The ages of the lunar seas.

Two ages are attributed to each circular lunar sea, the age of formation of the circular basin, probably by impact, and the age of the filling of the basin by lava-like effusions.Each lunar-sea surface displays a range of geomorphic indices. This is interpreted as being due to the presence of effusions of different ages on the surface of each sea. The landing sites of Apollo 11 and 12 have geomorphic indices of value 10.3 and 8.4. The radiometric ages of the rocks are, respectively, approximately 3.65 x 10(9) and 3.36 x 10(9) years. The range of geomorphic indices on sea surfaces is from less than 5 to more than 14, which indicates that the two Apollo crafts landed on surfaces formed about in the middle of the total span of time of sea-surface formation. Using four possible relationships between geomorphic index and age, I conclude that the age of the youngest effusions is less than 3 x 10(9) years and the age of the oldest effusions is more than 4 x 10(9) years. The results of the analyses of the Russian Luna 16 samples, although preliminary, fit in this interpretation.