imbrian series is defined by materials of the Orientale basin. Identification of geologic units is based mostly on Orbiter photographs, supplemented in places by Zond 7 and Mariner 10 images. Thus identification of geologic units on the far side is tentative, and location of contacts approximate. The low sun illumination angle on all pictures near the pole also complicates recognition of units.
Line geometry is topologically clean.
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