The text describes the major tectonic, volcanic, and fluvial episodes that have contributed to Mars' evolutionary history. The base used for compilation reflects improved imagery and photogrammetry and updated nomenclature. Relative ages of the units were established by stratigraphic and structural relations and by crater size-frequency distributions. Also, several units have been assigned todifferent time-stratigraphic systems. For example, the mottled plains material, formerly classified as Noachian in age, has been placed in the upper part of the Hesperian System on the basis of crater counts and stratigraphic position, and it is now considered to form part of the Vastitas Borealis Formation.
Line geometry is topologically clean.
During shaded relief portrayal, features on these bases were used to position details taken from Viking Orbiter pictures. The shading is not generalized and accurately represents the character of surface features. Shaded relief analysis and portrayal were made by Barbara J. Hall. Information has been obtained from sources believed to be reliable, but its accuracy and completeness and the opinions based thereon, are not guaranteed.