Each near-side lunar mare displays a range of geomorphic indices on its surface. This is interpreted to be due to the presence of effusions of different ages on the surface of each mare. This interpretation contradicts the theory that mare surfaces were formed contemporaneously. The landing sites of Apollo 11 and 12 have geomorphic indices of 10.3 and 8.4, respectively; the radiometric ages of the rocks are approximately 3.6 × 109 and 3.4 × 109 yrs. Geomorphic indices on mare surfaces range from less than 5 to more than 14, indicating that the two landing sites were formed approximately in the middle of the time span required for mare surface formation. Using four possible relationships between geomorphic index and age, the length of time between the beginning and end of mare effusion is inferred to be at least 1 × 109 yrs, and possibly twice as much. Some evidence is presented that effusive activity may have reached two maxima.

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