Moon in the middle of the Imbrian Period. This image of the Moon combines data obtained using remote-sensing equipment, with geologic and artistic interpretations to show the Moon as it looked 4 billion years ago (middle of the Imbrian Period). This is part of a sequence showing the Moon as it looked 3.3 billion years ago (end of the Imbrian Period, C004/3886) and today (C004/3885). The Imbrian Period was when the Imbrium and Orientale impact basins formed, before which (as seen here) the Moon lacked large lava plains (maria). This work was done by Don E. Wilhelms and Donald E. Davis, and published by the US Geological Survey in 1971.