Until recently, scientists since Galileo's time who studied the Moon were astronomers, not geologists. It wasn't until the late 1950s and early 1960s that Eugene Shoemaker and Robert Hackman of the U.S. Geological Survey (USGS) developed a lunar stratigraphic sequence based on telescopic and photographic studies of the spatial relations among lunar landforms. They chose the southeast quadrant of Mare Imbrium to determine sequences of events by mapping the overlap of deposits created by each event.
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