The probability is very great that the geology of the decade of Apollo will become one of the fundamental turning points in the history of all science. For the first time the scientists of the Earth have been presented with the opportunity to interpret their home planet through the direct investigations of another. Mankind can be proud and take heart in this fact.
The interpretive evolution of the Moon can be divided now into seven major stages beginning sometime near the end of the formation of the solar system. These stages and their approximate durations in time are as follows:
1. The Beginning—4.6 billion years ago
2. The Melted She11—4.6–4.4 billion years ago
3. The Cratered Highlands—4.4–4.1 billion years ago
4. The Large Basins—4.1–3 9 billion years ago
5. The Light-colored Plains—3.9–3.8 billion years ago
6. The Basaltic Maria—3.8–3.0 (?) billion years ago
7. The Quiet Crust—3.0 (?) billion years ago to the present
The Apollo explorations that permit us to study these stages of evolution each have contributed in progressive and significant ways. Through them we now can look with new insight into the early differentiation of the Earth, the nature of the Earth’s protocrust, the influence of the formation of large impact basins in that crust, the effects of early partial melting of the protomantle and possibly the earliest stages of the breakup of the protocrust into continents and ocean basins.