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.
Photographs courtesy of National Aeronautics and Space Administration. Information has been obtained from sources believed to be reliable, but its accuracy and completeness and the opinions based thereon, are not guaranteed. As every effort is made to provide accurate information in this database, USGS would appreciate it if users could call to our attention any errors that may occur by communicating with the contact person.